Investigations of Nanocrystal Phase Transformation in Glass Ceramics

In-situ and ex-situ TEM investigations of Fluorochlorozirconate (FCZ) glass have led to the discovery of previously unreported BaF₂ in the face-centered-cubic (FCC) and orthorhombic phases. These FCZ glasses are a class of material based on ZBLAN glasses, which are being developed for uses in advance mammography systems. The FCZs of interest have been doped with Eu (II) for use as either a scintillator or a storage phosphor material but need to be partially crystalline to show good optical properties. The photo-stimulated luminescence of this material, for use as storage phosphor, is attributed to the characteristic 5d-4f emission of Eu²⁺ present in the BaCl₂ nanocrystals. The crystals formed are known from XRD experiments to be hexagonal and orthorhombic BaCl₂ depending on the annealing temperature, 265 and 295°C respectively. In-situ and ex-situ TEM heating experiments were used to study the nucleation and growth process of the nanocrystals at the EMC. The nanocrystals nucleate and grow through-out the glass matrix when annealing FCZ glasses, therein producing a nanocomposite glass-ceramic system. The traditional BaCl₂ orthogonal phase in addition to the unreported FCC and orthogonal BaF₂ phase have been found in multiple ZBLAN compositions in which the content of Cl and F has been varied. This indicates that annealing FCZ glasses produces polymorphic crystals of both BaCl₂ and BaF₂, which vary in size from 10 nm to 100 nm.