Structure and Lattice Thermal Conductivity of Fractionally Filled Skutterudites: Solid Solutions of Fully Filled and Unfilled End Members

The structure of the skutterudite ${\mathrm{CoSb}}_3$ contains two large empty cages, or voids, per unit cell. Compounds obtained by filling these cages with rare earth atoms possess an enhanced thermoelectric figure of merit. Control of the electronic and thermal transport properties of these materials can be achieved over an extremely wide phase space by partial cage filling in conjunction with Fe alloying on the Co site. We show that such partially filled skutterudites can be rationalized as solid solutions of fully filled ${\mathrm{CeFe}}_4{\mathrm{Sb}}_{12}$ and $\square {\mathrm{Co}}_4{\mathrm{Sb}}_{12}$, where $\square$ is a vacancy.