Nucleation and crystallisation behaviour of transparent, erbium III doped,oxyfluoride glass ceramics for active photonic devices

A new Et3+ doped oxyfluoride glass ceramic composition is reported based on published compositions but with improved glass stability on reheating A se ries of heat treatments up to 168 h at 394 degreesC, close to T-g (glass tr ansformation temperature), has been carried out and yielded visually transp arent materials. X-ray diffractometry (XRD) has shown that a fluorite struc tured phase with lattice parameter 0.574 nm has crystallised after the 394 degreesC/168 h heat treatment. High resolution, transmission electron micro scope (HRTEM) images of the as-annealed glass prior to heat treatment revea led that the glass appeared not to have phase separated on melt quenching. However; after heat treatment for 6 h at 394 degreesC, ordered regions (bel ow 0.5 nm) which had a dark contrast with respect to the matrix were imaged . These regions are believed to be associated with the onset of nucleation of the fluorite structured phase. The regions grew with heat treatment time at 394 degreesC initially as approximately spherical crystals and at longe r times becoming dendritic with rounded arms protruding from a central nucl eus. After heat treatment of 0.25 h at 600 degreesC, glasses appeared visua lly pink and opaque, being heavily crystallised. XRD showed that again the fluorite structured phase had grown and HRTEM imaging indicated that the fl uorite 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 phas e and that the latter phase is PbF2 rather than a PbxCd[1-x]F2 solid soluti on as previously supposed. Near infrared spectroscopy indicated that after 48 h/394 degreesC the spectral absorption envelope due to Er3+ in the range 1495-1544 nm sharpened and shifted providing further evidence that the rar e earth local environment had changed.