Electronic and Structural Instabilities in GaV 4 S 8 and GaMo 4 S 8

GaV 4 S 8 was prepared by direct heating of the elements at 1123 K. The crystal structures were refined from single-crystal data at room temperature ( RT -GaV 4 S 8, GaMo 4 S 8 -type, F 4̄3 m, a = 9.661(1) Å, Z = 4) and by the Rietveld method at 20 K ( LT -GaV 4 S 8, R 3 m, a = 6.834(1) Å, α rh = 59.66(2)°, Z = 1). Magnetic measurements show weak paramagnetism with a temperature-dependent magnetic moment between 1.5μ B (50 K) and 1.9μ B (300 K) compatible with one unpaired electron per V 4 cluster. Ferromagnetic ordering was detected below T C = 10 K. DC electrical conductivity measurements between 30 and 320 K reveal semiconducting behavior. GaV 4 S 8 undergoes a structural phase transition at 38 K from cubic to rhombohedral symmetry, as known for GaMo 4 S 8, but with an opposite change of the rhombohedral angle α rh . This behavior is explained by self-consistent band structure calculations and simple cluster orbital schemes for GaV 4 S 8 in comparison with GaMo 4 S 8 . The results prove that the directions of the structural deformations are determined by orbital interactions and depend on the electron count in the tetrahedral metal cluster units.