Optical Properties of Mg ₂ GeO ₄ :Mn ⁴⁺ Phosphors: From Fine Structures of Vibronic Transitions to the Density of Phonon States

To address the spectral deficiencies in the red region of commercial white LEDs, developing red-emitting phosphors with reduced dependence on rare-earth elements with limited availability is crucial for spectral compensation. Herein, microstructures and optical characteristics of Mn4+-activated Mg2GeO4 phosphors are presented. While the phosphor exhibits a single orthorhombic phase, its cryogenic micro-Raman and photoluminescence (PL) spectra comprises 19 peaks and three groups of vibronic emissions with wealthy fine structures, respectively. Following the split zero-phonon lines, nine pairs of vibronic lines with an energy separation of 3.5 meV are identified. Furthermore, the overall line shape of the three groups of phonon sidebands can be reproduced by the fully symmetric electron–phonon coupling model, enabling determination of the total density of phonon states. The study sheds light on complicated luminescence fine structures in Mn4+-activated oxide phosphors, offering valuable insights into the design of red-emitting phosphors.