Surface Structures and Epitaxial Growths on AgBr Microcrystals

Abstract

Crystals of AgBr, cubes bounded by {200} surfaces or octahedra bounded by {111} surfaces, can be readily produced. All other shapes may be considered as built up from these two stable faces. Although flat surfaces of cubes are electrically neutral, flat surfaces of octahedra have a large unbalance of charge. The supposition that the electrical neutrality of an octahedral crystal bounded by bromide ions is maintained by a large excess of interstitial ions rather than by certain other surface structures is supported by the amount of lattice expansion observed for the octahedra relative to cubes. Growth hillocks of AgCl on AgBr were produced which mark surface sites of enhanced reactivity. In no case were the growths twinned relative to the substrate crystals, even though both AgCl and AgBr often twin on {111} planes during growth. Preferred reaction sites on the cubes were corners and edges, but the octahedral surfaces were generally reactive. With octahedra also present, the cubes were almost inert. The surfaces of twinned tabular crystals often had linear arrays of growths along 〈211〉 directions before mechanical deformation but along 〈110〉 directions after deformation. Points of enhanced reactivity corresponded to points where the most rapid etching would be expected.