Spherulitic crystallization of quartz-like GeO2 and correlated second harmonic generation in sodium tantalum germanate glasses

Complex sequential crystallization mechanisms were investigated in the glass composition 70GeO2-10Na2O-20Ta2O5 prepared by melt-quenching. In a first stage, classical volume-nucleated of sodium tantalate perovskite Na2Ta8O21 is induced under short time heat-treatments whereas longer annealing treatments promote formation surface spherical structures known as spherulites. These singular glass-ceramics deeply density measurements, differential scanning calorimetry (DSC), X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), Auger Electron Spectroscopy correlated Raman and second harmonic generation (SHG) microscopy well macro-SHG measurements. spherulites with average size up to 50 µm are richer tantalum but poorer germanium than surrounding material. However, measured SHG signals found be related quartz-like α-GeO2 grown inside Therefore these materials exhibit second-order non-linear optical response, symmetry point group D3 given quartz GeO2 for transparent glass-ceramic.