In situ study of the crystallization mechanism in LiNbO$_3$-SiO$_2$ glasses

International audience; A crystallization study of lithium niobium silicate glass with the molar composition 35Li$_2$O.25Nb$_2$O$_5$.40SiO$_2$ is reported using in situ techniques (x-ray diffraction, Raman spectroscopy and environmental electron microscopy) at high temperatures. Those techniques reveal that the surface crystallization begins at Tg with c-axis crystallites oriented perpendicular to the surface. Precipitation of nuclei, followed by growth and coalescence processes, induce the formation of particles and spherulites 0*5-4 μm diameter in size. A strong difference between surface and bulk crystallization has been observed with bulk crystallization yielding to 30 μm diameter size spherulites. Those original spherulite shapes could be the origin of the macroscopic bulk second harmonic generation (SHG) signal.