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Complex sequential crystallization mechanisms were investigated in the glass composition 70GeO(2)-10Na(2)O-20Ta(2)O(5) prepared by melt-quenching. In a first stage, a classical volume-nucleated crystallization of sodium tantalate perovskite Na2Ta8O21 is induced under short time heat-treatments whereas longer annealing treatments promote the formation of surface spherical structures known as spherulites. These singular glass-ceramics were deeply investigated by density measurements, differential scanning calorimetry (DSC), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), Auger Electron Spectroscopy (EDS), correlated Raman and second harmonic generation (SHG) microscopy as well as macro-SHG measurements. These surface spherulites with average size up to 50 mu m are richer in tantalum but poorer in germanium than the surrounding material. However, measured SHG signals were found to be related to quartz-like alpha-GeO2 grown inside the spherulites. Therefore these materials exhibit a second-order non-linear optical response, related to the symmetry of the point group D-3 given by the quartz GeO2 grown for the first time in a transparent glass-ceramic. (C) 2021 Elsevier B.V. All rights reserved.


Evangelista, Renato Oliveira;  Cardinal, Thierry;  Fargin, Evelyne;  Buffiere, Sonia;  Cam, Nithavong;  Dussauze, Marc;  Rodriguez, Vincent;  Adamietz, Frederic;  Marcondes, Lia;  Batista, Gislene;  Cassanjes, Fabia Castro;  Poirier, Gael

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