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

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 sub(0.5)Zr sub(0.5)NiSn has a distribution of Ti sub(1-x)Zr sub(x)NiSn compositions between 0.24 less than or equal to x less than or equal to 0.70. Similar variations are observed for Zr sub(0.50)Hf sub(0.5)NiSn and Ti sub(0.5)Hf sub(0.5)NiSn. 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. S super(2)/ rho = 1.8 mW m super(-1) K super(-2) for Ti sub(0.5)Zr sub(0.5)NiSn, compared to S super(2)/ rho = 1.5 mW m super(-1) K super(-2) for TiNiSn), demonstrating that the multiphase behaviour is not detrimental to electronic transport. Thermal conductivity measurements for Ti sub(0.5)Zr sub(0.5)NiSn sub(0.95) suggest that the dominant reduction comes from Ti/Zr mass and size difference phonon scattering with the multiphase behaviour a secondary effect.