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ABSTRACT. This paper discusses the preparation and crystal structure determination of a new ternary telluride RbA[g.sub.2][Te.sub.2] using the molten salt (flux,) method. The compound crystallizes in the tetragonal crystal system, space group 14/mmm (No. 139), a = 4.550(3) A, c=15.207(8) A, V=314.8(3) [A.sup.3], Z=2, with R1=0.0465, and wR2=0.0940 for observed reflections. Rb[Ag.sub.2][Te.sub.2] has a two-dimensional layered structure and is isostructural to Cs[Fe.sub.0.72][Ag.sub.1.28][Te.sub.2].


The molten salt method, also known as polychalcogenide flux growth method (Bloom,1967; Mamantov, 1969; Sunshine and Ibers, 1987; Kanatzidis, 1990) has been successfully adopted in the synthesis of metal chalcogenides known as materials potentially useful in various industrial applications. Employing this technique in an intermediate temperature region (300-600[degrees]C) we have synthesized a number of new compounds over the last several years. Examples include binary Ba[Te.sub.2] (Li et al., 1994), ternary [Ta.sub.3]Cu[Te.sub.4] (Li et al., 1995), Cs[Ag.sub.5][Te.sub.3] (Li et al., 1995), [Ba.sub.2]Sn[Te.sub.5] (Li et al., 1996), quaternary RbTa[Cu.sub.2][Te.sub.4] (Li et al., 1995), [K.sub.2][Ag.sub.2]Sn[Te.sub.4] (Li et al., 1995), [K.sub.2]BaSn[Te.sub.4] (Li et al., 1995), Cs[Fe.sub.0.72][Ag.sub.1.28][Te.sub.2] (Li et al., 1995), [K.sub.0.33][Ba.sub.0.67]Ag[Te.sub.2](Zhang et al., 1995), and KCuZn[Te.sub.2] (Heulings et al., 1998). In this article, we describe the synthesis of a new ternary telluride, Rb[Ag.sub.2][Te.sub.2], prepared from rubidium polytelluride melt at 450[degrees]C, its crystal structure determination and relation to previous reported AMM'[Te.sub.2] systems (M, M' = a transition or post-transition metal).



[Rb.sub.2]Te was prepared by reactions of rubidium metal and elemental tellurirm in a 2:1 ratio in liquid ammonia. Rb (99.5%, Aldrich Chemical Company), Ag (99.9%, Aldrich Chemical Company) and Te (99.8%, Strem Chemicals, Inc.)

Synthesis of Rb[Ag.sub.2][Te.sub.2]

The reactants, 0.0746 g of [Rb.sub.2]Te, 0.0539 g of Ag and 0.1595 g of Te were weighed in an inert argon-filled glove box. After thorough mixing the reactants were transferred to a 9 mm OD thin-walled Pyrex reaction tube. The sample was covered with parafilm before removal from the inert chamber. The sample was then immediately sealed under an approximately [10.sup.-3] torr vacuum. The reaction vessel was then placed in a furnace and brought up to 425[degrees]C within 8 hours. After heating at 425[degrees]C for 3 days, the container was slowly cooled to 150[degrees]C (2[degrees]C/hr) followed by naturally cooling to room temperature. Upon removal from the furnace, the sample was treated by an isolation procedure. The reaction mixture consisted of the final products imbedded in the excess alkali-metal polychalcogenide melt. In order to remove the remaining flux, several washes with DMF (N, N-Dimethylformamide) were performed in a nitrogen atmosphere. The sample was then washed twice with 95% ethanol and dried with diethyl ether. Black block-like crystals were isolated after this procedure. Microprobe analysis using a JEOL JXA-8600 Superprobe was performed on selected single crystals which determined the approximate elemental ratios of Rb, Ag, and Te.

