Na+-driven enhancement of the 5D₀ → 7F₄ emission in Eu3+-activated KCa₄(BO₃)₃: photoluminescence and Judd–Ofelt study

Abstract

A series of Eu³⁺-activated KCa₄(BO₃)₃ (KCOB) phosphors were synthesized using a conventional solid-state reaction method. The structural, morphological, and photoluminescence properties were systematically tuned via Li⁺ and Na⁺ co-doping. Rietveld-refined X-ray diffraction confirmed successful substitution of Ca²⁺ sites by Eu³⁺ and alkali ions without forming secondary phases. FTIR and Raman spectroscopy validated the stability of the [BO₃]³⁻ unitswhile also indicating local vibrational distortions caused by asymmetry in the crystal field environment. SEM images displayed uniform microstructures with enhanced surface densification upon alkali incorporation. Photoluminescence spectra exhibited strong red emission centered at ∼613 nm, attributed to the ⁵D₀ → ⁷F₂ electric dipole transition of Eu³⁺, with intensity enhancement under alkali co-doping due to increased site asymmetry. Judd–Ofelt analysis revealed significant increases in Ω₂ and Ω₄ parameters, particularly in Na⁺-doped samples, confirming the enhancement of electric dipole transition probabilities. CIE chromaticity analysis showed that Li⁺ co-doping at y = 0.02 yielded the highest color purity (84 %) and lowest correlated color temperature (CCT ≈ 1940 K), while Na⁺ co-doping provided moderate-to-high color purity (up to 79 %) with tunable CCT values between 1831and 2038 K. Temperature-dependent PL studies anomalous non-monotonic behavior, including partial intensity recovery at elevated temperatures, deviating from classical quenching models. This suggests the involvement of defect-mediated recombination pathways and strong local crystal field effects stabilizing emission under thermal stress. These findings demonstrate that Li⁺/Na⁺ co-doping enables precise tuning of crystal symmetry, emission intensity, chromaticity, and thermal resilience. Thus, KCOB:Eu³⁺ phosphors with optimized alkali content represent promising candidates for thermally robust, high-purity red emitters in near-UV-pumped warm-white solid-state lighting systems.