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High pressure x-ray crystallography with the diamond cell at NIST/NBS.

Scientists in the Crystallography Section at NIST/NBS made several outstanding contributions which greatly promoted the development and advancement of high pressure x-ray crystallography X-ray crystallography, the study of crystal structures through X-ray diffraction techniques. When an X-ray beam bombards a crystalline lattice in a given orientation, the beam is scattered in a definite manner characterized by the atomic structure of the lattice.  during the second-half of the 20th century. These milestone achievements or "firsts" included: (1) the invention of the lever-arm type diamond anvil cell A diamond anvil cell (DAC) is a device used by physicists to exert extreme pressures on a material. It consists of two opposing cone-shaped diamonds squeezed together. The resultant high pressures — in excess of a million atmospheres — are produced when force is applied  (DAC See D/A converter and discretionary access control.

DAC - Digital to Analog Converter
) in 1958; (2) the development of DAC technology for powder x-ray diffraction at high pressure in 1960; (3) the introduction of DAC methodology for single crystal x-ray diffraction at high pressure in 1964; (4) the invention of the optical fluorescence fluorescence (flrĕs`əns), luminescence in which light of a visible color is emitted from a substance under stimulation or excitation by light or other forms of electromagnetic  ruby method of pressure measurement in 1971; and (5) the discovery of hydrostatic hy·dro·stat·ic or hy·dro·stat·i·cal
Of or relating to fluids at rest or under pressure.


pertaining to a liquid in a state of equilibrium or the pressure exerted by a stationary fluid.
 pressure-transmitting media useful to unprecedented pressures for that time.

These achievements provided the spark that ignited the explosion of activity in high pressure research that occurred in laboratories throughout the world during the latter part of the 20th century. It is still going on, unabated un·a·bat·ed  
Sustaining an original intensity or maintaining full force with no decrease: an unabated windstorm; a battle fought with unabated violence.
, today. An estimated 5000 DACs were built during the last 40 years.

Key words: crystallography; diamond anvil cell (DAC); high pressure; hydrostaticity; polycrystalline Adj. 1. polycrystalline - composed of aggregates of crystals; "polycrystalline metals"
crystalline - consisting of or containing or of the nature of crystals; "granite is crystalline"
; ruby pressure measurement; single crystal.

1. Introduction

Two closely related world-class scientific and technological achievements occurred in the field of high pressure research at the National Bureau of Standards National Bureau of Standards: see National Institute of Standards and Technology.

National Bureau of Standards - National Institute of Standards and Technology
 (NBS (National Bureau of Standards) See NIST.

NBS - National Bureau of Standards: part of the US Department of Commerce, now NIST.
), now the National Institute of Standards and Technology National Institute of Standards and Technology, governmental agency within the U.S. Dept. of Commerce with the mission of "working with industry to develop and apply technology, measurements, and standards" in the national interest.  (NIST (National Institute of Standards & Technology, Washington, DC, The standards-defining agency of the U.S. government, formerly the National Bureau of Standards. It is one of three agencies that fall under the Technology Administration ( ), during the second-half of the 20th Century: (1) the invention of the lever-arm type diamond anvil anvil

Iron block on which metal is placed for shaping, originally by hand with a hammer. The blacksmith's anvil is usually of wrought iron (sometimes of cast iron), with a smooth working surface of hardened steel.
 high pressure cell in 1958, and (2) the invention of the ruby fluorescence method of pressure measurement in 1971. These two developments, probably more than any others, stimulated the profound advancement of high pressure research that evolved during the last 30 years both at NBS and other scientific laboratories throughout the world.

The work leading to these accomplishments took place in the Constitution and Microstructure mi·cro·struc·ture  
The structure of an organism or object as revealed through microscopic examination.


a structure on a microscopic scale, such as that of a metal or a cell
 Section (CMS (1) See content management system and color management system.

(2) (Conversational Monitor System) Software that provides interactive communications for IBM's VM operating system.
), renamed the Crystallography Section (CS) in 1964. Soon after the invention of the lever-arm type diamond anvil cell (DAC), a high pressure research program utilizing the DAC was initiated in CMS and was active for almost 37 years. During those years, milestones or "firsts" were made in several areas including: the development of DAC technology for obtaining x-ray powder diffraction Powder diffraction is a scientific technique using X-Ray or neutron diffraction on powder or microcrystalline samples for structural characterization of materials.

Ideally, every possible crystalline orientation is represented equally in a powdered sample.
 data at high pressure; use of a metal gasket to encapsulate en·cap·su·late
1. To form a capsule or sheath around.

2. To become encapsulated.

 liquids and other materials under pressure between the anvils of a DAC; the development of the methodology for determining crystal structures at high pressure in the DAC from single crystal x-ray diffraction intensity data (first application: growth and structure of a single crystal of ice VI); the invention of the ruby fluorescence technique for pressure measurement; the generation of ultrahigh ul·tra·high  
Exceedingly high: an ultrahigh vacuum. 
 pressures through improvements in the design of the original NBS DAC; the measurement of hydrostatic limits in pressure-transmitting media utilizing the ruby method; the application of the DAC to Raman spectroscopy Raman spectroscopy is a spectroscopic technique used in condensed matter physics and chemistry to study vibrational, rotational, and other low-frequency modes in a system.[1] ; and the measurement of the viscosity of liquids at high pressures in the DAC by the classical Stokes method.

In the spirit of the NIST Centenary and this Special Issue to honor it, the content of this article is limited to a description of the role CMS and CS played in influencing the development and advancement of high pressure x-ray crystallography during the early years of its infancy, from 1959 to 1975. The author acknowledges the many outstanding accomplishments with the DAC by other laboratories in the United States United States, officially United States of America, republic (2005 est. pop. 295,734,000), 3,539,227 sq mi (9,166,598 sq km), North America. The United States is the world's third largest country in population and the fourth largest country in area.  and abroad, including the attainment of megabar pressures and the application of the DAC to many scientific measurement techniques, too numerous to mention here. Certainly, all provided additional impetus for the further advancement of high pressure research with the DAC. To maintain focus within the framework and scope of this Special Issue, this report emphasizes the seminal nature of the work accomplished at NBS in the years immediately following the invention of the DAC when work with it in other laboratories was just beginning.

2. The Invention of the Diamond Anvil Cell (DAC)

The invention of the DAC took place in CMS (then headed by H. F. McMurdie) of the Mineral Products Division (I. C. Schoonover, Chief) in 1958. The NBS laboratories were then located in Washington, DC. From the beginning, the DAC was designed as a leverarm device with a 180[degrees] optical transmission path, and was conceived for use in infrared (ir) spectroscopy at high pressure. Indeed, its first application was to obtain ir absorption spectra for the first time on powders squeezed between two opposed diamond anvils (1).

A schematic cross section diagram of the original design of the instrument is shown in Fig. 1. The device contains two gem-cut type H diamonds, each about 1/3 carat CARAT, weights. A carat is a weight equal to three and one-sixth grains, in diamonds, and the like. Jac. L. Dict. See Weight.  in size, comprising the "squeezer" anvils (inset). The culet cu·let  
1. The small flat face at the bottom of a gem cut as a brilliant.

2. Armor consisting of overlapping plates used to protect the buttocks.
 of each diamond contains a small flat, parallel to the table. The powdered specimen is placed between these opposed flats, having an area of about 0.13 [mm.sup.2]. Each diamond, A, is seated on its tabular face which rests in a close-fitting recess in a stainless steel stainless steel: see steel.
stainless steel

Any of a family of alloy steels usually containing 10–30% chromium. The presence of chromium, together with low carbon content, gives remarkable resistance to corrosion and heat.
 piston, B. Each piston has a conical conical /con·i·cal/ (kon´i-k'l) cone-shaped.

con·i·cal or con·ic
Of, relating to, or shaped like a cone.
 hole to permit acceptance of the maximum flux from a convergent cone of radiation passing through both pistons, the diamonds, and the powdered specimen pressed between them. The pistons are free to slide in a cylindrical bearing, C, that screws into a large block of steel carrying the pressure-generating mechanism. The entablature entablature (ĕntăb`ləchr), the entire unit of horizontal members above the columns or pilasters in classical architecture—Greek, Roman or Renaissance.  piston is supported by a threaded thrust-bearing ring, D. The presser-plate. E, connected to a lever, pivoted in the block and activated by a calibrated cal·i·brate  
tr.v. cal·i·brat·ed, cal·i·brat·ing, cal·i·brates
1. To check, adjust, or determine by comparison with a standard (the graduations of a quantitative measuring instrument):
 sprin g, F, bearing against the upper end of the lever, pushes against the other piston. The spring is compressed by the manually operated screw, G. Because of the small sample size, the device had to be used with a set of beam condensing con·dense  
v. con·densed, con·dens·ing, con·dens·es
1. To reduce the volume or compass of.

2. To make more concise; abridge or shorten.

3. Physics
 parabolic par·a·bol·ic   also par·a·bol·i·cal
1. Of or similar to a parable.

2. Of or having the form of a parabola or paraboloid.
 mirrors in order to obtain sufficient ir energy throughput.

Some of the very first materials studied were [NaNO.sub.3], [KNO KNO Knobloch Syndrome .sub.3], [AgNO.sub.3], ferrocene Ferrocene is the chemical compound with the formula Fe(C5H5)2. Ferrocene is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. , ice, and [CaCO.sub.3]. These were pioneering experiments because they demonstrated for the first time that pressure-dependent shifts and changes in intensity of infrared absorption bands Noun 1. absorption band - a dark band in the spectrum of white light that has been transmitted through a substance that exhibits absorption at selective wavelengths
optical phenomenon - a physical phenomenon related to or involving light
 could be measured. The absorbance absorbance /ab·sor·bance/ (-sor´bans)
1. in analytical chemistry, a measure of the light that a solution does not transmit compared to a pure solution. Symbol .

 of most bands was observed to weaken with increasing pressure. In the case of calcite calcite (kăl`sīt), very widely distributed mineral, commonly white or colorless, but appearing in a great variety of colors owing to impurities.  (CaCO3), the pressure-shifts were related quantitatively to a decrease in the C-O C-O Coherent Orthogonal  bond length, a result compatible with the reported compressibility of calcite perpendicular to the trigonal trigonal /tri·go·nal/ (tri´go-nal)
1. triangular.

2. pertaining to a trigone.


1. triangular.

2. pertaining to a trigone.
 axis. In addition, large changes in spectra resulting from pressure-induced phase transformations were observed. Results such as these, opened up a whole new area of study with the DAC for infrared spectroscopy.

The invention was not the result of formally planned research. Rather, it was the result of an evolving interaction among three NBS scientists, C. E. Weir, A. Van Valkenburg and E. N. Bunting bunting, common name for small, plump birds of the family Fringillidae (finch family). Among the American buntings are the indigo bunting, in which the summer plumage of the male reflects sunlight as a rich, metallic blue; the painted bunting, or nonpareil ( , and a consultant from the University of Maryland University of Maryland can refer to:
  • University of Maryland, College Park, a research-extensive and flagship university; when the term "University of Maryland" is used without any qualification, it generally refers to this school
, E. R. Lippincott--four scientists with different interests, different research activities and different backgrounds, who interacted on a professional level and recognized an opportunity to make a significant contribution to scientific research. Together, they produced this remarkable instrument we call the diamond anvil cell.

In the early stages of the DAC's development, funds to support it directly were not available. However, NBS scientists had the freedom to pursue nonfunded personal interest research, provided it did not interfere with official duties and the work was related to goals of their research project. NIST Scientists still have the freedom to do this today. In the very early stages of the DAC developmental work, these four scientists used their own initiative and ingenuity to get things accomplished. Thus, the pressure cell was handmade by Weir, who fabricated fab·ri·cate  
tr.v. fab·ri·cat·ed, fab·ri·cat·ing, fab·ri·cates
1. To make; create.

