Crystallization Behaviour and Nanostructuring in Alkali Niobiosilicate Glasses

23K(2)O center dot 27Nb(2)O(5)-50SiO(2) (KNS), 13K(2)O center dot 10Na(2)O center dot 27Nb(2)O(5)center dot 50SiO(2) (KNaNS) and 15K(2)O center dot 12Li(2)O center dot 27Nb(2)O(5)center dot 46SiO(2) (KLiNS) transparent glasses were synthesized by melt-quenching technique, and studied by differential thermal analysis (DTA), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) to reveal the effect of the devitrification behaviour on transparent nanostructure. Just above the glass transition temperature T(g) in the KNS glass, an unidentified phase was formed, while in KNaNS and KLiNS, mixed-alkali niobate phases with tungsten bronze structure were obtained by bulk crystallization. Heat treatments at T(g) performed on the KNS glass resulted in the transparent nanostructure with second order harmonic generation (SHG) activity. Heat treatment for 10 h on KNaNS and KLiNS decreased the first DTA exothermic peaks (at least 24 degrees C), indicating the bulk nucleation, which was confirmed by the DTA in comparison with the powdered as-quenched samples. KNaNS and KLiNS showed similar XRD profiles as the K(3)Li(2)Nb(5)O(15) crystal with the five most intense peaks at 22.7, 29.4, 32.3, 46.3 and 52.0 deg. HRTEM micrograph showed clear-cut nano-sized circular domains and spherical nanocrystals dispersed into the amorphous matrix