Versatile luminescence of Eu^2+,3+-activated fluorosilicate apatites M_2Y_3[SiO_4]_3F (M = Sr, Ba) suitable for white light emitting diodes

Eu-doped fluorosilicate apatites M2Y3[SiO4]3F (M = Sr, Ba) are prepared by solid state reaction. Unlike conventional Eu-doped materials, coexistence of Eu3+ and Eu2+ ions is found from the photoluminescence of Eu-doped apatites M2Y3[SiO4]3F (M = Sr, Ba) which were prepared in reducing atmosphere. Eu2+ ions are converted from Eu3+ ions in the reduction process. It is suggested that Eu2+ ions occupy A(I) (4f) or A(II) (6h) crystallographic site in the apatite lattices. Intense emission lines due to Eu3+ are observed at 600-630 nm, while broad emission band due to Eu2+ is observed at 450-650 nm. These emissions combined with blue emission from LED are suitable to obtain white light, i.e., white LEDs for lighting and display. Different luminescence characteristics are obtained between Sr2Y3[SiO4]3F:Eu and Ba2Y3[SiO4]3F:Eu, which were prepared in reducing atmosphere.