Electronic Structure and Phase Stability of Yb-Filled CoSb₃ Skutterudite Thermoelectrics from First-Principles

Filling the large voids in the crystal structure of the skutterudite CoSb₃ with rattler atoms R provides an avenue for both increasing carrier concentration and disrupting lattice heat transport, leading to impressive thermoelectric performance. While the influence of R on the lattice dynamics of skutterudite materials has been well-studied, the phase stability of R-filled skutterudite materials and the influence of the presence and ordering of R on the electronic structure remain unclear. Here, focusing on the Yb-filled skutterudite Yb_xCo₄Sb₁₂, we employ first-principles methods to compute the phase stability and electronic structure. Yb-filled CoSb₃ exhibits (1) a mild tendency for phase separation into Yb-rich and Yb-poor regions and (2) a strong tendency for chemical decomposition into Co-Sb and Yb-Sb binaries (i.e., CoSb₃, CoSb₂, and YbSb₂). We find that, at reasonable synthesis temperatures, configurational entropy stabilizes single-phase solid solutions with limited Yb solubility, in agreement with experiments. Filling CoSb₃ with Yb increases the band gap, enhances the carrier effective masses, and generates new low-energy "emergent" conduction band minima, which is distinct from the traditional band convergence picture of aligning the energies of existing band extrema. The explicit presence of R is necessary to achieve the emergent conduction band minima, though the rattler ordering does not strongly influence the electronic structure. The emergent conduction bands are spatially localized in the Yb-rich regions, unlike the delocalized electronic states at the Brillouin zone center that form the unfilled skutterudite band edges.