Probing Eu2+ luminescence from different crystallographic sites in Ca10M(PO4)7:Eu2+ (M = Li, Na and K) with beta-Ca3(PO4)2-type structure

Abstract

Eu2+ local environments in various crystallographic sites ena-bles the different distribution 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 cations 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 wavelength is related with 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 refine-ments 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) keep 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 to probe the distribution of Eu2+ ions. It is found that the emission band 410 nm is as-cribed to the occupation of Eu2+ in Ca(1), Ca(2) and Ca(3) sites with similar local environments, while the long-wavelength band (466 nm or 511 nm) is attributed to Eu2+ at M(4) site (M = Na and K). We show that the crystal-site en-gineering approach discussed herein can be applied to probe the luminescence of the dopants and provide a new method for photoluminescence tuning.