Anisotropy of extinction: extrapolation to the kinematical limit by γ-ray diffraction.

A direct experimental approach to the problem of anisotropic extinction is presented. Structure-factor measurements from a vanadium and a niobium crystal, performed with four γ-ray wavelengths in the range 0.02-0.06 Å, substantiate the adequacy of Zachariasen's theory [Acta Cryst. (1967), 23, 558-564] in high-energy diffraction, which provides a theoretical basis for the extrapolation to zero extinction values. Fitting of the theoretical curve to the observed points, placed on a common scale, allows determination of the kinematical structure-factor value without the need for a particular model of anisotropy in the mosaic structure.