ON LONG-RANGE LATTICE PERFECTION IN NATURAL TYPE-1A DIAMOND

Several methods have been applied to examine the lattice perfection in a natural diamond that was uncommonly low in density of dislocations and other sources of long-range strains, but which belonged to the commonest optical type 1a. Infrared absorption microscopy and cathodoluminescence topography demonstrated an exceptionally uniform distribution of nitrogen impurity content, with a concentration of about 0.1%. Defect-imaging examinations by birefringence, X-ray single-crystal and double-crystal topography (which were facilitated by optically polished surfaces on the specimen) were applied to select the most strain-free volume, which was then probed by synchrotron radiation double-crystal diffractometry. A 220-reflection, symmetrical Laue case rocking curve only about 0.25 arc second wider than that calculated for perfect diamond was recorded. Analysis was expressed in terms of apparent value of epsilon = F(g)"/F0", the ratio of imaginary parts of the structure factor for orders g and 0. The observed profile for g = 220 and wavelength 0.154 nm was well fitted by apparent epsilon = 0.69 (theoretical epsilon = 0.96). Minor asymmetry in the flanks of the experimental profile was explained by the presence of a small component of incoherent scattering in the diffracted beam.