Diffusion in Binary Silicides of Iron and Molybdenum

Abstract

Studies of tracer diffusion in silicides of iron and molybdenum mainly from our laboratory are reviewed. For three compositions of the DO3-structured Fe3Si tracer diffusion data of Fe and Ge are available. Fe diffusion is relatively fast and increases with Si content. Ge diffusion, which mimics Si self-diffusion, is fairly slow and almost independent of composition. A Mössbauer study of Fe diffusion revealed nearest-neighbour jumps. Positron annihilation and high pressure diffusion data indicate that, in addition to isolated vacancies, vacancy pairs also contribute to Fe diffusion. Ge and Si diffusion is presumably restricted to the Si sublattice. Tracer diffusion of Fe and Ge has been studied for B20-structured FeSi with the conventional tracer technique. Tracer diffusion studies after implantation of 31Si (half-life 2.6 hours) performed at the IGISOL facility in Jyväskylä, Finland, show that Ge and Si diffusion have similar diffusivities. Fe diffusion rates are considerably slower than those of Si and Ge. Tracer data of Mo, Ge and Si diffusion are available for both principal directions of the tetragonal C11b-structured molybdenum disilicide (MoSi2). For all three kinds of atoms diffusion perpendicular to the tetragonal axis is faster than parallel to it. Mo diffusion in both principal directions is by orders of magnitude slower than Si and Ge diffusion. The huge asymmetry between Mo and Si or Ge suggests that diffusion of both constituents is restricted to their own sublattice. Positron annihilation studies show formation of thermal vacancies on the Si sublattice. This suggests that Si and Ge diffusion proceed by a vacancy mechanism in the Si sublattice of MoSi2. The anisotropy ratio of Si self-diffusion is attributed to a high mobility of vacancies in the Si double layers perpendicular to the tetragonal axis.