The wavelength dependence of extinction in a real cystal: γ-ray diffraction from NiF₂

The wavelength dependence of extinction in an NiF2 crystal of well known mosaicity has been examined by gamma-radiation of wavelengths 0.0205, 0.0265, 0.0392 and 0.0603 Angstrom. The results for seven strong low-order reflections, some of them symmetrically equivalent, are related to the extinction models of Becker & Coppens [Acta Cryst. (1975), A31, 417-425] and Sabine [International Tables for Crystallography (1999), Vol, C, pp, 530-533. Dordrecht: Kluwer]. In the considered wavelength region, the extinction-affected observed intensity is approximately a linear function of lambda 2, and secondary extinction is found to be dominant. Allowance for pure secondary extinction according to the Becker & Coppens formalism yields both a satisfactory description of the wavelength dependence and mosaicities close to the directly observed ones. With the Sabine model, the influence of the mosaic distribution has to be excluded in order to describe the data properly, Hypothetical assumption of pure primary extinction, however, leads to nonrealistic mosaic-block sizes between 50 and 100 mu m. This model is therefore not supported by experiment.