Grain‐Boundary Diffusion in MgO

Grain‐boundary diffusion of Ni 2+ was investigated in polycrystalline MgO and also in isolated grain boundaries in natural and prepared bi‐crystals of MgO. Concentration distributions were determined with the aid of both electron microbeam probe spectroscopy and X‐ray absorption analysis. Results from diffusion couples in which a surface was maintained at constant concentration during the diffusion annealing indicate that grain‐boundary diffusion is the predominant transport mechanism in polycrystalline MgO at temperatures below 1700°C. Lattice diffusion becomes increasingly important at higher temperatures and eventually becomes the dominant mechanism. Concentration distributions for diffusion couples in which a fixed amount of NiO was supplied to the surface of the couple as a thin initial plating resemble those for lattice diffusion but yield anomalously high values for apparent lattice diffusion coefficients. Grain‐boundary diffusion in MgO is not confined to a layer of atomic dimensions but extends over a zone of the order of microns. The activation energy for grain‐boundary diffusion is less than that for lattice diffusion and is between 1 and 2 ev. Grain‐boundary diffusion was observed even in tilt boundaries with a mismatch as low as 6°.