Blue Emission of Sr_2-xCa_xP₂O₇:Eu²⁺ for Near UV Excitation

Sr _2-xCa _xP ₂O ₇:Eu ²⁺ (x = 0 - 2) phosphors with intense and adjustable blue emission are prepared by solid state reaction. The crystal phase evolution and correlated photoluminescence properties are studied as a function of Ca ²⁺ content based on the experimental measurements of X-ray diffraction, photoluminescence and fluorescence decay. Our results indicate that Sr _2-xCa _xP ₂O ₇:Eu ²⁺ crystallize in single α-Sr ₂P ₂O ₇ type phase for x ≤ 0.75. As increasing x from 0.75, α-Ca ₂P ₂O ₇ type phase starts to form and increase followed by reduction of α-Sr ₂P ₂O ₇ type phase. The single α-Ca ₂P ₂O ₇ type phase is obtained only at x = 2. The emission band (band I) in α-Sr ₂P ₂O ₇ type phase and that (band II) in α-Ca ₂P ₂O ₇ type phase both shift to the red on increasing x due to enhancement of crystal field strength in case of Ca ²⁺ substitution for Sr ²⁺. This substitution also leads to remarkably inhomogeneous broadening of band I, within which energy transfer from the high energy Eu ²⁺ to the low energy one is observed. The superposition of band I and band II results in various distributions of intense emission band including the redshifted and more broadened ones suitable for white light generation.