Thermal properties of TiNiSn and VFeSb half-Heusler thermoelectrics from synchrotron x-ray powder diffraction

Abstract

Half-Heusler (HH) alloys are an important class of thermoelectric materials that combine promising performance with good engineering properties. This manuscript reports a variable temperature synchrotron x-ray diffraction study of several TiNiSn- and VFeSb-based HH alloys. A Debye model was found to capture the main trends in thermal expansion and atomic displacement parameters. The linear thermal expansion coefficient α(T) of the TiNiSn-based samples was found to be independent of alloying or presence of Cu interstitials with α av = 10.1 × 10−6 K−1 between 400 and 848 K. The α(T) of VFeSb and TiNiSn are well-matched, but NbFeSb has a reduced α av = 8.9 × 10−6 K−1, caused by a stiffer lattice structure. This is confirmed by analysis of the Debye temperatures, which indicate significantly larger bond force constants for all atomic sites in NbFeSb. This work also reveals substantial amounts of Fe interstitials in VFeSb, whilst these are absent for NbFeSb. The Fe interstitials are linked to low thermal conductivities, but also reduce the bandgap and lower the onset of thermal bipolar transport.

Electronic scattering in half-Heusler thermoelectrics from resistivity data

“…In the X IV NiSn system, the large ρ 0 could be linked to the interstitial Ni defect, which has been shown to reduce µ and cause reduced lattice thermal conductivities [26]. Minimising interstitial Ni to levels observed in the X V FeSb family (X V = Nb, Ta; < 1% Fe) [29] is likely to significantly increase the electronic performance. The temperature dependence of ρ(T) in the X IV NiSn and X V FeSb families is fundamentally different due to the larger E def in the X V FeSb materials, up to 14 eV, where n-type ZrNiSn based compositions have E def as low as 5 eV, leading to a weak temperature dependence in the latter.…”

“…By contrast, the Nb 0.8 Ti 0.2 FeSb sample shows a much stronger temperature dependence and has a much lower relative impact of ρ 0 (only ∼10% of ρ 800 K compared to ∼50% for TiNiCu 0.025 Sn). This strong temperature dependence is significant for the thermoelectric properties, with the power factor, S 2 /ρ typically decreasing with temperature in the X V FeSb system, whereas they increase in the X IV NiSn family [21,28,29]. The samples in figure 3 either have a T 1 or T 1.5 dependence of ρ(T).…”

Advances in half-Heusler alloys for thermoelectric power generation

“…Recent work using unfolded supercell calculations has shown that new bands can emerge in substituted HH compositions (e.g. in vacancy systems, 170 for interstitials 58 and complex compositions 233 ) and this is another possible route for performance improvements. This is an area where theory could lead experiments by identifying new compositions with promising electronic bandstructures.…”

“…As indicated by defect energy calculations, inversion of X and Y is uncommon (e.g. <1% in X IV NiSn), 57 but there is evidence that this occurs for V 0.8 CoSb and VFeSb, 58,59 where V/Fe and V/Co are (almost) adjacent in the periodic table and hence likely to have similar site preferences.…”

A mechanistic view of defect engineered VFeSb half-Heusler alloys

“…It can be inferred that off-stoichiometry in these defective compositions at low concentration is well accommodated within the sublattice implying limited yet considerable solubility of interstitials/vacancies within the HH matrix. 10,33,48,79 The refined XRD patterns corresponding to each composition are also shown in supplementary information Fig. S1-S9 with refined parameters enlisted in Table S1.…”