Phase formation, stability, and oxidation in (Ti, Zr, Hf)NiSn half‐Heusler compounds

Thermoelectric half‐Heusler compounds with the nominal composition (Ti 0.33 Zr 0.33 Hf 0.33 )NiSn were synthesized and exposed to operating conditions, i.e., elevated temperatures and oxidizing atmosphere. As‐prepared samples were found to be a stable multiphase system consisting of Ti‐ and (Zr,Hf)‐rich phases with half‐Heusler structure. The influence of an oxidizing atmosphere at elevated temperatures on the sample was studied in‐ and ex situ by optical and electron microscopy and thermal analysis. Exposure to air resulted in surface oxidation starting at T = 545 K. During a defined heat treatment the Ti‐rich phase decomposed while the (Zr,Hf)‐rich phase was stable. In situ Atomic Force Microscopy and Scanning Kelvin Probe Force Microscopy experiments confirmed that interfaces, especially grain boundaries, are most prone to phase decomposition.