Crystal structure of Cu2Cd2/3S2: Interlayer short-range order of Gd vacancies

摘要

The average structure of ternary sulfide Cu2Gd2/3S2 has been refined using single-crystal X-ray diffraction data: space group P (3) over bar, a = b =3.899(1), c = 6.421(2) Angstrom, Z = 1, R-F = 0.036, and wR(F) = 0.041 for 418 reflections and 8 structural parameters. The average structure is based on hexagonal close packing of S with Cu in tetrahedral sites and Gd in octahedral sites, the latter sites being occupied to 2/3. Diffuse streaks observed by electron diffraction, however, indicate a trigonal strict periodicity of A = 6.753 Angstrom = root 3a within (001) and vague period of C = 12.842 Angstrom = 2c, suggesting a layer structure with an intralayer order and interlayer short-range order of Gd vacancies. A model of the real structure has been proposed, and the diffuse scattering has been simulated by the matrix method for one-dimensional disorder. The calculated intensity distribution corresponds to the observed distribution of the diffuse streaks in the electron diffraction pattern and to the intensity distribution measured on an X-ray single-crystal diffractometer; the total X-ray powder diffraction pattern of Cu2Gd2/3S2 can be reproduced almost completely by calculation. (C) 2000 Academic Press. [References: 29]