Thermoelectric properties of the aliovalent half-Heusler alloy Zn_0.5Ti_0.5NiSb with intrinsic low thermal conductivity

Using mixtures of aliovalent elements to achieve a valence balanced electronic state is a rapidly emerging area in half-Heusler thermoelectric materials research. Here, we report on Zn_0.5Ti_0.5NiSb, a combination of ZnNiSb and TiNiSb, which by adjusting the Zn/Ti-ratio can be made p- and n-type, achieving peak zT = 0.18 at 793 K and zT = 0.33 at 700 K, respectively. These promising values are underpinned by a low lattice thermal conductivity, κ_L = 2.7 W m⁻¹ K⁻¹ at 340 K, similar for all samples, decreasing to 1.25 W m⁻¹ K⁻¹ at 793 K. Transport data reveal similar weighted electronic mobilities for p- and n-type samples, suggesting similar zT should be possible. For both polarities, a transition to degenerate conduction is observed, superposed on intrinsic semiconducting behaviour with a bandgap E_g = 0.4 eV. Neutron and synchrotron X-ray diffraction experiments, including total scattering, indicate the absence of interstitial metals and do not reveal strong local structural variations. The absence of substantial mass disorder and lattice strain points towards bond disorder as a possible origin for the low κ_L. This work describes a new materials system and provides further insight into the impact of aliovalent alloying in the half-Heusler structure.