ISSN:
1600-5724
Source:
Crystallography Journals Online : IUCR Backfile Archive 1948-2001
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
The equations of the spherical-wave dynamical diffraction theory for finite polyhedral crystals [Saka, Katagawa & Kato (1972). Acta Cryst. A28, 102-113, 113-120] that relate to the Borrmann-Lehmann interference effect have been cast into simple forms in order to display explicitly the leading periodicities and thereby facilitate comparison with experimental observations. The evolution of fringe profiles during passage from the low-absorption case to that of high absorption accompanied by strong anomalous transmission is discussed in detail and illustrated with series of computed profiles. Representative topograph patterns are compared with simulated images and exemplify the poor agreement between observed and calculated fringe spacings previously reported [Lang, Kowalski, Makepeace & Moore (1986). Acta Cryst. A42, 501-510]. The effect of lattice distortion in Borrmann-Lehmann interference is investigated by applying the ray-optical diffraction theory for mildly distorted crystals developed by Kato [J. Phys. Soc. Jpn (1963), 18, 1785-1791; (1964), 19, 67-77, 971- 985] with assumption of a constant strain gradient in the specimen. Two factors have been identified that can account for the apparent extreme sensitivity of Borrmann-Lehmann fringe spacings to lattice distortions. One factor arises as a geometrical consequence of the curvature of ray trajectories in the distorted crystals, the other derives from Kato's 'potential' term in the phase integrals of the crystal waves that recombine and interfere. Both factors depend upon the first power of the strain gradient. Under typical experimental conditions, strain gradients sufficiently small as to produce less than 1% contraction in Pendellösung fringe spacings can change Borrmann-Lehmann fringe spacings by more than a factor of two.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1107/S0108767389011670
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