2. To construct by combining or assembling diverse, typically standardized parts:
 it utilizing only equipment that was available in his laboratory, e.g., a lathe lathe (lāth), machine tool for holding and turning metal, wood, plastic, or other material against a cutting tool to form a cylindrical product or part. It also drills, bores, polishes, grinds, makes threads, and performs other operations. , drill press, hack saw, soldering gun A soldering gun is a tool for producing a weld in metals using lead-tin-based solder to achieve a highly conductive contact. The tool uses a pistol shape, and has a trigger style switch so it can be easily operated with one hand. , threading tools, files, and a high speed grinding wheel to polish down the culets of the brilliant-cut gem diamonds to form anvil faces. The gem diamonds were obtained by Van Valkenburg from the General Services Administration The General Services Administration (GSA) was established by section 101 of the Federal Property and Administrative Services Act of 1949 (40 U.S.C.A. § 751). The GSA sets policy for and manages government property and records.  (GSA (1) (Global mobile Suppliers Association, Sawbridgeworth, U.K., A membership organization of suppliers of GSM products and services. Its goal is to promote GSM as the worldwide mobile communications standard. See GSM Association and GSM. ), the custodian of diamonds confiscated con·fis·cate  
tr.v. con·fis·cat·ed, con·fis·cat·ing, con·fis·cates
1. To seize (private property) for the public treasury.

2. To seize by or as if by authority. See Synonyms at appropriate.

 from smugglers by customs agents. At that time, GSA contraband contraband, in international law, goods necessary or useful in the prosecution of war that a belligerent may lawfully seize from a neutral who is attempting to deliver them to the enemy.  diamonds we re available to other government agencies at no cost, if their use in supporting government programs could be justified. Without these gratis GRATIS. Without reward or consideration.
     2. When a bailee undertakes to perform some act or work gratis, he is answerable for his gross negligence, if any loss should be sustained in consequence of it; but a distinction exists between non-feasance and
 gem diamonds, the DAC probably would not have been perfected as rapidly as it was, because, during the early stages of its development, many diamond anvils were destroyed in the testing process. To replace them commercially would have been prohibitively expensive, an expenditure not easily justified.

A photograph of the original NBS instrument, small enough to fit in the palm of one's hand, is shown in Fig. 2. Close inspection confirms that the device was crudely made utilizing simple machine tools and fabrication fabrication (fab´rikā´shn),
n the construction or making of a restoration.
 procedures. Yet, the basic design shown here is the prototype for most DACs with 180[degrees] optical transmission even to the present day. It was truly an ingenious design, dictated by the limited fabrication equipment available to them. Indeed, what were considered to be great hindrances to its fabrication, turned out in the end to be a great boon. Because of its miniature size and simplicity in design and use, it was quickly adopted by spectroscopists worldwide. Not only was it used for its intended purpose, i.e., to study the effects of pressure on the infrared spectra of materials subjected to high pressures, but it also had a completely unexpected application in crime laboratories as a microsample holder for traces of evidence found at the scene of a crime. It is still used today for this purpose.

In what appears to be a case of almost simultaneous discovery, because neither group was aware of the other's work, a similar device was built by J. C. Jamieson and coworkers at the University of Chicago for x-ray powder diffraction studies (2). Their device, described in the literature 3 months after a detailed description of the NBS instrument had been published, also made use of diamonds in a Bridgman opposed anvil arrangement, but the fundamental difference between the two instruments was that the University of Chicago cell did not have 180[degrees] optical access and required a hydraulic press hydraulic press

Machine consisting of a cylinder fitted with a piston (see piston and cylinder) that uses liquid under pressure to exert a compressive force upon a stationary anvil or baseplate. The liquid is forced into the cylinder by a pump.
 to apply load to the anvils. It was designed specifically for use on a horizontal x-ray diffractometer A Diffractometer (Main Entry: dif·frac·tom·e·ter Pronunciation: di-"frak-'tä-m&-t&r Function: noun) is a measuring instrument for analyzing the structure of a usually crystalline substance from the scattering pattern produced when a beam of radiation or particles (as X rays or . The instrument did not enjoy widespread use because it lacked portability, ease of use, and adaptability to other scientific measurement techniques.

3. Crystallographic crys·tal·log·ra·phy  
The science of crystal structure and phenomena.

 Studies at High Pressures

Crystallographic studies at high pressure with the DAC began in 1960, just 2 years after it was first used in the laboratory for ir studies. The first advance was made in x-ray powder diffraction. Then, after the development of the gasket technique which permitted encapsulation (1) In object technology, the creation of self-contained modules that contain both the data and the processing. See object-oriented programming.

(2) The transmission of one network protocol within another.
 of liquids and crystals in liquids in the DAC, advances were made in single crystal x-ray diffraction. Both of these applications utilized film methods which were in prevalent use at that time. Also the DAC was easily adapted to the instrumentation available for film techniques. The powder and single crystal advances made at NBS were truly significant contributions to high pressure x-ray crystallography. Both demonstrated the power of the DAC in acquiring crystallographic information at high pressures. They served as the foundation for later work in both areas as advanced x-ray sources and detection technology came into being.

3.1 Powder Diffraction

In 1960, Weir had redesigned the lever-arm cell into a miniature hydraulically-loaded device for x-ray powder diffraction measurements. To demonstrate the new instruments capability, he spent the summer of that year in Prof. Leason H. Adams' x-ray laboratory at the University of California The University of California has a combined student body of more than 191,000 students, over 1,340,000 living alumni, and a combined systemwide and campus endowment of just over $7.3 billion (8th largest in the United States).  in Los Angeles Los Angeles (lôs ăn`jələs, lŏs, ăn`jəlēz'), city (1990 pop. 3,485,398), seat of Los Angeles co., S Calif.; inc. 1850. , where he succeeded in obtaining transmission powder patterns on a circular flat film. A schematic cross section diagram of his design of the x-ray powder camera for high pressure studies is shown in Fig. 3. The unit contains a hardened steel The term hardened steel is often used for a medium or high carbon steel that has been given the heat treatments of quenching followed by tempering. This is the most common state for finished articles such as tools and machine parts.  entablature (E) and a piston (H) actuated ac·tu·ate  
tr.v. ac·tu·at·ed, ac·tu·at·ing, ac·tu·ates
1. To put into motion or action; activate: electrical relays that actuate the elevator's movements.

 by hydraulic pressure generated by an external hand pump and introduced through a fitting (M). The hydraulic liquid, confined to the small volume between the rubber O-rings (J), exerts an axial axial /ax·i·al/ (ak´se-al) of or pertaining to the axis of a structure or part.

1. Relating to or characterized by an axis; axile.

 load on the shoulder of the piston (H). The entablature (E) is supported by the thrust washer (C) and the screw-cap (B). The entablature and the piston are both fitted with brilliant-cut gem-quality diamonds (F), approximately 0.15 carats. The diamonds are seated on their table faces in small close-fitting recesses. The piston diamond is seated in a small cylindrical steel insert (G) which can be tilted by three steel screws 120[degrees] apart (I) to permit parallel alignment of the opposed diamond faces. The x-ray beam x-ray beam,
n the spatial distribution of radiation emerging from a radiograph generator or source. The colloquial term for radiographic beam. See radiographic beam.
 enters through two collimating pinholes (K), 0.355 mm in diameter. Zirconium zirconium (zərkō`nēəm), metallic chemical element; symbol Zr; at. no. 40; at. wt. 91.22; m.p. about 1,852°C;; b.p. 4,377°C;; sp. gr. 6.5 at 20°C;; valence +2, +3, or +4.  filtered molybdenum molybdenum (məlĭb`dənəm) [Gr.,=leadlike], metallic chemical element; symbol Mo; at. no. 42; at. wt. 95.94; m.p. about 2,617°C;; b.p. about 4,612°C;; sp. gr. 10.22 at 20°C;; valence +2, +3, +4, +5, or +6.  radiation was used which traverses both diamonds and the film of powder compressed between the anvils. Diffraction rings from the powdered specimen diverge diverge - If a series of approximations to some value get progressively further from it then the series is said to diverge.

The reduction of some term under some evaluation strategy diverges if it does not reach a normal form after a finite number of reductions.
 through the diamond in the entablature and are recorded on the film (D), placed inside the cap and held securely between cap and thrust washer. The film is covered with a light protective paper (P) and a small beam stop (A) secures the film at the center. The entablature is machined out to form a conical surface conical surface: see cone.  with a 45[degrees] semivertex angle to permit a wide divergence of the diffraction rings. The sample-to-film distance was approximately 10 mm. Under the experimental cond itions used here diffraction rings with 2[theta Theta

A measure of the rate of decline in the value of an option due to the passage of time. Theta can also be referred to as the time decay on the value of an option. If everything is held constant, then the option will lose value as time moves closer to the maturity of the option.
] > 20[degrees] could not be recorded on the film.

On his return to NBS, Weir continued to work with the new powder camera, further exploring its potential uses. Shortly thereafter, I began a collaboration with him in that effort, and introduced several improvements in the design of the instrument which included: (1) a reduction in the x-ray beam divergence and collimation collimation /col·li·ma·tion/ (kol?i-ma´shun)
1. in microscopy, the process of making light rays parallel; the adjustment or aligning of optical axes.

 diameter to 0.15 mm to minimize the effect of pressure gradients In atmospheric sciences (meteorology, climatology and related fields), the pressure gradient (typically of air, more generally of any fluid) is a physical quantity that describes in which direction and at what rate the pressure changes the most rapidly around a particular location.  in the pressed sample; (2) an increase in the sample-to-film distance to include a range between 30 mm and 75 mm for improved accuracy in the measurement of diffraction ring diameters; (3) a corresponding increase in film-cassette diameter to 75 mm; (4) a modified entablature containing a wedge cutout cut·out  
1. Something cut out or intended to be cut out from something else.

2. Electricity A device that interrupts, bypasses, or disconnects a circuit or circuit element.

 to permit the recording of diffraction rings on film to 2[theta] [approximately equal to] 35[degrees]. The circular film cassette incorporated a split-field baffle permitting simultaneous exposure only of opposite quadrants of the film. A simple 45[degrees] rotation of the baffle permitted a second independent exposure on the same film. Because the thermal expansion thermal expansion

Increase in volume of a material as its temperature is increased, usually expressed as a fractional change in dimensions per unit temperature change.
 of the hydraulic fluid hydraulic fluid

toxic because of its high content of industrial triaryl phosphate.
 was sensitive to small changes in the ambient room temperature, the applied load on the anvils (sample pressure) varied significantly during the course of an experiment, often significantly broadening the width of the diffraction rings. To minimize this undesired temperature effect, water at constant temperature was circulated through a Cu tube coiled around the base of the instrument. These combined improvements produced a relatively large diameter, narrow width, well defined diffraction ring on film, ultimately yielding a more accurate measurement of its diameter. The fi rst report describing the x-ray powder camera along with results obtained with it, was published in 1962 (3). The instrument was used initially to study the effect of pressure on AgI, Bi, Tl, and KI, but later work included most of the alkali halides The alkali halides are the family of ionic compounds with simple chemical formula X+Y- or XY, where X is an alkali metal and Y is a halogen. One of the most well known of these is sodium chloride or common table salt.  and four rare earth elements “Rare earth” redirects here. For other uses, see Rare earth (disambiguation).

Rare earth elements and rare earth metals are a collection of sixteen chemical elements in the periodic table, namely scandium, yttrium, and fourteen of the fifteen lanthanoids
, La, Ce, Pr, and Nd. (4-6). Many of these materials were known to exhibit pressure-induced phase transitions, while others were thought to be good candidates for high pressure polymorphism polymorphism, of minerals, property of crystallizing in two or more distinct forms. Calcium carbonate is dimorphous (two forms), crystallizing as calcite or aragonite. Titanium dioxide is trimorphous; its three forms are brookite, anatase (or octahedrite), and rutile. . In many cases the crystallography of the high pressure phases was unknown and was the primary reason for studying them.

