Extinction in sodium fluoride

Abstract

X-ray intensity measurements for a single crystal of sodium fluoride obtained at four wavelengths by Howard & Jones [Acta Cryst. (1977). A33, 776-783] have been reanalysed using both the Cooper-Rouse and Becker-Coppens extinction formalisms in order to study the wavelength dependence of the extinction in this crystal, since the original analysis was unable to account for the wavelength dependence of the intensities of the strong reflections. The results indicate that the crystal is intermediate between type I and type II in nature and are consistent with the wavelength dependence predicted by the theories, although the extinction is not large enough at the shorter wavelengths to provide a useful test of the validity of the theoretical wavelength dependence. The analysis also demonstrates some of the possible consequences of the use of unbalanced weighting schemes and of inappropriate models for the scattering factors.