Na<SUP>+</SUP>-driven enhancement of the <SUP>5</SUP>D₀ → <SUP>7</SUP>F₄ emission in Eu<SUP>3+</SUP>-activated KCa₄(BO₃)₃: photoluminescence and Judd-Ofelt study

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 similar to 613 nm, attributed to the D-5(0) -> F-7(2) electric dipole transition of Eu3+, with intensity enhancement under alkali co-doping due to increased site asymmetry. Judd-Ofelt analysis revealed significant increases in Omega(2) and Omega(4) 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 approximate to 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.