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

TL;DR

This study combines NMR and transport measurements to analyze charge-carrier behavior in filled skutterudites, revealing a shallow defect level near the conduction band that influences electronic properties and thermoelectric performance.

Contribution

It introduces a defect model with a single density of states peak that explains carrier behavior and thermoelectric properties without adjustable parameters.

Findings

Shallow defect level dominates electronic behavior in filled skutterudites.

The model accurately fits NMR and Hall data across temperatures.

Calculated Seebeck coefficients align well with experimental trends.

Abstract

We report $^{59}$Co NMR and transport measurements on $n$-type filled skutterudites Ba$_x$Yb$_y$Co$_4$Sb$_{12}$ and $A$$_x$Co$_4$Sb$_{12}$ ($A$= Ba, Sr), promising thermoelectric materials. The results demonstrate consistently that a shallow defect level near the conduction band minimum dominates the electronic behavior, in contrast to the behavior of unfilled CoSb$_3$. To analyze the results, we modeled the defect as having a single peak in the density of states, occupied at low temperatures due to donated charges from filler atoms. We fitted the NMR shifts and spin-lattice relaxation rates allowing for arbitrary carrier densities and degeneracies. The results provide a consistent picture for the Hall data, explaining the temperature dependence of 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 that composite native defects induced by the presence of filler atoms can explain this behavior. These results provide a better understanding of the balance of charge carriers, of crucial importance for designing improved thermoelectric materials.