Bond Energy, Site Preferential Occupancy and Eu2+/3+ Co-doping System Induced by Eu3+ Self-reduction in Ca10M(PO4)7 (M=Li, Na, K) Crystals

Ling Li, Wenjun Wang, Yu Pan, Yuhan Zhu, Hai-Bing Xu, Liqun Zhou, Hyeon Mi Noh, Jung Hyun Jeong, Xiao Guang Liu

Index: 10.1039/C8DT00749G

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Abstract

The Ca10M(PO4)7: Eu (M=Li, Na, K) phosphors have been synthesized via solid-state reaction process, their phase purity was examined by XRD patterns, Rietveld refinement confirmed the Ca10Li(PO4)7, Ca10Na(PO4)7 and Ca10K(PO4)7 are pure phase. The photoluminescence property of the Ca10M(PO4)7: Eu (M=Li, Na, K) phosphors showed that the self-reduction of Eu3+ to Eu2+ can occur in the air atmosphere. Eu3+ ions can be reduced to Eu2+ ions when doped in Ca10Li(PO4)7, Ca10Na(PO4)7 and Ca10K(PO4)7 crystals, which was detected by photoluminescence spectra. In this work, the bond energy method was used to determine and explain the mechanism of the site occupation of Eu entering the host matrix. According to the calculated value of the deviation of bond energy for Eu3+ doped Ca10M(PO4)7 (M=Li, Na, K) crystals, the similar value between ∆〖E〗_(〖Eu〗^(2+)-O^(2-))^(〖Li〗^+ ) and∆〖E〗_(〖Eu〗^(3+)-O^(2-))^(〖Li〗^+ ) , ∆〖E〗_(〖Eu〗^(2+)-O^(2-))^(〖Na〗^+ ) and ∆〖E〗_(〖Eu〗^(3+)-O^(2-))^(〖Na〗^+ ), ∆〖E〗_(〖Eu〗^(2+)-O^(2-))^(K^+ ) and ∆〖E〗_(〖Eu〗^(3+)-O^(2-))^(K^+ ) can provide the conditions for the self-reduction of Eu3+ in Ca10M(PO4)7 (M=Li, Na, K) system. Meanwhile, the smaller deviation values of ∆〖E〗_(Eu-O)^Li, ∆〖E〗_(Eu-O)^Na, ∆〖E〗_(Eu-O)^K in Ca10Li(PO4)7, Ca10Na(PO4)7, Ca10K(PO4)7 crystals and ∆〖E〗_(Eu-O)^Ca in Ca10K(PO4)7 crystal indicated that the preferential occupancy of Eu ions in Ca10M(PO4)7 (M=Li, Na, K) lattices are Li, Na, K and Ca sites. The conclusion obtained from the calculated result of bond energy method is consistent with the Rietveld refinement and the photoluminescence spectra of Ca10M(PO4)7 (M=Li, Na, K).