Transport and magnetic properties of Co1-xNixSb3 with x less than 0.01

The filled skutterudite compounds based on the binary skutterudite CoSb₃ are currently being investigated for their potential application as thermoelectric materials. One route to optimization of these compounds is by doping on the Co site. An obvious candidate for an n-type dopant is Ni, because it has one more electron in its valence shell than Co. We present electrical resistivity, thermopower, Hall effect and magnetic susceptibility measurements on polycrystalline, n-type Co_1-xNi_xSb₃ with x less than 0.01. A model which takes into account conduction of electrons residing in the conduction band in addition to hopping conduction within an impurity "band" formed by the Ni atoms is consistent with the observed behavior. The doping dependence suggests Ni acts as an electron donor thus taking the tetravalent state Ni⁴⁺ and assuming the d⁶ electronic configuration. The measured magnetic susceptibility suggests a paramagnetic state implying that Ni may carry a moment at least at low temperatures. We attempt to tie together the transport and magnetic susceptibility data to provide a consistent picture of the overall behavior