Site Symmetries of Cerium Ions in BaWO4 Single Crystals Codoped with Sodium Ions

Resumo

Barium tungstate crystals are an interesting and relatively new medium for stimulated Raman scattering for applications in Raman shifters of laser radiation. Good quality BaWO₄ crystals can be grown by Czochralski technique and doped with rare-earth ions. Doping with trivalent ions requires charge compensation which may be provided, for example, by structural defects or proper codoping with alkaline metal ions. Crystals possess scheelite-like structure with the space group I4₁/a. Results of the electron paramagnetic resonance (EPR) studies of BaWO₄:Ce crystals and crystals codoped with Na are presented in this paper. EPR study was performed in the low temperature range 3 K < T < 50 K for all three planes of single crystals. The EPR spectra were attributed to cerium ions with a fictitious spin S = 1/2. The spin Hamilton (SH) parameters were determined. The values of SH parameters indicate the occurrence of paramagnetic centers in axial and low symmetry. The number of observed EPR lines depends on the selected plane. Based on the roadmap, we have found that one center with axial symmetry and at least two centers with low symmetry appear. The linewidth, ∆B, vs temperature dependence revealed increasing exponential tendency with increasing temperature. It shows 1 phonon at the lower temperatures and Raman + Orbach effect at the higher temperatures. Exponential change of the ΔB could be connected with the spin–lattice relaxation processes involving excited states of Ce³⁺ ions.