Typical examples of diffraction patterns diffraction pattern

The interference pattern that results when a wave or a series of waves undergoes diffraction, as when passed through a diffraction grating or the lattices of a crystal.
 recorded on film are shown in Figs. 4 and 5 for CsF and La, respectively. Both illustrate a pressure-induced phase transition. Cesium cesium (sē`zēəm) [Lat.,=bluish gray], a metallic chemical element; symbol Cs; at. no. 55; at. wt. 132.9054; m.p. 28.4°C;; b.p. 669.3°C;; sp. gr. 1.873 at 20°C;; valence +1.  fluoride fluoride, a salt of hydrofluoric acid; see hydrogen fluoride. See also fluoridation; fluorine.  normally has a NaCl-type structure and was shown to convert to a CsCl-type structure at approximately 2.0 GPa. Previous reports on the alkali halides indicated that pressure-induced polymorphism was absent in the fluorides, and that the fluorides were anomalous in this respect. However, in this work similar transformations were found also for the fluorides of potassium and rubidium rubidium (rbĭd`ēəm), metallic chemical element; symbol Rb; at. no. 37; at. wt. 85.4678; m.p. 38.89°C;; b.p. 686°C;; sp. gr. 1.53 at 20°C;; valence +1. . The transitions were confirmed by visual microscopic examination of powders pressed in a DAC. In La metal, the ambient pressure hexagonal phase A hexagonal lyotropic liquid crystal phase is formed by some amphiphilic molecules when they are mixed with water or another polar solvent. In this phase the amphiphile molecules are aggregated into cylindrical structures of indefinite length and these cylindrical aggregates are  transforms to a cubic-close-packed structure at about 3.3 GPa. The four elements studied, La, Ce, Pr, and Nd, have similar electronic configurations. Except for Nd, reported compression and electrical resistance Electrical resistance

Opposition of a circuit to the flow of electric current. Ohm's law states that the current I flowing in a circuit is proportional to the applied potential difference V.
 measurements indicated that pressure-induced phase transitions occurred in these metals, but the c rystallography of the high pressure phases was unknown. The results of our x-ray studies showed that La, Pr, and Nd transformed from the normal hexagonal hex·ag·o·nal  
1. Having six sides.

2. Containing a hexagon or shaped like one.

3. Mineralogy
 La-type structure to a face-centered cubic (fcc) structure. Cerium cerium (sēr`ēəm) [from the asteroid Ceres], metallic chemical element; symbol Ce; at. no. 58; at. wt. 140.12; m.p. 799°C;; b.p. 3,426°C;; sp. gr. 6.77 at 25°C;; valence +3 or +4.  metal, however, behaved quite differently from the other three. It was found to transform from a fcc structure to a "collapsed" fcc structure. at 0.75 GPa. The transition is thought to involve the promotion of a 4f electron to the 5d band.

This work was very important to crystallography because not only did it demonstrate that x-ray powder diffraction patterns could be obtained routinely from a DAC, but it also showed some interesting effects of pressure on materials, e. g., compressibility and pressure-induced phase transformations. High-pressure polymorphism in CsF, RbF, KF, and Nd was shown for the first time. The high pressure forms of these alkali fluorides all were shown to be simple cubic phases, while the high pressure modifications of the rare-earth metals rare-earth metals, in chemistry, group of metals including those of the lanthanide series and actinide series, usually yttrium, sometimes scandium and thorium, and rarely zirconium. , La, Ce, Pr, and Nd, exhibited a face-centered cubic structure. The unit cell dimensions for these rare-earth metals were reported for the first time. A photograph of the refined version of the high pressure x-ray powder diffraction camera is shown mounted on the table of an x-ray unit x-ray unit,
n See unit, radiographic.
 in Fig. 6.

3.2 Gasket Technique

In 1962, Van Valkenburg developed a gasket method to confine liquids in the DAC by placing a thin metal sheet with a small hole containing the liquid between the anvil faces as shown in Fig. 7 (7). Initially, the technique was used to study the freezing behavior of many liquids such as water, methanol methanol, methyl alcohol, or wood alcohol, CH3OH, a colorless, flammable liquid that is miscible with water in all proportions. Methanol is a monohydric alcohol. It melts at −97. , ethanol, carbon tetrachloride carbon tetrachloride (tĕ'trəklôr`īd) or tetrachloromethane (tĕ'trəklôr'əmĕth`ān), CCl4, colorless, poisonous, liquid organic compound that boils at 76. , benzene benzene (bĕn`zēn, bĕnzēn`), colorless, flammable, toxic liquid with a pleasant aromatic odor. It boils at 80.1°C; and solidifies at 5.5°C;. Benzene is a hydrocarbon, with formula C6H6.  and similar liquids by the application of pressure at room temperature (RT). Typical examples of this behavior as viewed through the diamond window of the pressure cell with the aid of a polarizing microscope po·lar·iz·ing microscope
A microscope in which the object viewed is illuminated by polarized light.

polarizing microscope  

A microscope in which the object viewed is illuminated by polarized light.
 are shown in Figs. 8a and 8b for single crystals of ice VI and carbon tetrachloride, respectively. The gasket technique was quickly extended to powders, mixtures of liquid and powder and liquid with single crystals of another material. As a result of these developments, one of the first applications of the gasket technique was in x-ray powder diffraction experiments with the hydraulic DAC in use at that time. Powders were confined with chemically inert liquids in a ga sketed cell to provide a hydrostatic environment for the sample under pressure. Introduction of this technique resulted in improved powder diffraction patterns owing to owing to
Because of; on account of: I couldn't attend, owing to illness.

owing to prepdebido a, por causa de 
 the absence of stress inhomogeneity in·ho·mo·ge·ne·i·ty  
n. pl. in·ho·mo·ge·ne·i·ties
1. Lack of homogeneity.

2. Something that is not homogeneous or uniform.

Noun 1.
 in the sample. To eliminate diffraction interference from the metal gasket, W collimator collimator (kol´imātur),
n a diaphragm or system of diaphragms made of an absorbent material and designed to define the dimensions and direction of a beam of radiation.
 tubes (0.2 mm ID) with internal diameters much less than the gasket diameter were used. This procedure resulted in fine-quality powder diffraction patterns out to 2[theta] [approximately equal to] 35[degrees].

3.3 Single Crystal Diffraction

The gasket technique was very important to crystallography, because, soon after we used it with chemically inert liquids to provide a hydrostatic pressure-transmitting environment for x-ray powder diffraction, we recognized almost immediately its potential application to single crystal x-ray diffraction. Chronologically, the high pressure single crystal diffraction technique evolved stepwise stepwise

incremental; additional information is added at each step.

stepwise multiple regression
used when a large number of possible explanatory variables are available and there is difficulty interpreting the partial regression
 as potential applications of the DAC became more apparent with use. Within two years of its inception, the original DAC for infrared spectroscopy evolved, as we saw, into the miniature hydraulically actuated device for x-ray powder diffraction. Almost simultaneously, the gasket technique for encapsulating liquids, coupled with optical polarizing microscopy for viewing the sample, was introduced. Shortly thereafter, optical microscopy observations with polarized A one-way direction of a signal or the molecules within a material pointing in one direction.  light in a gasketed cell revealed that defect-free single crystals could be grown readily from many liquids, e. g. , water, methanol, ethanol, benzene, carbon tetr achloride, etc., at room temperature (RT) by the application of pressure. Moreover, the crystals could be retained in the cell at pressure indefinitely. These early discoveries with the gasket technique were important because they encouraged Stanley Block Background of Stanley Block

Stanley Block, Ph.D., C.F.A., C.C.M., is a Professor of Finance and holds the Stan Block Endowed Chair in Finance at Texas Christian University, Fort Worth.
 (who became chief of the Crystallography Section in 1965), Gasper gasp·er  
n. Chiefly British Slang
A cigarette.
 Piermarmni, and Charles Weir to investigate the possibility of studying single crystals at high pressure by an x-ray diffraction technique. In 1964, all high pressure x-ray studies involved powder diffraction only, and, because of the limitations of the powder method Noun 1. powder method - a process for identifying minerals or crystals; a small rod is coated with a powdered form of the substance and subjected to suitably modified X-rays; the pattern of diffracted rings is used for identification , there was little promise of unequivocal structure determinations of high pressure phases, except for those of the utmost simplicity. Some high pressure polymorphs can be stabilized by quenching quenching

Rapid cooling, as by immersion in oil or water, of a metal object from the high temperature at which it is shaped. Quenching is usually done to maintain mechanical properties that would be lost with slow cooling.
 to low temperatures while still under pressure and then studied by powder and single crystal x-ray diffraction techniques. This technique was employed whenever possible, but only a limited number of known high pressure polymorphs are quenc hable in the metastable state metastable state

Excited state (see excitation) of an atom, nucleus, or other system that has a longer lifetime than the ordinary excited states and generally has a shorter lifetime than the ground state.
. So, there was a critical need for producing and studying stable single crystals of high pressure polymorphs by x-ray diffraction, which meant that the crystal had to be studied in its P, T stability field.

In 1964, single crystal x-ray diffraction methods were still basically film techniques because computer driven automatic diffractometers were just being developed and not at all in common use. This was the situation in the Crystallography Section, for single crystal work was performed with Weissenberg and Buerger-type precession cameras. The precession camera was chosen for our investigation because, unlike the Weissenberg instrument, it had space available to accommodate the pressure cell and its design was simple to modify.

To test our notion that single crystal studies might be possible at high pressures, we carried out initial experiments utilizing a typical lever-arm pressure cell with stainless steel pistons modified to include a 90[degrees] apex angle conical aperture, thus permitting a relatively large sampling of reciprocal space. At the same time, however, the cell's pressure capability was reduced to lessthan 5 GPa because the large conical aperture provided less load-bearing support for the anvils. The pressure cell, mounted on a crudely modified Buerger-type precession camera, is shown in Fig. 9. In its early stages of development, Weir altered the precession camera himself using the same basic machine tools he used several years earlier to fabricate the original infrared pressure cell, thus, its rudimentary appearance. The first experiments were carried out on a single crystal of ice VI grown from high purity distilled water Noun 1. distilled water - water that has been purified by distillation
H2O, water - binary compound that occurs at room temperature as a clear colorless odorless tasteless liquid; freezes into ice below 0 degrees centigrade and boils above 100 degrees centigrade;
 at about 0.9 GPa and RT. Palladium-filtered Ag radiation was used to minimize absorption effe cts (largely due to the diamond anvils) and also to decrease the size of the reciprocal lattice In crystallography, the reciprocal lattice of a Bravais lattice is the set of all vectors K such that

for all lattice point position vectors R.
 permitting more data to be obtained. These early experiments with the crudely modified precession camera demonstrated unequivocally that useful x-ray patterns could be obtained from the ice VI single crystal in a DAC (8,9). Fig. 10 shows one of the very first precession patterns (hk0 level) obtained for ice VI with this camera, and Fig. 11 is a photograph of C. E. Weir examining two precession films of ice VI taken at NBS in 1969. Precession patterns containing reciprocal lattice parameters, [a.sup.*] and [b.sup.*], were observed, but none containing [c.sup.*] were obtained because of the preferential direction of growth of the crystal and the restricted movement of the pressure cell itself. The cell constants for ice VI at 0.9 GPa and RT, derived from 30 observed unique reflections (the first ever obtained from a crystal in situ In place. When something is "in situ," it is in its original location. ) listed in Table 1, were a = (8.38[+ or -]0.05) A, b = (6.17[+ or -]0.05) A, c = (8.90[+ or -]0.15) A. Errors are smaller for a and b than for c because c was measured indirectly from cone-axis and mixed-index patterns. The observed systematic absences were hk0, h = 2n + 1 and 0k0, k = 2n + 1 and indicated the orthorhombic or·tho·rhom·bic  
Of or relating to a crystalline structure of three mutually perpendicular axes of different length.

 aspect [P.sup.**] a. The cell constants found in this work successfully indexed previously reported, but unindexed, reflections obtained from metastable met·a·sta·ble  
Of, relating to, or being an unstable and transient but relatively long-lived state of a chemical or physical system, as of a supersaturated solution or an excited atom.
 polycrystalline ice VI at 90 K and ambient pressure.

