The Limits of X-ray Diffraction Theory

Abstract

X-ray diffraction theory allows the interpretation of experiments to build a structural model that fits the collected data. As with any experimental science, the observations are subject to uncertainty through the instrument and user limitations. Similarly, the theory can never be perfectly complete; it will have limits, and therefore the resultant model will have uncertainties associated with it. This article discusses the limits of X-ray kinematical and dynamical diffraction theories. These are not the only theories, but are the most widely used. These theories are often extended to accommodate new findings, which can reach the stage at which their fundamental premise is clouded. At that point, the theory requires a rethink. There should be nothing sacrosanct about a theory; it should represent the best usable explanation that will allow a good interpretation of the data. Both kinematical and dynamical theories assume that the X-rays see an average structure, which is not what a photon experiences. The observed diffraction pattern is the average of the diffraction patterns created by all the photons, which is not the same as the diffraction pattern from the average structure. Accounting for this has a profound influence on the interpretation of the data.