Effect of Ni on the transport and magnetic properties ofCo1−xNixSb3

The filled skutterudite compounds based on the binary skutterudite ${\mathrm{CoSb}}_3$ are currently being investigated for their potential applications 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, since it has one more electron in its valence shell than Co. Up to now, however, only high concentrations of Ni in ${\mathrm{CoSb}}_3$ have been studied; and the valence of Ni in this compound and its influence on the transport and magnetic properties has been an open question. We present electrical resistivity, thermopower, Hall effect, magnetoresistance, and magnetic susceptibility measurements on polycrystalline, n-type ${\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Sb}}_3$ with $x=0,$ 0.001, 0.003, 0.005, 0.0075, and 0.01. Our results show that in these low concentrations Ni has a dramatic effect on the transport properties. A two-band model is proposed that takes into account transport in both the conduction band and within donor impurity states formed by the Ni impurities. This model provides a consistent explanation of both the anomalous low-temperature transport properties as well as the Curie-Weiss behavior of the magnetic susceptibility. We conclude that Ni takes the tetravalent ${\mathrm{Ni}}^{4+}$ state, assumes the $d^6$ electronic configuration for the lower energy nonbonding orbitals, and gives an electron to the conduction band.