Crystal Structure and Thermoelectric Properties of Mn-Substituted Ru 2 Si 3 with the Chimney-Ladder Structure
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1128-U05-01
Crystal Structure and Thermoelectric Properties of Mn-Substituted Ru2Si3 with the Chimney-Ladder Structure Tatsuya Koyama, Norihiko L. Okamoto, Kyosuke Kishida, Katsushi Tanaka and Haruyuki Inui Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ABSTRACT Phase relationships of manganese-substituted ruthenium sesquisilicide alloys have been investigated by using scanning and transmission electron microscopy. A series of chimneyladder phases Ru1-xMnxSiy are formed over a wide compositional range between Ru2Si3 and Mn4Si7. The thermoelectric properties of the directionally solidified alloys with the nominal compositions of Ru1-xMnxSiy (0.55 ≤ x ≤ 0.90) have been investigated as a function of the Mn content and temperature. The thermoelectric power factor and dimensionless figure-of-merit (ZT) for the alloys with high Mn contents (x ≥ 0.75) increase with the increase in the Mn content. The ZT value for the crystal with x = 0.90 is as high as 0.76 at 874 K. INTRODUCTION The high-temperature (HT) phase of ruthenium sesquisilicide (Ru2Si3) possesses the tetragonal Ru2Sn3-type structure, which is known as one of the chimney-ladder structures [1,2]. Some of the family of compounds with the chimney-ladder structures are known to exhibit a high Seebeck coefficient and low thermal conductivity simultaneously so that they have been investigated as a candidate for thermoelectric materials [3-5] because the thermoelectric performance is evaluated with the dimensionless figure-of-merit, ZT = α2T/(ρ·λ), where α, ρ, λ and T stand for Seebeck coefficient, electrical resistivity, thermal conductivity and temperature, respectively. The chimney-ladder compounds expressed with the general chemical formula of MnX2n-m (M: transition metal element, X: group 13 or 14 element, n, m: integers) possess a particular tetragonal crystal structure, in which the unit cell consists of M (Ru) subcell with the atomic arrangement of the β-Sn type (chimney) and X (Si) subcell with the atomic arrangement of a coupled helices (ladders) with both the chimney and ladder being aligned along the c-axis of the tetragonal unit cell [1,6,7]. We have recently found out that substitutions of Ru (group 8) in Ru2Si3 with Re (group 7) stabilize the HT phase with the chimney-ladder structure to appear at low temperatures so that a series of chimney-ladder phases, Ru1-xRexSiy are formed over a wide composition range so that the valence electron count (VEC) per M atom in the compounds is maintained (VEC=14) [8,9]. Of interest to note is that the X/M values for these chimney-ladder phases deviate from the ideal values expected from the VEC=14 rule and also that semiconducting behavior of the chimney-ladder phases depends on the extent of compositional deviation from the ideal VEC=14 values [10]. In the present study, we investigate the phase relationships and crystal structures in Mnsubstituted ruthenium sesquisilicide alloys by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to clar
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