Cation Substitution Induced Adjustment on Lattice Structure and Photoluminescence Properties of Mg14Ge5O24:Mn4+: Optimized Emission for w-LED and Thermometry Applications

Currently, Mn4+-activated red-emitting materials are the research hotspot due to their promising application in phosphor-converted white light-emitting diodes (w-LEDs). Here, a typical cation substitution strategy is reported to optimize Mn4+-doped germinate phosphor for improving luminescence performances. The effect of cation substitution for Ge4+ ions by Ti4+, Sn4+, and Si4+ ions on lattice structure, photoluminescence properties, and thermal stability of the as-prepared Mn4+-doped germinates is systematically investigated. A significant improvement of emission intensity with Ti4+ doping should benefit from the resonance emission enhancement effect. Some lattice distortion defects contribute to the thermal stability enhancement, which plays a role in driving the excited phonon traps to compensate the energy loss of nonradiative transition. Surprisingly, abnormal variation in intensity of anti-Stokes and Stokes emission peaks with changing temperature is first observed, revealing a potential application in thermometry. The electroluminescence performances of as-fabricated w-LED devices using Mg14Ge5Ti0.5O24:Mn4+ red phosphor are evaluated, which possess higher color rendering index (R-a = 87.3) and luminous efficiency (109.42 lm W-1) as well as lower color correlated temperature (3566 K) than those using Cs2GeF6:Mn4+ and CaAlSiN3:Eu2+ red phosphors. The results reflect the superiority of the studied phosphor in w-LEDs and optical thermometry areas.