SYNCHROTRON X-RAY AND NEUTRON-DIFFRACTION STUDIES IN SOLID-STATE CHEMISTRY

Neutron diffraction studies, especially with powders, play an important role in structural solid-state chemistry, making possible the precise determination of the location of light atoms, particularly hydrogen, and enabling a distinction to be made between certain neighboring elements in the periodic table that are difficult to distinguish in experiments with X-rays. Neutron diffraction investigations also make a unique contribution in the area of magnetic structure determination. The availability of intense synchrotron X-rays sources, however, is opening up new opportunities to the structural chemist, many of them complementary to the ''traditional'' strengths of neutron methods. The key features of synchrotron radiation in relation to structural studies are the wavelength tunability, which facilitates the use of resonant diffraction methods, and the high brightness and excellent vertical collimation of the source, which make possible the construction of diffractometers with unparalleled angular and spatial resolution. The following types of experiments are now possible with synchrotron X-ray diffraction: 1) The ab initio determination of structures from powder diffraction data. 2) The differentiation between different oxidation states of an element (valence contrast experiments) based upon the sensitivity of an absorption edge to the valence of the element in question. 3) The differentiation of elements adjacent to each other in the periodic table, which is now feasible with synchrotron X-rays for all elements beyond chromium. 4) Site-selective X-ray absorption spectroscopy. 5) The study of cation occupancies in materials where more than one element occupies a site that is, or may be, partially occupied. (Such problems are important in zeolite chemistry and high-temperature superconductors.) 6) The determination of crystal structures from microcrystals. 7) In situ and rapid, time-resolved diffraction studies. This review examines the roles played by X-ray and neutron diffraction studies in modern solid-state chemistry, and describes some recent examples in which the use of neutron radiation or synchrotron X-rays has been advantageous.