Electron-lattice coupling parameters and oscillator strengths of cerium-doped lutetium oxyorthosilicate

Abstract

Optical absorption of single-crystal, cerium-doped lutetium oxyorthosilicate has been carefully measured in the temperature interval 10--300 K. Prominent Gaussian absorption peaks occur at $3.432\ifmmode\pm\else\textpm\fi{}0.002\mathrm{eV}$ (peak a), $3.502\ifmmode\pm\else\textpm\fi{}0.002\mathrm{eV}$ (peak b), $4.236\ifmmode\pm\else\textpm\fi{}0.0002\mathrm{eV}$ (peak c), and $4.746\ifmmode\pm\else\textpm\fi{}0.0002\mathrm{eV}$ (peak d), in excellent agreement with previously reported excitation spectra. The second moments are well described by the usual linear model, yielding the Huang-Rhys parameter (S) and vibrational quantum energies for the individual peaks. All absorption bands are characterized by $S>5$ indicating strong coupling between the ${\mathrm{Ce}}^{3+}$ ion and lattice. Temperature dependence of the band centroids exhibits contrasting behavior that is dominated by higher-order coupling terms in the linear harmonic oscillator model or by crystal-field effects. Oscillator strengths of the $4\stackrel{\ensuremath{\rightarrow}}{f}5d$ transitions are calculated from Smakula's formula and knowledge of the ${\mathrm{Ce}}^{3+}$ distribution between the two crystallographically inequivalent sites. Values for peaks b, c, and d range from approximately 0.003 to 0.004, and peak a spans magnitude approximately 0.012 to 0.018. From the known correlation between average ${\mathrm{Ce}}^{3+}$-ion-ligand distance and oscillator strength, we tentatively conclude that peak a is correlated with the seven-oxygen-coordinated site, and peaks b, c, and d are associated with the six-oxygen-coordinated site. These results support the previously proposed two-activation-center model and identify the centers as the two crystallographically inequivalent substitutional sites.