Achieving Deep Red Emission in Mn4+-Activated K2MgGeO4 Phosphor with a Tetrahedral Network for Light-Emitting Diodes

Designing a thermally robust Mn4+-based red-emitting phosphor is essential for warm phosphor-converted light-emitting diodes (pc-LEDs) with improved CRI and indoor plant cultivation applications. However, the major constraint is selecting a host with octahedral crystallographic sites and poor thermal stability. Herein, we report the stabilization of Mn4+ ions in tetrahedral coordination in a K2MgGeO4 matrix. The combined experimental and theoretical results confirmed the location of Mn4+ in tetrahedral coordination, preferably occupied at GeO4 lattice sites of K2MgGeO4. The Mn4+-doped K2MgGeO4 phosphors emit deep red emission (662 nm), and the temperature-dependent photoluminescence study revealed good thermal stability of K2MgGeO4:Mn4+ phosphor, retaining 70% of the initial emission intensity at 423 K. The defect formation energies calculated for various defects using density functional theory, thermoluminescence measurement, and positron annihilation lifetime spectroscopy confirmed the presence of cation vacancies (VK, VMg) and VO with wide-ranging trap depths (∼0.8 to 1 eV). The thermal quenching mechanism and defect-assisted thermal stability of Mn4+ emission are discussed in detail. Finally, a red pc-LED device was fabricated using a K2MgGeO4:Mn4+ phosphor and a UV-LED chip to demonstrate its potential for indoor plant growth. This work explored the stabilization of Mn4+ in the tetrahedral crystal field, which will open new directions for matrix selection of multifunctional red-emitting phosphors.