Extinction-free measurements in crystallography

Abstract

OVER the past fifty or more years extinction has remained a major experimental problem in crystallography^1–3. Many strategies have been devised to reduce its magnitude for the accurate determination of physically-significant structure factors (F). Its elimination, as a practical possibility, has appeared remote because extinction is a concomitant of the production of diffraction intensity in the standard techniques. Phenomenologically, extinction is a manifestation of multiple diffraction (see ref. 4), and this being so, an obvious conclusion is that the experimental disposition to be sought is one where the recorded intensity must arise solely from single-scattering events^5–7. Then, by definition, the diffracted beam is not ‘extinguished’ and simple kinematical theory is applicable. Starting with the design of a ‘defocusing’ X-ray monochromator⁸, I have been investigating the intensity of asymmetric reflections from extended-face crystals over the regions of both positive and negative asymmetry, first with an abraded surface⁹ and then with an optically flat one. The results from this latter study, with an extrapolation of the work of Hirsch and Ramachandran⁵, show that it is feasible to establish values of integrated intensity which involve only single-scattering events.