X-ray and neutron attenuation correction factors for spherical samples

A method is derived to calculate the attenuation correction factors for elastic or inelastic X-ray or neutron scattering experiments using a spherical sample. The method can be applied to a sphere that is either fully or partially illuminated by an incident beam of rectangular cross-sectional area. The required input parameters are the energy-dependent attenuation coefficients, the radius of the sphere and the dimensions of the incident beam. In-plane scattering is assumed.</jats:p

Pressure-driven transformation of the ordering in amorphous network-forming materials

The pressure-induced changes to the structure of disordered oxide and chalcogenide network-forming materials are investigated on the length scales associated with the first three peaks in measured diffraction patterns. The density dependence of a given peak position does not yield the network dimensionality, in contrast to metallic glasses where the results indicate a fractal geometry with a local dimensionality of ~5/2. For oxides, a common relation is found between the intermediate-range ordering, as described by the position of the first sharp diffraction peak, and the oxygen-packing fraction, a parameter that plays a key role in driving changes to the coordination number of local motifs. The first sharp diffraction peak can therefore be used to gauge when topological changes are likely to occur, events that transform network structures and their related physical properties

High-pressure neutron diffraction apparatus for investigating the structure of liquids under hydrothermal conditions

A high-pressure setup is described for making neutron diffraction experiments on liquids under hydrothermal conditions. Designs are given for a modied Bridgman unsupported area seal, a fluid separator that keeps apart the liquid sample and pressurising fluid, and a pressure-cell made from the null-scattering alloy Ti0:676Zr0:324.Special attention is paid to the choice of construction materials used to avoid corrosion by the liquid sample under load at elevated temperatures. The apparatus is used to investigate the structure of heavy water at pressures up to 2 kbar and temperatures up to 250 degC

Materials under pressure

Pressure in manufacturing and life sciences is a key parameter in defining the state of matter. In this special issue of MRS Bulletin, we focus on several of the many and diverse domains of advanced materials research where the pressure (or stress) applied is used to alter or otherwise garner information on the material properties. We give an overview of research in which the application of high pressure – often combined with high temperatures and advanced analysis – has led to technological progress in the preparation of superhard materials, in discovering new chemistry for the dense forms of low-Z elements, and in the interplay and mimicking of chemical-induced versus pressure-induced structural and electronic changes to prepare new magnetic and energy materials. In addition, the response of materials such as glasses and perovskites to high stress conditions is discussed, where pressures in the gigapascal regime can easily be achieved in everyday usage. Finally, the structural, dynamical and phase behavior of biological systems is considered

A topological analysis of void spaces in tungstate frameworks:assessing storage properties for the environmentally important guest molecules and ions: CO₂, UO₂, PuO₂, U, Pu, Sr²+, Cs+, CH₄, and H₂

This is the author accepted manuscript. The final version is available from ACS via http://dx.doi.org/10.1021/acssuschemeng.5b00369The identification of inorganic materials, which are able to encapsulate environmentally important small molecules or ions via host-guest interactions, is crucial for the design and development of next-generation energy sources and for storing environmental waste. Especially sought after are molecular sponges with the ability to incorporate CO2, gas pollutants, or nuclear waste materials such as UO2 and PuO2 oxides or U, Pu, Sr2+ or Cs+ ions. Porous framework structures promise very attractive prospects for applications in environmental technologies, if they are able to incorporate CH4 for biogas energy applications, or to store H2, which is important for fuel cells e.g. in the automotive industry. All of these applications should benefit from the host being resistant to extreme conditions such as heat, nuclear radiation, rapid gas expansion, or wear and tear from heavy gas cycling. As inorganic tungstates are well known for their thermal stability, and their rigid open-framework networks, the potential of Na2O-Al2O3-WO3 and Na2O-WO3 phases for such applications was evaluated. To this end, all known experimentally-determined crystal structures with the stoichiometric formula MaM?bWcOd (M = any element) are surveyed together with all corresponding theoretically calculated NaaAlbWcOd and NaxWyOz structures that are statistically likely to form. Network descriptors that categorize these host structures are used to reveal topological patterns in the hosts, including the nature of porous cages which are able to accommodate a certain type of guest; this leads to the classification of preferential structure types for a given environmental storage application. Crystal structures of two new tungstates NaAlW2O8 (1) and NaAlW3O11 (2) and one updated structure determination of Na2W2O7 (3) are also presented from in-house X-ray diffraction studies, and their potential merits for environmental applications are assessed against those of this larger data-sourced survey. Overall, results show that tungstate structures with three-nodal topologies are most frequently able to accommodate CH4 or H2, while CO2 appears to be captured by a wide range of nodal structure types. The computationally generated host structures appear systematically smaller than the experimentally determined structures. For the structures of 1 and 2, potential applications in nuclear waste storage seem feasible.J. M. C. is indebted to the Fulbright Commission for a UK-US Fulbright Scholar Award hosted by Argonne National Laboratory where work done was supported by DOE Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357