Optical study of ion-defect clustering in CaF2:Er3+

The effect of ion-defect clustering in CaF2:Er3+ has been investigated by means of fluorescence spectroscopy and lifetime measurements. The fluorescence spectra of CaF2:Er3+ exhibit a sensitive dependence on the dopant concentration and on the temperature history of the crystal. At low concentrations (≲ 0.01 mole%), only fluorescence lines arising from single Er3+–F− interstitial pairs are observed. As the concentration increases, a large number of new lines are observed. From the explicit dependence of the line intensity on the dopant concentration, the new lines are attributed to the dimerization of the Er3+–F− interstitial pairs. At higher concentrations (> 0.5 mole%), the total observed fluorescence intensity departs dramatically from a linear increase with concentration, indicative of the formation of high-order Er3+–F− interstitial clusters. The dissociation of the dimeric clusters to single pairs occurs readily at ≳ 1200 K. The present findings help to clarify the origin of the frequently observed ``cubic'' site symmetry for the trivalent cation in fluorite crystals.