EVIDENCE OF HOPPING CONDUCTION IN THE V4-CLUSTER COMPOUND GAV4S8

The electrical resistivity and Seebeck coefficient of ferromagnetic se miconductors GaV4S8 and GaV4Se8 between 20 K and 350 K have been measu red. These compounds have a cation-deficient spinel composition with G allium-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 an d 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 substitu tion of sulphur by selenium or imperfect ordering of Ga vacancies resu lts in a constant E(a) of about 0.13 eV over a wide temperature interv al. The thermopower behaviour is very sensitive to the quality of the sample and indicates charge transport in a narrow band. These properti es are discussed in relation to the special structural features of clu ster compounds.