Defect Control and Defect Engineering of Transition-metal Silicides
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0980-II04-01
Defect Control and Defect Engineering of Transition-metal Silicides Haruyuki Inui, Katsushi Tanaka, and Kyosuke Kishida Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto, 606-8501, Japan ABSTRACT The microstructure, defect structure and thermoelectric properties of two different semiconducting transition-metal silicides, ReSi1.75 and Ru2Si3 upon alloying with a substitutional element with a valence electron number different from that of the constituent metal have been investigated in order to see if the crystal and defect structures of these silicides and thereby their physical properties can be controlled through defect engineering according to the valence electron counting rule. The Si vacancy concentration and its arrangement can be successfully controlled in ReSi1.75 while the relative magnitude of the metal and silicon subcell dimensions in the chimney-ladder structures can be successfully controlled in Ru2Si3. As a result, the improvement in the thermoelectric properties and the p- to n-type conduction transition are successfully achieved respectively for these semiconducting transition-metal silicides. INTRODUCTION Transition-metal disilicides have attracted considerable interest due to their applications in silicon microelectronics technology in the last two, three decades [1,2]. Although most of them are metallic, some of them such as CrSi2 and FeSi2 are reported to be semiconductors. Many semiconducting transition-metal silicides such as CrSi2 and FeSi2 have been known to exhibit nice thermoelectric properties but their properties are generally believed to be the subject of further improvements for practical usage [1,2]. In view of the crystal structure of these transition-metal silicides, their thermoelectric properties cannot obviously be interpreted in terms of the concept of “phonon glass-electron crystal (PGEC)”, which has been recently developed to describe nicely the thermoelectric properties of many new thermoelectric materials with particular crystal structures such as of the Skutterudite-type and clathrate-type [3,4], where the “rattling” motion of guest atoms in oversized cages, which scatters heat-carrying phonons, results in low thermal conductivity while electrical conductivity remains relatively high because electronic conduction mainly takes place through the cage framework. Therefore, the development of some other strategy to improve thermoelectric properties of semiconducting transition-metal silicides is definitely needed. While extensive work has been devoted to some particular transition-metal silicides such as CrSi2, FeSi2 and MnSiX, many other silicides have been awaiting in-depth studies as a thermoelectric material. Of particular interest to note in these semiconducting transition-metal silicides is the complex crystal structures and their affordability for the wide range of metal/silicon ratios [1,2]. The affordability of the crystal structures for the wide range of metal/silicon ratios stems from the fact that the crystal structures
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