Phase transformations in Na2O–K2O–Nb2O5–SiO2 glasses

Abstract Phase transformations in glasses of compositions xNb2O5 · (66 − x)SiO2 · 19Na2O · 11K2O · 2B2O3 · 2BaO (mol%) (x = 5–39) at temperatures 610–700 °C are studied by small-angle X-ray scattering and X-ray diffraction analysis. In glasses with x ⩾ 15, a metastable phase separation takes place which is followed by the formation of NaNbO3 crystals in regions with a high niobium content. In every region the structure consists of NaNbO3 microcrystals separated by high-silicate layers. These layers hinder diffusion processes and provide the stability of the structure at temperatures up to 750–800 °C. Through this study, transparent glass ceramics showing high values of electrooptical constant, non-linear refractive index and two-photon non-linear absorption coefficient were developed.