Diffusion of isotopes in surface layers of some oxides

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Abstract
The mechanism of the near surface diffusion of isotopes in MgO, NiO, and Fe3O4 was investigated by the edge source method. The divalent Co ion diffuses more rapidly than the trivalent Cr ion in the surface layer of MgO, which is also true for the lattice diffusion. This was explained by the fact that the trivalent ion in a divalent matrix has a larger impurity-vacancy bonding energy than does a divalent ion. The activation energy for the localized surface diffusion of 57Co isotope on the Ni (110) surface is 139 ± 12 kJ mol−1 compared with those for grain boundary and volume diffusion of 180 and 227 kJ mol−1 respectively. The surface diffusion of Co2+ ion at 800°C seems to be approximately 1-1.5 and six orders or magnitude faster than grain boundary and volume diffusion respectively. The influence of the point defects on near surface diffusion of 57Co in Fe3O4 and NiO is very similar to the influence on volume diffusion. It was concluded that similar mechanisms were involved in both near surface and lattice diffusion.

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