Mechanistic Insights into the Formation of Thermoelectric TiNiSn from In Situ Neutron Powder Diffraction

Abstract

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 TiNi_1+ySn only occurring near 600 °C, followed by the emergence of TiNi and full-Heusler TiNi_2y'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_2y'Sn reacts with TiNi and molten Ti₂Sn₃ and Sn to form half-Heusler TiNi_1+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.