Single Crystal Structure Determination

A black block-like crystal of dimensions 0.20 x 0.20 x 0.40 [mm.sup.3] was mounted on a glass fiber at room temperature (294 [+ or -] 1[degrees]K). The single crystal x-ray diffraction experiments were performed on an Enraf-Nonius CAD4 deffractometer equipped with a graphite monochromatized Mo-K[alpha] radiation. The unit cell parameters were obtained from 25 well-centered reflections within the range 4.6[degrees] [less than] [theta] [less than] 13.9[degrees]. Table 1 illustrates the results from the full data collection and refinement of the unit cell data. Data collection was monitored by three standard reflections every two hours. No decay was observed with the exception of statistic fluctuation in the range of [+ or -] 0.9%. Raw intensities were corrected for Lorentz and polarization effects, and for absorption by empirical method based on [psi]-scan data (Kopfmann and Huber, 1968). Direct phase determination yielded the positions of Rb, Ag, and Te atoms and all were subjected to anisotropic refinement. T he final full-matrix least-square refinement on [F.sub.2] converged with R1=0.0465 and Wr2=0.0940 for observed reflections. The data collection was performed by the CAD4/PC program. Calculations were performed using the SHELX97 program package (Sheldrick, 1997) on an IBM PC 586 computer. Analytic expressions of atomic scattering factors were employed, and anomalous dispersion corrections were incorporated (International Tables for X-ray Crystallography, 1989). The crystal drawings were produced by SCHAKAL 92 (Keller, 1992). Final atomic coordinates, average temperature factors, selected bond lengths and angles are listed in Tables 2 and 3.


The crystal structure of [RbAg.sub.2][Te.sub.2] was analyzed and determined by single crystal X-ray diffraction method. The compound was found to crystallize in the tetragonal crystal system, space group 14/mmm (No. 139). As shown in Figure 1, the structure is a two-dimensional layered network with the Ag ions being tetrahedrally coordinated to Te. The Ag-Te interatomic distance, 2.858(1)A compared well with those observed in [CsFe.sub.0.72][Ag.sub.1.28][Te.sub.2], 2.7853(4)A (Li et al., 1995). The silver atoms form a square lattice with a Ag-Ag distance of 3.2173(7) A, also similar to that reported for [CsFe.sub.0.72][Ag.sub.1.28][Te.sub.2], 3.1861(3) A. The two-dimensional layers of ([Ag.sub.2][Te.sub.2]) are separated by [Rb.sup.+] counterions. The coordination of Rb atoms to Te is eight-fold (cubic) with a unique distance of 3.827(1) A, as shown in Figure 2. While the title compound is isostructural with [CsFe.sub.0.72][Ag.sub.1.28][Te.sub.2] (Li et al., 1995) and [KcuZnTe.sub.2] (Heulings et al., 1998), it d iffers from the two previous reported structures in that the Wycoff position 4d is solely occupied by Ag ions, which are shared by Fe and Ag in [CsFe.sub.0.72][Ag.sub.1.28][Te.sub.2] and by Cu and Zn in [KCuZnTe.sub.2]. [RbAg.sub.2][Te.sub.2] can be considered as a mono-telluride since there is no short Te-Te contact in this compound. An oxidation state assignment of Rb(+1), Ag(+1) leaves Te a -1.5 charge which suggests that the 5p orbitals of Te are only partially occupied. It is therefore likely that the title compound exhibits metallic behavior.


Financial support from the National Science Foundation (Grant DMR9553066) is greatly appreciated. JL is grateful to the Camille and Henry Dreyfus Foundation for the Henry Dreyfus Teacher-Scholar Award.

* This paper is based on an award-winning student presentation by the first author at the Annual Meeting of the New Jersey Academy of Science, 1998.


