Enhanced photoluminescence, thermal stability, and anti-thermal quenching in Li plus /Na plus Co-doped K2B4O7:Dy3+ phosphors for solid state lighting

Abstract

In this work, Li+- and Na+-co-doped Dy3+-activated K2B4O7 phosphors were synthesized via a conventional hightemperature solid-state reaction route and systematically investigated. Rather than proposing a universal alkaliion co-doping effect, this study elucidates a host-specific physicochemical regulation mechanism in the K2B4O7 tetraborate lattice, governed by charge compensation and local structural asymmetry. It is demonstrated that K2B4O7 can act as an efficient photoluminescent host for Dy3+ ions, owing to its wide bandgap, low phonon energy, and structurally rigid tetraborate framework. By introducing Li+ (or Na+) as charge compensators for Dy3+→K+ substitution, a marked enhancement of emission intensity and thermal stability is achieved. Structural, morphological, and optical characterizations were carried out using XRD, SEM/EDS, FTIR/Raman, and photoluminescence spectroscopy. Under 364 nm excitation, the optimized composition K2B4O7:0.03Dy3+,0.01Li + exhibits intense blue and yellow emissions originating from the 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2 transitions of Dy3+, together with a high activation energy for thermal quenching (Ea ≈ 0.21 eV). Concentration-quenching analysis (Blasse and Dexter–Van Uitert models), temperature-dependent PL (300–550 K), and lifetime measurements reveal a pronounced host-dependent anti-thermal-quenching behavior over a wide temperature range (300–550 K), in which the emission intensity increases with temperature due to thermally assisted detrapping and enhanced radiative recombination. Judd–Ofelt analysis further provides quantitative evidence that Li+ codoping enhances the local asymmetry and polarizability of the Dy–O coordination environment, leading to strengthened electric-dipole transitions a high internal quantum efficiency (~96.8 %). These results demonstrate a host-tailored quantitative regulation approach based on charge compensation and local-field engineering, highlighting Li+/Na + modified K2B4O7:Dy3+ as a thermally robust yellow-emitting phosphor for high-power solid-state lighting applications.