Defect equilibria in fluorite structure crystals

Abstract

The defect distribution of charge compensated sites in Er³⁺:CaF₂ crystals has been measured by laser spectroscopy over a concentration range from 10⁻⁴ to 1.0 mol %. The measurements show the transition from the low concentration region, where the site distribution is described by simple pairing of Er³⁺ and fluoride interstitial ions to a high concentration region dominated by clustering of the simple pairs. The simple pairs undergo a decrease in the absolute concentration in this region in direct conflict with all equilibrium models of solid state defect equilibria. A model is proposed which describes the measured site distribution and also explains the anomalous behavior observed in many other experiments. The model describes the distribution by requiring the cation equilibria to be frozen at a high temperature and the fluoride equilibria at a low temperature, in agreement with the temperature dependence of their mobilities. The excess fluoride interstitial ions are then ’’gettered’’ by the clustered pairs forcing dissociation of the simple pairs.