Impact of Excess Cu on Phase Separation and Thermoelectric Properties of Arc Melted Ti_0.5Zr_0.5NiCu_ySn

Alloys based on XNiSn (X = Ti, Zr or Hf) are leading n-type half-Heusler thermoelectrics.¹ They have large power factors <i>S²σ</i>, but are limited by an inherently high lattice thermal conductivity, <i>κ_lat</i>. Alloying Ti, Zr and Hf on the X-site in the crystal structure affords significant reductions of <i>κ_lat</i>. However, Ti and Zr/Hf mix poorly during materials synthesis, typically resulting in the presence of multiple HH phases in the final product.^{2, 3} The presence of multiple HH phases has been attributed to phase segregation, and has been linked to low <i>κ_lat</i>, below values expected from alloying.<br/>In this contribution, the impact of excess Cu on the HH phase distribution of Ti_0.5Zr_0.5NiCu_ySn (y = 0.025, 0.1) is discussed. Structural characterisation of samples treated at varying temperatures reveals minimal impact on the phase distribution for y = 0.025, compared to not having Cu. By contrast, samples with y = 0.1 show improved homogeneity and can be made single phase at high temperature. The homogeneous y = 0.1 sample has the lowest <i>κ_lat</i> of all samples, confirming that alloying is the dominant phonon scattering effect. A highest <i>zT</i> = 0.7 is found for multiphase Ti_0.5Zr_0.5NiCu_0.025Sn, with a lower <i>zT</i> = 0.5 observed for single phase Ti_0.5Zr_0.5NiCu_0.1Sn. This lower value is due to over-doping and a compromised <i>S²σ</i>.<br/><br/>This work demonstrates that (1) the poor mixing of Ti and Zr/Hf in XNiSn alloys is predominantly a kinetic effect and not driven by thermodynamic phase segregation. (2) That there is no evidence that multiphase behaviour leads to significant reductions of <i>κ_lat</i> in our samples.<br/><br/><b>References</b><br/>1. R. J. Quinn and J.-W. G. Bos, <i>Materials Advances</i>, 2021, <b>2</b>, 6246-6266.<br/>2. M. Schwall and B. Balke, <i>Materials</i>, 2018, <b>11</b>, 649.<br/>3. A. Page, A. Van der Ven, P. F. P. Poudeu and C. Uher, <i>Journal of Materials Chemistry A</i>, 2016, <b>4</b>, 13949-13956.