Equilibrium and nonequilibrium distribution of aliovalent light-impurity ions in simple oxides

The effects of atmosphere and cooling rate from elevated temperatures on the distribution of Li impurities in MgO, and Mg impurities in Al₂O₃ crystals are reported. At sufficiently high temperatures, regardless of the atmosphere, Li⁺ ions from Li₂O precipitates in MgO, and Mg²⁺ ions from MgAl₂O₄ precipitates in Al₂O₃ are dispersed around the precipitates, forming complex-oxide regions rich in these impurities, referred to as microgalaxies. Fast cooling of these crystals freezes the nonequilibrium distribution of these impurities surrounding the precipitates. However, if the crystals are cooled sufficiently slowly, the system has ample time to establish an equilibrium distribution, with most of the Li and Mg ions returning to the Li₂O and MgAl₂O₄ precipitates respectively. Based on the microgalaxy model, it is not unrealistic to expect that oxygen vacancies would be formed within the microgalaxy, regardless of the atmosphere.