Photoluminescence and abnormal reduction of Eu³⁺ ions in CaAl₂O₄:EU nanophosphors calcined in air atmosphere

Precursor of Europium doped calcium aluminate (CaAl₂O₄: Eu) was prepared by the Pechini technique. The synthesized samples were calcined at 900–1500 °C for 2 h in air atmosphere. The crystal structures of calcined samples at low and high temperatures were found to be the metastable hexagonal and stable monoclinic systems by X-ray powder diffraction (XRD) analysis, respectively. The effects of Eu content and valence state on the luminescence characteristics of CaAl₂O₄: Eu were studied by luminescence spectrometer (LS). With the increase in calcination temperature, the phosphors exhibited color change from bright red to violet upon excitation by UV-light and an abnormal reduction of Eu³⁺→Eu²⁺ was observed in monoclinic phase of CaAl₂O₄: Eu that calcined in air atmosphere at high temperature. The reduction mechanism was explained reasonably by a rigid three-dimensional framework structure of the stable monoclinic phase of CaAl₂O₄: Eu and a charge compensation model