Thermoelectric performance of multiphase XNiSn (X = Ti, Zr, Hf) half-Heusler alloys

Abstract

Quantitative X-ray powder diffraction analysis demonstrates that mixing Ti, Zr and Hf on the ionic site in the half-Heusler structure, which is a common strategy to lower the lattice thermal conductivity in this important class of thermoelectric materials, leads to multiphase behaviour. For example, nominal Ti_0.5Zr_0.5NiSn has a distribution of Ti_1−xZr_xNiSn compositions between 0.24 ≤ x ≤ 0.70. Similar variations are observed for Zr_0.50Hf_0.5NiSn and Ti_0.5Hf_0.5NiSn. Electron microscopy and elemental mapping demonstrate that the main compositional variations occur over micrometre length scales. The thermoelectric power factors of the mixed phase samples are improved compared to the single phase end-members (e.g. S²/ρ = 1.8 mW m⁻¹ K⁻² for Ti_0.5Zr_0.5NiSn, compared to S²/ρ = 1.5 mW m⁻¹ K⁻² for TiNiSn), demonstrating that the multiphase behaviour is not detrimental to electronic transport. Thermal conductivity measurements for Ti_0.5Zr_0.5NiSn_0.95 suggest that the dominant reduction comes from Ti/Zr mass and size difference phonon scattering with the multiphase behaviour a secondary effect.