Anomalous 5D₀ → 7F₄ transition induced by alkali ion co-doping in Eu3+-activated K₇SrY₂(B5O10)3 borates: A Judd–Ofelt and photoluminescence analysis for pc-WLEDs

Abstract

In this study, red-emitting K₇SrY₂(B5O10)3:Eu3+ 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 5 D₀ → 7 F₄ tran sition, which is typically parity-forbidden. This behavior is further supported by the high Judd–Ofelt Ω₆ parameter (5.42 × 10− 20 cm2 ), 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:Eu3+,Li+ as a promising red phosphor for pc-WLEDs.