ABSTRACT
Diffraction methods have provided the large majority of solved atomic-scale structures for both the bulk materials and their surfaces, mainly in the crystalline state. Crystallography by diffraction tends to filter out defects and focus on the periodic part of a structure. By adding contributions from very many unit cells, diffraction gives results that are, in effect, averaged over space and time. This is excellent for investigating stable states of solid matter as they occur in well crystallized samples; some forms of disorder can also be analysed reasonably well. Diffraction, however, is much less appropriate for examining inhomogeneous and time-dependent events such as transition states and pathways in chemical reactions.
