Structures and thermoelectric properties of double-filled (Ca _xCe_1-x)Fe₄Sb₁₂ skutterudites

The structures and thermoelectric properties of the double-filled (Ca _xCe_1-x)Fe₄Sb₁₂ series (x=0, 0.25, 0.5, 0.75, and 1) have been studied using a combined experimental and computational methods. Compounds of (Ca_xCe_1-x)Fe ₄Sb₁₂ were obtained only for x=0, 0.5, and 1. Composition with x=0.25 was found to be a mixture of x=0 and 0.5 compounds, and composition with x=0.75 was found to be a mixture of x=1 and 0.5 compounds, respectively. Our conclusions on phase formation are supported by density functional theory (DFT) calculations. In Ca_0.5Ce_0.5Fe₄Sb _12, Ca substitution in the Ce site of CeFe₄Sb₁₂ leads to high hole concentrations, resulting in stronger semimetal transport as compared to CeFe₄Sb₁₂. Ca_0.5Ce _0.5Fe₄Sb₁₂ yields a slightly higher ZT value than that of CeFe₄Sb₁₂, which is attributed to its lower lattice thermal conductivity. Phonon mode calculations adopting a three-particle bending model suggest that thermal conductivity is reduced upon Ca substitution because of an additional vibration mode which involves both Ca and Ce atoms.