Structural transition of the tetrahedral metal cluster: nuclear magnetic resonance study of GaV4S8

The small rhombohedral distortion (at TS ∼ 44 K), observed for the V4 tetrahedral cluster involved in the cubic compound GaV4S8, has been investigated by means of nuclear magnetic resonance (NMR). 51V NMR experiments successfully monitored the considerable change in the electronic state inside the cluster. The isotropic term of 51V hyperfine coupling is reduced markedly, and instead large anisotropy appears below TS. The 51V nuclear spin-lattice relaxation rate is reduced markedly below TS, suggesting a reduction of the density of states in the spin excitation spectrum. The structural transition can be interpreted as the Jahn-Teller instability resulting from the orbital degeneracy of the highest occupied 3d cluster orbitals in the V4 tetrahedral cluster. The drastic change in the NMR quantities at TS is explained as a transition from the electron occupation in nearly spherical degenerate orbitals to that in a strongly asymmetric non-degenerate orbital