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 Ni3Sn4, Ni3Sn2, and Ni3Sn phases
form upon heating. Ti remains inert with formation of Ti2Ni and small amounts of half-Heusler TiNi1+ySn only occurring
near 600 °C, followed by the emergence of TiNi and full-Heusler
TiNi2y’Sn phases. Heusler phase
formation is greatly accelerated by a second melting event near 750–800
°C. During annealing at 900 °C, full-Heusler TiNi2y’Sn reacts with TiNi and molten Ti2Sn3and Sn to form half-Heusler TiNi1+ySn 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.