One-dimensional "pencil" disorder discovered for a class of intermetallic compounds. (General Developments).

Deviations of atoms from an average crystal structure caused by structural distortions, chemical ordering, or the presence of defects cause diffuse intensity in scattering experiments. The diffuse scattering is usually difficult to detect by x-ray or neutron diffraction, but it is readily observed in electron diffraction. Recently, scientists at NIST discovered an unusual planar diffuse scattering for a group of intermetallic compounds, tetragonal [Zr.sub.9][M.sub.11] (M = Pd, Pt, and Ni) and hexagonal [Co.sub.3][Y.sub.4], by using transmission electron microscopy. The near two-dimensional intensity planes are parallel to each other, and the inter-planar distance is incommensurate with fundamental translations of the average crystal structure.

By combining electron microscopy with powder neutron diffraction, the researchers determined that these compounds could be described as having a host periodic structure, but with one-dimensional structural channels. The channels are filled with strings of atoms, and a one-dimensional arrangement of the atoms along the strings has a periodicity that differs from the host structure. There is a weak correlation between the relative positions (phase) of the strings, i.e., the strings are largely independent of each other. In previous x-ray diffraction studies the possible existence of such disorder was proposed in order to explain unusually high temperature factors; however, the unambiguous demonstration of the phenomenon was presented in this NIST work for the first time. It is possible that the presence of the strings could impose special one-dimensional properties in a three-dimensional crystal.

CONTACT: Leonid Bendersky, [phone]; leonid. [e-mail] or Judith Stalick, [phone]; judith.stalick@ nist.gov.