Diffusion of 71Ge and 99Mo in molybdenum disilicide

Diffusion studies of heterodiffusion of germanium and self-diffusion of molybdenum in molybdenum disilicide were carried out using the radiotracer serial sectioning technique. Ge-71 was used to simulate silicon self-diffusion due to the lack of a suitable silicon tracer. Diffusivities were measured for two different orientations of single crystals with the tetragonal C11(b) structure. Diffusion annealings were carried out in the temperature range from 830 K to 1970 K. We found a surprisingly fast germanium diffusion and a relatively high anisotropy between both principal directions of the tetragonal lattice. The activation enthalpies for Ge-71 are Q = 200+/-2 kJ/mol for <100> and Q = 169+/-2 kJ/mol for <001>. Diffusion of molybdenum is by orders of magnitude slower than germanium. The activation enthalpy for Mo-99 is Q = 495+/-14 kJ/mol for <001>. Diffusion in both sublattices is decoupled and occurs very likely via an independent sublattice vacancy mechanism.