Fluorescence and absorption studies ofSr0.999−xGd0.001CexF2.001+x

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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 ($\ensuremath{\lesssim}0.1$ 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 ${C}_{4\ensuremath{\nu}}$ 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 \ifmmode\pm\else\textpm\fi{} 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 ${O}_{h}$, ${C}_{4\ensuremath{\nu}}$, and ${C}_{3\ensuremath{\nu}}$ 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 ${\mathrm{F}}^{\ensuremath{-}}$ 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.