Crystallization behavior of potassium niobium silicate (KNS) glasses having compositions expressed by the general formula xK(2)O . xNb(2)O(5).(1-2x)SiO2, with x = 0.167, 0.182, 0.200, 0.220, and 0.250, has been studied by DTA, X-ray diffraction, second harmonic optical generation (SHG), and electron microscopy. Bulk crystallization of potassium niobates in glasses with compositions near K2O . Nb2O5. 2SiO(2), as well as surface crystallization of KNbSi2O7 phase, has been established. Transparent glass-ceramics, based on potassium niobates with remarkable SHG signal values, can be obtained from glasses with the lowest silica content, by heat treatment at temperatures just above T-g, while at higher temperatures from all of the glasses under investigation the main crystallizing phase is KNbSi2O7 ferroelectric. Applying a dc electric field, grain-oriented crystallization is produced in KNS glasses with development of significantly anisotropic arrangements of KNbSi2O7 crystallites.
