This work concerns a novel red emitting Mn⁴⁺ doped phosphor showing an extraordinarily high thermal quenching temperature. The composition Y₂Mg₃Ge₃O₁₂:Mn⁴⁺,Li⁺ exhibits efficient red photoluminescence peaking at 658 nm, which can be assigned to the ²E_g → ⁴A_2g intraconfigurational transition of Mn⁴⁺ ([Ar]3d³ configuration) located at the octahedral site in the garnet structure. Photoluminescence properties, such as temperature dependence of the luminescence intensity and luminescence lifetime are presented. Additionally, the band structure of the undoped host material were treated with Density Functional Theory (DFT) and evaluated with UV-reflectance spectroscopy. Furthermore, the energy levels of the Mn⁴⁺ ion in the Y₂Mg₃Ge₃O₁₂ host were calculated with the exchange charge model of crystal field theory and compared to those obtained experimentally from luminescence studies.

The VUV $4f^{n-1}5d\text{−}4f^n$ luminescence and luminescence excitation spectra of ${\mathrm{Gd}}^{3+}$ $(n=7)$ in ${\mathrm{LiGdF}}_4$, ${\mathrm{GdF}}_3$, ${\mathrm{LiYF}}_4:{\mathrm{Gd}}^{3+}$, and ${\mathrm{YF}}_3:{\mathrm{Gd}}^{3+}$, and of ${\mathrm{Lu}}^{3+}$ $(n=14)$ in ${\mathrm{LiLuF}}_4$, ${\mathrm{LuF}}_3$, and ${\mathrm{LiYF}}_4:{\mathrm{Lu}}^{3+}$ have been analyzed with high spectral resolution. In systems with intermediate electron-phonon coupling, zero-phonon lines, and phonon sidebands were observed. The excitation spectra of dilute systems exhibit rich fine structure originating from electronic origins of transitions and their phonon replica. Theoretical calculations explicitly taking into account a microscopic model of the crystal field and the crystal lattice vibrational spectra agree well with experimental data and are the basis for a safe analysis of the spectra.