Probing Eu2+ Luminescence from Different Crystallographic Sites in Ca10M(PO4)(7):Eu2+ (M = Li, Na, and K) with beta-Ca-3(PO4)(2)-Type Structure

Chen, Mingyue; Xia, Zhiguo; Molokeev, Maxim S.; Lin, Chun Che; Su, Chaochin; Chuang, Yu-Chun; Liu, Quanlin CHEMISTRY OF MATERIALS, 29 (17):7563-7570; 10.1021/acs.chemmater.7b02724 SEP 12 2017

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.