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A glass with the composition BaO•Al 2 O 3 •B 2 O 3 was thermally annealed at temperatures from 700 to 780 °C for 1−8 h. This resulted in the crystallization of [0001]-oriented rhombohedral BaAl 2 B 2 O 7 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 BaSO 4 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.

Polycrystalline ZnS samples are studied using X‐ray diffraction and scanning electron microscopy including electron backscatter diffraction (EBSD). The material is industrially produced by a chemical vapor deposition process (CVD). Near the substrate, crystal growth leads to grains smaller than 50 μm in the cut plane. Elongated crystals with visible lengths of up to 400 μm are formed further from the substrate. These crystals are heavily twinned and exhibit Σ3 grain boundaries (i.e., the orientation of one {111}‐plane is constant while rotations of 60° around its normal occur). About 1000 μm from the substrate, the grain size shrinks to about 20 μm along an abrupt border; a continuous grain size transition is not observed. Gradual orientation changes within single grains occur and in some cases lead to the fragmentation of grains parallel to the direction of growth. This is preferably observed in smaller grains more than 1000 μm from the substrate. Twinning, on the other hand, predominantly occurs in the large grains near the substrate. Both mechanisms should contribute to stress minimization in the sample. Textures of the analyzed surfaced indicate a general <001>‐orientation perpendicular to the substrate and thus parallel to the direction of crystal growth.

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