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We report $^{59}$Co NMR and transport measurements on $n$-type filledskutterudites Ba$_x$Yb$_y$Co$_4$Sb$_{12}$ and $A$$_x$Co$_4$Sb$_{12}$ ($A$= Ba,Sr), promising thermoelectric materials. The results demonstrate consistentlythat a shallow defect level near the conduction band minimum dominates theelectronic behavior, in contrast to the behavior of unfilled CoSb$_3$. Toanalyze the results, we modeled the defect as having a single peak in thedensity of states, occupied at low temperatures due to donated charges fromfiller atoms. We fitted the NMR shifts and spin-lattice relaxation ratesallowing for arbitrary carrier densities and degeneracies. The results providea consistent picture for the Hall data, explaining the temperature dependenceof the carrier concentration. Furthermore, without adjusting model parameters,we calculated Seebeck coefficient curves, which also provide good consistency.In agreement with recently reported computational results, it appears thatcomposite native defects induced by the presence of filler atoms can explainthis behavior. These results provide a better understanding of the balance ofcharge carriers, of crucial importance for designing improved thermoelectricmaterials.

Charge-carrier behavior in Ba-, Sr- and Yb-filled CoSb$_3$: NMR and transport studies