Reduction of Europium Ions in SrAl2O4 Glass Composites Using Aluminum Nitride

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Frozen sorbet technique has applied to the SrO-Al2O3-B2O3 glass system to prepare glass composites including Eu2+, Dy3+:SrAl2O4 (SA) microcrystals1. The glass composites show remarkable light-storage ability based on the natural properties of divalent europium in SA crystals, and this provide various photo-functionalities, such as long-persistent luminescence, photo-induced conductivity, and mechano-luminescence. Their unique optical properties of SA glass composites are expected to become novel light-storage materials for future energy applications. To control the valence states of europium (Eu3+ or Eu2+) is one of the important factors for the development of photo-functional glasses2. Generally, industrially mass-produced glasses have been prepared by melting raw materials at high temperature in air (oxidizing atmosphere), and then the valence of multivalent ions is determined by redox equilibrium in glass melt. For some special applications, photo-functional glasses have been prepared under reduction atmosphere using H2 or CO gas to design low valence state of ions or metal formation in glasses. Low valance lanthanide ions, such as Eu2+, Yb2+, Sm2+, and Ce3+, for example, have been used as functional centers for laser materials, phosphors, and magnetic-optical memory devices. Considering the difficulty and safety in H2 or CO gases use, more convenient and simple glass preparation methods are strongly required.Here, a new method for the reduction of Eu3+ to Eu2+ in SrO-B2O3-Al2O3-Eu2O3 (-AlN; reducing agent) system was proposed using a Frozen sorbet technique in air. Effective reduction of Eu3+ in SA crystals during the melting at 1500oC was demonstrated using differential thermal analysis (DTA), thermogravimetric analysis (TGA), and the optical properties. The reduction of Eu3+ to Eu2+ in AlN added glasses was proposed to proceed by the reaction of 2AlN + Eu2O3 + O2 (in air) → Al2O3 + 2EuO + N2. In this study, we have attempted to prepare the SA glass composites under various conditions using reducing agent; AlN for the frozen sorbet technique. Highly transparent SA glass ceramics were successfully synthesized and characterized using X-ray diffraction (XRD) and field emission-scanning electron microscopy (FE-SEM) with energy-dispersive X-ray analysis (EDX). The emission spectra and lifetime decays were observed using a steady-state and time-resolved spectrofluorophotometer system. Light-storage glass ceramics using the frozen sorbet method under oxidation control overcomes the limitation of Eu activated materials and the results of this study should be a significant contribution to the fields of inorganic chemistry and material design.