Although a complete space group and structure had not been determined, the pioneering work with ice VI, and later ice VII, demonstrated for the first time that useful x-ray intensity data could be obtained at high pressures from single crystals grown and maintained in a diamond cell at RT. However, the results also indicated a critical weakness of the method, i.e., all of reciprocal space could not be examined because of interference from opaque (to x rays) parts of the pressure cell. To observe all of reciprocal space, it was necessary to re-grow the crystal, perhaps several times, in order to obtain enough orientations to achieve this. Through this tedious procedure, it might be possible to examine all of reciprocal space to permit a complete structure analysis of an unknown high pressure polymorph polymorph /poly·morph/ (pol´i-morf) colloquial term for polymorphonuclear leukocyte.


a colloquial term for a polymorphonuclear leukocyte.
. Despite this limitation the results appeared promising, so we proceeded to modify our design and to implement new procedures to improve the quality and quantity of data obtained by the method.

Our first improvement involved replacing the DAC with one fabricated almost entirely from Be metal (except for the load-generating spring and screw) to minimize absorption effects and to increase the range of orientation of the crystal sampled by the x-ray beam. The Be DAC, shown disassembled in Fig. 12, closely followed the original lever-arm design. Also, a new precession camera with a massive goniometer goniometer /go·ni·om·e·ter/ (go?ne-om´e-ter)
1. an instrument for measuring angles.

2. a plank that can be tilted at one end to any height, used in testing for labyrinthine disease.
 head was built to support the weight of the Be cell and to permit large translational and angular ranges for centering and orienting the crystal (10). This new precession camera with the Be cell mounted on the very large goniometer head is shown in Fig. 13. To obtain more accurate intensity data, a procedure, originally developed by A. Santoro for evaluating the absorption corrections for complex cases, i.e., where other objects besides the crystal take part in the absorption, was applied to the Be cell (11). The validity of the procedure was tested on intensity data obtained from a single crystal of bromin e at 1.0 GPa and RT. Fig. 14 shows a precession pattern for bromine bromine (brō`mēn, –mĭn) [Gr.,=stench], volatile, liquid chemical element; symbol Br; at. no. 35; at. wt. 79.904; m.p. –7.2°C;; b.p. 58.78°C;; sp. gr. of liquid 3.12 at 20°C;; density of vapor 7.  (0 level, [mu] = 25[degrees]) taken with this new equipment. The calculated absorption corrections were considered satisfactory and should be applied. The residual R index (a measure of the correctness of the structure) was 0.091 and the overall conclusion was that, in the case of simple structures such as bromine, useful structural information could be obtained. The first crystal structure determination of an unknown utilizing this method was a high pressure form of benzene ([C.sub.6][H.sub.6] II) at 2.5 GPa and RT (12). Fig. 15 shows a single crystal of benzene II with well defined morphology in equilibrium with its liquid in a gasketed DAC at about 3 GPa and 310 [degrees]C, the P,T conditions necessary for growing the crystal. A total of seventy-two reflections were observed from thirteen zero-level and one upper-level photographs of the same crystal which could be indexed as a monoclinic mon·o·clin·ic  
Of or relating to three unequal crystal axes, two of which intersect obliquely and are perpendicular to the third.


 unit cell with a = (5.417[+ or -]0.005) A, b = (5.376[+ or -]0.019) A, c = (7.532[+ or -]0.007) A, [beta] = 110.00[degrees][+ or -]0.08[degrees]. After applying the absorption correction procedure for the Be cell, the 72 corrected intensities were reduced to a single relative scale by the use of common reflections. Equivalent reflections were averaged to give a set of 19 unique intensities for the same crystal. The observed extinctions are consistent with the space group [P2.sub.1]/c with two molecules per unit cell. Assuming Z = 2, the density of benzene II determined at RT and 2.5 GPa was 1.258 g [cm.sup.-3]. As demonstrated in Table 2, a comparison of the crystal data for benzene I and II, there is a significant increase in density of benzene II because of the pressure effect.

Because of the limited number of unique reflections (only 19), conventional strategies to solve the structure were not possible. Consequently, a different approach was devised by A. Mighell to solve the structure. The procedure involved developing a computer program to generate all physically possible structures (excluding those with unreasonable closest-approach distances) by rotating the benzene ring benzene ring
The hexagonal ring structure in the benzene molecule and its substitutional derivatives, each vertex of which is occupied and distinguished by a carbon atom.

benzene ring,
n See aromatic ring.
 located on a special position in the unit cell. Three orthogonal At right angles. The term is used to describe electronic signals that appear at 90 degree angles to each other. It is also widely used to describe conditions that are contradictory, or opposite, rather than in parallel or in sync with each other.  rotation axes, [alpha], [beta], and [gamma], ([alpha] and [beta] coincident co·in·ci·dent  
1. Occupying the same area in space or happening at the same time: a series of coincident events. See Synonyms at contemporary.

 with a and b unit cell parameters, respectively, and [gamma], 20[degrees] from the c parameter in the XZ plane) were used to generate ring orientations and the structure was built-up on the basis of the space group symmetry requirements. To minimize computer runtime, a unique angular range much less than 0-2[pi] was determined for the rotations in the order [gamma], [alpha], and [beta]. The ring was rotated throughout the unique range in 4[degrees] increments. To further minimize computer run-time, exclusion parameters were set to eliminate structures with unacceptable intermolecular Adj. 1. intermolecular - existing or acting between molecules; "intermolecular forces"; "intermolecular condensation"  distances of closest-approach. For each orientation, a scale factor and 2 reliability factors (R) were calculated. One ([R.sub.1]) was the conventional value, and the other ([R.sub.2]), a value which equally weights the reflections because the limited amount of data may give a low [R.sub.1] value with some very poor individual agreements, especially for weaker reflections. Both R values gave minima with the same angular coodinates, but [R.sub.2] was more sensitive and gave a sharper minimum. The final structure was determined with a 1[degrees] angular scan of [alpha], [beta], and [gamma]. Observed and calculated structure factors, listed in Table 3, show reasonably good agreement along with additional reflections for which the intensity was less than the background. The b-axis projection of the monoclinic structure ([P2.sub.1]/c) with some intermolecular distances of closes t approach is shown in Fig. 16.

Following the structure work on benzene II, the technique was used at NBS to obtain unit cell and space group data on high-pressure forms of [C.sub.6][], [CS.sub.2], [Br.sub.2], [CCl.sub.4], and [KNO.sub.3] (13). The results of those studies were reported for the first time and are listed in Table 4. We made several useful observations during those experiments. In most instances, for liquid-solid transitions, crystals grew in a fixed orientation with respect to the cell. When the crystal was small, it could and often did move around in its liquid, but, as it grew larger, it reoriented and the final orientation of the large crystal was invariably in·var·i·a·ble  
Not changing or subject to change; constant.

 the same. It appeared that growth in a prescribed crystallographic direction predominated and effectively controlled the orientation in the confining volume. For crystals grown from solid-solid transitions, however, this behavior did not apply, as in the case of [KNO.sub.3]. In all of these cases, however, the pressure was not of primary importance and was consi dered as little more than an environmental parameter required to produce the phase of interest.

The situation, however, was quite different in later single crystal work involving the measurement of anisotropic Refers to properties that differ based on the direction that is measured. For example, an anisotropic antenna is a directional antenna; the power level is not the same in all directions. Contrast with isotropic.  and volume compressibilities. For that work, a knowledge of pressure was essential. Six inorganic azides were studied: [alpha] lead azide azide

inhibitor of cytochrome c oxidase (or complex IV) of the respiratory electron-transfer chain.
, [beta] lead azide, barium barium (bâr`ēəm) [Gr.,=heavy], metallic chemical element; symbol Ba; at. no. 56; at. wt. 137.33; m.p. 725°C;; b.p. 1,640°C;; sp. gr. 3.5 at 20°C;; valence +2.  azide, potassium azide, sodium azide sodium azide NaN3 Microbiology A toxic salt added–concentration, 0.01%, to a transport medium of lab specimens–eg, urine for culturing bacteria, which prevents oxidative phosphorylation and bacterial overgrowth  and thallium azide Thallium azide, TlN3, is a yellow-brown crystalline solid poorly soluble in water. Although it is not nearly as sensitive to shock or friction as lead azide, it can easily be detonated by a flame or spark. It can be stored safely dry in a closed non-metallic container. , all highly energetic materials Energetic materials are a class of materials with high amount of stored chemical energy that can be released.[1]

Typical classes of energetic materials are e.g. explosives, pyrotechnic compositions, propellants (e.g.
 (14). Compressibility measurements on such unstable materials had never been done before and were not seriously considered because of the danger of explosion until the development of this single crystal high pressure x-ray technique. The small crystal required in this method made this danger almost negligible. Prior to the advent of the DAC, measurements to obtain compressibility data could only be made with relatively large sample-volume presses and such measurements are rare because the potential for violent detonation is too great. Thus, these measurements on the azide compounds at NBS utilizing the DAC made a great impact in the study of energet ic materials at high pressures. Although the equipment was intended primarily for structure studies, it seemed to us that it could be used also to measure lattice parameters as a function of pressure, provided the pressure could be defined adequately. The pressures were estimated using a fixed-point scale based on the known freezing points freezing point

Temperature at which a liquid becomes a solid. When the pressure surrounding the liquid is increased, the freezing point is raised. The addition of some solids can lower the freezing point of a liquid, a principle used when salt is applied to melt ice on
 of n-hexane and ethanol, 1.04 GPa and 2.22 GPa, respectively. The desired crystal was cemented in place on one of the diamond anvils and the gasket was filled with a hydrostatic liquid and sealed by the application of pressure. The pressure was increased slowly until the liquid partially crystallized crys·tal·lize also crys·tal·ize  
v. crys·tal·lized also crys·tal·ized, crys·tal·liz·ing also crys·tal·iz·ing, crys·tal·liz·es also crys·tal·iz·es
 as shown in Fig. 17 for a single crystal of Pb[([N.sub.3]).sub.2] at 2.2 GPa and RT. The equilibrium between liquid and solid served to specify the pressure from the reported P-T P-T Pressure-Temperature (thermodynamics diagram)  behavior of the liquidus curve. The loaded cell was mounted on the precession camera at RT and as many reciprocal lattice planes as possible were recorded on film. A phase transition in thallium azide was discovered at a pressure between the freezing pressure of chloroform chloroform (klôr`əfôrm) or trichloromethane (trī'klôrōmĕth`ān), CHCl3  (0.54 GPa) and n-decane (0.30 GPa). Table 5 lists the results of those measurements which were being reported for the first time on the azide compounds.

The second unknown structure determined by this method was a high pressure form of carbon tetrachloride ([CCl.sub.4] III) at 1.0 GPa and RT (15). A typical zero level pattern (Zr-filtered Mo radiation, [mu] = 15[degrees], 23 h exposure) taken of [CCl.sub.4] III at 1 GPa and RT is shown in Fig. 18. Carbon tetrachloride III crystallizes in the monoclinic system ([P2.sub.1]/c, Z= 4) with unit cell dimensions a = (9.079[+ or -]0.012)A, b = (5.764[+ or -]0.003)A, c = (9.201[+ or -]0.004)A, and [beta] - 104.29[degrees][+ or -]0.05[degrees]. Using four molecules per unit cell, the calculated density was 2.19 g [cm.sup.-3] and agreed well with reported values for those conditions of pressure and temperature. The R value (0.0951) for [CCl.sub.4] III ([SnBr.sub.4]-type structure) was approximately the same as that found for benzene II. Table 6 lists the 52 unique observed and calculated structure factors for the phase. The b-axis projection of the monoclinic cell of the [CCl.sub.4] III struct ure is shown in Fig. 19. It is interesting to note that the R-value obtained for the CC14 III structure based on 52 unique reflections was no better than that obtained for benzene II which used a set of 19 unique intensities. It appeared that R values in the 9 % range were all that one could expect from this film technique. The crystal structure of [CCl.sub.4] III is isostructural with the structures reported for [SnBr.sub.4], [TiCl.sub.4], [TiBr.sub.4], and probably [SnCl.sub.4]. Table 7 lists the lattice constants The lattice constant refers to the constant distance between unit cells in a crystal lattice. Lattices in three dimensions generally have three lattice constants, referred to as a, b, and c.  of these five tetrahalogens exhibiting the [SnBr.sub.4]-type structure. Similarities exist in the unit cell dimensions as well as their ratios with the differences reflecting the relative size difference in the atoms of the various substances.

