Optical studies on diamond cleavage surfaces

Abstract

A review is given of the properties of diamond, including the optical absorption, X-ray and counting characteristics, as well as the growth and etch features. A summary of the theories of diamond structure based on these observations leads to the conclusion that the lattice is rarely, if ever, perfect, and that in general two types of imperfections are present:(i) isolated vacancies or interstitial atoms.(ii) irregularities on a much larger scale, which can be regarded as regions of lattice disorder extending over many planes. Cleavage surfaces of diamond were studied, using phase contrast microscopy, multiple-beam interferometry and the light-profile, which techniques are described. In agreement with previous observations, the cleavage is seen to be quite imperfect on an interferometric scale. The surfaces are crossed by many steps, radiating from the origin of cleavage, and usually converging with increasing distance, to form the typical 'river' system. A mechanism is suggested for their origin and character. Two types of features not previously reported are described. Curved lines can be seen, lying across the cleavage direction, and are shown to be abrupt changes in height of the surface. An analogy is drawn with the lines observed on the surfaces of broken glass rods. The lines occur through the interaction of stress waves, arising at in homogeneities in the stone, with the advancing cleavage front. The velocity of the front is calculated, and shown to increase from values at the origin of the order of one tenth of the transverse acoustic wave velocity to maximum values of one fifth of this velocity. The total duration of the cleavage process is estimated. The shape of the cleavage front is derived for some stones, and found to be an arc of a circle, except for discontinuities near large steps, where the front is retarded. An explanation is given. Also discussed are lines visible on the surfaces, lying in crystallographic directions. Such lines are parallel to the three directions of intersection of 111 planes with the (111) cleavage surface; and are shown, from their displacements across large cleavage steps, to be sections of these planes. Their occurrence during cleavage is explained on the assumption that weak 111 planes exist within the diamond. Various mechanisms are discussed and the conclusion reached from the evidence that such planes must arise during growth. The sum of all the lines on one cleavage face is related to the growth history of the stone. It is suggested that these planes occur under irregular growth conditions, and are layers of lattice disorder. That such regions are necessary to explain many of the other properties of diamond has been seen, and it is believed that their lower cohesive energy accounts for the highly preferred 111 cleavage.