Selective oxidation of thermoelectric TiNiSn

Multiscale modelling, involving thermodynamic assessment and molecular dynamics based on density functional theory, was employed to unravel oxidation mechanisms pertinent to half-Heusler TiNiSn (space group F-43 m), in particular counterintuitive Ni inertness. O2 molecules dissociate and chemisorb onto TiNiSn(0 0 1) and TiNiSn(1 1 0), which is followed by ingress of O. Both Ti and Sn egress, while Ni is less mobile. Such diffusion processes yield point defects (vacancies and interstitials) and give rise to Ti and Sn binary oxide formation, while Ni is inert, which may be corelated to its relatively low mobility. Based on the Mulliken analysis and thermodynamics at 900 K, the Ti oxide formation sequence is suggested to be Ti2O3 → Ti3O5 → TiO2 → TiO. These data explain the experimental observations on the Ni inertness during oxidation of TiNiSn.