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

In this work, Li + - and Na + -co-doped Dy 3+ -activated K 2 B 4 O 7 phosphors were synthesized via a conventional high-temperature solid-state reaction route and systematically investigated. Rather than proposing a universal alkali-ion co-doping effect, this study elucidates a host-specific physicochemical regulation mechanism in the K 2 B 4 O 7 tetraborate lattice, governed by charge compensation and local structural asymmetry. It is demonstrated that K 2 B 4 O 7 can act as an efficient photoluminescent host for Dy 3+ ions, owing to its wide bandgap, low phonon energy, and structurally rigid tetraborate framework. By introducing Li + (or Na + ) as charge compensators for Dy 3+ →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 K 2 B 4 O 7 :0.03Dy 3+ ,0.01Li + exhibits intense blue and yellow emissions originating from the 4 F 9/2 → 6 H 15/2 and 4 F 9/2 → 6 H 13/2 transitions of Dy 3+ , together with a high activation energy for thermal quenching ( E a ≈ 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 + co-doping 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 K 2 B 4 O 7 :Dy 3+ as a thermally robust yellow-emitting phosphor for high-power solid-state lighting applications.