Crystallization of the K2O - Nb2O5 - 2SiO2 glass: evidences for existence of bulk nanocrystalline structure

The crystallization of the K2O . Nb2O5. 2SiO2 (KNS-50) glass has been studied by DTA, XRD, SEM and FTIR. The as-quenched glass crystallizes in two steps during a DTA run. In the first step only potassium niobates crystallize while at the higher temperature step KNbSi207 ferroelectric crystallizes. Heat treatments of the bulk glass at temperatures not far from T-g produce a nanocrystalline structure penetrating the whole volume of the glass and clearly observed by high resolution SEM as isometric nuclei. The observed nanostructure is the result of two processes occurring during the heat treatment amorphous phase separation followed by the crystallization of the high alkali and niobium content glassy phase. The heat-treated glass, in which the nanostructure is grown, shows an additional DTA peak at about 900 degrees C related to the crystallization of an unknown crystalline phase, nor obtained in the as-quenched glass. The existence of the nanostructure clarifies the origin of SHG discovered recently in potassium niobium silicate glasses. (C) 2000 Elsevier Science B.V. All rights reserved.