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.

Half-Heusler alloys are leading contenders for application in thermoelectric generators. However, reproducible synthesis of these materials remains challenging. Here, we have used in situ neutron powder diffraction to monitor the synthesis of TiNiSn from elemental powders, including the impact of intentional excess Ni. This reveals a complex sequence of reactions with an important role for molten phases. The first reaction occurs upon melting of Sn (232 °C), when Ni 3 Sn 4 , Ni 3 Sn 2 , and Ni 3 Sn phases form upon heating. Ti remains inert with formation of Ti 2 Ni and small amounts of half-Heusler TiNi 1+y Sn only occurring near 600 °C, followed by the emergence of TiNi and full-Heusler TiNi 2 y ’ Sn phases. Heusler phase formation is greatly accelerated by a second melting event near 750–800 °C. During annealing at 900 °C, full-Heusler TiNi 2 y ’ Sn reacts with TiNi and molten Ti 2 Sn 3 and Sn to form half-Heusler TiNi 1+ y Sn on a timescale of 3–5 h. Increasing the nominal Ni excess leads to increased concentrations of Ni interstitials in the half-Heusler phase and an increased fraction of full-Heusler. The final amount of interstitial Ni is controlled by defect chemistry thermodynamics. In contrast to melt processing, no crystalline Ti–Sn binaries are observed, confirming that the powder route proceeds via a different pathway. This work provides important new fundamental insights in the complex formation mechanism of TiNiSn that can be used for future targeted synthetic design. Analysis of the impact of interstitial Ni on the thermoelectric transport data is also presented.

Materials with the TiNiSi structure have recently been highlighted as potential thermoelectric materials. Here we report the thermoelectric properties of TiNiX (X=Si and Ge). Both materials behave as defective metals or heavily doped degenerate semiconductors. Room temperature Seebeck coefficients are −45 μV K−1 (Si) and −20 μV K−1 (Ge) with electrical resistivities of 0.5–1 mΩ cm. The lattice thermal conductivities are 8 W m−1 K−1 (Si) and 6 W m−1 K−1 (Ge) at 360 K, which is promising in the absence of alloying. The calculated power factors and figures of merit remain small, with the largest S2/ρ=0.17 mW m−1 K−2 and peak zT=5×10−3 seen in TiNiSi near 300 K. Both compositions show Kondo behaviour at low‐temperatures, linked to the emergence of local moment magnetism, and have substantial magnetoresistance effects at 2 K. This work provides property characterisation for two members of this large class of intermetallic materials.