CLUSTERING AND PERCOLATION OF DEFECTS IN PR3+-SRF2 USING SITE-SELECTIVE SPECTROSCOPY

Site-selective laser spectroscopy has been used to study Pr3+-doped SrF2 in order to identify whether the dopant substitution is random or whether preferential clusters of several Pr3+ ions form along with their charge-compensating fluoride interstitial ions. At very low dopant concentrations, a single locally compensated Pr3+ site dominates the spectrum. It corresponds to the simple dipoles that are observed by ionic thermal current and dielectric relaxation experiments. At concentrations of 1 mol %, three centers are observed that are associated with preferential clusters. There are many additional centers that are also observable at this concentration. These observations provide clear evidence that the dopant substitution in SrF2 is not random and that models for the percolation of fluoride interstitial ions must be modified to accommodate preferential clusters.