Rare-earth free p-type filled skutterudites: Mechanisms for low thermal conductivity and effects of Fe/Co ratio on the band structure and charge transport

A key driver of cost for a thermoelectric generator is the thermoelectric materials themselves, and the heavy reliance on rare earth (RE) elements as fillers, particularly for p-type formulations, represents a strategic risk. In the present study, we have investigated fully filled p-type skutterudites with nominal compositions CaFe_xCo_4−xSb₁₂ (2 ⩽ x ⩽ 3.5) using both experimental and theoretical methods. High purity samples were successfully synthesized using a combined melt-spinning (MS) and spark plasma sintering (SPS) technique. Structural analysis confirmed phase-pure samples with high filling fraction (>90%), homogeneous filler and transition metal distribution, and exceptionally low levels of oxygen contamination. Electrical and thermal transport property measurements revealed large power factors and unexpectedly low thermal conductivities. Electronic band structure calculations found a rapid increase in the density of states at the Fermi level with increasing Fe content. The large power factors observed in this system are attributable to this effect. Maximum zT of 0.9 was achieved in CaFe₃CoSb₁₂ at 773 K. To understand the low lattice thermal conductivity in these compounds, lattice dynamics calculations were performed. The small size of Ca atoms and their concomitantly weak interactions with the surrounding Sb atoms result in a small force constant between fillers and the host, giving rise to a heretofore unreported low frequency optical phonon mode. This low frequency mode at 7 meV is in addition to the previously described mode at 17 meV identified by modeling the Ca as a simple harmonic oscillator. The lower energy phonon mode imparted by Ca filling results in a comparable lattice thermal conductivity reduction to early lanthanide species such as La