HRTEM OF (100) PLATELETS IN NATURAL TYPE-1AA DIAMOND AT 1.7A RESOLUTION - A DEFECT STRUCTURE REFINEMENT

The majority of naturally occurring gem-quality diamonds contain {100} platelet defects, the presence of which was first detected by X-ray diffraction. In this paper a detailed comparison is made, between 1.7 angstrom resolution HRTEM lattice images of diamond platelets in the <110> and <100> orientations, and images calculated for six different platelet models. It is found that the different platelet models are clearly distinguishable in thin crystal, and close to optimum defocus. The projected structure of the platelet in the [110] orientation is different to that in the [110BAR] orientation, whereas four of the six models have the same projected structure in both <110> directions. It was necessary to derive a new model for the diamond platelet in order to match the experimental images. It is highly probable that the new model, which is called the disordered-nitrogen-fretwork model, represents the true structure of the diamond platelets.