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abstracts
Polymorphism refers to the ability of a solid to exist in more than one crystal structure. Apart from being of scientific interest, it is of practical importance in the pharmaceutical and chemical manufacturing industries. In pharmaceuticals the polymorphic form of the substance can affect the ease of manufacture or the rate of uptake by the human body [1]. There is consequently a great need to be able to understand, predict and control polymorphism. This work is part of a larger study using diffuse scattering methods to investigate the role of molecular flexibility and disorder in polymorphism. Diffuse scattering is sensitive to two-body correlations so can provide information about the intermolecular interactions that cannot be obtained from the Bragg peaks, such as how the displacement or orientation of a molecule is correlated with that of its neighbours. p-(N-methylbenzylidene)-p-methylaniline (MeMe) is a model system for studying polymorphic behaviour. The system is trimorphic with all three polymorphs exhibiting highly structured diffuse scattering patterns [2]. The short-range order has been modelled using the program ZMC in which the molecules are allowed to interact via Hooke's law springs and brought to thermal equilibrium using a Monte Carlo algorithm [3]. Here we present a comparison of the diffuse scattering in the different forms of MeMe and assess how successfully different models for the intermolecular interactions reproduce the observed diffuse scattering data.
abstracts
Many minerals (and materials more widely) show evidence of strong and complex local structural ordering. This local ordering can affect a material's mechanical properties, its transport properties (for example, how vacancies relate to oxygen transport through the structure) and its thermodynamics, and so is clearly of prime importance. For crystalline materials the analysis of single crystal diffuse scattering (SCDS) is the most definitive way of determining local structure but for many minerals (and materials more widely) single crystals of a sufficient size for such studies are often not readily obtainable and powder diffraction data must suffice. While conventional powder XRD (e.g. using Rietveld refinement) can provide information about the average crystal structure, total scattering (TS) - which includes both Bragg peaks and diffuse scattering - is needed if information about the local structure and short-range order is to be gained. The pair distribution function (PDF) analysis of such total scattering data has become a widely used technique for extracting such local structural information from a wide variety of materials including crystalline powders, nano-materials, amorphous materials, glasses and liquids. The aim of the present work is to explore the sensitivity of the PDF methodology to various aspects of disorder and short-range order for a mineral system for which the local structure has been characterised previously using SCDS. The system chosen for this study is the non-stoichiometric iron oxide wüstite, (Fe1-xO, x = 0.057). The X-ray diffraction patterns obtained from a single crystal of wüstite show strong and richly structured diffuse scattering (see Fig. 1). This has enabled a detailed model of the defect structure to be established. The aim is to assess to what extent this defect structure model could have been established using PDF analysis of powder diffraction data.