In the [CCl.sub.4] III structure, chlorine atoms occupy a distorted hexagonal-close-packed arrangement with the distortion arising from the marked difference between inter- and intra-molecular nonbonded chlorine-chlorine distances which are 3.49 A and 2.88 A, respectively. Because this structure deviates significantly from a close-packed arrangement, it was thought that a more compact phase of [CCl.sub.4] must exist and should be found at higher pressures. Indeed, its existence was confirmed by optical polarizing microscopy observations with the DAC at pressures above 1.5 GPa at RT, and the new phase was named [CCl.sub.4] IV. The crystal under polarized light exhibited no detectable birefringence Birefringence

The splitting which a wavefront experiences when a wave disturbance is propagated in an anisotropic material; also called double refraction. In anisotropic substances the velocity of a wave is a function of displacement direction.
 suggesting that it belonged to the cubic system. Further work with the single crystal high pressure x-ray precession camera identified the space group (Pa 3) with a = (9.334 [+ or -] 0.008) A, determined from 27 reflections. The data were obtained at 2.4 GPa and RT. The calculated density, assuming eight molecules per unit cell, was 2.513 g [cm.sup.-3], and, as expected, was greater than the value for [CC1.sub.4] III (2.190 g [cm.sup.-3]) measured at 1.0 GPa. By analogy, we assumed the structure to be an isotype i·so·type
An antigenic marker that occurs in all members of a subclass of an immunoglobulin class.

 of the Sn[I.sub.4] structure. No further work was done on [CC1.sub.4] IV because more pressing priorities arose related to the development of a spectroscopic spec·tro·scope  
An instrument for producing and observing spectra.

 method to measure sample pressure in the DAC.

The high pressure x-ray precession film method was not used widely because of the Be cell's large size and expensive fabrication costs. Also, the special camera requirements and large goniometer head demanded the construction of an expensive precession instrument, a task which many scientists declined to undertake. Another contributing factor to its lack of acceptance was the absence of a reliable, convenient pressure measurement capability for the sample in a DAC. However, the method demonstrated its usefulness and served as the foundation for later more advanced work in the early-to-mid 1970s in other laboratories. The basic opposed-anvil cell design underwent miniaturization min·i·a·tur·ize  
tr.v. min·i·a·tur·ized, min·i·a·tur·iz·ing, min·i·a·tur·iz·es
To plan or make on a greatly reduced scale.

 and further simplification and was adapted to automatic diffractometers for single crystal x-ray diffraction studies, rapidly displacing the earlier precession film method. Another contributing factor to the acceleration of high pressure single crystal x-ray diffraction studies was the introduction of the ruby fluorescence technique fo r pressure measurement by NBS in 1971.

4. The Ruby Fluorescence Method of Pressure Measurement

By 1971, the DAC had undergone several stages of refinement and had been adopted by other laboratories in the U.S. and abroad. Still, it was not fully appreciated by many scientists because there remained a serious deficiency with the instrument. There was no convenient, rapid and reliable method for measuring the sample pressure. This situation is reminiscent of the statement made by the famous 19th century physicist, William Thomson (better known to us as Lord Kelvin kelvin, abbr. K, official name in the International System of Units (SI) for the degree of temperature as measured on the Kelvin temperature scale.

A unit of measurement of temperature.
). In one of his many lectures, he said: "When you can measure what you are speaking about, and express it in numbers in numbered parts; as, a book published in numbers.

See also: Number
, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager mea·ger also mea·gre  
1. Deficient in quantity, fullness, or extent; scanty.

2. Deficient in richness, fertility, or vigor; feeble: the meager soil of an eroded plain.

 and unsatisfactory kind: it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science" (16).

Such was the case with the DAC before 1971. Pressures were either calculated (force per unit area) or measured from compression data (utilizing an equation of state, e.g., sodium chloride sodium chloride, NaCl, common salt. Properties

Sodium chloride is readily soluble in water and insoluble or only slightly soluble in most other liquids. It forms small, transparent, colorless to white cubic crystals.
) obtained by an x-ray powder diffraction film method. The former procedure was extremely inaccurate because it was difficult to ascertain the distribution of the applied load over the gasket/sample area. The latter procedure, although reliable, was tedious and time consuming, often requiring as many as 15 h to make one pressure measurement. In that time frame, many factors could influence and change the assumed conditions under which the desired measurement was made. Consequently, acceptance of the DAC as a tool in high pressure research was somewhat limited in 1970. It was used primarily in laboratories engaged in research of geological interest where very high pressures simulating the earth's interior were desired, while the accuracy in the value of pressure was not yet considered of primary importance.

This situation changed dramatically when the high pressure group in the CS, under constant encouragement to develop a rapid, convenient and reliable method for measuring pressures in the DAC, achieved a breakthrough in 1971. The problem was discussed one day in the NBS cafeteria among John B. Wachtman, Jr. (Chief of the Inorganic Materials Division), Stanley Block (Chief of the Crystallography Section), and J. Dean Barnett Dean Barnett (b. July 13, 1967) is an American conservative blogger and occasional fill-in host for Hugh Hewitt. He is a graduate of Harvard University and Boston University Law School. , a guest scientist at NBS on sabbatical leave Noun 1. sabbatical leave - a leave usually taken every seventh year

leave, leave of absence - the period of time during which you are absent from work or duty; "a ten day's leave to visit his mother"
 from Brigham Young University Brigham Young University, at Provo, Utah; Latter-Day Saints; coeducational; opened as an academy in 1875 and became a university in 1903. It is noted for its law and business schools. . While having lunch together, they discussed possible techniques for measuring pressure in the DAC. Wachtman suggested various methods, all of which had been considered already and found to be unsuitable. Finally, Wachtman asked, "Have you considered fluorescent spectroscopy" As it happened, they had not, but replied they would look into it. Encouraged by this meeting, Stanley Block, Dean Barnett, Gasper Piermarini, and Richard Forman began measuring the pressure-dependence of several fluorescing materials that happ ened to be on the shelves in Forman's spectroscopy laboratory and in H. Parker's and W. Brauer's crystal growing laboratory. Those materials included ruby ([Al.sub.2][O.sub.3]), [YAlO.sub.3], YAG YAG  
A hard synthetic yttrium aluminum garnet used in laser technology and as a gemstone.

[y(ttrium) + a(luminum) + g(arnet)1.]
, MgO, and a few others. While several materials were found to have a readily measurable pressure dependence, ruby exhibited by far the most promising results. Ruby's main fluorescence lines (the [R.sub.1][R.sub.2] doublet dou·blet
A pairing of two lenses to optically correct a chromatic and spherical aberration.
) were intense and sharp, and the lines shifted measurably toward the red with increasing pressure (17). Fig. 20 shows the effect of 2.2 GPa (hydrostatic) and 4 GPa (nonhydrostatic) compared with ambient P, T conditions. Significantly, it was observed that pressure could be detected in situ using only a very small amount of ruby crystal as the internal pressure sensor A pressure sensor measures the pressure, typically of gases or fluids. Pressure is an expression of the force required to stop a gas or fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor generates a signal related to the pressure imposed. , comprising only 1 % of the available volume under pressure. Further, because ruby is generally chemically inert, it can be present in the sample chamber as an internal pressure sensor without interfering with the desired measurement on the specimen.

4.1 Pressure Calibration of the Ruby [R.sub.1] Line-Shift

Recognizing the significance of these observations, the group concluded it was of paramount importance to calibrate To adjust or bring into balance. Scanners, CRTs and similar peripherals may require periodic adjustment. Unlike digital devices, the electronic components within these analog devices may change from their original specification. See color calibration and tweak.  the R-line shift against reliable pressure values. The calibration was accomplished using an x-ray powder diffraction film method to measure the compression of NaCl for a measured shift in the wavelength of the ruby [R.sub.1]-line (18). The corresponding pressure was calculated from the compression data utilizing an established equation of state for NaCl. The [R.sub.1]-line shift was found to be linear with pressure, (0.2740[+ or -]0.0016 GPa [A.sup.-1]), up to about 19.5 GPa, shown graphically in Fig. 21. In subsequent work, the linearity was confirmed to about 29 GPa (19). In 1970, a fixed-point pressure scale, based on a series of well-characterized phase transitions, was widely used and accepted for estimating pressures in most high pressure experiments performed at that time. A test of the validity of the fixed-point scale was made by measuring the transition pressures of the fixed -points using the linear scale of the ruby technique and comparing these measured pressures with the accepted values. The results showed that the fixed-point scale required serious downward revision by a factor of two at 50 GPa. The divergence of the two scales is shown graphically in Fig. 22. The success of this calibration quickly established the ruby scale as a secondary pressure standard, and, it soon became the de facto standard Hardware or software that is widely used, but not endorsed by a standards organization. Contrast with de jure standard.

de facto standard - A widespread consensus on a particular product or protocol which has not been ratified by any official standards body, such as ISO,
 for measuring pressures in a DAC in all kinds of experiments including both powder and single crystal x-ray diffraction.

Shortly after its calibration, the pressure-dependence of the ruby-shift was found to be essentially independent of temperature within the limits of uncertainty of the measurement at that time, so it was possible to use it to measure pressure at temperatures other than RT (20). It was particularly useful at elevated temperatures where many high pressure phenomena were observed. Consequently, a specially designed DAC for use at elevated temperatures was made at NBS in 1972. A cross section of the design of that instrument is shown in Fig. 23. It was first used at NBS to study many materials to temperatures as high as 600 [degrees]C, but the quality of the pressure measurement rapidly deteriorated above 300 [degrees]C.

4.2 Evaluating Stresses in Solid and Liquid Pressure-Transmitting Media

In the years following the introduction of the ruby technique, the group at NBS also developed techniques to obtain useful information from the pressure-dependence of the ruby R-lines concerning the stress conditions in the gasketed and ungasketed sample configurations, both widely used in a DAC at that time. Two methods to measure the nature of the stressed environments Stressed environment: In radiocommunications, an environment that is under the influence of extrinsic factors that degrade communications integrity, such as when (a) the benign communications medium is disturbed by natural or man-made events (such as an intentional nuclear burst),  in compressed liquids and solids in a DAC were developed which provided extremely useful information on pressure-transmitting media used in x-ray diffraction experiments at high pressure, particularly for single crystal work. Method I uses finely powdered ruby crystals dispersed in a powdered sample. It was used to study stress distributions in both the gasketed or un-gasketed configurations. The pressure is measured at several localized areas over the extent of the sample. This spatially sensitive technique, although time consuming, yields quantitative measurements of pressure gradients at different locations in the sample. Method II, genera genera, in taxonomy: see classification.  lly used to study stress distributions in liquids, employs several small ruby crystals or just one large crystal fragment in the gasketed sample configuration. The spectral line-width of the [R.sub.1] peak emitted from an extended region of the sample chamber is measured. Under hydrostatic conditions, the line-width decreases slightly with increasing pressure, but increases dramatically when the ruby senses nonhydrostaticity in the medium. Method II yields only a qualitative measure of the pressure gradient, but is much more rapid that Method I.

