Non-diffraction structural probes

Abstract

Conventional diffraction techniques suffer from three distinct disadvantages when used in the study of the structure of amorphous materials: they generally yield only information concerning pair correlations; they are chemically non-specific, in that the diffraction pattern obtained is a structural and chemical average; and they are insensitive to dilute constituents. These limitations are particularly severe for the case of amorphous materials containing several components, and although neutron scattering studies employing isotopic substitution etc. do allow individual pair correlation functions to be extracted for binary systems, such methods are impractical for more complex systems. In this paper two non-diffraction structural probes are discussed which circumvent some of these difficulties and are therefore very useful techniques complementary to conventional neutron or X-ray diffraction methods. The two techniques are X-ray absorption spectroscopy (including extended X-ray absorption fine structure (EXAFS) and magic-angle spinning nuclear magnetic resonance (MASNMR). Both techniques are chemically specific in that the local structural environment around a given type of atom in a multicomponent system can be probed. Furthermore, information concerning structural conformations, i.e. involving higher order (e.g. triplet) correlation functions, can in principle be obtained both from the X-ray absorption near-edge and pre-edge structure and from MASNMR spectra. The principles underlying both techniques are described, and some examples of their application are given.