Versatile luminescence of Eu²⁺,³⁺-activated fluorosilicate apatites M₂Y₃[SiO₄]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.