X-ray study of thermal expansion and phase transformation in β-In2S3

A study of thermal expansion of β-In2S3 is made using X-ray powder diffraction technique over the temperature range 30 to 800°C. The tetragonal β-phase due to vacancy ordering in the basic spinel structure is retained up to a temperature of 750°C as revealed by the presence of specific tetragonal cell reflections (l ≠ 3n) such as 215, 314, 321 in the high temperature X-ray photographs. A first order reversible phase change β ⇌ γ is observed in the 750 to 800°C range. High temperature (800°C) Laue photographs show a 3-fold symmetry axis perpendicular to the (013) cleavage plane of β-In2S3, and the X-ray powder pattern of the γ-phase at 800°C is indexible on a hexagonal cell with lattice parameters a = 3.85 Å, c = 9.15 Å. The mode of phase change is considered in terms of corrugations mainly of the S atoms existing in the (013) plane of β-In2S3. The structure of the γ-phase is confirmed as trigonal, space group P3ml with a sequence of atoms (S–In–S–In–S), along the c-axis.