Stacking Fault Contrast in X-Ray Diffraction: a High-Resolution Experimental Study

Stacking fault fringe patterns and related dynamical diffraction effects have been studied by X-ray section topographs in the case of a fault about 1 mm^2 in area contained in an unusually perfect and almost dislocation-free type I a natural diamond cut and polished into a plate 1 mm thick. The fault, a growth accident, lay on an octahedral plane inclined to the local octahedral growth surface and was bounded by a pair of partial dislocations parallel to < 110 >. Images of the fault were recorded with AgKα_1, MoKα_1 and CuKα_1 radiations. Those obtained with AgKα_1 and MoKα_1 represented the low absorption case, and were compared with computer simulated images. In the case of moderately high absorption (CuKα_1) both diffracted beam (K_g-beam) and direct beam (K_0-beam) section topographs were recorded. The sign of contrast of the first fringe (i.e. of that fringe generated closest to the outcrop of the fault with the specimen surface) was observed in both K_0-beam and K_g-beam patterns. These and other combinations of first-fringe contrast observations showed that the fault was intrinsic. Several departures from perfect-crystal diffraction behaviour were noted, and interpreted in terms of a long-range curvature of the lattice (which produced deviations in the trajectories of interbranch scattered rays) and short-range strains which reduced the strength of anomalous transmission of Bragg reflexions and superimposed a background intensity upon the dynamically-diffracted image. From microdensitometric scans across images of faulted and unfaulted regions of the specimen, estimates of the background were extracted, and amounted to about 1/3 the intensity of the 'perfect-crystal' pattern at the centre of the section topograph image in the case of AgKα_1 and MoKα_1 radiations.