Nucleation and crystallisation of transparent, erbium III-doped, oxyfluoride glass-ceramics

A new Er3+-doped oxyfluoride glass-ceramic composition is reported, based on published compositions but with improved glass stability on reheating. A series of heat treatments up to 168 h at 394°C, close to Tg (glass transformation temperature), has been carried out and yielded visually transparent materials. X-ray diffractometry (XRD) has shown that a fluorite-structured phase with lattice parameter 0.574 nm has crystallised after the 394°C/168 h heat treatment. High-resolution, transmission electron microscope (HRTEM) images of the as-annealed glass prior to heat treatment revealed that the glass appeared not to have phase separated on melt-quenching. However, after heat treatment for 6 h at 394°C, ordered regions (0.5 nm) which had a dark contrast with respect to the matrix were imaged. These regions are believed to be associated with onset of nucleation of the fluorite phase. The regions grew with heat treatment time at 394°C initially as approximately spherical crystals and at longer times becoming dendritic with rounded arms protruding from a central nucleus. After heat treatment of 0.25 h at 600°C, glasses appeared visually pink and opaque, being heavily crystallised. XRD showed that again the fluorite-structured phase had grown and HRTEM imaging indicated that the fluorite-type crystals had coalesced into comparatively large spheres of 203±34 nm diameter in a glassy matrix. Energy dispersive spectroscopy (EDS) proved Er3+ had preferentially separated into the fluorite-structured phase, and that the latter phase is PbF2 rather than a PbxCd[1−x]F2 solid solution as previously supposed.