Low Temperature Synthesis of Eu-Doped Cubic Phase BaAl2S4 Blue Phosphor Using H3BO3 or B2O3 Flux

A H3BO3 or B2O3 flux was added to a mixture of BaS, EuS, Al, and S to synthesize a Eu-doped cubic BaAl2S4 phosphor. The formation temperature of the cubic BaAl2S4 phase was lowered to 600 degrees C, which was 60 degrees C below the temperature of a case without a flux. To improve the crystallinity and color purity, a two-step annealing process that involved low temperature annealing (130 degrees C for H3BO3 and 400 degrees C for B2O3) was proposed for a uniform immersion of a flux into a starting mixture and a high temperature annealing (600 degrees C) for the formation of BaAl2S4. The phosphor synthesized by the two-step process with a 2 wt % H3BO3 flux showed a distinct cubic BaAl2S4 phase and a strong photoluminescence (PL) intensity at 475 nm. The PL intensity was 6.6 times stronger than that of the phosphor without a flux. The phosphor synthesized with a B2O3 flux showed only an orthorhombic phase, while direct annealing at 600 degrees with the flux showed a mixture of cubic and orthorhombic phases. The role of the fluxes in the formation mechanism of the cubic and orthorhombic BaAl2S4 phases was described.