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 di ffraction for thin (mu t sec theta similar to 0.30) and perfect crystal we demonstrate the presence of forward diffracted beam when the crystal is ori ented for Laue diffraction. Recently, these features have been demonstrated experimentally by LaI et al, (Solid State Comm, 81 (1992) 461-465 & 96 (19 95) 33-36) in the case of diamond crystals. We also show that a residual di rect beam is implicitly built in the expression for forward diffracted beam intensity. It has been shown that there is a substantial decrease in absor ption coefficient, mu, of thin perfect crystals for exploring X-ray beam ar ound Laue diffraction maximum. The computed fractional change in mu, Delta mu/mu, at the (2) over bar 20 diffraction peak of a thin (Ill) diamond crys tal for MoKa1 exploring radiation is found to be in reasonably good agreeme nt with the experimentally observed value (LaI et al. Acta Crystallogr. A52 (1996) C-493). The results reported here are of general nature and applica ble to all crystals.