Anomalous D₀ → F₄ transition induced by alkali ion co-doping in Eu-activated K₇SrY₂(BO) borates: A Judd-Ofelt and photoluminescence analysis for pc-WLEDs.

In this study, red-emitting K₇SrY₂(BO):Eu phosphors were synthesized via high-temperature solid-state reaction. The effects of Li and Na co-doping on structural and photoluminescent properties were investigated. XRD and Rietveld refinement confirmed a phase-pure trigonal structure. Under 394 nm excitation, Li co-doping induced significant local lattice asymmetry, leading to a remarkable ∼58-fold enhancement of D₀ → F₄ transition, which is typically parity-forbidden. This behavior is further supported by the high Judd-Ofelt Ω₆ parameter (5.42 × 10 cm), indicating enhanced electric dipole character due to symmetry breaking. CIE chromaticity coordinates shifted toward deeper red emission with color purity up to 97.7 % and CCT as low as 2253 K. Temperature-dependent PL confirmed high thermal stability (Eₐ = 0.190 eV). These results demonstrate that Li-induced symmetry distortion is a powerful strategy to enhance anomalous red emission, positioning KSYBO:Eu,Li as a promising red phosphor for pc-WLEDs.