f → f transition luminescence of Eu2+ in barium–aluminum–borate oxyfluoride glass ceramics

Novel transparent luminescent glass ceramics with the ability to generate the line emission of Eu2+ ions were successfully prepared. First, Eu2+-doped barium–aluminum–borate oxyfluoride glasses were prepared by high-temperature melting in a reducing atmosphere, and then transparent glass ceramics were obtained by crystallization heat treatment. Fluorescence spectroscopy showed that Eu2+ exhibited only broadband luminescence (FWHM ≈ 136 nm) of the 4f65d1 → 4f7 transition in the glasses. However, after heat treatment, a 359 nm line emission (FWHM ≈ 2.8 nm) from the 4f7 → 4f7 transition of Eu2+ appeared in the glass ceramics. To explore the mechanism of transformation of the optical transition of Eu2+ from 4f65d1 → 4f7 to 4f7 → 4f7, the glasses and glass ceramics were analyzed by fluorescence spectroscopy, X-ray diffraction (XRD), Raman spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy measurements. The results showed that the alteration in the optical transition of Eu2+ was a result of the Eu2+ ions entering the Ba2+ sites with weak crystal field strength in BaAlBO3F2 (BABF) crystals from the strong crystal field strength glass phase.