Reactions between Vacancies and Impurities in Magnesium Oxide. I. Cr3+-Ion Impurities

Abstract

Reactions between cation vacancies and Cr3+ impurity ions in single crystals of magnesium oxide are studied using optical techniques to determine the concentrations of the reaction components. Several new features of the Cr3+-ion luminescence spectrum are reported. The spectrum of these impurity ions is very sensitive to local variations of crystal structure. The effect of heat treating MgO crystals, and quenching them to room temperature, on the photoluminescence and cathodoluminescence spectra of the Cr3+ ions is investigated. The variation of intensities of the spectral lines with heat treatment is accounted for by the variation in concentration of impurity-ion—vacancy associates in the lattice. It is found that below 400°C cation vacancies are essentially immobile in MgO, whereas between 400° and 900°C the impurity—vacancy interaction may be accounted for in terms of simple mass-action theory. Above 900°C MgO reacts with the surrounding atmosphere, and the diffusion of defects into the crystal is observed. The experiments considerably aid the interpretation of the spectra, and in several cases the entropy and enthalpy of association of the defects is evaluated.