Crystal structure determination by powder neutron diffraction at the spallation neutron source, ISIS

Abstract

The power and scope of powder diffraction methods have advanced dramatically in recent years, partly as a consequence of improved X-ray and neutron instrumentation¹, and partly due to the develop-ment of Rietveld profile analysis² for the refinement of low-symmetry structures. Emphasis has focused on neutron studies, with both constant wavelength and time-of-flight techniques, but progress has also been made in X-ray methods³, where synchrotron sources promise to play an important part⁴. At present, structural models with more than 100 variable parameters can be refined^5,6, but the determination of structures from powder data has remained an empirical, rather than exact, science. The principal difficulty is that the overlapping of adjacent reflections introduces ambiguity into the assignment of integrated intensities, which are required to solve the phase problem. Nevertheless, significant advances have been made in this area^7,8. With the advent of a new generation of ultrahigh-resolution powder diffractometers, at the Rutherford–Appleton Laboratory, ILL Grenoble, and the Brookhaven National Light Source, a new era in structure determination is signalled, with the prospect that the structures of new materials that cannot be obtained as single crystals might be determined by powder methods. Here we illustrate the practicability of this approach with an ab initio study of the structure of ferric arsenate, FeAsO₄.