Structural phase transition in GaMo4Se8 and AlMo4S8 by X-ray powder diffraction

GaMo4Se8 (I) and A1MO 4 S 8 (Π) transform at Tx = 45(2) Κ and 24(2) K, respectively, from a cubic high-temperature modification with ordered defect spinel structure (space group F43m, a = 10.1747(2) A (I), 9.7289(2) Α (Π) at 300 K) to a rhombohedrally distorted low-temperature modification (R3m, ARH = 7.1537(3) A, 6.8500(3) Α (Π), = 60.572(5)°, 60.374(5)° (Π) at 10 K). Structure parameters for both modifications are refined by the Rietveld method from X-ray powder data recorded at Τ = 300 Κ (Ι, Π), 60 Κ (I), 50 Κ (Π) and 10 Κ (I, Π) on a Guinier diffractometer. In the cubic modification the tetrahedral Mo4 clusters have 33m symmetry and intracluster bond lengths of [Mo-Mo] = 2.881(2) A (I), 2.829(2) A (II). During the phase transition their symmetry is reduced to 3M as they expand each by about 0.05 A at one of their triangular bases.