Abnormal reduction of Eu3+ to Eu2+ in Sr5(PO4)3Cl:Eu phosphor and its enhanced red emission by the charge compensation

A series of Eu doped and charge compensated Sr-5(PO4)(3)Cl phosphors have been synthesized via a solid-state reaction in air atmosphere. The XRD patterns and the Rietveld refinement confirmed that the obtained samples were pure phases. The photoluminescence properties of Sr-5(PO4)(3)Cl:Eu phosphors exhibited that Eu3+ could be reduced to Eu2+ in an air atmosphere, and the corresponding mechanism of abnormal reduction of Eu3+ to Eu2+ had been discussed. Meanwhile, the enhancement in the PL intensity of red emission and the thermal stability of phosphors was observed upon co-doping via the charge balance strategies of (a) 2Sr(2+) = Eu3+ + R+ (R = Li, Na and K), (b) Eu3+ + Si4+ = Sr2+ + P5+ and (c) 3Sr(2+) = vacancy + 2Eu(3+), among which Na+ ions had the optimized charge compensation effect. The mechanisms between different charge compensation methods and the improvement in PL intensities of Eu3+ were also analyzed.