Crystal Structures and Thermoelectric Properties of Ru 1-x Re x Si y Chimney-Ladder Compounds
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0980-II05-37
Crystal Structures and Thermoelectric Properties of Ru1-xRexSiy Chimney-Ladder Compounds Akira Ishida, Norihiko L. Okamoto, Kyosuke Kishida, Katsushi Tanaka, and Haruyuki Inui Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto, 6068501, Japan
ABSTRACT The phase relationship of Re-alloyed Ru2Si3 and the variations of the crystal structures and thermoelectric properties of the Ru1-xRexSiy chimney-ladder phases have been studied as a function of the Re concentration. The Ru1-xRexSiy chimney-ladder phases are formed in a wide compositional range. Compositional formula of the chimney-ladder phases are determined to be Ru1-xRexSi1.539+0.178x (0.14 ≤ x ≤ 0.76), which are systematically deviated from the idealized composition conforming the valence electron counting rule: VEC=14. Measurements of thermoelectric properties of single crystals with different compositions reveal that the chimneyladder phases exhibit n-type semiconducting behavior at low Re concentrations and p-type semiconducting behavior at high Re concentrations. These results suggest that the VEC=14 rule can be used for predicting the semiconducting behavior of the chimney-ladder phases through the adjustment of VEC values by substituting elements, producing p-type semiconductors for VEC14, with the carrier concentration related to the deviation from VEC=14. INTRODUCTION Semiconducting Ruthenium sesquisilicide, Ru2Si3 has been attracting attentions as a high-temperature thermoelectric material because of its high Seebeck coefficient and low thermal conductivity [1-5]. Ru2Si3 crystallizes into two different types of structures: the tetragonal-type high-temperature (HT) phase and the orthorhombic-type low-temperature (LT) phase. The HT-Ru2Si3 phase is a member of the so-called chimney-ladder phases (fig.1). The chimney-ladder phase MnX2n-m has a structure composed of transition metal (M) atoms in a tetragonal β-Sn arrangement (a “chimney”) and group 14 or 13 atoms (X) in a coupled helical arrangement (a “ladder”), with both the chimney and ladder being aligned along the c-axis of the tetragonal unit cell [6-10]. The chimney-ladder structure can be also described as a subcell structure composed of n M subcells and m X subcells stacking along the c-axis with the c-axis dimensions of cM and cX, respectively (fig.1b). The c-axis dimension of the total unit cell (c) of the chimney-ladder structure is then calculated as the least common multiple of cM and cX, i.e., c = n cM = m cX. The HT-Ru2Si3 possesses the simplest type of the chimney-ladder unit cell (with n=2 and m=1) with a short c-axis of c = 2cM = cX (fig.1a).
Fig. 1. (a) Unit cell of HT-Ru2Si3 and (b) schematic illustration of the subcell structure in the MnX2n-m chimney-ladder phase. Since the chimney-ladder compounds are known to be electron compounds following an valence electron counting rule: VEC=14 [6-10], the crystal structure and therefore intrinsic physical properties are expected to be controlled by alloying with a substitutional element, especially
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