Thermally robust dual-band anti-thermal quenching in alkali-co-doped K₇SrGd₂(B₅O₁₀)₃:Tb³⁺ phosphors

In this study, Tb³⁺-activated K₇SrGd₂(B₅O₁₀)₃ (KSGBO) phosphors, with and without alkali ion (Li⁺/Na⁺) co-doping, were synthesized via a combustion method and systematically investigated for their structural and photoluminescence properties. Rietveld-refined XRD analysis confirmed the formation of a trigonal pentaborate phase (space group R32), with Tb³⁺ ions substituting at Gd³⁺ sites. Alkali co-doping induced a slight unit cell contraction (∼0.22%for Li⁺ and ∼ 0.26% for Na⁺), indicating lattice relaxation. Under 378 nm excitation, the phosphors exhibited intense dual-band Tb³⁺ emission-green (⁵D₄ → ⁷F₅, 541 nm) and red (⁵D₄ → ⁷F₄, 672 nm)-with maximum intensity at 3 wt% Tb³⁺, beyond which concentration quenching occurs. Remarkably, both emission bands showed anti-thermal quenching behavior, with the 541 nm emission intensity increasing by nearly twofold between 300 and 550 K, demonstrating exceptional thermal stability. For Li⁺ and Na⁺ co-doped samples, thermally activated enhancement followed by quenching was supported by activation energy values of 0.349 eV (Li⁺) and 0.304 eV (Na⁺). Additionally, alkali co-doping significantly enhanced the emission intensity, with ∼4.5-fold (Li⁺) and ∼ 3-fold (Na⁺) increases compared to the undoped sample. Time-resolved PL measurements revealed millisecond-scale decay lifetimes, with the average lifetime slightly decreasing from 1.25 ms (Tb³⁺ only) to 1.15 ms (Li⁺) and 1.20 ms (Na⁺), while maintaining efficient radiative recombination. Furthermore, CIE analysis demonstrated improved color purity, reaching up to 92% in Li⁺ co-doped samples, along with tunable chromaticity. These combined effects-dual-band emission, quantified anti-thermal enhancement, and co-doping-induced intensity amplification-highlight KSGBO:Tb³⁺ phosphors as promising candidates for high-temperature optoelectronic and solid-state lighting applications.