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

Abstract

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 Ca₁₀Li(PO₄)₇, Ca₁₀Na(PO₄)₇ and Ca₁₀K(PO₄)₇ are pure phases. The photoluminescence properties of the Ca₁₀M(PO₄)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 Ca₁₀M(PO₄)₇ (M = Li, Na, K) crystals, the similar value between and , and , and and can provide the conditions for the self-reduction of Eu³⁺ in the Ca₁₀M(PO₄)₇ (M = Li, Na, K) system. Meanwhile, the smaller deviation values of , , and in Ca₁₀Li(PO₄)₇, Ca₁₀Na(PO₄)₇, and Ca₁₀K(PO₄)₇ crystals and in Ca₁₀K(PO₄)₇ crystals indicated that the preferential sites of Eu ion occupancy in the Ca₁₀M(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 Ca₁₀M(PO₄)₇ (M = Li, Na, K).