Structural and magnetic phase transitions of the V4-cluster compound GeV4S8

The phase transitions of the strongly correlated tetrahedral V-4-cluster compound GeV4S8 have been studied by low-temperature powder x-ray diffraction, magnetic susceptibility, and specific heat measurements. The crystal structure is cubic at room temperature (space group F43m) and transforms to orthorhombic (space group Imm2) at T-S=30 K. A Jahn-Teller distortion reduces the symmetry of the V-4-cluster from 43m to mm2. The second transition at 18 K is the onset of antiferromagnetic ordering without symmetry change but with a certain increase in the distortion. The latter reflects a strong magnetoelastic coupling at T-N. Specific heat anomalies at 30 and 18 K confirm the two phase transitions.