Dual layered surface crystallization of 30BaO-15TiO₂-55GeO ₂ glass by stepwise heat treatment

In the crystallization process of 30BaO-15Ti O2 -55Ge O2 (BTG55), we demonstrated by an x-ray diffraction analysis that a stepwise heat treatment enables a selective crystallization of fresnoite and benitoite, two crystalline phases obtained from the BTG55 mother glass and that these two phases form a dual surface layer. The result indicates that the stepwise heat treatment generates a compositional heterogeneity inside the glass matrix, which facilitates the nucleation of the fresnoite phase. The mechanism for the generation of the dual surface layer is discussed. Due to a marked difference in optical nonlinearity of the two phases, the selective crystallization may open the possibility of an optimized heat-treatment condition to achieve a large optical nonlinearity in crystallized glass. Also, two types of fresnoite, tetragonal and orthorhombic, were observed in the crystallized phase after a stepwise heat treatment of 690720 °C.