Charge-compensated LiCa4O(BO3)3:Tb3+ phosphors: K+/Na+-assisted lattice engineering for thermally robust green emission and luminescent thermometry

Abstract

Tb3+-activated LiCa₄O(BO₃)₃ (LiCBO) borates are promising phosphors for high-power near-UV LEDs and optical thermometry, but charge imbalance and lattice defects limit their thermal stability and efficiency. This work reports a comprehensive structural–optical study of LiCBO:Tb3+ phosphors with K+/Na+ charge-compensating co-dopants synthesized by combustion reaction. X-ray diffraction and Rietveld refinement show that all com positions remain single-phase LiCBO, with Tb3+ preferentially substituting Ca2+ sites while K+/Na+ ions act as charge compensators, inducing only marginal, monotonic changes in lattice parameters. Multi-model XRD linebroadening analysis reveals that alkali co-doping increases crystallite size from ~60 to ~90 nm and reduces microstrain and dislocation density, indicating defect suppression through charge-compensated lattice engi neering. FTIR/Raman spectroscopy confirms preservation of the mixed BO₃/BO₄ framework, with subtle band shifts and increased I(BO₄)/I(BO₃) ratios evidencing local network reorganization rather than phase segregation. Under 377 nm excitation, LiCBO:Tb3+ exhibits intense green 5 D₄ → 7 F₅ emission at 542 nm with an optimum Tb3+ content of x = 0.05, above which concentration quenching proceeds via multipolar Tb3+–Tb3+ interactions (Rc ≈ 10.6 Å). K+/Na+ co-doping boosts the green emission by up to ~2–2.25× at y = 0.01, mainly by sup pressing non-radiative defect channels and tuning local symmetry, as supported by biexponential lifetime shortening and increased Judd–Ofelt Ω₂ parameters. Temperature-dependent PL (300–550 K) demonstrates robust thermal stability, retaining ~40–45% of the initial 542 nm intensity at 423–450 K with a single activation energy Ea ≈ 0.33 eV. Finally, a fluorescence intensity ratio thermometer based on the 680/542 nm Tb3+ emissions delivers a maximum relative sensitivity of ~1.3% K− 1 near 500 K, demonstrating competitive per formance among single-center Tb3+-activated phosphors for high-temperature operation. Overall, K+/Na+- assisted charge-compensated lattice engineering is shown to simultaneously enhance green emission efficiency, thermal robustness, and luminescent thermometry performance in LiCBO:Tb3+ phosphors.