Low thermal conductivity and high thermoelectric performance via Cd underbonding in half-Heusler PCdNa

Abstract

Half-Heusler compounds are a very large class of materials that have received attention as promising high-temperature thermoelectric materials due to their favorable electrical transport behavior and other properties beneficial for device fabrication. A particular challenge is that while half-Heusler compounds show an exceptionally large range of thermal conductivities, these are still generally higher than other state-of-the-art thermoelectric materials. Here, we investigate PCdNa, which was reported to be a material that may form in the half-Heusler structure and have an unusually low thermal conductivity. This is in order to understand its very low thermal conductivity and elucidate its electronic properties in relation to thermoelectricity. We find that the low thermal conductivity of 2.6 W/mK at 300 K is a consequence of underbonding of the Cd ions within the network formed by the P. This underbonding leads to anharmonicity and relatively low frequency phonons and may be regarded as a generalized effective rattling phenomenon. The electronic structure shows a high valence band degeneracy, which we find leads to favorable good electrical transport properties. The combination of the low lattice thermal conductivity and favorable electrical transport properties results in high $p$-type thermoelectric performance. The figure of merit is predicted to reach $ZT=3.3$ at 900 K for optimum $p$-type doping. This suggests that PCdNa is a promising $p$-type high-temperature half-Heusler thermoelectric material.