Probing Eu²⁺ Luminescence from Different Crystallographic Sites in Ca₁₀M(PO₄)₇:Eu²⁺ (M = Li, Na, and K) with β-Ca₃(PO₄)₂-Type Structure

Eu²⁺ local environments in various crystallographic sites enable the different distributions of the emission and excitation energies and then realize the photoluminescence tuning of the Eu²⁺ doped solid state phosphors. Herein we report the Eu²⁺-doped Ca₁₀M(PO₄)₇ (M = Li, Na, and K) phosphors with β-Ca₃(PO₄)₂-type structure, in which there are five cation crystallographic sites, and the phosphors show a color tuning from bluish-violet to blue and yellow with the variation of M ions. The difference in decay rate monitored at selected wavelengths is related to multiple luminescent centers in Ca₁₀M(PO₄)₇:Eu²⁺, and the occupied rates of Eu²⁺ in Ca(1), Ca(2), Ca(3), Na(4), and Ca(5) sites from Rietveld refinements using synchrotron power diffraction data confirm that Eu²⁺ enters into four cation sites except for Ca(5). Since the average bond lengths d(Ca-O) remain invariable in the Ca₁₀M(PO₄)₇:Eu²⁺, the drastic changes of bond lengths d(M-O) and Eu²⁺ emission depending on the variation from Li to Na and K can provide insight into the distribution of Eu²⁺ ions. It is found that the emission band at 410 nm is ascribed to the occupation of Eu²⁺ in the Ca(1), Ca(2), and Ca(3) sites with similar local environments, while the long-wavelength band (466 or 511 nm) is attributed to Eu²⁺ at the M(4) site (M = Na and K). We show that the crystal-site engineering approach discussed herein can be applied to probe the luminescence of the dopants and provide a new method for photoluminescence tuning.