Method I was used to study the stress distribution in an ungasketed powdered sample. The ungasketed sample is the simplest configuration, and was the first used with the DAC. It was found to be least desirable for achieving homogeneous stress. In this configuration, a powder dispersed with numerous minute crystals (1 [micro] to 10 [micro]) of ruby is simply squeezed between the two anvil flats, thinning down the powder until frictional forces prevent further flow. The distribution of stress in the compressed layer of powder can be determined from shift and broadening properties of the ruby R-lines, To illustrate, R-line spectra were measured from individual ruby particles in a NaCI/ruby powder pressed between the anvil flats using a microscope coupled to a specially constructed fluorescence wavelength measuring instrument. The instrument, the first of its kind, was designed and constructed by Barnett, Piermarini, and Block specifically to measure the wavelength-shift of the ruby R-lines (203). It contained a special Eschelle grating which could measure pressure (wavelength) with a precision of [+ or -]0.025 GPa under ideal conditions. A photograph with Block and Piermarini operating the instrument is shown in Fig. 24. Results, shown in Fig. 25, depict a parabolic distribution of stress along an anvil diameter for two different applied loads (21). As expected, the results demonstrated clearly that in x-ray powder diffraction experiments where the sample is ungasketed, a large stress distribution is experienced by the sample, producing significant broadening in the diffraction rings. To minimize this undesired effect, a very small diameter (0.2 mm) collimated In a straight line. Collimated light beams are parallel rays of light.  x-ray beam is used, allowing only the very center of the sample (top portion of the parabola where stresses are maximized and over a narrow range) to contribute to diffraction. With conventional x-ray sources available at that time, such finely collimated beams See collimated.  required long exposure times to get measurable diffraction patterns on film. Later, this situation c hanged dramatically with the advent of synchrotron radiation sources Coordinates:  The Synchrotron Radiation Source (SRS) at the Daresbury Laboratory in Cheshire, United Kingdom is a second generation[1] synchrotron radiation source. .

Methods I and II were used to determine the nature of the stress environment in powders, liquids, and combinations of both compressed in a gasketed DAC. Fig. 26 shows results obtained with Method I (line-shift) for increasingly applied loads on some important pressure-transmitting media materials: water, powdered NaCl, powdered AgCl, and a 4:1 (by volume) mixture of methanol:ethanol. Two, NaCl and AgCl, were studied because they were widely used at that time as a pressure-transmitting medium in solid-media high pressure systems (presses) because they were thought to produce a desired quasi [Latin, Almost as it were; as if; analogous to.] In the legal sense, the term denotes that one subject has certain characteristics in common with another subject but that intrinsic and material differences exist between them.  hydrostatic environment. Water was studied because of general geological interest in this fundamental material. The methanol:ethanol mixture was studied because of its potential importance as a truly hydrostatic medium, exceeding by a large amount the limit for that time, (6.5 to 7.0) GPa, provided by 1:1 (by volume) pentane pen·tane  
Any of three colorless, flammable isomeric hydrocarbons, C5H12, derived from petroleum and used as solvents.
:isopentane. The results (Fig. 26) indicate that powdered NaCl is the least desirable of the four mat erials studied because it shows deviations in pressure at very low values, with differences becoming quite prominent above 4.0 GPa, while AgCl appears to be much better in achieving quasi hydrostatic conditions to much higher pressures. No gradient in pressure is indicated below 6.5 GPa. The results for [H.sub.2]O indicate an unexpected low gradient in pressure at 10 GPa. Water appears to be at least as good as and perhaps better than AgCl for providing a quasi hydrostatic environment in the 10 GPa range. No measurable pressure gradient was observed in 4:1 methanol:ethanol mixture until the pressure exceeded approximately 9.5 GPa, extending the hydrostatic limit significantly beyond the then current limit of (6.5 to 7.0) GPa. The work clearly demonstrated, as expected, that the gasketed configuration is far superior to the ungasketed sample for producing a hydrostatic or quasi hydrostatic environment in a DAC.

Method II (line broadening) was used to study the stress properties in many liquids. Fig. 27 shows line-broadening results for four liquids that do not crystallize crys·tal·lize also crys·tal·ize  
v. crys·tal·lized also crys·tal·ized, crys·tal·liz·ing also crys·tal·iz·ing, crys·tal·liz·es also crys·tal·iz·es
 easily at high pressures: isopropyl alcohol isopropyl alcohol: see isopropanol. , 1:1 (by volume) pentane:isopentane, methanol, and 4:1 (by volume) methanol:ethanol. The data are plotted relative to the ambient pressure line-width. The general features of the curves are similar for each liquid. There is a slight decrease in line-width with increasing pressure followed by a sharp discontinuity dis·con·ti·nu·i·ty  
n. pl. dis·con·ti·nu·i·ties
1. Lack of continuity, logical sequence, or cohesion.

2. A break or gap.

3. Geology A surface at which seismic wave velocities change.
 which characterizes the pressures ([P.sub.1], [P.sub.2], [P.sub.3], and [P.sub.4]) at which the ruby senses inhomogeneous Adj. 1. inhomogeneous - not homogeneous

heterogeneous, heterogenous - consisting of elements that are not of the same kind or nature; "the population of the United States is vast and heterogeneous"
 stress. These discontinuities have been related to the glass-transition pressure for the given material. Because the liquid does not crystallize, its viscosity increases with pressure until it approaches [10.sup.13] P, the value generally accepted as the glass transition. These concepts were correlated by measurements of the pressure-dependence of viscosity of 4:1 methanol:eth anol and other liquids utilizing the DAC and a simple falling-ball Stokes technique as shown graphically in Fig. 28 (22, 23).

From a practical standpoint, the significance of the work on liquids established the fact that hydrostatic conditions could be achieved to pressures near 10 GPa, far in excess of the previously attainable limit of 7.0 GPa with 1:1 pentane:isopentane. However, it needs to be emphasized that even in liquids, time-dependent shear stresses shear stress
See shear.

shear stress

A form of stress that subjects an object to which force is applied to skew, tending to cause shear strain.
 do, indeed, exist and the rate of change in pressure must be carefully considered when the viscosity of these hydrostatic fluids In atmospheric dynamics and fluid dynamics, a hydrostatic fluid is a fluid in which fluid stresses act isotropically and fluid elements are in local equilibrium with one another.  exceeds roughly [10.sup.10] P. If changes in pressure are too large, then under these circumstances it is likely that plastic deformation plastic deformation,
n any irreversible deformation of tissues.
 will occur in low-shear-strength solid specimens as a result of time-dependent low magnitude stresses which are insensitive to the ruby monitor. This was an important result for x-ray diffraction experiments, particularly for single crystal work where the crystal must remain intact and without defects. While quality x-ray data could be obtained over a much larger pressure range with these fluids than previously, it was also important to be aware of the potential consequences of approaching the hydrostatic limit for a given medium too rapidly. Because the 4:1 methanol:ethanol mixture was easy to prepare and load into a gasketed DAC both for powder and single crystal x-ray diffraction work, it was widely used as a pressure-transmitting medium in early x-ray diffraction experiments, especially for single crystal x-ray diffraction where hydrostatic environments are essential for preventing shear in the crystal. In fact, for the reasons cited above, it is still often used today. Line broadening studies also led to the discovery of another pressure-transmitting liquid medium, 16:3:1 (by volume) methanol:ethanol:water, which extended the hydrostatic limit to 14.4 GPa (24). It, too, was easy to use and consequently enjoyed wide popularity. It is still used today.

5. Other X-Ray Diffraction Techniques Used With the DAC

In the years following the development of the ruby technique, the high pressure group in the Crystallography Section developed two additional techniques for x-ray diffraction studies with the DAC: (1) an energy dispersive dispersive /dis·per·sive/ (-per´siv)
1. tending to become dispersed.

2. promoting dispersion.
 x-ray technique and (2) a single crystal Bond method. The energy dispersive technique was used in numerous applications including compressibility measurements, detection of pressure-induced phase transformations, identification of high pressure phases, and even some innovative applications. A few noteworthy examples follow. It was shown possible, for the first time, to determine atomic radial distribution functions In computational mechanics and statistical mechanics, a radial distribution function (RDF), g(r), describes how the density of surrounding matter varies as a function of the distance from a distinguished point.  (RDF (Resource Description Framework) A recommendation from the W3C for creating meta-data structures that define data on the Web. RDF is designed to provide a method for classification of data on Web sites in order to improve searching and navigation (see Semantic Web). ) of amorphous materials contained in a DAC. The pressure dependencies to 10.5 GPa of RDFs for two amorphous materials, Fe-W and Ni-P were reported and critically evaluated (25). However, it was noted that the limitations on the data imposed by the operating conditions were significant and correspondingly limited the results derived from that data. It was estimated that the di stance of the first nearest-neighbor shell of atoms can be determined with an absolute accuracy of not better than 3 %. Energy dispersion was also used for the first time to study the effect of pressure on the crystal structure of poly(tetrafluoroethylene Noun 1. tetrafluoroethylene - a flammable gaseous fluorocarbon used in making plastics (polytetrafluoroethylene resins)
fluorocarbon - a halocarbon in which some hydrogen atoms have been replaced by fluorine; used in refrigerators and aerosols
) homo- and copolymers in the high pressure phase (26). In this case the experimental conditions needed to obtain intensity data successfully were extremely difficult to achieve. For example, an unusually large sample volume of polymer was needed to get measurable diffraction intensities. This required the use of large diamond anvils (1 carat gems) with 1 mm (edge-to-opposite edge) octagonal oc·tag·o·nal  
Having eight sides and eight angles.

oc·tago·nal·ly adv.

Adj. 1.
 flats. To prevent the sample, in the form of a rod 0.2 mm long and 0.36 mm in diameter, from being crushed, a special high yield-strength gasket material, Vascomax 300 (#), capable of reaching 5 GPa while still retaining sufficient thickness to prevent pinching the specimen, was used. The gasket diameter was 0.3 mm with a thickness of 0.56 mm. To provide a hydrostatic env ironment to 5 GPa, a 4:1 methanol:ethanol mixture was used. Under these critical experimental conditions, it was possible to obtain intensity data successfully from a very low-scattering polymer material. The single crystal bond method was employed to measure anisotropic compressibilities of Si and [alpha]-Pb[([N.sub.3]).sub.2] (27). The method provided high sensitivity in determining peak positions and eliminated the effect of centering errors on measured values of 2[theta]. Under optimum conditions diffraction angles were measured with an accuracy of [+ or -]0.001[degrees] in 2[theta].

In 1978, NBS underwent a restructuring of its organization in favor of a Laboratory concept and the Section no longer appeared in the organizational charts An organizational chart is a chart which represents the structure of an organization in terms of rank. The chart usually shows the managers and sub-workers who make up an organization. . Laboratories consisted of Divisions and the former Crystallography Section became part of the Ceramics, Glass, and Solid State Science Division, part of which later became the Ceramics Division. Because of the existence of a pressing National need to help U.S. industry, priorities changed toward new directions because of substantial new responsibilities added by the U.S. Congress to the NBS mission. To help meet these needs the activity of the high pressure group turned to utilizing the DAC to fabricate new dense ceramic materials from nanosize ceramic powders. That activity was ongoing to 1995 and resulted in many advances.

6. Conclusions

The invention of the diamond anvil cell (DAC) and the ruby technique were, truly, outstanding achievements of the Crystallography Section. Both had a lasting impact on the advancement of high pressure research in the latter part of the 20th century. The DAC is a unique instrument, unsurpassed in its simple design and mode of operation. It provides the means to generate ultrahigh pressures in a small, portable, simple mechanical device, relatively inexpensive to manufacture and small enough to fit in the palm of one's hand. Also, it is readily adaptable to a variety of scientific measurement techniques. Because it features a diamond window with 180[degrees] transmission, it is particularly suited for x-ray diffraction, spectroscopy, and optical polarizing microscopy. In x-ray crystallography, the DAC was used at NBS, for the first time, for powder diffraction in 1960, and later, after the introduction of the gasket technique, for single crystal diffraction in 1964. Both were benchmark events in the early devel opment of high pressure x-ray crystallography.

Even with its great successes in high pressure x-ray crystallography, the DAC had a serious deficiency--the lack of a rapid, convenient, and reliable method to measure sample pressure. This fault hindered its acceptance as a world-class scientific instrument. The invention of the ruby fluorescence technique in 1971 changed this view dramatically and the device began to be increasingly used world-wide. In 1974, an improved NBS design of the DAC attained pressures (assuming a linear ruby shift) in the 50 GPa range for the first time. The ultimate pressure capability of the DAC had not been established, but the potential for achieving even higher pressures by further improvements in design was recognized. Transition pressures in several semiconductors used as markers in the revised 1970 fixed-point scale were measured by the ruby technique. The results indicated that the generally accepted and widely used fixed-point scale diverged from the ruby scale above 13.5 GPa and disagreed by as much as a factor of two at 50 GPa with the ruby scale defining the lower pressure. As a result of these studies, the fixed-point scale was revised downward, and the ruby scale, widely used to measure pressure in the DAC, became the de facto [Latin, In fact.] In fact, in deed, actually.

