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 ...