Effects of the spin-triplet states mixture and electron-phonon coupling in Y₃Al₅O₁₂:Cr⁴⁺

The exchange charge model of the crystal field is applied to calculate the complete energy level scheme of the Cr⁴⁺ ions in Y₃Al ₅O₁₂ using D_2d symmetry at the Cr⁴⁺ position. Mixture of the spin-triplet states arising from the ³F and ³P terms of the 3d²-electron configuration is shown to have a profound effect on the energy levels sequence. Large splittings of the orbital triplets originating from the "parent" T_d symmetry are explained by large contribution of the low-symmetry component of the crystal field. Huang-Rhys parameter and effective phonon energy are evaluated from the experimental spectroscopic data available in the literature. Temperature-dependent probability of the nonradiative transition between the ground and first excited states is drawn; the emission band shape is obtained using the single configuration coordinate model.