Vacancies Substitution Tuning Photoluminescence and Distortion Triggered Eu Migration in NASICON-Type Phosphors

Abstract

Phosphor-converted white light-emitting diodes (PC-WLEDs) are an energy-saving alternative to traditional light sources, for which efficient and thermal stable phosphors are in high demand. Here, the NASICON-type Na₃₊ₓ□₁–ₓSc₂–ₓMgₓ(PO₄)₃:Eu²⁺ (N□SMₓPO:Eu²⁺) phosphor is synthesized for Na vacancies repair. The photoluminescence quantum yield (PLQY) and the emission morphology of solid solution are dramatically affected by the composition, with the PLQY improving by 27% and the emission spectra red-shifting by 11 nm going from x = 0.0 to x = 0.3. These highlights likely stem from the enhanced structural rigidity and the distortion of NaO₆₍₈₎ polyhedra, respectively. Temperature-dependent emission spectra reveal the thermal stable luminescence of the series. A luminescence intensity of ≥90% relative to the room-temperature value is observed at 425 K. The LED device with the blending of N□SM₀.₃PO:Eu²⁺ and commercial components generates CRI (color-rending indices) = 88 and CCT (correlated color temperatures) = 5209 K, characteristic of high-quality white light. More interestingly, the distortion of crystal lattice could switch to the migration of Eu ion to (Sc/Mg) site at x = 0.4. The phenomenon leads to the oxidation of Eu²⁺, optically rapidly weakening the photoluminescence. Modifying the lattice distortion in some type phosphors may be a very useful approach to improve the efficiency of luminescence materials.