Local structure investigation of Ga and Yb dopants in Co4Sb12 skutterudites

Abstract

We report comprehensive x-ray absorption spectroscopy studies at both the Ga $K$ edge and Yb ${L}_{2}$ edge to elucidate the local structure of Ga and Yb dopants in ${\mathrm{Yb}}_{x}{\mathrm{Ga}}_{y}{\mathrm{Co}}_{4}{\mathrm{Sb}}_{12}$. Our extended x-ray absorption fine structure (EXAFS) data confirm that Ga atoms occupy two crystallographic sites: one is the $24g$ site replacing Sb, and the other is the $2a$ site in the off-center void position. We find that the occupancy ratio of these two sites varies significantly as a function of the filling fraction of additional Yb, which exclusively occupies the $2a$ on-center site. At low concentrations of Yb, ${\mathrm{Ga}}_{24g}$ and ${\mathrm{Ga}}_{2a}$ dopants coexist and they form a charge-compensated compound defect proposed by Qiu et al. [Adv. Funct. Mater. 23, 3194 (2013)]. The ${\mathrm{Ga}}_{24g}$ occupancy increases gradually with increasing Yb concentration, and almost all Ga occupies the $24g$ site for the highest Yb concentration studied ($x=0.4$). In addition to the local structural evidence provided by our EXAFS data, we also present x-ray absorption near-edge structure (XANES) spectra, which show a small Ga $K$-edge energy shift as a function of Yb concentration consistent with the change from predominantly ${\mathrm{Ga}}_{2a}$ to ${\mathrm{Ga}}_{24g}$ states. Our result suggests that the increased solubility of Yb in Yb-Ga co-doped ${\mathrm{Co}}_{4}{\mathrm{Sb}}_{12}$ skutterudites is due to the increased ${\mathrm{Ga}}_{24g}$ electron acceptor, and thus provides an important strategy to optimize the carrier concentration in partially filled skutterudites.