Nanoscale observations of the epitaxial growth of hashemite on barite (0Â 0Â 1)

The heteroepitaxial growth of hashemite BaCrO4 on barite BaSO4(0 0 1) from supersaturated aqueous solutions was observed in situ using an atomic force microscope (AFM). It was shown that the first hashemite layer grows via two-dimensional nucleation easily forming a complete epitaxial layer, which is likely to have a low level of intrinsic stress. Two-dimensional nucleation of the second and subsequent layers proceeds with significantly lower rates, and growth occurs with lower step velocities. These layers seem to have significant level of intrinsic stress and tend to reduce it via the formation of free surface normal to the growth layer (holes in the layer, dendrite-like shape of nuclei and steps, preferable formation of nuclei at the step edges). As a result, the initially flat surface becomes rough. The process described corresponds to the Stranski-Krastanov epitaxial growth mode, which is well known for growth of semiconductor and metal films but not previously recognised for crystals grown from aqueous solutions.