SURFACE LAYERS OF CRYSTALS 151 



micrometer measurements. Since the disturbance is still discernible it 

 appears that its depth in this plate was greater than .07 mm. 



With the discovery that the disturbed material may be more than 70 

 microns thick, it becomes apparent that great care must be taken to remove 

 the disturbance produced by all previous lapping and sawing prior to mea- 

 surement of disturbance produced by a particular lapping technique. This 

 may require more than 48 hours' etching with 48% hydrofluoric acid which 

 will produce a rugged surface. An alternative procedure is to use a natural 

 crystal which has never been cut or worked. Since the natural faces of some 

 crystals do show disturbance, preliminary tests should be made with the 

 various techniques for detecting the presence of any disturbed material. A 

 transmission Laue photograph of a small quartz crystal from Herkimer 

 County, N. Y., taken by C, J. Davisson, showed no disturbed material. If 

 a natural face of such a crystal were lapped under carefully controlled con- 

 ditions and the resulting disturbance measured by the various techniques, a 

 reliable picture of the disturbance produced by that lapping procedure would 

 be obtained. 



In many of these photographs some darkening occurs between the two 

 lines that mark the outer surfaces of the plate. In most cases it appears as 

 well-defined streaks, as in 1 Ic, but may be irregular, as in 1 la. Such streaks 

 appear to be due to disturbed zones within the quartz plate whose disturb- 

 ance is related to the growth history of the crystal. Where they adjoin the 

 disturbed surface layer they may be responsible for erroneous measurements 

 of the misorientation and depth of the surface layer. 



Photograph He was taken prior to any etching, like 11a, but through a 

 different part of the plate and shows difi'erent internal imperfections. Pho- 

 tograph lid was taken through the same part of the plate after only 20 

 minutes' etching and shows very little surface disturbance. To get a picture 

 of the distribution of the surface disturbance and internal imperfections of a 

 plate a series of exposures should be taken with the plate translated a short 

 distance relative to the beam between each exposure. 



Measurements of the depth of the disturbed layer have given widely 

 different results with difi'erent techniques. The Laue photographs just 

 described indicate the depth to be more than .07 mm. or 70 microns in some 

 cases, even with a 303^ emery finish." This is in accord with the 0.1 mm. 

 depth assigned by Y. Sakisaka on the basis of two sets of measurements 

 made by him with two widely different techniques.'^ This value is also in 



" Since adequate precautions for the removal of all previous disturbance were not taken, 

 this value may he erroneous. . f t. a • c 



'■> Sakisaka, Y., "The Effects of the Surface Conditions on t__he Intensity of Refle.xion of 

 X-rays by Quartz, Japanese Journal of Plivsics 4 (1927) p. 171-181. ^^ 



Sakisaka, Y., "Refle.xion of Monochromatic X-rays from Some Crystals, Froc. Fhys.- 

 Math. Soc. of Japan, Ser. Ill, v. 12 (1930), p. 189-202. 



