Low Temperature Crystal Structures and Magnetic Properties of the V4-Cluster Compounds Ga1-xGexV4S8

The solid solution Ga1-xGexV4S8 (x = 0 - 1) was synthesized by solid state reactions and characterized by temperature-dependent x-ray powder diffraction and static magnetic susceptibility measurements. The compounds crystallize in the cubic GaMo4S8-type structure (space group F-43m), built up by heterocubane-like [V4S4](5-x)+ cubes and [Ga1-xGexS4](5-x)- tetrahedra arranged in a NaCl-like manner. The successive substitution of Ga3+ by Ge4+ increases the electron count in the molecular orbital (MO) of the V4-cluster gradually from seven to eight. We observe an almost linear increase of the magnetic moments, connected with a transition from ferromagnetic to antiferromagnetic ordering around x <= 0.5. Remarkably, the low temperature structural phase transitions as known from the ternary compounds were also detected in the solid solution. The gallium-rich compounds (0 < x < 0.5) undergo rhombohedral distortions like GaV4S8 (space group R3m), whereas distortions to orthorhombic symmetry (space group Imm2) as known from GeV4S8 occur in the germanium-rich part of the solid solution (0.5 <= x < 1).