Vibronic Structure in Luminescence Spectra of Rare Earth Ions In SrTiO₃, BaTiO₃ and Other Perovskites

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In the luminescence spectra of Eu³⁺ in SrTiO₃, two kinds of vibronic structure, i.e., rather sharp satellite lines (vibronic lines) and asymmetric bands (vibronic tails), were found to exist. The tails were observed also in the spectra of Tb³⁺ and Pr³⁺ in SrTiO₃. The tail strongly appears associated only with transitions allowed by forccd electric dipole. A comparison of the luminescence spectra of Eu³⁺ in SrTiO₃ and BaTiO₃ has suggested the existence of a correlation between the strength of vibronic structure and the site symmetry of rare earth ions. To investigate this, the luminescence spectra of Eu³⁺ were studied in various perovskite type oxides in which the site symmetry of Eu³⁺ varies from O_h to very low symmetries. It has been clarified that the vibronic lines strongly appear when the site symmetry is O_h or very close to it. However, the vibronic tail was observed only in SrTiO₃. In order that the vibronic tail appears, some special properties of host material seem to be required.