Studies of stacking-fault contrast by synchrotron X-ray section topography

The periodicities, profiles and contrast of stacking-fault fringes observed in high-resolution synchrotron X-ray section topographs have been comprehensively studied with controlled changes of diffraction parameters such as wavelength and polarization mode, which allowed observations under diffraction conditions inaccessible with conventional sources of characteristic radiation. Comparisons with simulated patterns and computed fringe profiles are presented. The fringe patterns were generated by a single intrinsic stacking fault of area 1.3 mm² contained in a polished parallelepiped specimen of natural diamond unusually free from lattice defects producing long-range strains. The principal X-ray wavelengths employed were 0.08, 0.10, 0.12 and 0.15 nm, providing values of μ₀t₀ ranging from 0.25 to 1.5 (μ₀ = normal photoelectric absorption coefficient, t₀ = specimen thickness). Fringe orders up to about 100 were observed at the longer wavelengths. Controlled fractional changes of Pendellösung interference order were obtained by wavelength fine-tuning (Δλ ≃ 10⁻³ nm). Matching of simulations with the resultant patterns is discussed in detail. Fringe patterns were recorded with pure σ, pure π and mixed polarization modes. Change of polarization mode was employed in order to vary the ratio of anomalous to normal absorption without having to change other diffraction variables. Particular attention was devoted to exploring the range of diffraction conditions under which fault vector sign could be experimentally determined from the sign of `first-fringe' contrast, and to examining how this range can be extended with the aid of comparison with pattern simulations.