Valence of Cr in skutterudites: Electrical transport and magnetic properties of Cr-doped CoSb₃

We investigated the effect of Cr doping in the binary skutterudite compound cobalt triantimonide using electrical resistivity, the Hall effect, x-ray powder diffraction, electron probe microanalysis, and magnetic susceptibility measurements. We find that the solubility limit for Cr in the crystal lattice of the skutterudites Co_1-xCr_xSb₃ is about x=0.01 for induction melted and annealed samples, that all samples are p-type with a doping rate much less than one hole per Cr, that Cr takes the Cr³⁺ charge state and a paramagnetic high-spin d³ electronic configuration associated with three unpaired electrons in the low-energy nonbonding d orbital, that the electrical resistivity is drastically affected by Cr doping, and that the hole mobility is very sensitive to the crystal microstructure.