Thermal Properties of TiNiSn and VFeSb Half-Heusler Thermoelectrics from Synchrotron X-Ray Powder Diffraction

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 alpha(T) of the TiNiSn-based samples was found to be independent of alloying or presence of Cu interstitials with alpha(av) = 10.1 x 10(-6) K-1 between 400 and 848 K. The alpha(T) of VFeSb and TiNiSn are well-matched, but NbFeSb has a reduced alpha(av) = 8.9 x 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.