Vibronic Spectrum ofEu3+in Strontium Titanate

Vibrational satellite structure is observed in the fluorescence spectrum of ${\mathrm{Eu}}^{3+}$ in SrTi${\mathrm{O}}_3$. Selection rules for one-phonon vibronic transitions involving phonons at k=0 and other special points in the Brillouin zone are derived and used in assigning the vibronic peaks to various modes in the lattice vibrational spectrum. All branches appear to contribute to the vibronic structure. The frequencies of peaks in the vibronic spectrum agree generally with known or predicted lattice vibrational frequencies of SrTi${\mathrm{O}}_3$. By observing the vibronic line corresponding to the lowest transverse-optical branch, the frequency shift with temperature of this branch was measured between 130 and 350°K and is discussed. The structural phase transition of SrTi${\mathrm{O}}_3$ is evident from the appearance of additional fluorescence lines for temperatures below ∼110°K. The absence of additional crystal-field splitting of the spectrum indicates that the noncubic distortions associated with the phase transition are small.