Influence of the annealing temperatures on the photoluminescence of KCaBO3:Eu3+ phosphor

Abstract

Novel red emitting KCaBO₃:Eu phosphors have been synthesized by solid-state reaction at various temperatures. Systematic studies on annealing effects and consequent structural evolution and optical properties were investigated by various structural and photoluminescence studies. With an increase in annealing temperature (from 700 °C to 950 °C), these phosphors show a gradual change from a mixed low crystalline phase to a highly crystalline single phase, with minimized volatile impurities. Photoluminescence studies revealed that the low-temperature annealed phosphors showed distinct mixed emission composed of blue and red emissions upon UV excitation. Such dual emission is due to the coexistence of Eu³⁺ and Eu²⁺ ions, wherein the reduction of Eu³⁺ into Eu²⁺ was attributed to the presence of volatile impurities. Relatively high-temperature annealed phosphors exhibit strong red color photoluminescence due to homogeneously occupied Eu³⁺ ions in the host crystal charge-compensated (with K⁺ ions) sites of Ca²⁺ ions. The dominant red-to-orange emission intensity ratios and Judd–Ofelt parameters of Eu³⁺ ions support the strong covalent nature and site-occupation of higher asymmetry sites of K⁺ and Ca²⁺ ions. High emission life times and efficient and stable photoluminescence at different excitation wavelengths make these phosphors suitable for white LEDs and other display applications.