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 (V_K, V_Mg) and V_O 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.