This phrase is used to characterize an officer, a government, a past action, or a state of affairs that must be accepted for all practical purposes, but is illegal or illegitimate.
 secondary pressure standard. Moreover, it was soon determined that the pressure-shift was essentially independent of temperature within the uncertainty of the measurement at that time, so pressures could be measured at temperatures other than RT. Particularly useful were studies at elevated temperatures where many high pressure physical phenomena occur. There were significant "spin offs" derived from the ruby technique. For x-ray crystallography, an important one, particularly for single crystal work, is the facility to determine the nature of stresses in a pressure-transmitting medium from both line-shift and line-broadening measurements. As early as 1973, such measurements led to the discovery of several pressure-transmitting media that extended the hydrostatic limit to unrivaled pressures for th at time. Indeed, all of these advances were milestone events with NBS playing a pivotal role, for, combined, they ignited and fueled an explosion of activity, not only in crystallography, but in many areas of high pressure research. In a relatively short time, the DAC's versatility in applications to high pressure studies was shown to be unequaled. The worldwide acceptance of the device spurred laboratories in the U.S. and abroad towards the further development of the full potential of this remarkable instrument. That activity is still going on unabated today. Its been estimated conservatively that the number of DAC instruments in use worldwide during the last 40 years is approximately 5000 (28).







Table 1

Observed and calculated d spacings for iceVI at 0.9 GPa and RT (8)

   d - spacing ([Angstrom])     hkl
Obs.           Calc.

4.3            4.3            111 (*)(a)
3.6            3.6            012
3.4            3.5            210
3.12           3.08           020
2.91           2.93           021 (*)
2.75           2.75           121
2.63           2.67           301
2.51           2.51           022
2.43           2.42           203
2.21           2.22           004 (*)
2.10           2.09           400
2.01           2.00           031 (*)
1.97           1.97           204
1.85           1.85           230
1.75           1.76           124
1.64           1.64           205
1.56           1.57           502 (*)
1.48           1.48           006
1.44           1.44           142 (*)
1.41           1.42           513
1.37           1.37           424
1.34           1.34           035
1.30           1.30           612
1.26           1.26           107
1.21           1.21           151
1.14           1.14           712
1.104          1.104          108
1.084          1.081          615 (*)
1.050          1.049          535 (*)
1.010          1.009          642

(a)Those indices marked with an asterisk represent multiple lines and
only one possible index is given. These date represent the first ever
obtained in situ from a single crystal under high pressure.
Table 2

A Comparison of crystal data for benzene I and II (12)

Benzene I                           Benzene II
(21 [degrees]C, 0.07 GPa)           (21 [degrees]C 2.5 GPa)

a = 7.17                            a = 5.417 (5) [Angstrom] (a)
b = 9.28                            b = 5.376 (19)
c = 6.65                            c = 7.532 (7)
                                    [beta] = 110.00 (8)[degrees]

Space group Pbca                    Space group [P2.sub.1]/c
Z = 4                               Z = 2
[[rho].sub.c] = 1.18 g [cm.sup.-3]  [[rho].sub.c] = 1.258 g [cm.sup.-3]

(a) The numbers in parantheses are standard deviations on the reported
values in units of the last place, obtained from least-squares
refinement from 20 experimental values of 2[theta]. The increase in
density ([[rho].sub.c]) for benezene II illustrates the effect of 2.5
Table 3

Observed and calculated structure factors for benzene II (12)

h  k  l       [F.sub.obs]  [F.sub.calc]

1  0   0          26.43        22.55
2  0   0          13.47        14.58
0  0   2          20.53        18.94
1  0   2          12.86        13.54
2  0   2           9.60        13.19
1  0  -2          33.39        33.32
2  0  -2 (L)       5.15         2.62
3  0  -2 (L)       6.98         4.99
0  0   4           6.26         6.73
1  1   0 (L)       3.98         4.20
3  1   0           7.87         8.57
0  1   1          24.37        25.12
1  1   1 (L)       4.98         7.02
2  1   1          10.41         7.54
1  1  -1          16.81        17.05
2  I  -1          11.69        11.29
0  1   2          17.42        18.50
1  I   2 (L)       5.98         1.05
2  1   2 (L)       6.98         2.24
1  1  -2          27.35        27.26
2  1  -2          12.58        11.06
1  1  -3          12.30        12.85
3  1  -3           8.18         8.29
0  1  -4 (L)       6.98         1.42
1  1  -4          11.85        13.60
2  1  -4          11.05        10.43
0  2   0 (*)       7.35         7.77
0  4   0 (*)       5.29         5.95

(a)Reflections marked by an "L" were unobserved because their intensity
did not exceed the background. They were assigned an intensity slightly
less than the smallest observed value. Those marked by an asterisk were
measured on a second crystal which was in a different orientation in the
cell. These data represent the first ever obtained in situ from a high
pressure polymorph of a completely unknown structure.
Table 4

Original unit cell and space group data for some high pressure
polymorphs obtained from single crystal x-ray diffraction at high
pressure and RT (13)

Substance             Crystal system

[C.sub.6][H.sub.6]-I   orthorhombic
[CS.sub.2]             orthorhombic
[Br.sub.2]             orthorhombic
[CCl.sub.4] I          rhombohedral
[CCl.sub.4] II         monoclinic

[CCl.sub.4] III        orthorhombic

[KNO.sub.3] III        rhombohedral

[KNO.sub.3] IV(?)      orthorhombic

Substance             Unit cell dimensions, [Angstrom] (a)

[C.sub.6][H.sub.6]-I  a = 7.17,  b = 9.28,  c = 6.65
[CS.sub.2]            a = 6.16,  b = 5.38,  c = 8.53
[Br.sub.2]            a = 8.54,  b = 6.75,  c = 8.63
[CCl.sub.4] I         a = 14.27, [alpha] = 90[degrees]
[CCl.sub.4] II        a = 22.10, b = 11.05, c = 25.0,
                       [beta] = 114.0[degrees]
[CCl.sub.4] III       a = 11.16, b = 14.32, c = 5.74

[KNO.sub.3] III       a = 14.31, [alpha] = 78[degrees] 54'

[KNO.sub.3] IV(?)     a = 5.58,  b = 7.52,  c = 6.58

Substance             Space group

[C.sub.6][H.sub.6]-I  Pbca
[CS.sub.2]            Cmca
[Br.sub.2]            Cmca
[CCl.sub.4] I
[CCl.sub.4] II        Cc or C2/c

[CCl.sub.4] III       [C222.sub.1]

[KNO.sub.3] III

[KNO.sub.3] IV(?)     [P2.sub.1]nb or Pmnb

(a)All unit cell dimensions are given in [Angstrom] with estimated
uncertainties of [+ or -] 2 in the last decimal place given and
uncertainties of [+ or -] 5[degrees] in angles.
Table 5

Lattice parameters of some inorganic azides obtained at various
pressures by single crystal x-ray diffraction at RT (14)

Substance              Crystal system    Pressure (bar)

pb[([N.sub.3]).sub.2]  Orthorhombic               1
                                             10 395
                                             22 210
pb[([N.sub.3]).sub.2]  Monoclinic                 1
                                             10 395
Ba[([N.sub.3]).sub.2]  Monoclinic                 1
                                             10 395
                                             22 210
[KN.sub.3]             Tetragonal                 1
                                             10 395
                                             22 210
[TIN.sub.3]            Tetragonal                 1
                                              2 990
[NaN.sub.3]            "Monoclinic" (b)           1
                                             10 395

Substance              a ([10.sup.-8] cm)  b ([10.sup.-8] cm)

pb[([N.sub.3]).sub.2]     11.31               16.25
                          11.08(5) (a)        16.16(3)
                          10.83(1)            16.14(1)
pb[([N.sub.3]).sub.2]     18.46(8)             8.909(8)
                          18.01(9)             8.774(8)
Ba[([N.sub.3]).sub.2]      5.435(4)            4.401(1)
                           5.395(4)            4.345(3)
                           5.375(9)            4.316(3)
[KN.sub.3]                 6.072
[TIN.sub.3]                6.196(8)
[NaN.sub.3]                6.630(2)            3.640(2)
                           6.098               3.593

Substance              c ([10.sup.-8] cm)  [beta](deg)

pb[([N.sub.3]).sub.2]  6.63
pb[([N.sub.3]).sub.2]  5.093(6)             106.2(2)
                       5.065(5)             105.9(2)
Ba[([N.sub.3]).sub.2]  9.611(4)              99.67(8)
                       9.553(6)              99.8(2)
                       9.47(2)              101.2(5)
[KN.sub.3]             7.144
[TIN.sub.3]            7.376(7)
[NaN.sub.3]            5.299(2)             111.5(5)
                       5.288                106.0

Substance              V([10.sup.-24] [cm.sup.3])

pb[([N.sub.3]).sub.2]    1218.0
pb[([N.sub.3]).sub.2]     804.4(4)
Ba[([N.sub.3]).sub.2]     226.2(2)
[KN.sub.3]                263.4
[TIN.sub.3]               282.2(6)
[NaN.sub.3]               118.9(2)

(a)Number in parentheses represent deviations in the last significant
figure shown resulting from least- squares fitting process. Where
standard deviations are not shown either the parameters were ottained
from the literature or there was insufficient data to do a meaningful
least-squares refinement.

(b)[NaN.sub.3] assumed monoclinic at 9 bar and the parameters listed
derived on that basis.
Table 6

Observed and calculated structure factors for [CCl.sub.4] III (15)

 h  k  l         [F.sub.obs]  [F.sub.cale]

 0  1  1           26.575        26.000
 0  1  2           18.833        19.072
 0  1  3           46.223        42.693
 0  1  5           31.326        33.628
 0  1  6           25.074        22.580
 0  2  4           23.671        22.330
 0  2  3           18.115        18.058
 0  2  1           25.379        22.896
 0  2  2           17.702        21.996
 0  3  1           20.073        19.217
 0  3  4           21.095        27.875
 0  4  1           11.308         6.888
 0  1  4           24.031        28.600
 0  0  2           33.955        27.809
 0  0  4 (*)(a)    15.091        13.549
 0  0  6 (*)       33.846        22.551
 0  2  0 (*)       83.291        80.627
 0  4  0           19.974        23.681
-1  1  3           88.900        95.104
-1  1  1           52.463        53.041
 1  1  4           30.050        31.799
 1  1  5           28.924        29.708
 1  1  7           29.800        27.337
 3  3  3           19.032        17.331
 2  2  4           14.399        15.029
 2  2  6           20.409        17.126
 1  1  3           15.151        13.167
 1  1  6           15.526        14.143
-2  2  1           17.154        11.715
 2  2  3           27.797        32.311
-2  2  4           33.556        37.779
 3  3  1           13.773        13.357
 3  3  2            7.513         9.314
-3  3  2           24.416        25.312
-2  2  7           11.770        14.422
-3  3  4           14.149        14.452
-3  3  7           11.645         7.326
 4  4  2           14.024         9.977
 2  1  5           28.285        31.299
-2  1  1           39.141        40.119
-2  1  2           68.795        80.704
-2  1  3           36.280        44.797
-2  1  5           20.399        19.128
-2  1  6           13.978         9.701
 2  1  1           18.181        17.012
 2  0  2          136.471       133.854
 1  1  1           37.950        37.705
 3  1  3           31.748        32.344
 1  2  1          115.847       114.322
 1  3  1           17.797        13.575
 2  2  2 (*)       36.732        40.364
 3  2  3           39.749        42.664

(a)The observed structure factors are for the combined sets of data
(reduced to 52 unique reflections) and the calculated structure factors
are based on the atomic coordinates of the reference structure. Those
reflections marked with an asterisk have averaged observed structure
factor values which resulted from combining duplicate reflections from
the separate groups of data. These data are shown to illustrate the full
potential of the method in attempting an analysis of an unknown high
pressure polymorph using a relatively high x-ray scattering molecule
compared to those materials studied earlier.
Table 7

