Structural and kinetic analysis of BaCl2 nanocrystals in fluorochlorozirconate glass-ceramics

The presence of BaCl₂ nanocrystals and the crystallographic phase that they adopt controls the optical behavior of fluorochlorozirconate glass-ceramics. We have used in situ X-ray diffraction heating experiments and ex situ transmission electron microscopy to follow the BaCl₂ nanocrystal nucleation and growth processes as a function of heating rate and isothermal hold temperature. The BaCl₂ nanocrystals nucleate with the hexagonal crystal structure and grow as spherical particles to a size of ∼10 to 20 nm. They then undergo a structural transformation to the orthorhombic phase and their shape changes to rounded disks, with diameters ranging from 150 to 250 nm, and thicknesses ranging from 80 to 120 nm. The change in size results from Ostwald ripening of the hexagonal BaCl₂ nanocrystals to form the orthorhombic BaCl₂ nanocrystals.