Judd-Ofelt analysis and photoluminescence behavior of Tb<SUP>3+</SUP>-activated K₇SrY₂(B₅O₁₀)₃ phosphors modified with alkali co-dopants for solid-state lighting applications

In this study, the structural and luminescent properties of alkali-modified K7SrY2(B5O10)(3) phosphors activated with Tb3+ ions were systematically investigated for solid-state lighting and photonic applications. A combination of X-ray diffraction (XRD) with Rietveld refinement, vibrational spectroscopy (FTIR and Raman), and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) confirmed that the trigonal R32 structure of the host lattice is retained upon doping and co-doping, with Tb3+ ions preferentially occupying Y3+ sites. Photoluminescence (PL) studies revealed intense green emission centered at 540 nm (D-5(4) -> F-7(5) transition), with maximum intensity observed at 3 wt% Tb3+, beyond which concentration quenching occurred. The critical interaction distance (similar to 28 & Aring;) and interaction parameter (theta approximate to 5.55) confirmed that dipole-dipole interactions govern the quenching mechanism. Co-doping with Li+ and Na+ resulted in a significant enhancement in both emission intensity and decay lifetime, with Li+ co-doping at 3 wt% yielding an similar to 11.7-fold enhancement and increasing the average lifetime from 1.48 ms to 1.95 ms. This corresponds to a high radiative efficiency of similar to 93 %, indicating effective suppression of non-radiative losses. These enhancements were attributed to improved crystallinity, suppression of non-radiative defects, and modulation of the local crystal field symmetry. Judd-Ofelt analysis of the well-resolved emission bands yielded Omega(4) and Omega(6) intensity parameters of 0.96 x 10(-20) and 1.12 x 10(-20) cm(2), respectively, indicating moderate asymmetry in the ligand environment. The theoretical radiative lifetime (similar to 1.10 ms) showed excellent agreement with experimental values, indicating high radiative efficiency with minimal non-radiative losses. CIE chromaticity coordinates reached (0.3782, 0.6081) for 3 wt% Tb3+ and shifted toward the blue-green region (e.g., (0.2791, 0.4284) with Li+), demonstrating tunable and stable green emission. This work demonstrates the potential of Li+/Na+ co-doped KSYBO:Tb3+ phosphors as efficient, color-stable, and structurally robust green-emitting components for phosphor-converted white LED and optical display devices.