Abstract
This is a review of atomic pair distribution function (PDF) analysis as applied to the study of molecular materials. The PDF method is a powerful approach to study short- and intermediate-range order in materials on the nanoscale. It may be obtained from total scattering measurements using X-rays, neutrons, or electrons, and it provides structural details when defects, disorder, or structural ambiguities obscure their elucidation directly in reciprocal space. While its uses in the study of inorganic crystals, glasses, and nanomaterials have been recently highlighted, significant progress has also been made in its application to molecular materials such as carbons, pharmaceuticals, polymers, liquids, coordination compounds, composites, and more. Here, an overview of applications toward a wide variety of molecular compounds (organic and inorganic) and systems with molecular components is presented. We then present pedagogical descriptions and tips for further implementation. Successful utilization of the method requires an interdisciplinary consolidation of material preparation, high quality scattering experimentation, data processing, model formulation, and attentive scrutiny of the results. It is hoped that this article will provide a useful reference to practitioners for PDF applications in a wide realm of molecular sciences, and help new practitioners to get started with this technique.
Highlights
Molecular materials are profoundly useful, with applications from packaging food, to curing disease, to producing and storing energy
Scattering intensities often attributed to impurity γ polyamide 6 (PA6) were shown to arise from only short-range rather than long-range ordered structural states. These results suggest the utility for revisiting many semicrystalline polymers systems in the unoriented state using modern real-space methods
A future direction for fingerprinting and phase matching is through large-scale data mining, allowing for a more comprehensive search for, and identification of, potentially useful models. This has been implemented in the pair distribution function (PDF) in the cloud (PDFitc) project,[527] for searching open source databases and in the Crystallography Open Database (COD) and Material Project (MP), for candidate structures
Summary
Molecular materials are profoundly useful, with applications from packaging food, to curing disease, to producing and storing energy. It is an increasingly important tool within contemporary inorganic materials communities, for instance in studying nanoparticles,[21−23] magnetic structures,[24] strongly correlated electron systems,[25] cultural heritage objects,[26] and other functional materials.[27−29]. These methods of local structure characterization have been extended to the characterization of molecular materials, though along different trajectories within various communities of researchers. We attempt to provide descriptions that will help users to connect attributes of the measured signal to the structural properties that they encode
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