Structure-property correlation and tunable luminescence in Dy3+-activated YBa3B9O18 phosphors via alkali co-doping

Abstract

Borate-based hosts are attractive candidates for phosphor-converted white light-emitting diodes (pc-WLEDs) owing to their wide band gaps, chemical stability, and structural flexibility. In this work, Dy3+-activated YBa3B9O18 (YBBO) phosphors co-doped with Na+ and K+ ions were synthesized to investigate the influence of alkali-induced structural modification on photoluminescence behavior. X-ray diffraction and Rietveld refinement confirmed the formation of a single-phase hexagonal structure, while crystallite-size and microstrain analyses indicated that K+ co-doping introduces relatively higher lattice distortion than Na+. Photoluminescence spectra under near-UV excitation exhibited characteristic Dy3+ emissions at similar to 576, 660, and 760 nm, corresponding to yellow, red, and deep-red transitions, respectively. The optimum Dy3+ concentration was found to be x = 0.05, beyond which concentration quenching occurs mainly through long-range multipolar interactions, as supported by the critical transfer distance of similar to 8.8 & Aring;. Alkali co-doping enhanced the PL intensity, with Na+ giving the highest enhancement (similar to 1.8-fold), whereas K+ increased the relative contribution of the 660 nm red emission band. Emission-derived Judd-Ofelt-type analysis indicated an increased Omega(4,eff) trend, which is discussed only as a comparative indicator of the enhanced relative contribution of the 660 nm red band rather than as an absolute Judd-Ofelt parameter. Temperature-dependent PL analysis showed improved thermal stability for the co-doped samples, with activation energies of similar to 0.202 eV for Na+ and similar to 0.189 eV for K+. At 550 K, the co-doped phosphors retained approximately 40% of their room-temperature emission intensity, compared with similar to 23% for the singly doped sample. Chromaticity analysis placed the emission in the warm-white to yellow-red region, mainly governed by the relative contributions of the 576 nm yellow and 660 nm red bands. Overall, the results suggest an indirect structure-luminescence relationship in YBBO:Dy3+ phosphors and demonstrate that alkali co-doping is an effective strategy for tuning emission intensity, color output, and thermal stability.