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

Abstract

In this study, Tb3+-activated K7SrGd2(B5O10)3 (KSGBO) phosphors, with and without alkali ion (Li+/Na+) codoping, 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 ( 5 D₄ → 7 F₅, 541 nm) and red ( 5 D₄ → 7 F₄, 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 hightemperature optoelectronic and solid-state lighting applications