Correlated Electron Paramagnetic Resonance and Optical Study of Cd${\mathrm{F}}_2$: ${\mathrm{Er}}^{3+}$. II. $C_{3v}$ Local-Site Symmetry

A high concentration of a single type of trigonal ${\mathrm{Er}}^{3+}$ site has been generated in Cd${\mathrm{F}}_2$: Er crystals that were fired in oxygen subsequent to growth. More than 86% of the noncubic ${\mathrm{Er}}^{3+}$ sites observed were found by EPR measurements to be in trigonal sites with $g$ values ($g_{\parallel }=2.875\mathrm{}$ and $g_{\perp }=8.344\mathrm{}$) similar to those for the $C_{3v}\left(\mathrm{II}\right)$ spectrum examined by Ranon and Low in Ca${\mathrm{F}}_2$: Er. This selective generation of $C_{3v}\left(\mathrm{II}\right)$ sites has permitted an unambiguous determination of the structure of the ${}_{}{}^4{}_{}{}^{}I_{\frac{15}{2}}^{}{}_{}{}^{}$ ground state of ${\mathrm{Er}}^{3+}$ ions residing in these sites, using low-temperature optical-absorption and -emission data. The crystal field splitting of the ${}_{}{}^4{}_{}{}^{}I_{\frac{15}{2}}^{}{}_{}{}^{}$ multiplet in $C_{3v}$ symmetry is in reasonably good agreement with that expected from the cubic-field approximation of Lea, Leask, and Wolf for crystal field parameters $A_4〈r^4〉=-226\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and $A_6〈r^6〉=37\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. Cd-vapor and water-vapor treatments were found to generate not only the $C_{3v}\left(\mathrm{II}\right)$ site, but also a $C_{3v}\left(\mathrm{I}\right)$ site ($g_{\parallel }=3.231\mathrm{}$ and $g_{\perp }=8.344\mathrm{}$). Vacuum treatment was found to lead to the $C_{3v}\left(\mathrm{I}\right)$ site only. The implications of this study in terms of recently proposed models for oxygen-compensated (trigonal) rare-earth-ion sites are discussed.