Self-diffusion of silicon in molybdenum disilicide

Abstract

The self-diffusion of silicon in single crystal MoSi₂ was studied by means of a radiotracer technique using the short-lived radioisotope ³¹Si (half-life ), which was produced and implanted into the samples at the ion-guide isotope separator on-line device at the University of Jyväskylä in Finland. Diffusion annealing and subsequent serial sectioning of the specimens were performed immediately after the radiotracer implantation. In the entire temperature region investigated (835–1124 K), the ³¹Si diffusivities in both principal directions of the tetragonal MoSi₂ crystals obey Arrhenius laws, where the diffusion perpendicular to the tetragonal axis is faster than parallel to it. In previous studies the same features were observed for the ⁷¹Ge diffusivities in MoSi₂, except that these are somewhat higher than those of ³¹Si. Furthermore, it is noteworthy that in MoSi₂ the diffusivities of ³¹Si and ⁷¹Ge are orders of magnitude faster than the diffusivity of ⁹⁹Mo. This large difference suggests that silicon diffusion and molybdenum diffusion are completely decoupled and that silicon diffusion takes place exclusively on the silicon sublattice. Literature data on the phase growth of MoSi₂ are in accordance with the present results on the ³¹Si diffusivities; Monte Carlo simulations of the correlation effects of vacancy-mediated diffusion on the silicon sublattice of MoSi₂ lead to their rationalization.

Acknowledgements

We are grateful to Dr K. Ito and Professor M. Yamaguchi from Kyoto University, Japan, for the production of the MoSi₂ single crystals, to M. Hennemann from Stuttgart University, Germany, and J. Huikari and A. Nieminen from the University of Jyväskylä, Finland, for their assistance in the radiotracer experiments, to Dr W. Sprengel from Stuttgart University, Germany, for making available positron annihilation data on vacancies in MoSi₂ prior to publication, and to Professor F. J. J. van Loo from the University of Eindhoven, The Netherlands, for helpful comments on the relation between phase growth and tracer diffusivities. Financial support by the Deutsche Forschungsgemeinschaft (research project Me 480/41-1) is gratefully acknowledged.

Notes

¶ Present address: University of Helsinki, Accelerator Laboratory, PO Box 43, FIN-00014 Helsinki, Finland.

† For example, at 1400 K, and ; thus, according to equation (21), .

‡ For molybdenum, Mo_5/8Si_3/8, Mo_1/3Si_2/3 and silicon the molar volumes are 0.94, 0.85, 0.81 and 1.21 × 10⁻⁵ m³ respectively.