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A new force field has been empirically derived that is transferable across the YPO4, Y2O3, and P2O5 phases, utilizing a reverse Monte Carlo method. This method employs a simulated annealing technique with a logarithmic quench to fit potential parameters to observed crystallographic and mechanical properties, producing a force field suitable for simulating radiation damage events in an atomistic molecular dynamics regime. These potentials are used to investigate the defect properties of xenotime, where a wide range of intrinsic defects including Schottky, Schottky-like, Frenkel pairs, and anti-site defects have been investigated, both at infinite dilution and as defect clusters. A common feature in the lowest energy defect configurations was the presence of polymerized phosphate tetrahedra, forming P2O7 units. The trend in the formation energies for the Frenkel pair defects at infinite dilution was in good agreement with previously published simulations. However, the binding energy associated with the aggr...

A computer program, Activity, was developed to predict the activity and gamma lines of materials irradiated with an ion beam. It uses the TENDL (Koning and Rochman, 2012) [1] proton reaction cross section database, the Stopping and Range of Ions in Matter (SRIM) (Biersack et al., 2010) code, a Nuclear Data Services (NDS) radioactive decay database (Sonzogni, 2006) [2] and an ENDF gamma decay database (Herman and Chadwick, 2006) [3]. An extended version of Bateman’s equation is used to calculate the activity at time t, and this equation is solved analytically, with the option to also solve by numeric inverse Laplace Transform as a failsafe. The program outputs the expected activity and gamma lines of the activated material. Program summary Program title: Activity Catalogue identifier: AFBS_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AFBS_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: GNU GPL v3 No. of lines in distributed program, including test data, etc.: 688828 No. of bytes in distributed program, including test data, etc.: 71056048 Distribution format: tar.gz Programming language: Fortran. Computer: PCs or HPCs. Operating system: Linux (tested on Debian). Has the code been vectorized or parallelized?: OpenMPI RAM: 250MB per process + 200MB overhead Classification: 2.2, 17.8. Nature of problem: To calculate the predicted activity of an ion irradiated target. The expected range of ion energies is between 1MeV and 200MeV; this is the range of the available ion cross section data. Solution method: The program loads cross section data from the TENDL database and trajectory data from a SRIM [1] simulation exyz data file. It uses this data to calculate the production/loss rate of each isotope in the simulated target. Radioactive decay equations are used to calculate the amounts and activity of each radioactive isotope at the set time. Running time: Typically the Activity program runs each input from seconds to no more than several minutes. References: [1] SRIM — The stopping and range of ions in matter (2010). Ziegler, James F., Ziegler, M.D. and Biersack, J.P. 2010, Nuclear Instruments and Methods in Physics Research Section B, Vol. 268, pp. 1818–1823.

A new potential has been derived for ceria and used to calculate its lattice and defect properties. The Ce 4 + ⋯ O 2 - potential is obtained via a combination of empirical fitting to crystal structural data and parametric fitting to additional physical properties, while the O 2 - ⋯ O 2 - potential is transferred from earlier publications on UO 2 and PuO 2 . The overall potential is subsequently verified and validated by calculation of elastic and dielectric constants, whose values agree favourably with those measured experimentally. The potential is then employed to calculate intrinsic defect formation energies and predict the most favourable type of intrinsic disorder.

Solid-state optical materials are a continuing field of study that has many important applications including lasers. YLiF4 is one material has been studied in detail however, no studies into the surface properties or crystal morphology have been carried out. This paper summaries the key findings of a bulk study into the material and extends it to consider the morphology of pure YLiF4 before looking at the segregation of rare earth dopants to the low index surfaces. The presence of these dopants at the surfaces is shown to have a stabilising effect on some of the indices resulting in a lowering of the surface energy. By considering the optimum dopant coverage at all surfaces, a prediction for the doped La:YLiF4 crystal is made. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Atomistic computer simulations were performed for the actinide oxides (UO2, PuO2 and MOX) in the temperature range 300 K-2000 K, in order to elucidate the physical and defect properties of these nuclear fuel materials. Parameterised cation-anion Buckingham potentials, coupled to a standard Coulomb function, were used to describe the short range and long range interatomic interactions respectively. Mott-Littleton methodology was employed for accurate defect modelling and to calculate intrinsic defect formation energies. This paper summarises the key findings of a bulk study into these materials and extends this briefly to consider surface effects and morphologies. In addition to highlighting the thermodynamic stability of aggregated defects over isolated point defects, it is shown that the empirical potentials themselves are thermally stable beyond the standard centreline temperatures experienced within a Light Water Reactor (LWR). Finally, minimum energy pathways (MEP) for oxygen migration in UO2 have been investigated, indicating that at least one of the mechanisms, the vacancy migration, is non-linear. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The aim of this research on the laser material yttrium lithium fluoride (YLiF4) is to provide the first study of its surface structure, morphology and the properties of rare-earth doped surfaces. The study utilises the METADISE computer code to model the surfaces using interatomic potentials. Equilibrium and growth morphologies are predicted and a detailed profile of segregation of rare-earth dopants to the three dominating surfaces is given. The (001), (011) and (112) surfaces dominate the morphology. Rare-earth dopants La to Sm show segregation to these surfaces, with the (112) surface being of particular interest. As this work demonstrates, it is likely to have dopants clustered towards it.

