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Effect of Vacancy-Site Occupation in Half-Heusler Compound ZrNiSn on Phase Stability and Thermoelectric Properties Yoshisato Kimura, Toshiyasu Tanoguchi, Yasuhiro Sakai, Yaw-Wang Chai and Yoshinao Mishima Tokyo Institute of Technology, Interdisciplinary Graduate School of Science and Engineering, Department of Materials Science and Engineering, 4259-G3-23 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. ABSTRACT The half-Heusler compound ZrNiSn has a quite small solubility for Ni from the stoichiometric composition towards the Ni-rich direction since Ni atoms are not supposed to occupy the vacancy-site. Nevertheless, Co and Ir atoms preferably occupy the vacancy-site of ZrNiSn, which is contrary to the prediction that they would substitute for Ni sites. This implies that the phase stability of the compound gradually changes toward that of the Heusler compound Zr(Ni,M)2Sn (M = Co, Ir). It has been confirmed that there exists a two-phase field between halfHeusler Zr(Ni,Cox)Sn and Heusler Zr(Ni,Co)2Sn. The n-type thermoelectric property of ZrNiSn can be converted to p-type by the addition of Co and Ir within the compositional range of the half-Heusler phase. The occupation of vacancy sites by Co and Ir atoms leads to a drastic reduction in the thermal conductivity owing to the enhancement of phonon scattering. With further Co addition, the Heusler phase Zr(Ni,Co)2Sn alloys show metallic behavior. INTRODUCTION Thermoelectric generation is one of appealing solutions that help us reduce the amount of greenhouse gas and conserve the global environment. In order to convert the waste heat directly into clean electrical energy at relatively elevated temperature, we focus on half-Heusler compounds which can be used at around 1000 K. The ordered crystal structure of half-Heusler, C1b type ABX (ZrNiSn), consists of four interpenetrating fcc sub-lattices of elements A, B, X and vacancy, where A and B are typically transition metal elements and X is Sn, Sb and Pb. Note that a half of the B sites in (full-)Heusler, L21 type AB2X (ZrNi2Sn), are vacant in half-Heusler ABX (ZrNiSn). Unit cells of both, half-Heusler type and Heusler type ordered crystal structures, are represented in Figure 1. Many research groups have been working on the application of halfHeusler compounds at high temperatures around 1000 K [1-11]. The most well-known and studied half-Heusler compounds are n-type MNiSn (M = Ti, Zr, Hf), not only because MNiSnbased alloys show potentially high thermoelectric performance but also because these elements are eco-friendly and they eliminate the need for rare or toxic elements. In order to understand the actual thermoelectric properties of half-Heusler compounds, our research group fabricated various nearly single-phase half-Heusler alloys by unidirectional solidification using optical floating zone melting; (Zrx,Hf1-x)NiSn (x = 0~1), (Ti,Zr)NiSn 0.04, 0.13, 0.15) and (Ti0.04,Hf0.96)NiSn [4-7]. For example, (Ti0.15,Zr0.85)NiSn exhibits a high power factor exceeding 5.0 mW m-1 K-2 while ZT (dimensionless thermoelectric figur