Photoluminescence characteristics and Judd–Ofelt analysis of YBa3(BO3)3:Tb3+ phosphors co-doped with Li+, Na+, and K+

This study presents a systematic investigation of the photoluminescent properties of Tb³⁺-doped YBa₃(BO₃)₃ (YBBO) phosphors, synthesized via a microwave-assisted sol-gel combustion (MA-SG) method and co-doped with monovalent alkali ions (Li⁺, Na⁺, K⁺). Structural, vibrational, and morphological analyses were performed using XRD with Rietveld refinement, Raman and FTIR spectroscopy, and SEM–EDS. These analyses confirmed the successful incorporation of dopants without compromising the borate lattice. Photoluminescence (PL) measurements under near-UV excitation (377 nm) showed intense green emission attributed to the ⁵D₄ → ⁷F₅ transition of Tb³⁺, with Li⁺ co-doping producing the greatest enhancement (∼2.7 × ) in emission intensity. Lifetime measurements revealed longer decay times with co-doping, suggesting reduced non-radiative relaxation processes. Judd–Ofelt (J–O) analysis confirmed strong radiative transitions and high internal quantum efficiency (η ≈ 48.4 %). The internal quantum efficiency (IQE) of 48.4 % was estimated using Judd–Ofelt theory, which is based on radiative transition probabilities derived from emission spectra. This method, while theoretical, is widely accepted for powdered phosphors and provides insight into the intrinsic radiative efficiency of the activator ions. Although absolute quantum yield (QY) measurements are typically obtained using integrating sphere systems to account for all optical losses, in this study, such measurements were not performed. Nonetheless, the strong agreement between radiative parameters and observed photoluminescence behavior supports the reliability of the calculated efficiency. In this study, J–O parameters were derived from the integrated emission spectra of Tb³⁺ transitions, following an emission-based approach that has been increasingly employed for powdered phosphors due to its experimental feasibility. Colorimetric analysis using CIE chromaticity diagrams validated the tunable green emission behaviour of the phosphors. Minor deviations from ideal green were linked to background blue emission from the host matrix, a feature that may offer spectral advantages in multifunctional optical applications. Furthermore, the phosphor exhibited a rare negative thermal quenching (NTQ) behavior, maintaining or enhancing emission intensity up to 550 K, which is superior to many commercial green phosphors.These results highlight the crucial role of alkali ion co-doping in tuning the local crystal field, enhancing emission efficiency, and paving the way for the development of efficient green-emitting phosphors for solid-state lighting applications.