We report on the application of Density Functional Theory (DFT) methodologies to investigate the degradation of polymer materials and provide new insights into the effects of degradation on molecular geometry. The temperature- and radiation-assisted degradation of a cross-linked ethylene–vinyl acetate–vinyl alcohol (EVA-OH) elastomer was studied both experimentally and theoretically, in order to correlate observable parameters with theoretically calculated electronic properties. Experiments showed ‘yellowing’ of the material, outgassing of acetic acid, and attenuated IR deformation modes upon exposure to increased levels of gamma radiation or increased temperatures, consistent with the de-acetylation model in which acetic acid is abstracted from the vinyl acetate group via a molecular rearrangement involving the displacement of a hydrogen atom from the ethylene backbone toward the acetyl group, and the propagation of the mechanism to adjacent vinyl acetate groups, forming polyenes in the polymer backbone. DFT modeling predicted the molecular structures of the cross-linked EVA-OH polymer for various degrees of de-acetylation, with corresponding IR and UV–visible absorption spectra. Theoretical attenuated IR deformation modes matched experimental observations, and the theoretical absorption spectrum of polyenes with 5 double bonds matched the optical absorption data, shedding light onto the final chemical structure of the polymer fragment. In addition, DFT unveiled precise and local effects of de-acetylation on the geometry of both the remaining polymer chain and cross-linker, which could only be detected as methylene deformation mode reflectance changes by IR spectroscopy.

Previously reported forms of the cation–anion Buckingham potential provide a significantly greater contribution than the repulsive Coulombic component at short-range thus predicting an unphysical attraction between the pair of ions. A detailed reappraisal of the computer modelling of uranium dioxide (UO2) employing atomistic simulation techniques is presented. An improved set of interatomic potentials is derived in order to describe the lattice correctly under conditions subsequent to radiation damage with the creation of Frenkel pair defects. Novel methodology is employed in the derivation of potentials ensuring applicability over the entire region of interest. The cation–anion potential is obtained via a combination of empirical fitting to crystal structural data and parametric fitting to additional physical properties. These potentials are subsequently verified and validated by calculation of additional bulk lattice properties, whose values agree favourably with those measured experimentally. Atomistic computer simulation techniques are then used to investigate the defect properties of UO2. The theoretical techniques are based upon efficient energy minimization procedures and Mott–Littleton methodology for accurate defect modelling and employed to calculate intrinsic defect formation energies and enable predictions of the expected type of intrinsic disorder to be made.

Solid-state optical materials are a continuing field of study that has many important applications including lasers. BaMgF4 is one material has been studied in detail however, no studies into the surface properties or crystal morphology have been carried out. This paper summarizes the key findings of a bulk study into the material and extends it to consider the morphology of BaMgF4 before looking at the segregation of rare earth dopants defect clusters to the low index surfaces. It is shown that there is strong segregation to all of the three morphology-dominating surfaces. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Computer modelling techniques are used to investigate the surface properties and defect chemistry of the La2NiO4 material. Relaxed surface structures and energies are calculated for the low index planes which are used to predict the equilibrium crystal morphology. The {111} surface is calculated to dominate in the absence of impurities, water or surface irregularities, with significant contributions from the {100} and {001} surfaces. Isovalent doping of the Ni site by Fe and Cu is found to affect the crystal morphology by increasing the expression of the {001} surface, although Fe doping is predicted to create the {011} face which is not present in the undoped crystal. The Sr dopant at the La site is calculated to be the most soluble of the alkaline earth metals, in accord with observation. Charge compensation is predicted to occur via the formation of Ni(III), which is consistent with bulk calculations and catalytic models in which Ni(III) species are correlated to the observed catalytic activity.

Atomistic computer simulation techniques are used to investigate the surface properties and defect chemistry of the LaCoO3 perovskite. The theoretical techniques are based upon efficient energy minimisation routines, a ‘two-region’ strategy and the Mott–Littleton methodology for the accurate modelling of surface and bulk defects. Sr and Ca dopants are calculated to be the most soluble of the alkaline earth metals, in accord with observation. Charge compensation is predicted to occur via oxygen ion vacancies which are believed to be key sites with regard to catalytic activity. Relaxed surface energies are calculated for the low index surfaces and the order of stability is found to be {110} > {100} > {111}. The equilibrium morphology of LaCoO3 is predicted from the surface energies, in which the {110} surface is calculated to dominate in the absence of impurities or surface irregularities, with a lesser contribution from the {100} surface. The surface defect energies are generally lower than in the bulk crystal implying that the dopants and oxygen vacancies will segregate to the surfaces, thus enhancing their catalytic and electrochemical activity.

