Abstract
GaMo4Se8 (I) and AlMo4S8 (II) transform at T1 = 45(2) K and 24(2) K, respectively, from a cubic high-temperature modification with ordered defect spinel structure (space group F[unk]3m, a = 10.1747(2) Å (I), 9.7289(2) Å (II) at 300 K) to a rhombohedrally distorted low-temperature modification (R3m, arh = 7.1537(3) Å, 6.8500(3) Å (II), αrh = 60.572(5)°, 60.374(5)° (II) at 10 K). Structure parameters for both modifications are refined by the Rietveld method from X-ray powder data recorded at T = 300 K (I, II), 60 K (I), 50 K (II) and 10 K (I, II) on a Guinier diffractometer. In the cubic modification the tetrahedral Mo4 clusters have [unk]3m symmetry and intracluster bond lengths of [Mo – Mo] = 2.881(2) Å (I), 2.829(2) Å (II). During the phase transition their symmetry is reduced to 3m as they expand each by about 0.05 Å at one of their triangular bases.
