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

Ca10M(PO4)7:Eu (M = Li, Na, K) phosphors have been synthesized via a solid-state reaction process, their phase purity was examined using XRD patterns, and Rietveld refinement confirmed that the Ca1oLi(PO4)7, Ca1oNa(PO4)7 and Ca10K(PO4)7 are pure phases. The photoluminescence properties of the Ca10M(P0.4)7: Eu (M = Li, Na, K) phosphors showed that the self -reduction of Eu3' to Eu2+ can occur in an 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 using photoluminescence spectra. In this work, the bond energy method was used to determine and explain the mechanism of site occupation of Eu entering the host matrix. According to the calculated value of the deviation of bond energy for Eu3+-doped Ca1oM(P0.4)7 (M = Li, Na, K) crystals, the similar value between LIE' and AEL'',-. and AEI':. E.- 0 [Le' U_and AE',`,* 02_ and AEER,c,* 02_ can provide the conditions for the self -reduction of Eu3+ in the Cato M(PO4)7 (M = Li, Na, K) system. Meanwhile, the smaller deviation values of AEL_0, AEr, 0, and AEL,.___o in Cal0Li(P0.4)7, CaloNa(P0.4)7, and Cai0K(P0J7 crystals and AELa, in Cai0K(PO4)7 crystals indicated that the preferential sites of Eu ion occupancy in the Cal0M(PO4)7 (M - Li, Na, K) lattices are Li, Na, K and Ca sites. The conclusions obtained from the calculated results of the bond energy method are consistent with the Rietveld refinement and the photoluminescence spectra of CaioM(PO4)7 (M = Li, Na, K).