A new interpretation of reduction in absorption coefficient at and near Laue diffraction maximum in thin and perfect crystals on the basis of dynamical theory of X-ray diffraction

By analysing in detail the existing plane wave dynamical theory of X-ray diffraction for thin (μt sec θ ∼ 0.30) and perfect crystal we demonstrate the presence of forward diffracted beam when the crystal is oriented for Laue diffraction. Recently, these features have been demonstrated experimentally by Lal et al. (Solid State Comm. 81 (1992) 461-465 & 96 (1995) 33-36) in the case of diamond crystals. We also show that a residual direct beam is implicitly built in the expression for forward diffracted beam intensity. It has been shown that there is a substantial decrease in absorption coefficient, μ, of thin perfect crystal for exploring X-ray beam around Laue diffraction maximum. The computed fractional change in μ, Δμ/μ, at the 220 diffraction peak of a a thin (111) diamond crystal for MoK α1 exploring radiation is found to be in reasonably good agreement with the experimentally observed value (Lal et al. Acta Crystallogr. A52 (1996) C-493). The results reported here are of general nature and applicable to all crystals.