Refinement of Crystallographic Parameters in Refractory Metal Disilicides
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Refinement of Crystallographic Parameters in Refractory Metal Disilicides K. Tanaka, K. Nawata, H. Inui, M. Yamaguchi and M. Koiwa Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan. ABSTRACT The crystallographic structures of seven refractory metal (Ti, V, Cr, Nb, Mo, Ta and W) disilicides with the C11b, C40 and C54 structures have been refined through analysis of single-crystal X-ray diffraction data. Crystallographic parameters refined are lattice constants, atomic parameters and the space group of the C40 disilicides. In most of previous studies, silicon atoms have been considered to locate at the ideal positions so that the refractory metal atoms are perfectly six-fold coordinated in RSi2 layers prevailing in all the three structures. The present analysis shows that the silicon atoms are displaced from the ideal positions. The magnitude of such displacement is found to be closely related to the interatomic distance in these pseudo-hexagonally arranged RSi2 layers. The space group of three of the four C40 disilicides, VSi2, CrSi2 and TaSi2, is determined to be P6422, which is of chirality with respect to that (P6222) assigned in the previous studies.
INTRODUCTION There is a growing interest in refractory metal disilicides in recent years not only as candidate materials for very high-temperature structural applications [1, 2] but also as materials used in microelectronic applications [3, 4]. These disilicides include MoSi2 and WSi2 with the tetragonal C11b structure, VSi2, CrSi2, NbSi2 and TaSi2 with the hexagonal C40 structure, and TiSi2 with the orthorhombic C54 structure. The space group of C11b structure is I4/mmm and the basis has refractory metal atoms in 2a positions and Si atom in 4e positions with the atom position parameter z. The space group of the C40 structure can be either P6222 (prototype: CrSi2) or P6422 (prototype: NbSi2), depending on the chirality related to the six-fold axis parallel to the c-axis. For the former case, the basis consists of refractory metal atom in 3d positions and Si atoms in 6j positions, while for the latter case, it consists of refractory metal atoms in 3c positions and Si atoms in 6i positions. For both cases, the position of Si atoms is expressed with the parameter x. The basis of the C54 structure with the space group Fddd consists of refractory metal atoms in 8a positions and Si atoms in 16e positions with the atom position parameter x. According to Pearson’s Handbook [5], the Si atoms in most of these disilicides are located the ideal positions. Recent first principle calculations, however, have indicated that the total energy of both MoSi2 [6-8] and WSi2 [6, 8] with the C11b structure is minimized when the position is slightly shifted from the ideal one, which has very recently been confirmed experimentally for MoSi2 by the Rietveld analysis of X-ray powder diffraction data [9]. Similar deviation of the Si atom position from the ideal one has been reported also for NbSi2 with the C40 structure [10] and for TiSi2 with the C54 str
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