Electron Diffraction and Surface Structure

Abstract

IN opening the discussion on electron diffraction and surface structure before a joint meeting of the Physical and Chemical Societies held on March 17, Prof. G. I. Finch compared electron diffraction with other methods of surface structure examination, and cited cases where the microscope or X-rays may give a wrong impression of the structure of deposits and of the size, shape and chemical nature of thin crystalline films. He described, among others, a striking experiment on the orientation of fatty acids by friction, which seems to have a direct bearing on the nature of the mechanism of lubrication. Thus the previously unorientated molecules in a stearic acid layer deposited on a metal surface can be made to point in a common direction by rubbing. If the surface be rubbed in all manner of ways, it is the last stroke only which determines the final direction of orientation of the stearic acid chains. Prof. Finch also gave an account of new experiments which confirm the conclusions previously arrived at by electron diffraction as to the structure of the polish layer on calcite. Sodium nitrate crystals were found to orientate in such a way as virtually to continue the structure of calcite when deposited on the polish layer on a cleavage face and which electron diffraction had shown to be of single crystal structure integral with the main crystal. A calcite surface cut and polished in such a manner as to be steeply inclined to all cleavage planes, however, gives a halo pattern characteristic of an amorphous layer, and on such a surface the nitrate crystals do not orientate but point in all directions just as they do when grown, for example, on a glass surface. After heating, which causes this amorphous polish layer to recrystallize, the sodium nitrate crystals orientate once more.