Microstructure investigation and fluorescence properties of europium-doped scheelite crystals in glass-ceramics made under different synthesis conditions

Abstract

Glasses of the composition 8 BaO·8 SrO·34 ZnO·46 SiO2·4 WO3 doped with 1·1020 Eu3+/cm³ were melted either in air or under continuous bubbling with forming gas (90% N2 and 10% H2). UV–vis spectra of both glasses show absorption lines of Eu3+. In the glass melted under reducing conditions, an additional broad absorption band in the 300–600 nm range is observed, typical for the occurrence of Eu2+. Thermal treatment of the samples lead to the crystallization of (Sr, Ba)WO4 and LEAZit (Low expansion alkaline earth zinc silicate with the composition (Sr, Ba)Zn2Si2O7). The incorporation of rare earth oxides into glass-ceramics containing LEAZit have not been described before. As shown by scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy, europium is incorporated in (Sr, Ba)WO4, to a lesser extend in (Sr, Ba)Zn2Si2O7 crystals, and also in the residual glass phase. Fluorescence spectra showed solely emissions due to Eu3+. Emissions by Eu2+ were not observed, neither in the crystallized samples nor in glass samples bubbled with forming gas. In the samples melted in air, the emission intensities due to Eu3+ were ten times larger than in the samples bubbled with forming gas. This is supposedly due to the higher absorption of Eu2+ in the reduced samples which does not result in Eu3+ luminescence. In comparison to the emission spectra of the glasses, the intensity ratios of the Eu3+ emission peaks change during the crystallization process. This indicates the incorporation of Eu3+ into the (Sr, Ba)WO4 crystals, most probably at (Sr, Ba) sites.