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 ⁵D₄ → ⁷F₅ 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. [Display omitted] •Green emission at 542 nm via the ⁵D₄ → ⁷F₅ transition in Tb³⁺ ions.•3 wt% Tb3+ is optimal; higher concentrations cause quenching.•NTQ observed up to 470 K with an activation energy of 0.96 eV.•Na⁺ co-doping enhances energy transfer and luminescence via lattice distortions.•K₃Gd(BO₂)₆:Tb³⁺ phosphors show promise in solid-state lighting and display.