Photoluminescence properties and structural analysis of Tb3+-doped K3Gd (BO2)6: A first study on negative thermal quenching

In this study, Tb3+-doped K3Gd(BO2)6 phosphors were synthesized using the microwave-assisted sol-gel method to explore their photoluminescence (PL) properties and thermal stability. XRD and Rietveld refinement confirmed the incorporation of Tb3+ions, without secondary phases. PL analysis revealed a strong green emission near 542 nm, attributed to the 5 D 4 -> 7 F 5 transition of Tb3+ions. An optimal Tb3+concentration of 3 wt% was identified, beyond which concentration quenching significantly reduced luminescence intensity. Radiative energy transfer, occurring via reabsorption, was observed at lower concentrations, facilitating efficient energy migration. Conversely, at higher concentrations, non-radiative processes such as cross-relaxation dominated. Remarkably, negative thermal quenching (NTQ) was observed up to 470 K, with an activation energy of 0.96 eV. Additionally, Na+ co- doping introduced lattice distortions that enhanced energy transfer between Tb3+ions and improved luminescence efficiency. The chromaticity diagram highlighted a shift towards the yellow-green region with increasing the Tb3+concentration, demonstrating tunable emission properties for solid-state lighting applications.