X-Ray Studies of Surface Layers of Crystals

W H E N a crystalline substance is sawed, ground, lapped or polished, the crystal structure adjacent to the worked surface is distorted and ruptured. Since the selective diffraction of X-rays by a crystal is a result of the orderly arrangement of the planes of atoms of the crystal, disturbance of this arrangement is detectable by X-ray diffraction. Research on aging of quartz oscillator plates seems to indicate that changes which are accelerated by high humidity take place in this disturbed material resulting in changes in the frequeny and activity 1 of the crystal plate. A knowledge of the nature and extent of this disturbed layer is essential to an understanding of the changes that are taking place in it and may contribute to the improvement of the quality of crystal plates, apart from the problem of aging. The purpose of this paper is to present a review of X-ray techniques that have been or may be useful tools for the examination of the nature of the surface layers of crystals. Each technique is also discussed from the standpoint of the kind of evidence which it seem best suited to bring to light. Familiarity with the general principles of X-ray diffraction as outlined in this Journal, volume xxii, number 3, pages 293 and 297, is assumed. It has long been known that the nature of the surface preparation of a crystalline substance affects the intensity of the reflected X-rays. As early as 1913, about a year after the first X-ray diffraction experiments with crystals, de Broglie and Lindemann 2 noticed that the spots in Laue photographs of certain crystals were inhomogeneous and suggested the interpretation that the darker parts of the spots might result from disturbed material.