Defects in natural diamonds: Recent observations by new methods

Abstract

The observations presented concern both grown-in defects and those which develop after growth. They will deal principally with impurity platelets precipitated on {100} planes and with dislocations. The former defects are a notable feature in the most common variety of natural diamond (Type Ia.) The experimental techniques used are of the direct-imaging, topographic type and include ultraviolet absorption topography, X-ray topography, cathodoluminescence topography (both with visible and near infrared wavelengths) and transmission electron microscopy at 100 kV, 120 kV and 1 MeV. Spectrography and colour photography are informative in cathodoluminescence studies. The cathodolumisnecent images of individual dislocation lines can be photographed, whether the dislocations be grown-in or arise from plastic deformation post growth, provided that they lie in matrices sufficiently free from point defects acting as strong electron-hole recombination centres. In practice this requirement implies a low concentration of nitrogen impurity much of which is distributed in “small clusters” which give rise to the bright blue cathodoluminescent emission characteristic of Type Ia diamonds. The luminescence from individual dislocation lines may be either a violet-coloured emission, or an emission which is dominantly a system (known as “H3”) having a zero-phonon line at 2.46 eV and stretching from green to orange in the visible spectrum. The violet dislocation emission is strongly polarised with E vector parallel to the dislocation line. New observations on platelets have been mainly concerned with those attaining diameters of 1 μm or more which can be observed individually by X-ray topography and cathodoluminescence topography. However, evidence from electron and X-ray diffraction contrast indicates that these “giant platelets” produce the same matrix lattice displacements as the more familiar submicrometre-sized platelets in Type Ia diamond and may be presumed to have essentially the same structure and composition. Near infrared cathodoluminescence has been recorded from individual large platelets and it has been discovered that this emission is highly polarised with the E vector in the platelet plane. Available evidence on the structure and properties of the platelets weighs against a currently mooted hypothesis that they are composed of interstitial carbon, and does not conflict with an older idea, that they are composed of nitrogen impurity.