Structural and luminescence properties of Tb3+-doped Na2Gd2B2O7 phosphors for LED applications: Judd-Ofelt analysis and alkali co-doping effects

Abstract

In this study, we report the structural and luminescent properties of Tb3+-activated Na2Gd2B2O7 (NGBO) phosphors synthesized via a microwave-assisted gel combustion method. The effect of alkali ion co-doping (K+, Li+) on emission efficiency, crystal structure, and color purity was systematically investigated under near-UV excitation (λex = 377 nm). X-ray diffraction confirmed the formation of a single-phase monoclinic structure, while Fourier-transform infrared (FTIR) spectroscopy and Raman spectra revealed preserved BO3/BO4 units with minor vibrational shifts. The NGBO:0.02 Tb3+ sample showed strong green emission at 542 nm ( 5 D4 → 7 F5), with the highest intensity at 2 wt% Tb3+, prior to concentration quenching. Judd–Ofelt (J-O) analysis gave Ω4 = 0.41 × 10- 20 cm2 and Ω6 = 0.64 × 10- 20 cm2 , indicating favorable asymmetric environments. Notably, temperaturedependent PL studies exhibited negative thermal quenching up to 500 K, attributed to thermally released trapped carriers. Time-resolved photoluminescence (PL) revealed lifetimes up to 1.27 ms, enhanced by alkali doping due to reduced non-radiative losses. Co-doped samples showed tunable Commission Internationale de l’Eclairage ´ (CIE) coordinates (x = 0.2728–0.3110, y = 0.3061–0.4508), approaching white-light emission. These results position NGBO:Tb3+ as a promising, thermally stable phosphor for solid-state lighting and optical sensing