Thermoelectric properties control of Half-Heusler compounds by lattice defects and interfaces introduced based on the cl
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Thermoelectric properties control of Half-Heusler compounds by lattice defects and interfaces introduced based on the close relationship with Heusler compounds Yoshisato Kimura, Yaw-Wang Chai, Toshinori Oniki, Takahiko Itagaki, and Shinya Otani Tokyo Institute of Technology, Materials Science and Engineering, 4259-J3-19 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan. ABSTRACT Half-Heusler MNiSn (M=Ti, Zr, Hf) compounds are well-known, excellent n-type thermoelectric materials. The n-type Seebeck coefficients of ZrNiSn are reduced because of the precipitation of the metallic Heusler ZrNi2Sn phase. An excellent n-type Seebeck coefficient can be converted to p-type based on the vacancy site occupation by the solute Co atoms in the halfHeusler TiNiSn phase as well as ZrNiSn. The Heusler phase precipitates, including their precursor nano-structure in the half-Heusler matrix and the vacancy site occupation of the halfHeusler phase, are regarded as lattice defects based on the crystallographically and thermodynamically close relationship between half-Heusler and Heusler phases. INTRODUCTION Thermoelectric power generation is an appealing approach for conserving energy and preserving the global environment. The present authors focus on half-Heusler MNiSn (M=Ti, Zr, Hf) compounds [1,2] that are well-known as high potential thermoelectric materials for elevated temperature applications. The ordered structure of Half-Heusler MNiSn is shown in figure 1 together with that of Heusler MNi2Sn. These structures are quite similar to each other; a half of the Ni-site in the Heusler structure is replaced by vacancies in the half-Heusler structure. The half-Heusler structure consists of four interpenetrating fcc sublattices: the M-site (Ti and Zr in the figure) at (1/4, 1/4, 1/4), Ni-site at (0, 0, 0), Sn-site at (3/4, 3/4, 3/4), and vacancy-site at (1/2, 1/2, 1/2). This structure is also considered to be composed of two interpenetrating NaCl type (B1 type) sublattices, represented as a combination of M and Sn sites, and that of Ni and vacancy sites. In M–Ni–Sn alloy systems, MNiSn and MNi2Sn exist as individual stable phases at the ground state, where MNiSn is an excellent n-type thermoelectric material, and MNi2Sn is a metallic phase. By assuming a structural vacancy to be an atom (Va), Ni atoms and vacancies are ordered in the Ni–Va sublattice. A half-Heusler MNiSn compound can be represented by the chemical formula of a Heusler compound as M(Ni,Va)2Sn at finite temperatures, which indicates the possibility of a half-Heusler phase formation via chemical ordering between B and Va sites in the Heusler phase with an appropriate accompanying compositional change. Intermediate compositions between half-Heusler and Heusler phases, i.e., M(Ni,Va)2Sn or AB2+yX1−y, are thermodynamically allowed as off-stoichiometric compositions. However, in the M–Ni–Sn system, the solubility of Ni in the vacancy site tends to be restricted to be small at high temperatures up to the melting temperature. Localized supersaturation of Ni atoms easily causes
