Transition metal silicides have been investigated due to the applications in several fields, such as: structural materials, electronic devices, nuclear industry. The physical properties including mechanic, electric, magnetic, thermal, optical, etc. of these compounds are strongly dependent of the metallic atoms and their atomic bonds. The family of silicon-based compounds of Me5Si3stoichiometry, known as Nowotny phases, where Me is a IV, V or IV transition metal element crystallize in three different structures: 1) Ti5Si3, Zr5Si3and Hf5Si3compounds crystallize in a hexagonal structure obeying P6/mcm symmetry and (Me: 4d and 6g, Si: 6g) Wyckoff positions; 2) Compounds such as V5Si3, Cr5Si3, Mo5Si3, W5Si3, βNb5Si3and βTa5Si3crystallize in so-called T1structure with I4/mcm space group and (Me: 4b and 16k, Si: 4a and 8h) Wyckoff positions and 3) Compounds such as αNb5Si3and αTa5Si3crystallize also in a tetragonal structure, called T2, in same I4/mcm space group but different Wyckoff positions (Me: 4c and 16l, Si: 4a and 8h). The measurement of the thermal expansion coefficients of these compounds by high temperature X-ray diffraction shown that they are strongly dependent of the metallic atoms. Since these compounds crystallize in hexagonal or tetragonal symmetry, the thermal expansion is anistropic. The anisotropy of the thermal expansion in these materials have been controlled by the following ways: 1) by partial substitution of the metallic atom by another metallic atom to promote the formation of solid solutions, or 2) by formation of ternary compounds, by partial substitution of silicon by boron in the structures, for example. Since these compounds present high melting temperatures, they are prepared in polycristalline form by arc-melting process followed by heat-treatment at temperatures above 1400 K. The thermal expansion coefficients of Me5Si3compounds have been analyzed taking into account the crystal structure of these compounds.
Thermal expansion in isomorphic Me5Si3(Me=transition metal) compounds
