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

A new Er3+-doped oxyfluoride glass-ceramic composition is reported, based o n published compositions but with improved glass stability on reheating. A series of heat treatments up to 168 h at 394 degreesC, close to T-g (glass transformation temperature), has been carried out and yielded visually tran sparent materials. X-ray diffractometry (XRD) has shown that a fluorite-str uctured phase with lattice parameter 0.574 nm has crystallised after the 39 4 degreesC/168 h heat treatment. High-resolution, transmission electron mic roscope (HRTEM) images of the as-annealed glass prior to heat treatment rev ealed that the glass appeared not to have phase separated on melt-quenching . However, after heat treatment for 6 h at 394 degreesC, ordered regions (0 .5 nm) which had a dark contrast with respect to the matrix were imaged. Th ese regions are believed to be associated with onset of nucleation of the f luorite phase. The regions grew with heat treatment time at 394 degreesC in itially as approximately spherical crystals and at longer times becoming de ndritic with rounded arms protruding from a central nucleus. After heat tre atment of 0.25 h at 600 degreesC, 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 crystal s had coalesced into comparatively large spheres of 203 +/- 34 nm diameter in a glassy matrix. Energy dispersive spectroscopy (EDS) proved Er3+ had pr eferentially separated into the fluorite-structured phase, and that the lat ter phase is PbF2 rather than a PbxCd[1-x]F2 solid solution as previously s upposed. (C) 2001 Elsevier Science B.V. All rights reserved.