Phase transformations in Na2O-K2O-Nb2O5-SiO2 glasses

Phase transformations in glasses of compositions xNb(2)O(5) (.) (66 - X)SiO2 (.) 19Na(2)O (.) 11K(2)O (.) 2B(2)O(3) (.) 2BaO (mol%) (x = 5-39) at temperatures 610-700degreesC are studied by small-angle X-ray scattering and X-ray diffraction analysis. In glasses with x greater than or equal to 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 750-800degreesC. 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. (C) 2004 Elsevier B.V. All rights reserved.