Composition dependence of the optical and structural properties of Eu-doped oxyfluoride glasses

Europium doped oxyfluoride glasses were prepared by the melt quenching method for the light emitting diodes applications. The optical and structural properties of these glasses were studied by means of photoluminescence spectra, Commission Internationale de L'Eclairage chromaticity coordinates, X-ray diffraction, and Fourier transform infrared spectra. We find that the spontaneous reduction of Eu3+ to Eu2+ is realized by Eu3+ ions occupying the sites of Ca2+ in the glasses prepared in air atmosphere. The Eu3+ to Eu2+ conversion efficiency, optical performances and structure of the glasses strongly depend on the base glass compositions. For certain base glass compositions, CaF2 crystals can form during the melt cooling process, and thereby enhance the conversion from Eu3+ to Eu2+. The formation of CaF2 crystals can be suppressed by adding CaO, Al2O3 and B2O3, but enhanced by adding Na2O and K2O in glass compositions. Finally, we propose a mechanism to explain how the glass structure affects the reduction of Eu ions as well as optical properties of the glasses.