Judd–Ofelt analysis and negative thermal quenching behavior of Tb³⁺-activated Ca₃La₃(BO₃)₅ phosphors co-doped with alkali ions for high-temperature photonic applications

This work presents a systematic investigation of the structural, optical, and thermal properties of Tb³ ⁺-doped Ca₃La₃(BO₃)₅ (CLBO) phosphors synthesized via sol-gel method and co-doped with alkali ions (Li⁺, Na⁺, K⁺). XRD confirmed successful dopant incorporation into the rhombohedral CLBO lattice without secondary phases. FTIR and Raman spectroscopy validated borate network stability and revealed local lattice distortion due to co-doping. Photoluminescence (PL) analysis under variable excitation demonstrated intense green emission (543 nm), with 3 wt% Tb³ ⁺ yielding optimum output. Remarkably, alkali co-doping enhanced both intensity and decay lifetime, with CLBO:3 wt% Tb³ ⁺, 1 wt% Na⁺ achieving a record 1.786 ms lifetime. Most notably, an extended negative thermal quenching effect was observed up to 550 K, exceeding previous thermal stability limits for Tb³ ⁺ phosphors. Additionally, a spectrally distinct 672 nm red emission was selectively observed under 377 nm excitation, suggesting excitation-dependent population mechanisms. Judd–Ofelt analysis of ⁵D₄ → ⁷F_J transitions yielded Ω₂, Ω₄, and Ω₆ parameters, along with radiative rates and optical gain, supporting the role of local symmetry in enhancing emission efficiency. Close agreement between theoretical (τ_rad = 0.071 ms) and experimental (0.980 ms) lifetimes confirmed model accuracy. CIE chromaticity analysis showed tunable color output via alkali co-doping, indicating strong potential for CLBO:Tb³ ⁺ materials in thermally stable photonic applications, including LEDs, displays, and luminescent thermometry.