The topography of crystal faces - I. The topography of a (100) face of a left-handed quartz crystal

Abstract

A description is given of a multiple beam interferometric procedure which can be used as a powerful method for revealing the details of the surface topography of approximately flat crystal planes. The optical conditions are critical and are discussed. Details of structure are revealed which cannot be detected by the microscope or the goniometer. The interference fringes which are employed can be used either in transmission or in reflexion, the latter type being well suited for the examination of translucent or opaque crystals. Face angles, between small faces or large faces, which are no more than 1/50th of a minute of arc can be measured. The topographical features of a highly lustrous (100) face of a left-handed quartz crystal are studied by this procedure. Large vicinal faces, inclined at angles varying from $0\cdot 50$ to $9\cdot 00$ min. of arc, are found to cover the (100) face. The surfaces of most of these vicinal faces are curved, with radii of curvature varying from 20 to 60 m. The angles of some of the vicinal faces in contact are found to vary considerably along the length of the common edge. The characteristic visual markings of a (100) face, namely striations, $\widehat{{}}$ marks and triangles, reveal their influence upon the fringe patterns and their topographical features are determined. The striations are shown to be either minute ridges or ruts, the height or depth being only 100 A (20 silica molecules). The $\widehat{{}}$ markings reveal small discontinuities in level in their neighbourhood. The triangular markings are shown to be submicroscopic tetrahedral projections of some 450 A in height (90 molecules), with vicinal face angles of the order of 2 or 3 min. The nature of the markings is discussed. The experimental method described can be considered to function in a region intermediate between the microscope and X-rays.