Structural Phase Formation and Tunable Luminescence of Eu2+-Activated Apatite-Type (Ca,Sr,Ba)5(PO4)2(SiO4)

Abstract

The samples on the base of the substitutions on Sr-cation in apatite Sr5(PO4)2(SiO4) were prepared by conventional solid-state reaction method: I) Sr4.75-5xCa5x(PO4)2(SiO4):Eu0.252+ (0.01 ≤ x ≤ 0.95) and II) Sr4.75-5yBa5y(PO4)2(SiO4):Eu0.252+ (0.01 ≤ z ≤ 0.5). The crystal phase formations were investigated by the X-ray powder diffraction patterns. The tunable green-yellow color was detected due to the substitution of cation Sr2+ in the host lattice by either Ca2+ or Ba2+. The relationship between the structure and luminescence properties was reported. The site-occupation of Eu2+ ions and energy transfer were discussed on the base of crystal structure. Partial substitution of Sr2+ by either Ca2+ or Ba2+ all results in a blue-shifted Eu2+ emission. The luminescence blue-shift is a common phenomenon in Ba2+-substition for Sr2+ due to a decreased crystal field splitting for Eu2+ on the bigger Ba2+ cation site. However, in the Ca2+-substitution for Sr2+, the luminescence blue-shifted Eu2+ emission is unusual. The detailed luminescence spectra of Eu2+ ions in relation with the micro-structure were first reported in one of the end compound Ca5(PO4)2(SiO4). This is helpful to improve this series of materials for a potential application as a biomaterial or a phosphor for the lighting.