Cation Diffusion in Single Crystal MgO up to 2500 °C

The diffusion of Ca2+ and Ge4+ in single crystal MgO has been studied over the temperature range 1850 to 2400 °C using a sectioning technique. The activation energy for the diffusion of Ca2+ (3.2 eV) is similar to the values obtained for other divalent cations at the same temperatures, but the value for Ge4+ (4.0 eV) is significantly greater. For ions of a given valence, it is concluded that the activation energy for diffusion in MgO is independent of the “hard‐sphere” ionic radius of the diffusing ion, while the pre‐exponential factor, Do, appears to depend on its mass and electronic polarisability. If results for aliovalent ion diffusion are interpreted in terms of the association theory developed for the alkali halides, an empirical relation between the charge of the diffusing ion and its enthalpy of migration emerges.