Anomalous resistivity and thermopower of the spinel-type compoundsCuIr2S4and

Resistivity (ρ) and thermopower (S) of spinel-type compounds CuIr2S4 and CuIr2Se4 have been measured at temperatures from 2 to 900 K under magnetic field from 0 to 15 T. The thermopower is positive in the metallic phase of both compounds at high temperatures, as well as in the low-temperature insulating state of CuIr2S4. The positive thermopower of the insulating phase implies p-type charge carriers, in agreement with the recent photoemission results. The low-temperature resistivity of CuIr2S4 is in good agreement with the Efros-Shklovskii variable-range hopping conductivity mechanism: ρ=ρ0exp[(T*/T)1/2]. The most striking result is that the resistivity of the metallic phases is well described by an exponential-type temperature dependence in a wide temperature range from 2 K to at least 900 K. This unusual result for metals type of the resistivity temperature dependence, as well as other features in the transport properties, imply a nonconventional conductivity mechanism. The magnetoresistivity Δρ is positive and proportional to H2, while magnetothermopower ΔS=S(H,T)-S(0,T) is very small for both compounds at all temperature