Effects of Sb-doping on electrical transport properties of Co-based half-Heusler compound
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Effects of Sb-doping on electrical transport properties of Co-based half-Heusler compound Yasuhiro Ono1, Shingo Inayama1, Hideaki Adachi2, Satoshi Yotsuhashi2, Yuzuru Miyazaki1 and Tsuyoshi Kajitani1 1 Department of Applied Physics, Graduate School of Engineering, Tohoku University, Aobayama-05, Aoba-ku, Sendai, 980-8579, Japan. 2 Advanced Technology Research Laboratories, Matsushita Electric Industrial Co., LTD., 3-4 Hikaridai, Seika, Souraku, Kyoto, 619-0237, Japan ABSTRACT Electrical transport properties of NbCoSn1-xSbx (x=0, 0.01, 0.02 and 0.05), a half-Heusler compound and its alloys, have been studied in the temperature range from 80 K to 850 K. As-prepared samples exhibit metallic conduction and similar Seebeck coefficients near 300 K (S= –100 µVK-1). Except for NbCoSn0.95Sn0.05, both electrical resistivity, ρ, and the absolute value of S appreciably increase during the annealing for 6 days at 1123 K. Unusual increase in ρ of the annealed NbCoSn sample is found at about 200 K. ρ-T curves of the other annealed samples remain metallic over the measured temperature range and the ρ value noticeably decreases with increasing Sb content, x. Among the annealed samples, the high power factor, 25×10-4 Wm-1K-2 at 850 K, is obtained for NbCoSn0.95Sb0.05. The band structure of NbCoSn is calculated based on the determined crystal structure, indicating that NbCoSn is an indirect transition-type semiconductor with a band gap of approximately 1 eV. This is not consistent with the metallic behavior of ρ observed for the annealed NbCoSn sample above 400 K. Partial disordering of Nb and Co atoms is a conceivable answer. INTRODUCTION The ternary intermetallics with the formula, MNiSn (M=Ti, Zr, Hf), frequently referred to the half-Heusler compounds, have attracted much attention due to their potential thermoelectric properties [1-7]. M and Sn atoms form rock-salt type arrangement and Ni atoms occupy one half of the tetrahedral interstitial positions in an ordered array [8], i.e., MgAgAs-type structure (space -
symmetry, F4 3m). These compounds have high Seebeck coefficient, S, and low electrical resistivity, ρ, yielding high power factor, S2/ρ, in the range from 5 to 20×10-4 W m-1K-2 at 450 K [1]. Sb-doping to one of the half Heusler compounds, MNiSn (M=Ti, Zr, Hf), greatly enhances the power factors, e.g., 69×10-4 Wm-1K-2 at 650 K for TiNiSn0.95Sb0.05 [3] and 34×10-4 Wm-1K-2
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at 700 K for ZrNiSn0.99Sb0.01 [5]. The half-Heusler compounds such as MNiSn (M=Ti, Zr, Hf) and TiCoSb become the narrow-gap indirect transition-type semiconductors because they have 18 valence electrons per formula unit (VEC=18), while the other half-Heusler compounds with VEC=17 and 19 are metals [9,10,11]. Despite of the fact, VEC=18, NbCoSn is a semimetallic compound with significantly lower electrical resistivity than those of TiNiSn and TiCoSb [10,11]. In this paper, we report the transport properties, including the Seebeck coefficient, of NbCoSn and Sb-doped NbCoSn. EXPERIMENTAL DETAILS Polycrystalline samples, NbCoSn1-xSbx (x=0, 0.
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