Electronic and structural instabilities in GaV4S8 and GaMo4S8

GaV4S8 was prepared by direct heating of the elements at 1123 K. The crystal structures were refined from single-crystal data at room temperature (RT-GaV4S8, GaMo4S8-type, F (4) over bar 3m, a = 9.661(1) Angstrom, Z = 4) and by the Rietveld method at 20 K (LT-GaV4S8, R3m, a = 6.834(1) Angstrom, alpha (rh) = 59.66(2)degrees, Z = I). Magnetic measurements show weak paramagnetism with a temperature-dependent magnetic moment between 1.5 mu (B)(50 K) and 1.9 mu (B)(300K) compatible with one unpaired electron per V-4 cluster. Ferromagnetic ordering was detected below T-c = 10 K. DG electrical conductivity measurements between 30 and 320 K reveal semiconducting behavior. GaV4S8 undergoes a structural phase transition at 38 K from cubic to rhombohedral symmetry, as known for GaMo4S8, but with an opposite change of the rhombohedral angle alpha (rh). This behavior is explained by self-consistent band structure calculations and simple cluster orbital schemes for GaV4S8 in comparison with GaMo4S8. 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.