Reactivity of Different Faces of a Copper Single Crystal

Abstract

BY using single-crystal metal spheres, Hausser¹, Tamman and Sartorius², and in particular A. T. Gwathmey³ and his collaborators, have demonstrated the marked differences in the physico-chemical properties of the surfaces of a crystal. I have extended their techniques to reactions in which crystals may be etched by vapours. A single crystal of the substance under examination is allowed to react with gaseous reagent under conditions of temperature and pressure such that the product is volatilized away as fast as it is formed. For example, a single crystal of pure copper was electrolytically polished and subjected to the action of a halogen vapour at 444° C. and about 1 mm. pressure. Under the conditions of the experiment the cuprous halide sublimed away continuously, leaving a clean, etched copper surface. Etch pits were produced with their reflecting surfaces (micro-facets) parallel to certain crystallographic planes, so that, on rotating the sphere in a parallel beam of light with the eye looking along the beam, reflexions were seen at certain orientations. It was thus possible to identify easily the facets produced and, by roughly comparing the intensities of the reflected beams, to judge which predominated.