The wavelength dependence of secondary extinction effects

The wavelength dependence of extinction effects in X-ray and neutron diffraction predicted by recent theories is considered and compared with the results obtained for recent accurate diffraction measurements. It is shown that it is not usually possible to determine reliable values of the domain radius and mosaic spread parameters separately from data measured at only one or two conventional wavelengths. The most significant experimental test of the wavelength dependence is at present that for secondary extinction in a spherical crystal arising from neutron diffraction measurements for SrF₂ at three different wavelengths. These have been analysed on both the Cooper-Rouse and the Becker-Coppens formalisms, both of which predict a simple form for the wavelength dependence. Although both these formalisms give excellent agreement with the data when those for different wavelengths are analysed separately the results indicate, in both cases, that the wavelength dependence is not accounted for adequately by the theory.