Lattice constants for some tetrahalogens with the [SnBr.sub.4]-type
structure (space group), [P2.sub.1]/c) [15]

Substance                     [a.sub.1] [Angstrom]  [b.sub.1] [Angstrom]

[SnBr.sub.4] (20 [degrees]C)  10.59[+ or -]0.03     7.10[+ or -]0.02
[TiCl.sub.4] (-32 [degree]C)   9.70                 6.48
[SnCl.sub.4] (-39 [degree]C)   9.85                 6.75
[TiBr.sub.4] (20 [degrees]C)  10.17[+ or -]0.02     7.09[+ or -]0.01
[CCl.sub.4] III (10 kbar)      9.079[+ or -]0.021   5.764[+ or -]0.003

Substance                     [c.sub.1] [Angstrom]  [beta][degrees]

[SnBr.sub.4] (20 [degrees]C)  10.66[+ or -]0.03     103.6[+ or -]0.2
[TiCl.sub.4] (-32 [degree]C)   9.75                 102.67
[SnCl.sub.4] (-39 [degree]C)   9.98                 102.25
[TiBr.sub.4] (20 [degrees]C)  10.41[+ or -]0.01     102.0[+ or -]0.2
[CCl.sub.4] III (10 kbar)      9.201[+ or -]0.003   104.29[+ or -]0.05

Substance                     a:b:c

[SnBr.sub.4] (20 [degrees]C)  1.492:1:1.501
[TiCl.sub.4] (-32 [degree]C)  1.495:1:1.501
[SnCl.sub.4] (-39 [degree]C)  1.460:1:1.480
[TiBr.sub.4] (20 [degrees]C)  1.434:1:1.468
[CCl.sub.4] III (10 kbar)     1.575:1:1.596


I would like to thank the editors of this special issue, Winnie Wong-Ng and Alan Mighell, for their constant encouragement during the writing this paper. I'm particularly indebted to Alan Mighell and Vern Bean for their many suggestions and recommendations concerning the accuracy of the content of the paper and for their thorough and careful reading of the manuscript.

Accepted: August 22, 2001

Available online:

(#.) Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

7. References

(1.) C. E. Weir, E. R. Lippincott, A. Van Valkenburg, and E.N. Bunting, Infrared Studies in the 1- to 15-Micron Region to 30,000 Atmospheres, J. Res. Natl. Bur. Stand. (U.S.) 63A, 55 (1959).

(2.) J. C. Jamieson, A. W. Lawson, and N. D. Nachtrieb, New Device for Obtaining X-Ray Diffraction Patterns from Substances Exposed to High Pressure, Rev. Sci. Instrum. 30, 1016 (1959).

(3.) G. J. Piermarini and C. E. Weir, A Diamond Cell for X-ray Diffraction Studies at High Pressures, J. Res. Natl. Bur. Stand. (U.S.) 66A, 325 (1962).

(4.) G. J. Piermarini and C. E. Weir, High--Pressure Transition in RbF, J. Chem. Phys. 37, 1887 (1962).

(5.) C. E. Weir and G. J. Piermarini, Lattice Parameters and Lattice Energies The lattice energy of an ionic solid is a measure of the strength of bonds in that ionic compound. It is given the symbol U and is equivalent to the amount of energy required to separate a solid ionic compound into gaseous ions.  of High Pressure Polymorphs of Some Alkali Halides, J. Res. Natl. Bur. Stand. (U.S.) 68A, 105 (1964).

(6.) G. J. Piermarini and C. E. Weir, Allotropy allotropy (əlŏ`trəpē) [Gr.,=other form]. A chemical element is said to exhibit allotropy when it occurs in two or more forms in the same physical state; the forms are called allotropes.  in Some Rare-Earth Metals at High Pressures, Science 144, 69 (1964).

(7.) A. Van Valkenburg, Visual Observations of High Pressure Transitions, Rev. Sci. Instrum. 33, 1462 (1962).

(8.) S. Block, C. E. Weir, and G. J. Piermarini, High Pressure Single Crystal Studies of Ice VI, Science 148, 947 (1965).

(9.) C. E. Weir, S. Block, and G. J. Piermarini, Single Crystal X-ray Diffraction at High Pressures, J. Res. Natl. Bur. Stand. (U.S.) 69C, 275 (1965).

(10.) C. E. Weir, G. J. Piermarini, and S. Block, Instrumentation for Single Crystal X-ray Diffraction at High Pressures, Rev. Sci. Instrum. 40, 1133 (1969).

(11.) A. Santoro, C. E. Weir, S. Block, and G. J. Piermarini, Absorption Corrections in Complex Cases. Application to Single Crystal Diffraction Studies at High Pressure, J. Appl. Cryst. 1, 101 (1968).

(12.) G. J. Piermarini, A. D. Mighell, C. E. Weir and S. Block, Crystal Structure of Benzene II at 25 Kilobars, Science 165, 1250 (1969).

(13.) C. E. Weir, G. J. Piermarini, and S. Block, Crystallography of Some High-Pressure Forms of [C.sub.6][H.sub.6], [CS.sub.2], [Br.sub.2], [CCl.sub.4], and [KNO.sub.3], J. Chem. Phys. 50, 2089 (1969).

(14.) C. E. Weir, S. Block, and G. J. Piermarini, Compressibility of Inorganic Azides, J. Chem. Phys. 53, 4265 (1970).

(15.) G. J. Piermarini and A. B. Braun, Crystal and Molecular Structure of CCI CCI Chambre de Commerce et d'Industrie (France)
CCI CAM (Complementary and Alternative Medicine) Citation Index
CCI Chamber of Commerce and Industry (Western Australia) 
4 Ill: A High Pressure Polymorph at 10 kbar, J. Chem. Phys. 58, 1974 (1973).

(16.) Sir William Thomson, Lecture to the Institution of Civil Engineers Founded on 2 January 1818, the Institution of Civil Engineers (ICE) is an independent professional association, based in central London, representing civil engineers. Like its early membership, the majority of its current members are British engineers, but it also has members in , May 3, 1883, in Popular Lectures and Addresses, Vol. 1, Macmillan and Co., London, New York New York, state, United States
New York, Middle Atlantic state of the United States. It is bordered by Vermont, Massachusetts, Connecticut, and the Atlantic Ocean (E), New Jersey and Pennsylvania (S), Lakes Erie and Ontario and the Canadian province of
 (1891) p. 80.

(17.) R. A. Forman, G. J. Piermarini, J. D. Barnett J.D. Barnett is a former head basketball coach at several Division I institutions, the most high-profile being the University of Tulsa. He is also well known for being a mentor of Tubby Smith, the current head coach at the University of Minnesota. , and S. Block, Pressure Measurement Made by the Utilization of Ruby SharpLine Luminescence luminescence, general term applied to all forms of cool light, i.e., light emitted by sources other than a hot, incandescent body, such as a black body radiator. , Science 176, 284 (1972).

(18.) G. J. Piermarini, S. Block, J. D. Barnett, and R. A. Forman, Calibration of the Pressure Dependence of the Rl Ruby Fluorescence Line to 195 kbar, J. AppI. Phys. 46, 2774 (1975).

(19.) G. J. Piermarini and S. Block, Ultrahigh Pressure DiamondAnvil Cell and Several Semiconductor Phase Transition Pressures in Relation to the Fixed Point Pressure Scale, Rev. Sci. Instrum. 46, 973 (1975).

(20.) J. D. Barnett, S. Block and G. J. Piermarini, An Optical Fluorescence System for Quantitative Pressure Measurement in the Diamond-Anvil Cell, Rev. Sci. Instrum. 44, 1 (1973).

(21.) G. J. Piermarini, S. Block, and J. D. Barnett, Hydrostatic Limits in Liquids and Solids to 100 kbar, J. Appl. Phys. 44, 5377 (1973).

(22.) G. J. Piermarini, R. A. Forman, and S. Block, Viscosity Measurements in the Diamond Anvil Pressure Cell, Rev. Sci. Instrum. 49, 1061 (1978).

(23.) R. G. Munro, S. Block, and G. J. Piermarini, Correlation of the Glass Transition and the Pressure Dependence of Viscosity in Liquids, J. Appl. Phys. 50, 6779 (1979).

(24.) I. Fujishiro, G. J. Piermarini, S. Block, and R. G. Munro, Viscosities and Glass Transition Pressures in The Methanol-Ethanol-Water System, Proceedings of the 8th AIRAPT AIRAPT International Association for the Advancement of High Pressure Science and Technology  Conference on High Pressure in Research and Industry, C. M. Backman, T. Johannison, and L. Tegner, University of Uppsala, Sweden, Vol. 2, August 17-22, 1981.

(25.) R. G. Munro, F. A. Mauer, G. J. Piermarini, and S. Block, Radial Distribution Studies of Amorphous Fe-W and Ni-P at high Pressure, J. Appl. Phys. 54, 5698 (1983).

(26.) R. K. Eby, E. S. Clark, B. L. Farmer, G. J. Piermarini, and S. Block, Crystal Structure of poly(tetrafluoroethylene) homo- and copolymers in the high pressure phase, Polymer 31, 2227 (1990).

(27.) F. A. Mauer, C. R. Hubbard, C. J. Piermarini, and S. Block, Measurement of Anisotropic Compressibilities by a Single Crystal Diffractometer Method, in Advances in X-Ray Analysis, Vol. 18, W.L. Pickles Pickles may refer to
  • Pickled cucumber
  • Other vegetables that have been pickled
  • Pickles (comic strip), a comic strip by Brian Crane
  • Pickles (dog), the dog that found the World Cup trophy in 1966
  • "Pickles" (
, C. S. Barrett, J.B. Newkirk, and C. O. Ruud, eds., Plenum In a building, the space between the real ceiling and the dropped ceiling, which is often used as an air duct for heating and air conditioning. It is also filled with electrical, telephone and network wires. See plenum cable.  Publishing, New York, NY (1975) p. 437.

(28.) Private communication (2001), Eric Van Eric M. Van (b. May 8 1954, Boston, Massachusetts) is an American sabermetrician, science fiction convention organizer and critical public speaker, and rock music critic.

Raised in Natick, he graduated from Northfield Mt.
 Valkenburg, High Pressure Diamond Optics Co., Tucson, Arizona Tucson (pronounced /ˈtusɑn/, Spanish: Tucsón [tuk'son] .

About The Author: The author has been associated with NIST/NBS since 1957. Much of his professional career was devoted to the development and application of the diamond anvil pressure cell to numerous scientific measurement techniques including powder and single crystal x-ray diffraction, infrared spectroscopy, Raman scattering Raman scattering or the Raman effect is the inelastic scattering of a photon.

When light is scattered from an atom or molecule, most photons are elastically scattered (Rayleigh scattering).
, polarizing microscopy, and viscosity. His research also focused on the development and calibration of the ruby-fluorescence method of pressure measurement and the utilization of the technique in many high pressure applications. Among these were the first use of the technique to measure pressures at elevated temperatures, and the determination of hydrostatic limits in many liquid and solid pressuretransmitting media. During his professional career, Dr. Piermarini collaborated with a small group of NIST/NBS scientists and also occasionally with scientists from universities and other government laboratories. Dr. Piermarini retired from NIST in 1995 and is presently a Guest Scientist in t he Ceramics Division of the NIST Materials Science and Engineering Materials science and engineering

A multidisciplinary field concerned with the generation and application of knowledge relating to the composition, structure, and processing of materials to their properties and uses.
 laboratory. The National Institute of Standards and Technology is an agency of the Technology Administration, U.S Department of Commerce.
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Title Annotation:National Institute of Standards and Technology/National Bureau of Standards
Author:Piermarini, Gasper J.
Publication:Journal of Research of the National Institute of Standards and Technology
Geographic Code:1USA
Date:Nov 1, 2001
Previous Article:Crystallographers at NBS/NIST.
Next Article:Past and present crystallographic work at the NBS/NIST reactor.

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