Photoluminescent properties and site occupation preference in Bi³⁺, Eu³⁺ doped CaY₄(SiO₄)₃O phosphor

Bi³⁺, Eu³⁺ doped CaY₄(SiO₄)₃O phosphors were synthesized through high temperature solid state reaction. Their photoluminescent properties were investigated and site occupation preference of Bi³⁺ in cationic sites was analyzed. The structure of CaY₄(SiO₄)₃O is characterized by three non-equivalent cationic sites with different coordination and cation-oxygen distances. By means of dielectric theory of the chemical bond for complex crystals, several kinds of chemical bond parameters like fractional covalence of CaY₄(SiO₄)₃O were calculated and integrated to yield environmental factor h_e. According to quantitative equations between the transition energy of Bi³⁺ and environmental factor h_e, the excitation bands at 308 and 226 nm were assigned to ¹S₀→³P₁ transition of Bi³⁺ in Y(6h) and Y(4f) site, respectively. Another excitation band centered at 210 nm should be the overlap of Bi³⁺ A-band in Ca site and C-band in Y(6h) site. Optical band gap of pure CYSO was calculated using Kubelka–Munk method from diffuse reflectance spectra. Red emission can be realized in CaY₄(SiO₄)₃O:Bi³⁺, Eu³⁺ under UV light excitation because of efficient energy transfer from Bi³⁺ to Eu³⁺ and decay behaviors of Bi³⁺ and Eu³⁺ emission were investigated. Without optimization, the internal quantum efficiency of CYSO:2%Bi³⁺, 7%Eu³⁺ at 310 and 393 nm excitations were 31.563%, 74.252%, respectively.