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 Data Collection and Processing parameters
Molecular formula Rb[Ag.sub.2] [Te.sub.2]
Molecular weight 556.41
Color and habit black block
Crystal size 0.20 x 0.20 x 0.40 [mm.sup.3]
Crystal system tetragonal
Space group 14/mmm (No. 139)
Unit cell parameters a = 4.550(3) A, c = 15.207(8) A
 v=314.8(3) [A.sup.3], Z=2
F(000) 470
Density (calcd) 5.870 g/[cm.sup.3]
Diffractometer Enraf-Nonius CAD4
Radiation graphite-monochromatized Mo
 K[alpha], [lambda]=0.71073 A
Refs used for cell 25, 4.6[degrees] [less than] [theta]
 measurement [less than] 13.9[degrees]
Standard reflections (-1,0,3); (0,-2,0);(1,-1,0)
Intensity variation [+ or -]0.9%
Absorption coefficient 22.832 [mm.sup.4]
Transmission factor 0.4965-0.9991
Scan type and rate [omega] scan
Scan range (0.60 + 0.35 tan [theta])[degrees]
Data collection range -6[less than]h[less than]6,0[less than]
 k[less than]6,0[less than]/[less than]
 20; 2[degrees][less than][theta]
 [less than]28[degrees]
Reflections measured
 total: 440
 unique (n): 141
 observed [I[greater than
 or equal to] 2[sigma](I)]: 131
No. of variables, p 10
Weighting scheme w = 1/[[sigma].sup.2]([F.sup.2].sub.o])
 +25.00pl, p = ([[F.sup.2].sub.o]) +
R1 [a] 0.0512 (all data) 0.0465 (observed data)
wR2 [b] 0.0977 (all data) 0.0940 (observed data)
Goof (S) [c] 1.190
Largest and mean [delta]/[sigma] 0.084, 0.025
Largest peak and hole +2.400 to -3.296 e [A.sup.-3]
(a.)R1 = [sigma]\[F.sub.o]
[absolute val. of -][F.sub.c]\/,
[sigma][absolute val. of [F.sub.o]]
(b.)wR2 = [square root][sigma][w
[([[F.sup.2].sub.o] - [[F.sup.2].sub.c]).sup.2]]/
(c.)Goof = S = [square root][sigma][w
[([[F.sup.2].sub.o] - [[F.sup.2].sub.c]).sup.2]]/n - p
 Atomic coordinates and equivalent isotropic
 temperature factors [*] ([A.sup.2])
Atoms x y z [U.sub.eq]
Rb(1) 0.0000 0.0000 0.5000 0.0374(7)
Ag(1) 0.0000 0.5000 0.2500 0.0701(7)
Te(1) 0.0000 0.0000 0.13622(9) 0.0346(3)
(*.)[U.sub.eq] defined as one third of the trace of the orthogonalized U
 Bond lengths (A) and bond angles ([degrees])
Rb(1)-Rb(1)X4 4.550(1) Ag(1)-Ag(1)(4 3.2173(7)
Rb(1)-Te(1)X8 3.827(1) Ag(1)-Te(1)(4 2.858(1)
Te(1)-Ag(1)-Te(la) 111.50(2) Ag(1)-Te(1)-Ag(la) 68.50(2)
Te(1)-Ag(1)-Te(lb) 111.50(2) Ag(1)-Te(1)-Ag(lb) 68.50(2)
Te(1)-Ag(1)-Te(lc) 105.49(5) Ag(1)-Te(1)-Ag(ld) 105.49(5)
Te(la)-Ag(1)-Te(lb) 105.49(5) Ag(la)-Te(1)-Ag(lb) 105.49(5)
Te(la)-Ag(1)-Te(lc) 111.50(2) Ag(1a)-Te(1)-Ag(ld) 68.50(2)
Te(lb)-Ag(1)-Te(lc) 111.50(2) Ag(1b)-Te(1)-Ag(ld) 68.50(2)
Symmetry transfomrations:
a (0.5-x, 0.5-y, 0.5-z)
b (-0.5-x, 0.5-y, 0.5-z)
c (x, 1+y, z)
d (x,-1+y,z)
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Publication:Bulletin of the New Jersey Academy of Science
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Date:Mar 22, 1999

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