Influence of CoO addition on phase separation and crystallization of glasses of the ZnO–Al2O3–SiO2–TiO2 system

The evolution of structure, phase composition and spectroscopic properties of CoO-doped (up to 5 mol%) titania-containing zinc aluminosilicate glasses with their heat-treatment has been studied using Raman scattering, small angle X-ray scattering, X-ray diffraction analysis and optical absorption spectra. Addition of cobalt oxide was observed to facilitate amorphous phase separation of the parent glass and gahnite, ZnAl2O4, crystallization. Cobalt oxide entered phases formed during low-temperature heat-treatments (720 °C), i.e., amorphous phase, enriched in ZnO, Al2O3 and TiO2 and crystalline phase of gahnite. The absorption of these glass-ceramics was defined mainly by tetrahedral Co2+ ions located in gahnite nanocrystals. As the temperature was increased further, traces of anosovite solid solution appeared and then decomposed. Even after high-temperature heat-treatments, a certain portion of Co2+ ions remained in amorphous zinc aluminotitanate phase and in octahedral sites of inversed gahnite spinel. In glass-ceramics, the residual high silica amorphous phase contained a small quantity of [TiO4] centers, which content was smaller in Co:ZAS samples as compared with non-doped glass-ceramics.