X-ray Bragg reflexion, ‘spike’ reflexion and ultra-violet absorption topography of diamonds

Abstract

The relation between X-ray diffracting properties and ultra-violet absorption has been studied point by point on polished plates of diamond having thicknesses from 0-1 mm to more than 1 mm and edge lengths from 2 mm to more than 1 cm. Most specimens exhibit marked non-uniformity of ultra-violet absorption. The intensity of the ‘spike’ X-ray reflexions was found to be roughly proportional to ultra-violet absorption, point by point, a t least in the range of weak and moderate spike intensities. Hence it is concluded that the platelets responsible for the ‘spikes’ are also responsible for the characteristic type I absorptions. However, high values of spike intensity and ultra-violet absorption have only been observed in relatively perfect crystal regions with a low density of grown-in dislocations as determined by X-ray topography; and it is suggested that in imperfect regions containing many grown-in dislocations impurity precipitation in the form of platelets coherent with the matrix may have been inhibited or pre-empted by precipitation in some other way. Thus the absence of ‘ spikes ’ and characteristic type I absorptions may not necessarily imply absence of impurity. On the other hand, some regions of high ultra-violet transparency can be highly perfect; hence lattice imperfection is not necessarily associated with type II optical characteristics. A comparison of ‘spike’ intensity in equivalent cube directions, made on a specimen selected for the high definition of its ‘spike’ topograph features, showed that where platelet precipitation was present it had occurred equally on all cube planes. The sharpness of boundaries between regions of high and low ultra-violet absorption and the association of such boundaries with growth stratifications revealed by Bragg reflexion topographs suggest that they correspond to variations of concentration of grown-in impurity in the crystal, no detectable subsequent diffusion having taken place.