A model for the AgBr (111) surface, based on the symmetry of nucleation sites for evaporated metal

Abstract

Metallic nuclei vacuum-deposited on the (111) faces of epitaxial silver bromide films are highly ordered, compared with those formed on inactive substrates. Low-angle electron diffraction reveals a pattern from this deposit consisting of twelve spots, in the directions of the 642 silver bromide reflections, and at radial distances corresponding to a 70 Å spacing. The nuclei are assumed to form on preferential sites of some unidentified type. Their symmetry presumably reflects the symmetry of the surface layer of ions on the silver bromide film. By analysis of the optical Fourier transform of an electron microscope image, it is shown that the deposit is a mosaic of two types of domain in crystallographically equivalent rotational orientations, each having hexagonal symmetry. A model for the surface ionic layer is proposed, having the same symmetry. This layer contains somewhat less than the number of ions which would be required for electrical neutrality, namely, half the number in a (111) lattice plane. The charge discrepancy is similar to the charge density of the diffuse surface space charge layer, as determined from measurements of ionic conductivity.