Growth-front hopping via stress-induced nucleation illustrated for the crystallization of Ba1-xSrxZn2Si2O7 from a glass in the BaO-SrO-ZnO-SiO2 system

The effect of P2O5 and Ce2O3 on the crystallization behavior of a BaO-SrO-ZnO-SiO2 glass was investigated using thermal analyses, X-ray diffraction, scanning electron microscopy including electron backscatter diffraction as well as laser scanning microscopy. Heat treatments lead to the precipitation of Ba1-xSrxZn2Si2O7, Zn2SiO4, and CeO2 at the surface. While Zn2SiO4 crystals form 1 µm deep immersions in the surface, CeO2 crystals originate from near-surface areas. The star-shaped CeO2 crystals show mostly three different orientations, among which the <001>- or <111>-orientation perpendicular to the surface are clearly preferred. In the bulk, many (Ba, Sr)5(PO4)3Cl crystals are observed acting as nucleation sites for Ba1-xSrxZn2Si2O7. This nucleation process is clearly facilitated by the occurring stresses. Moreover, an epitaxial growth of Ba1-xSrxZn2Si2O7 on (Ba, Sr)5(PO4)3Cl is indicated. If the surface-crystallized layer consisting of Ba1-xSrxZn2Si2O7 crystals approaches the (Ba, Sr)5(PO4)3Cl crystals in the bulk, nucleation is induced and the thus provoked growth in the same growth direction as the (Ba, Sr)5(PO4)3Cl crystal leads to a shift of the growth front towards the sample volume and hence growth-front hopping occurs.