STRUCTURAL AND OPTICAL-PROPERTIES OF THE SYSTEM (SR,EU)(5)(PO4)(3)(CL,F)

Mixed crystals of the composition Sr5-yEuy(PO4)(3)Cl1-xFx with 0 less than or equal to x less than or equal to 1 and 0 less than or equal to y less than or equal to 0.05 were prepared by solid state reaction. T he crystal structure, optical properties at room temperature, and the emission spectra of the polycrystalline materials at liquid nitrogen t emperature are investigated. The structure parameters are refined by t he Rietveld method. The space group of the mixed crystals is P6(3)/m. The position of the halide ions in the unit cell depends on the fluori de content in the materials. Eu2+ ions occupy preferentially 6h cation positions of the unit cell. The lattice constant a(0) decreases but c (0) increases with increasing fluoride content in the samples and the emission band due to transitions between 4f(6)5d and 4f(7) configurati ons of the Eu2+ ions is shifted to higher energies. The shape of the e xcitation spectrum and the fluorescence lifetime change when the fluor ide content in the mixed crystals exceeds the chloride content.