The crystal structure of CuAsSe 0.8S 0.2 and related compounds

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The structure of CuAsSe x S 1− x in the composition range 0.5 < x < 0.9 has been determined by single-crystal X-ray and electron diffraction and high-resolution electron microscopy. CuAsSe 0.8S 0.2 is found to be orthorhombic with space group Pbcn and unit cell dimensions a = 11.66 ± 0.02 åA, b = 6.73 ± 0.01 Å, and c = 25.41 ± 0.03 Å. The structure was inferred by comparing the electron and X-ray diffraction patterns with those from CuAsSe, and was isotropically refined to R = 0.16 for 287 observed X-ray reflections. High-resolution electron microscopy was used to confirm the structure by matching observed and calculated images and to show the presence of stacking faults in the crystals. The atomic positions of CuAsSe 0.8S 0.2 are related to the 8 HZnS polytype structure. In Jagodzinski nomenclature it has hccchccc stacking. Powder X-ray and single crystal electron diffraction patterns of CuAsSe 0.75S 0.25 and CuAsSe 0.9S 0.1 were consistent with the same structure-type. Single-crystal X-ray diffraction of CuAsSe 0.6S 0.4 showed it to be isostructural with CuAsS with orthorhombic space group Pnam and unit cell dimensions a = 11.59 ± 0.02 Å, b = 5.56 ± 0.01 Å, and c = 3.827 ± 0.005 Å. The same structure-type was found for CuAsSe 0.5S 0.5 and CuAsSe 0.67S 0.33 using X-ray powder diffraction.