STRUCTURAL PHASE-TRANSITION IN GAMO4S8 BY X-RAY-POWDER DIFFRACTION

GaMo4S8 transforms at T(l) = 45(5) K from a cubic high-temperature modification with ordered defect spinel structure (space group F43mBAR, a = 9.7356(2) angstrom at 300 K) to a rhombohedrally distorted low-temperature modification (R3m, a(rh) = 6.8506(2) angstrom, alpha-rh = 60.533(1)-degrees at 8 K). The transition is of first order, shows a hysteresis of about 4 K, and leaves about 30% of the cubic phase untransformed. Lattice parameters as a function of temperature are reported between 8 K and 300 K, and structure refinements are performed by the Rietveld method from data recorded on a Guinier diffractometer at 8 K and 300 K. The tetrahedral Mo4 clusters expand during the phase transition at one of their triangular bases, [Mo-Mo] DELTA = 2.814(3) angstrom at T = 300 K; 2.89(1) angstrom at T = 8 K, while the other intra-cluster distances, [Mo-Mo DELTA] = 2.814(3) angstrom at T = 300 K; 2.81(2) angstrom at T = 8 K, and inter-cluster distances, [Mo4-Mo4] = 4.070(3) angstrom at T = 300 K; 4.02-4.04(2) angstrom at T = 8 K, remain approximately constant.