Crystallization Behaviour and Nanostructuring in Alkali Niobiosilicate Glasses

23K 2 O(27Nb 2 O 5 (50SiO 2 ((KNS), 13K 2 O(10Na 2 O(27Nb 2 O 5 (50SiO 2 (KNaNS) and 15K 2 O(12Li 2 O(27Nb 2 O5(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°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.