Zn_0.5Ti_0.5NiSb – A New Aliovalent Half-Heusler Alloy with Intrinsic Low Thermal Conductivity

Half Heusler (HH) alloys are leading contenders for thermoelectric power generation and cooling. Traditional XYZ HH materials are characterised by large electronic power factors, S²σ, and are limited by high lattice thermal conductivity, κ_lat, which limits the achievable figure of merit, zT.¹

Recently a new class of HH materials has emerged that uses mixtures of aliovalent elements. The archetypal example is the TiFe_0.5Ni_0.5Sb family, which is a mixture of 17 electron TiFeSb and 19 electron TiNiSb. Hence, an equal mixutre of these ternary systems achieves the required average 18 valence electron count required for semiconducting behaviour. In comparison to regular XYZ materials, these complex compositions are characterized by low κ_lat and modest S²σ, leading to zT approaching unity.² This approach is general, and can also be applied to the X and Z sites, leading to work on X’_0.5X’’_0.5YZ and XYZ’_0.5Z’’_0.5 compositions by multiple groups.³

As part of this recent new direction of research, this contribution is focused on the aliovalent Zn_1-xTi_xNiSb system, which links 17 electron ZnNiSb with 19 electron TiNiSb. The x = 0.5 composition has exactly 18 valence electrons. Samples were prepared between 0.40 ≤ x ≤ 0.65; with x < 0.4 not accessible under the used conditions, whilst Ti-rich samples can likely be prepared over the full range (to x = 1). This difference occurs because TiNiSb forms with Ti vacancies and is close to an 18-electron system, hence removing the electronic driving force for insolubility.
In terms of thermoelectric properties, the samples are characterised by very low κ_{lat, 340 K} = 2.5 W.m^-1.K^-1, comparable for all investigated samples. The electronic response suggests a small bandgap, E_g = 0.4 eV, with evidence for bipolar transport. Varying the composition away from x = 0.5 leads to a transition towards p- (x < 0.5) or n-type (x > 0.5) degenerate semiconducting behaviour. The best observed performance is zT = 0.18 for p-type Zn_0.6Ti_0.4NiSb and zT = 0.33 for n-type Zn_0.4Ti_0.6NiSb.

The origin for the low κ_lat in these materials is not fully resolved. Typically, mass and strain disorder are the main drivers of phonon scattering. However, for the aliovalent HH materials, atomic mass and size differences are typically small, whilst the velocity of sound is only about 10-20% reduced compared to the XYZ materials. We have used synchrotron X-ray total scattering and pair distribution function analysis to probe the local structure of Zn_0.5Ti_0.5NiSb. This confirms that there is not a large source of local lattice strain, e.g. the Ti-Ni and Zn-Ni distances are comparable. Hence, the structure does not substantially deviate from the average unit cell obtained from diffraction. This suggests that differences in bond strength (bond disorder) may play a crucial role in the low κ_lat in the aliovalent HH materials.

References
1. R. J. Quinn and J.-W. G. Bos, Materials Advances, 2021, 2, 6246-6266.
2. Z. Liu, S. Guo, Y. Wu, J. Mao, Q. Zhu, H. Zhu, Y. Pei, J. Sui, Y. Zhang and Z. Ren, Advanced Functional Materials, 2019, 29, 1905044.
3. P.-F. Luo, S. Dai, Y. Zhang, X. Liu, Z. Li, J. Zhang, J. Yang and J. Luo, Journal of Materials Chemistry A, 2023, 11, 9125-9135.