Evolution of the electronic structure in partially filled skutterudites: Cax Co4 Sb12 studied using NMR

C 59 o nuclear magnetic resonance (NMR) investigations are presented for the partially filled skutterudites Cax Co4 Sb12 with x≤0.2. For low Ca concentrations (x≤0.13), the temperature-dependent isotropic Knight shifts and spin-lattice relaxation rates are accounted for by semiconducting characteristics, with a trend of the reduction in the band gap as increasing the Ca content. For a higher Ca concentration (x=0.2), the NMR features can be described well in terms of a semimetallic response as the corresponding Fermi level falling within a pseudogap formed by nearby bands. With these respects, the present NMR results provide a microscopic picture that a semiconducting-semimetallic transition occurs as the filling fraction 0.13<x<0.2 in Cax Co4 Sb12.