Diffusion of isotopes in surface layers of some oxides

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.