EVIDENCE OF HOPPING CONDUCTION IN THE V4-CLUSTER COMPOUND GAV4S8

The electrical resistivity and Seebeck coefficient of ferromagnetic semiconductors GaV4S8 and GaV4Se8 between 20 K and 350 K have been measured. These compounds have a cation-deficient spinel composition with Gallium-vacancy ordering and clustering of vanadium atoms into metallic bonded tetrahedra. For the stoichiometric phase, the Arrhenius plots of resistivity show a strong downward curvature; the activation energy E(a)rho of GaV4S8 varies from 0.14 eV at room temperature to 0.04 eV at 80 K. We also report AC conductivity measurements between 400 Hz and 200 kHz on stoichiometric GaV4S8. We find that the sigma(AC)(omega, T) behaviour is similar to that of a Fermi glass and sigma(AC)(omega)) approximately omega(s); s = 0.3-0.8 between 50 and 20 K. The substitution of sulphur by selenium or imperfect ordering of Ga vacancies results in a constant E(a) of about 0.13 eV over a wide temperature interval. The thermopower behaviour is very sensitive to the quality of the sample and indicates charge transport in a narrow band. These properties are discussed in relation to the special structural features of cluster compounds.