Fluorescence and absorption studies of ${\mathrm{Sr}}_{0.999-x}{\mathrm{Gd}}_{0.001}{\mathrm{Ce}}_x{\mathrm{F}}_{2.001+x}$

Cerium-concentration-dependence studies of the optical spectra of doubly doped Sr${\mathrm{F}}_2$ with 0.1-mole% ${\mathrm{Gd}}^{3+}$ for eight ${\mathrm{Ce}}^{3+}$ concentrations ranging from 0.001 to 1.0 mole% were carried out. A new four-line pattern was observed in the ${}_{}{}^6{}_{}{}^{}P_{\frac{7}{2}}^{}{}_{}{}^{}$ fluorescence spectra of these samples. In addition, an unusual single-line spectrum was found in the ${}_{}{}^6{}_{}{}^{}P_{\frac{7}{2}}^{}{}_{}{}^{}$ region. These and other new features studied using intensity and lifetime measurements were tentatively identified as arising from various Gd-Ce mixed double and triple clusters. From the enhancement of the Gd fluorescence due to Ce at low Ce concentrations ($\lesssim 0.1$ mole%) and other data, energy transfer from the Ce ion to the Gd ion in these clusters by exchange is proposed. The ${}_{}{}^6{}_{}{}^{}P_{\frac{7}{2}}^{}{}_{}{}^{}$ fluorescence excitation and decay scheme of the usual $C_{4\nu }$ site was found to be the usual three-level scheme. The lifetime of the ${}_{}{}^6{}_{}{}^{}I$ pump state was observed to be 13.5 msec ± 9% while for the ${}_{}{}^6{}_{}{}^{}P_{\frac{7}{2}}^{}{}_{}{}^{}$ decay we observed radiation trapping giving lifetimes as much as four times the radiative value of 10 msec. Analysis of the absorption spectra gave a self-consistent set of site concentrations vs Ce concentration for the $O_h$, $C_{4\nu }$, and $C_{3\nu }$ sites of Gd. We observed an anomalous increase in the $O_h$ site concentration with a corresponding decrease in the axial site concentration with increasing Ce concentration starting at about 0.1-mole% Ce. An ad hoc gettering model is proposed which assumes that the Ce ions extract extra interstitial ${\mathrm{F}}^{-}$ ions from the Gd population producing the observed site-concentration changes. The calculations based on this model provide a satisfactory fit to the observed data.