A Global Glassy Layer on BaAl2B2O7 Crystals Formed during Surface Crystallization of BaO·Al2O3·B2O3 Glass

A glass with the composition BaO·Al2O3·B2O3 was thermally annealed at temperatures from 700 to 780 °C for 1–8 h. This resulted in the crystallization of [0001]-oriented rhombohedral BaAl2B2O7 at the surface of the glass. The crystals grow fastest perpendicular to their c-axes (parallel to the surface) and cannot circumvent barriers in the surface. The surface of samples encapsulated in alumina ceramics during annealing was covered by a thin layer of glass, indicating nucleation occurred below the surface. If the samples were not encapsulated in alumina, around 85% of the surface was glass covered. This was the result of surface contaminations such as cubic BaSO4 detected on these samples. Crystal growth occurs toward the surface until the enrichment of impurities in the glassy layer is large enough to inhibit further crystal growth. The coinciding enrichment of impurities on the bulk side of the oriented surface crystals probably initiates the secondary nucleation, leading to the second layer of oriented crystals previously described.