Incommensurate structure of a lillianite-type samarium chromium sulfide Sm2/3Cr2S4

The structure of Sm2/3Cr2S4 has been solved from single-crystal X-ray diffraction data. This compound presents a 3 x a superstructure along a and an incommensurate superstructure along b close to 3 x b. The basic structure is described in the space group Pmnb with a = 3.5030(3) Angstrom, b = 10.9199(8) Angstrom, c = 12.7987(8) Angstrom, Z = 4, R = 0.046 for 916 observed reflections. The modulated structure refinement, taking into account the two superstructures, was carried out in a (3 + 2)D superspace group and leads to R = 0.063 for 1706 reflections. This structure consists of staggered double columns of [CrS6] octahedra connected by samarium atoms in a trigonal prismatic bicapped environment. This structure is reminiscent of that found in the lillianite mineral family. The statistical occupancy of samarium atoms (1/3), in agreement with the charge equilibrium (Sm3+, Cr3+, S2-), is related to the superstructure. (C) 1996 Academic Press.