Correlated Electron Paramagnetic Resonance and Optical Study of CdF2: Er3+. II. C3v Local-Site Symmetry
A high concentration of a single type of trigonal Er3+ site has been generated in CdF2: Er crystals that were fired in oxygen subsequent to growth. More than 86% of the noncubic Er3+ sites observed were found by EPR measurements to be in trigonal sites with g values (g∥=2.875 and g⊥=8.344) similar to those for the C3v(II) spectrum examined by Ranon and Low in CaF2: Er. This selective generation of C3v(II) sites has permitted an unambiguous determination of the structure of the 4I152 ground state of Er3+ ions residing in these sites, using low-temperature optical-absorption and -emission data. The crystal field splitting of the 4I152 multiplet in C3v symmetry is in reasonably good agreement with that expected from the cubic-field approximation of Lea, Leask, and Wolf for crystal field parameters A4<r4>=-226 cm-1 and A6<r6>=37 cm-1. Cd-vapor and water-vapor treatments were found to generate not only the C3v(II) site, but also a C3v(I) site (g∥=3.231 and g⊥=8.344). Vacuum treatment was found to lead to the C3v(I) site only. The implications of this study in terms of recently proposed models for oxygen-compensated (trigonal) rare-earth-ion sites are discussed.