The Phase Transition and Photoluminescence Properties of (Ca9−x Sr x )Mg1.5(PO4)7:Eu2+ Solid-Solution Phosphors

Abstract

A series of color-tunable (Ca9−x Sr x )Mg1.5(PO4)7:Eu2+ (0 ≤ x ≤ 9) phosphors are designed by using mineral-inspired structural design strategy and synthesized by solid-state reaction. The mineral structure β-Ca3(PO4)2-type compounds, Ca9Mg1.5(PO4)7 and Sr9Mg1.5(PO4)7 are chosen as structural prototypes. Both of them are hexagonal structure but have different space groups. The former has R3c space group, while the latter possesses R3m space group. When Ca9Mg1.5(PO4)7 compound is used as maternal materials, the powder X-ray diffraction results demonstrate that the successive substitution of Ca2+ with Sr2+ ions expands and distorts the lattice parameters and unit cell volume of Ca9Mg1.5(PO4)7 compound, rendering phase transition from the (Ca9−x Sr x )Mg1.5(PO4)7:Eu2+ (phase 1, 0 ≤ x < 3) with R3c space group to (Ca9−x Sr x )Mg1.5(PO4)7:Eu2+ (phase 2, 5 < x ≤ 9) with R3m space group. The phase transition causes noticeable spectral redshift from the original blue-violet emission of x = 0 to bright yellow emission of x = 9 sample. Importantly, at x = 1, a bright white-light emission with a full width at half maximum of ∼255 nm is discovered in Ca8SrMg1.5(PO4)7:Eu2+ phosphor, which is beneficial to achieve phosphor-converted white-light-emitting diodes with high color-rendering index. These findings indicate that the mineral-inspired structural design strategy is very helpful for the development of new phosphors.