Crystal structure and thermoelectric properties of ReSi 1.75 silicide
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Crystal structure and thermoelectric properties of ReSi1.75 silicide J-J Gu, K. Kuwabara, K. Tanaka, H. Inui, M.Yamaguchi, A. Yamamoto1, T. Ohta1 and H. Obara1 Department of Materials Science & Engineering, Kyoto University, Sakyo-ku, Kyoto, 606-8501, Japan 1 National Institute for Advanced Industrial Science and Technology, AIST Central 5, Tsukuba, Ibaraki, 303-8568, Japan ABSTRACT The crystal structure of the defect disilicide formed with Re (ReSi1.75) has been refined by transmission electron microscopy combined with first-principles calculation. The crystal structure is monoclinic with the space group Cm (mc44) due to an ordered arrangement of vacancies on Si sites in the underlying (parent) C11b lattice. The thermoelectric properties of ReSi1.75 are highly anisotropic. Its electrical conduction is of n-type when measure along [001] while it is of p-type when measured along [100]. Although the value of Seebeck coefficient along [100] is moderately high (150-200 µV/K), it is very high along [001] (250-300 µV/K). As a result, a very high value of dimensionless figure of merit (ZT) of 0.7 is achieved at 1073 K when measured along [001]. INTRODUCTION There is a renewed interest in thermoelectric materials for applications in power generation and refrigeration in environmental safety manners [1]. Many new thermoelectric materials such as of skutterudite-type and clathrate-type have been found in recent years to exhibit nice performance [1]. Semiconducting transition-metal silicides such as CrSi2 and FeSi2 have also been regarded as nice thermoelectrics but it is generally believed that further improvement in thermoelectric properties is badly needed for these disilicides to be widely used [2]. Although a disilicide formed with Re is know to be a semiconductor, a little is known about its crystal structure and thermoelectric properties [3]. Early investigations found that the disilicide had a body-centered tetragonal C11b (MoSi2-type, space group I4/mmm) structure with lattice parameters a=0.3132 nm and c=0.7681 nm [4]. The compound was reported to have a composition of ReSi1.8 with vacancies on Si sites [5]. Later, Siegrist et al.[6] reported a body-centered orthorhombic structure with a=0.3128 nm, b=0.3144 nm and c=0.76776 nm (space group Immm) but found the composition to be close to ReSi2. More recent investigations by Gottlieb et al. [7] reported that the stoichiometry of the disilicide is ReSi1.75 and the unit cell is monoclinic (space group P1) with a=0.3139 nm, b=0.3121 nm, c=0.7670 nm and β=89.87°. In their assessment of the crystal structure, the occupancy of half the Si sites is assumed to be 75 %, resulting in the displacement of both Re and Si atoms from their positions corresponding to the underlying C11b lattice sites. The semiconducting nature of the disilicide was first noted by Nesphoret al. [5], who deduced an energy gap of 0.13 eV from electrical resistivity measurements. Siegrist et al. [6] estimated an energy gap of 0.21 eV from optical reflectivity measurements. More recently, Long et
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