Charge-compensated Eu³ ⁺ luminescence in BaZr(BO₃)₂: Effect of Li⁺/Na⁺/K⁺ Co-doping on local symmetry, defect chemistry and radiative properties

Abstract

BaZr(BO3)2:Eu3+ red phosphors were prepared via a microwave-assisted sol-gel auto-combustion approach, in which Li+, Na+, and K+ ions were systematically varied and introduced as alkali co-dopants to enhance luminescence performance and provide effective charge compensation. Rietveld refinement of X-ray diffraction (XRD) data confirms the formation of a phase-pure trigonal BaZr(BO3)2structure (space group R3c), while Fourier-transform infrared (FTIR) and Raman spectroscopy analyses reveal that the BO3-ZrO6 framework remains largely intact, with only slight local distortions induced by doping. Under 393 nm excitation, the phosphors display the characteristic Eu3+ (5D0 -> 7FJ) emission transitions, predominantly governed by the hypersensitive 5D0 -> 7F2 transition centered at approximately 612 nm, resulting in intense red emission with high color purity. The optimal Eu3+ concentration is determined to be 3 wt%, with concentration quenching attributed to longrange dipole-dipole interactions. Alkali-ion co-doping markedly enhances photoluminescence intensity by facilitating charge compensation and mitigating defect states. Among the introduced alkali ions, Na+ yields the highest emission enhancement (approximately 12-fold relative to the Eu3+ doped sample) along with comparatively improved thermal stability, whereas K+ effectively extends the 5D0 excited-state lifetime, and Li+ promotes increased local structural asymmetry and stronger Eu-O covalency, as evidenced by the Judd-Ofelt intensity parameter Omega 2. Temperature-dependent photoluminescence studies further suggest that the improved thermal stability in co-doped samples is associated with reduced defect-assisted non-radiative recombination pathways. These findings highlight that Na+/K+/Li+-compensated BaZr(BO3)2:Eu3+ phosphors represent promising red-emitting candidates for near-UV/blue-excited white light-emitting diodes (LEDs), where performance is critically governed by the interplay between local symmetry modulation and defect suppression mechanisms.