Influence of Al³⁺ co-doping on the spectral properties of europium doped Ca₉Y(PO₄)₇

A series of luminescent materials based on a calcium yttrium phosphate matrix doped with europium and different concentrations of aluminum ions (0, 5, 10% of mole) was synthesized using the Pechini method. A two-step strategy of synthesis was applied. Phase composition analysis and spectroscopic measurements were performed to characterize the obtained phosphors. The XRD patterns show that in all cases the obtained materials consist of a pure phase of Ca₉Y(PO₄)₇. Emission spectra of the materials obtained after the first step of the synthesis consist of narrow bands attributed to ⁵D₀-⁷F_J transitions in Eu³⁺ ions. Independently of the aluminum concentration, europium ions are incorporated into at least two different cationic sites. Considering the asymmetric ratio (R), the sites are characterized by the presence/absence of inversion symmetry. The emission intensity of Eu³⁺ introduced into the more symmetric site decreases with increasing aluminum concentration. The emission spectra of the materials after the reduction process are characterized by intensive broad bands located at 420 and 488 nm attributed to the d-f transitions in Eu²⁺; however, the line shape of the spectra depends on the aluminum concentration. Moreover, incorporation of aluminum ions causes the stabilization of the Eu³⁺ ions under a reductive atmosphere.