Anomalous resistivity and thermopower of the spinel-type compounds CuIr2S4and CuIr2Se4

Resistivity (rho) 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 insu lating state of CuIr2S4. The positive thermopower of the insulating phase i mplies p-type charge carriers, in agreement with the recent photoemission r esults. The low-temperature resistivity of CuIr2S4 is in good agreement wit h the Efros-Shklovskii variable-range hopping conductivity mechanism: rho = rho(0)exp[(T*/T)(1/2)]. The most striking result is that the resistivity o f 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 unu sual result for metals type of the resistivity temperature dependence, as w ell as other features in the transport properties, imply a nonconventional conductivity mechanism. The magnetoresistivity Delta rho is positive and pr oportional to H-2, while magnetothermopower Delta S = S(H, T) - S(O,T) is v ery small for both compounds at all temperatures.