Identification of the crystallographic sites of Eu2+ in Ca9NaMg(PO4)7: structure and luminescence properties study

Abstract

The substituted activator ions Eu²⁺ in the β-Ca₃(PO₄)₂-type structure, Ca₉NaMg(PO₄)₇ compound, have been studied as emission centers, as well as structural probes. The crystal structure and crystallographic sites of Eu²⁺ in Ca₉NaMg(PO₄)₇ have been identified by Rietveld refinements, electron paramagnetic resonance (EPR) spectroscopy analysis and combined photoluminescence spectral analysis, respectively. Ca₉NaMg(PO₄)₇:Eu²⁺ phosphor exhibits a broad emission band in the visible light region with three distinguished peaks centered at 415, 458, and 615 nm, which are ascribed to different Eu²⁺ emission centers occupied in three different Ca crystallographic sites. The concentration quenching behavior, variation in lifetimes and the temperature dependent luminescence properties have been investigated as a function of different emission centers, 415, 458, and 625 nm for Ca₉NaMg(PO₄)₇:Eu²⁺ phosphor. Accordingly, the relationship among three different Eu²⁺ luminescence centers originating from three different Ca²⁺ crystallographic sites has been found and verified.