Ordered Structures and Thermoelectric Properties of MNiSn (M = Ti, Zr, Hf)-Based Half-Heusler Compounds Affected by Close Relationship with Heusler Compounds

Half-Heusler compounds are excellent thermoelectric materials. A characteristic of the half-Heusler-type ordered structure is the vacancy site that occupies one-fourth of all the lattice points. Therefore, a half-Heusler ABX phase (where A and B are typically transition metal elements, such as Ti, Zr, and Hf, and X represents a half-metal element such as Sn or Sb) has a crystallographically close relationship with a Heusler AB(2)X phase in the sense that the vacancy site in the half-Heusler phase is filled with B atoms in the Heusler phase. The thermoelectric properties are improved or affected by point lattice defects related to the vacancy site and the B site, such as the antisite atom B in the vacancy site, vacancies in the B site, and vacancy-site occupancy by quaternary C atoms. A modulated-like nanostructure due to point defects regarding vacancies and Ni atoms is formed for an instance in ZrNiSn alloys even close to the stoichiometric composition. Ni-rich nanoclusters are locally formed by excessive Ni antisite atoms in the vacancy site, which work as precursors of Heusler precipitates (TiNi2Sn, ZrNi2Sn, and so forth). The vacancy-site occupation in ZrNiSn with Co and Ir results in the drastic conversion of thermoelectric properties from n type to p type, and the effective reduction of the lattice thermal conductivity.