On the geometrical condition for X-ray scattering by thermal waves in crystals

Using a highly monochromatic beam at the CuKα1, maximum X-ray diffraction studies are made on single crystals of KCl, Si, Ge, and V3Si and shown that selection rules exist which restrict diffraction by thermal waves. The general rule requires that incident, diffracted, thermal wave vectors and the vector of the reciprocal lattice have to be coplanar. A further limitation is connected with the low angle Bragg reflections. Thermal reflections, which are placed exactly in the Bragg position, correspond to a very narrow bundle of thermal wave vectors. Thermal reflections associated with high angle Bragg reflections are slightly broadened with a maximum corresponding to the shortest wave vector. These results are not predicted by the present theory of X-ray scattering by thermal waves. (author)