X-Ray Scattering from Deformable Ions

Abstract

Measurements of x-ray diffuse scattering by phonons in three alkali halides show that the usual scattering theory which assumes rigid ions is not adequate. The generalization of the theory to account for the deformation of the ions is given, and leads, in the case of x-ray scattering, to intensities consistent with those observed. The new terms in the scattering are due to the x-ray Raman effect. The measurements indicate that for NaCl a change of about 2% in the scattering factor of the negative ion occurs for a 0.1 Å change in the distance of its nearest-neighbor ion. It is shown that the core-shell displacements of the shell model are too small to explain the observed magnitude of the deformation, and that changes in the radial charge density of an ion are required.