Laser spectroscopic measurement of point-defect dynamics in Eu3+:CaF2.

Abstract

We have developed a site-selective laser method to study the aggregation kinetics of defect sites as a result of heat treatment in ${\mathrm{EuF}}_{3}$-doped ${\mathrm{CaF}}_{2}$ crystals. The crystals contain ${\mathrm{Eu}}^{3+}$ in four dominant defect sites and one minor site that differ in the number of nearby dopant and fluoride interstitial ions. A nonequilibrium site distribution was created by quenching the high-temperature distribution. The kinetics of the reequilibration to a lower-temperature distribution was followed by measuring the absolute concentrations of the defect sites with site-selective laser spectroscopy. The functionality of the temporal changes, the absolute concentrations, and the concentration and temperature dependence of the rate constants have been used to definitively identify the four major sites and to quantitatively describe the entire behavior of the defect distributions. The four sites are identified as an isolated ${\mathrm{Eu}}^{3+}$; a single-pair site of a ${\mathrm{Eu}}^{3+}$ ion and a compensating fluoride interstitial; a dimer consisting of two ${\mathrm{Eu}}^{3+}$ ions, no fluoride vacancies, two displaced fluoride ions, and three fluoride interstitials (2\ensuremath{\Vert}0\ensuremath{\Vert}2\ensuremath{\Vert}${3}_{1}$); and a trimer of three ${\mathrm{Eu}}^{3+}$ ions and four fluoride interstitials (3\ensuremath{\Vert}0\ensuremath{\Vert}2\ensuremath{\Vert}${4}_{1}$). It is suggested that the trimer and dimer are related to the hexamer clusters observed with heavier rare-earth dopants and that the hexamers contain extra fluoride interstitials that form a series of clusters with the largest possible cluster being 6\ensuremath{\Vert}0\ensuremath{\Vert}32\ensuremath{\Vert}12. We also determined activation energies of 0.49\ifmmode\pm\else\textpm\fi{}0.08 eV and 1.5\ifmmode\pm\else\textpm\fi{}0.4 eV for the formation of the dimer and trimer, respectively. This work presents the first complete microscopic picture for the solid-state defect chemistry of a fluorite material. It is in agreement with recent predictions of hades (Harwell Automatic Defect Examination System) models for fluorite defect centers.