Atomistic computer simulation techniques are used to investigate the defect properties of the La2Ni1-xMxO4 (M = Mn, Fe, Co, Cu) layered perovskite which are related to the mode of operation of the catalyst. The theoretical techniques are based upon efficient energy minimization procedures and Mott−Littleton methodology for accurate defect modeling. Effective ionic pairwise interatomic potentials correctly reproduce the tetragonal crystal structure. The formation energy of intrinsic atomic defects of the Schottky and Frenkel type are not particularly favorable. The oxidation of La2NiO4+δ was found to be an exothermic process with charge compensation occurring via hole formation preferentially on the Ni site. The highest solubility, for a range of dopants, is calculated for Sr and Ca, in accord with observation. Hole formation was most favorable for Mn > Fe > Co > Ni(undoped) > Cu, demonstrating that Mn and Fe enhance Ni(III) hole formation, which is believed to be an important factor in the observed cataly...

Atomistic computer simulation techniques are used to investigate the defect properties of high temperature superconductors, in an attempt to illustrate the type of information that can be obtained from the modelling of these complex materials. Attention is focused on two systems: the mercury cuprate HgBa2CuO4+δ, which is the first member of the homologous series HgBa2Can-1CunO2n+2+δ, and the copper oxychloride Ca2CuO2Cl2, which is part of a new family of oxyhalide superconductors. Effective interatomic potentials are used to reproduce accurately their observed crystal structures. A range of redox and dopant substitution reactions relevant to high TC behaviour have been examined. The results are consistent with the experimental observation that HgBa2CuO4+δ becomes superconducting by oxygen incorporation at interstitial sites, while Ca2CuO2Cl2 shows superconductivity only on doping with alkali metal ions at the calcium site. We also consider the energetics of dopant substitution on the copper sublattice of the oxychloride. Our study indicates the value of computer simulation methods in examining the nature of defect reactions at the atomic level.

Since the inception of the personal computer, the interface presented to users has been defined by the monitor screen, keyboard, and mouse, and by the framework of the desktop metaphor. It is very different from a physical desktop which has a large horizontal surface, allows paper documents to be arranged, browsed, and annotated, and is controlled via continuous movements with both hands. The desktop metaphor will not scale to such a large display; the continuing profusion of paper, which is used as much as ever, attests to its unsurpassed affordances as a medium for manipulating documents; and despite its proven manual and cognitive benefits, two-handed input is still not used in computer interfaces. I present a system called the Escritoire that uses a novel configuration of overlapping projectors to create a large desk display that fills the area of a conventional desk and also has a high resolution region in front of the user for precise work. The projectors need not be positioned exactly—the projected imagery is warped using standard 3D video hardware to compensate for rough projector positioning and oblique projection. Calibration involves computing planar homographies between the 2D co-ordinate spaces of the warped textures, projector framebuffers, desk, and input devices. The video hardware can easily perform the necessary warping and achieves 30 frames per second for the dual-projector display. Oblique projection has proved to be a solution to the problem of occlusion common to front-projection systems. The combination of an electromagnetic digitizer and an ultrasonic pen allows simultaneous input with two hands. The pen for the non-dominant hand is simpler and coarser than that for the dominant hand, reflecting the differing roles of the hands in bimanual manipulation. I give a new algorithm for calibrating a pen, that uses piecewise linear interpolation between control points. I also give an algorithm to calibrate a wall display at distance using a device whose position and orientation are tracked in three dimensions. The Escritoire software is divided into a client that exploits the video hardware and handles the input devices, and a server that processes events and stores all of the system state. Multiple clients can connect to a single server to support collaboration. Sheets of virtual paper on the Escritoire can be put in piles which can be browsed and reordered. As with physical paper this allows items to be arranged quickly and informally, avoiding the premature work required to add an item to a hierarchical file system. Another interface feature is pen traces, which allow remote users to gesture to each other. I report the results of tests with individuals and with pairs collaborating remotely. Collaborating participants found an audio channel and the shared desk surface much more useful than a video channel showing their faces. The Escritoire is constructed from commodity components, and unlike multi-projector display walls its cost is feasible for an individual user and it fits into a normal office setting. It demonstrates a hardware configuration, calibration algorithm, graphics warping process, set of interface features, and distributed architecture that can make personal projected displays a reality.