Crystal Field Asymmetry and Radiative Properties of Eu³⁺ Ions in LiNaSO₄ Phosphor: A Judd-Ofelt Approach

Eu³⁺-activated LiNaSO₄ phosphors were synthesized via a sol-gel technique and investigated to understand their structural, morphological, and luminescent properties. X-ray diffraction and Rietveld refinement confirmed the formation of a single-phase trigonal LiNaSO₄ structure with high crystallinity and successful incorporation of Eu³⁺ ions without disturbing the host framework, whereas FTIR spectra verified the presence of characteristic sulfate vibrational modes, and SEM analysis revealed agglomerated polycrystalline grains with well-defined boundaries. Photoluminescence measurements under 395-nm excitation showed intense red emission dominated by the hypersensitive ⁵D₀ → ⁷F₂ transition at 614 nm, indicating that Eu³⁺ ions occupy noncentrosymmetric lattice sites. The emission intensity increased with dopant concentration up to 0.8 mol% and then decreased due to concentration quenching governed by multipolar interactions. CIE chromaticity analysis demonstrated deep red emission with high color purity and reduced correlated color temperature, supporting suitability for warm white LED applications. Judd-Ofelt analysis yielded an asymmetry ratio of 2.07 and an intensity parameter Ω₂ = 1.43 × 10^-20 cm², confirming strong crystal field asymmetry and enhanced Eu-O covalency, which contribute to the observed intense red luminescence. These results establish LiNaSO₄:Eu³⁺ as a promising red-emitting phosphor for solid-state lighting and display technologies.