Small-angle X-ray scattering and low-frequency Raman scattering study of liquid phase separation and crystallization in titania-containing glasses of the ZnO-Al2O3-SiO2 System

The paper presents a study of phase transformations upon heat--treatment of three titania--containing zinc aluminosilicate glasses with equal contents of ZnO and Al2O3 by small--angle X--ray scattering and X--ray powder diffraction methods. Additionally the glass with the lowest SiO2 content was studied by both traditional and low--frequency Raman spectroscopy. It was found that upon phase decomposition a bimodal structure is developed in glasses under study. There are two types of nanometer sized inhomogeneous regions different in composition and in dimension: small zinc aluminate regions (~6--10nm in size) and large zinc aluminotitanate ones (~10--50nm in size depending on the composition of the initial glass). In zinc aluminate regions, aluminozinc spinel, gahnite, crystallizes starting from heat--treatment at 750 deg C. Zinc aluminotitanate phase remains amorphous until the material is heat--treated at 950 deg C, whereupon on heat--treatment up to 1050 deg C, crystals of two metastable forms of TiO2, anatase and brookite, are formed in this phase. At higher temperatures they transform into a stable form of TiO2, rutile. The origin of the low--frequency Raman scattering in titania--containing zinc aluminosilicate glasses has been associated with the formation of amorphous zinc aluminate nanometer sized inhomogeneous regions. It has been found that zinc aluminotitanate inhomogeneous regions do not reveal themselves in low--frequency Raman scattering.