NMR investigation of filled skutterudites Ba_xYb_yCo₄Sb₁₂ and A_xCo₄Sb₁₂ (A = Ba, Sr)

Considered as one of the most promising thermoelectric families, skutterudite CoSb₃ shows a high ZT value when prepared with filler atoms such as Ba, Sr, Yb, etc. We report ⁵⁹Co NMR and transport measurements on Ba_xYb_yCo₄Sb₁₂ and A_xCo₄Sb₁₂ (A = Ba, Sr) as promising thermoelectric candidates. To analyze the experimental results, we developed a formalism for the NMR shifts and T₁, allowing for arbitrary carrier densities rather than treating the extreme metallic or non-degenerate limit. We find that a model in which a large density of defect states located just below the conduction band edge dominate the electrical behavior works very well, providing a consistent picture of both the NMR and transport results. The carrier effective masses and g-factors estimated from NMR fitting results are in close agreement with each other. Additional contributions observed in shift and 1/T₁ measurements are shown to correspond to the susceptibility of electrons in localized states.