Laser spectroscopy of ion-size effects on point-defect equilibria inPbF2:Eu3+

Recent experiments have suggested a change occurs in the type of defects present in ${\mathrm{PbF}}_2$ when trivalent lanthanide ions larger than gadolinium are doped in the crystals. Site-selective spectroscopy has been used to study ${\mathrm{PbF}}_2$:${\mathrm{Eu}}^{3+}$ as an example of such a system. It is found that the types of defects and the behavior of the site distribution are very similar to those of the smaller ions. In particular, there is no evidence for an electron-compensated site. It is found that there are only two important sites at low concentrations, a cubic site and a single-pair site. The single-pair site is shown to correlate with the dielectric relaxation peak observed for ${\mathrm{PbF}}_2$:${\mathrm{Eu}}^{3+}$. Clustering becomes important at higher concentrations. There is extensive dissociation of the locally compensated defect sites near the superionic transition temperature that is direct evidence for the importance of strain interactions causing large nonideality corrections in the defect equilibria. The nonidealities also may prevent one from using unique values for the site association energies.