Performance improvement of Sr4Al14O25:Mn4+ red emission phosphor via Na+ doping

The absence of high-efficiency oxide red phosphors restricts the development of WLEDs. In this work, Na ⁺-doped Sr ₄Al ₁₄O ₂₅:Mn ⁴⁺ (SAO-Na,Mn) series oxide red phosphors effectively excited by blue chips were successfully prepared. The theoretical calculation, structure and luminescence properties were systematically analyzed. Theoretical calculations indicate that the Na-Mn codoping system exhibits a more stable structure as doping Mn into octahedron Al1 sites and Al1 and Al2 sites (octahedron) are possibly simultaneously occupied. The calculation results also show that Na ⁺ significantly improves the luminescence intensity and thermal quenching resistance and that the red emission only originates from Mn ⁴⁺ occupying the octahedron. The PL intensity of SAO-Mn increases and then decreases as the Na ⁺ doping content increases, and 0.5 mol is the optimal value. Compared with the PL intensity of SAO-Mn at 150 ℃, that of SAO-0.5Na,Mn increased by approximately 200 %, and the fitting thermal activation energy was 0.84 eV. The fluorescence lifetime continuously decreases with increasing Na ⁺ content. The pure phase and superior morphology were obtained in the 0.5 mol Na ⁺-doped sample. The WLED devices encapsulated with as-prepared SAO-Na,Mn and YAG:Ce ³⁺ phosphors show a correlated color temperature of 4085 K, a color rendering index of 89, and a luminous efficiency of 137.12 lm/W. The above results indicate that the use of SAO-Na,Mn phosphors is expected in warm WLEDs.