Abstract

The influence of thermal phonons on the dynamical diffraction 0H near a third reciprocal-lattice point L, in an otherwise perfect crystal, is investigated theoretically. It is shown that in the first-order modified two-beam description Juretschke (1984). Acta Cryst. A40, 379-389 all effects due to phonon transitions are governed by the usual Bessel functions, but only of arguments involving H. With this proviso, the first-order modified two-beam description of diffraction near L incorporates phonon coupling entirely in the standard manner of the strict two-beam case. Therefore typical phonon influences, such as the Debye-Waller factor or thermal diffuse scattering, can be discussed directly in the neighborhood of n-beam diffraction merely by using the modified parameters, i.e. structure factors, absorption coefficients etc. in a traditional two-beam formulation. Some additional implications of this result about the effect of other deviations from crystal perfection on the modified two-beam description are also pointed out.