X-Ray Studies of Surface Layers of Crystals 



By ELIZABETH J. ARMSTRONG 



1. Introduction 



WHEN a crystalline substance is sawed, ground, lapped or polished, the 

 crystal structure adjacent to the worked surface is distorted and rup- 

 tured. 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^ 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 stand- 

 point of the kind of evidence which it seem best suited to bring to light. 

 Familiarity with the general principles of X-ray diffraction as outUned 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- noticed that the spots in Laue photo- 

 graphs of certain crystals were inhomogeneous and suggested the interpreta- 

 tion that the darker parts of the spots might result from disturbed material. 

 A. H. Compton*, using a double crystal spectrometer in 1917, found that the 

 reflection from a ground surface of a calcite crystal was three times that from 

 a cleavage face. 



1 One plate is said to have greater activity than another similar plate if its amplitude of 

 oscillation is greater when the two are tested under identical conditions. The activity of a 

 plate is reduced by friction with its mountings or with particles on its surface, either of 

 quartz or of a foreign material. 



2 de Broglie, M. and Lindemann, F.-A., "Sur les P/ienomenes Optiques Presentes par les 

 Rayons de Rdntgen Renconiranl des Milieux Crislallins, Comples Rendus, 156 (1913), pp. 

 1461-1463. 



^ Compton, A. H., "T/ie Reflection Coefficient of Monochromatic X-Rays from Rock Salt 

 and Calcite," Pliys. Rev., 10 (1917j, pp. 95-96. 



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