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We report calculated energetics (at the GGA, and at the B3LYP, HSE06 hybrid density functional levels of theory) and electronic properties (B3LYP, HSE06) of Li and N co-doped diamond containing LiNx clusters (x = 1–4). Defect energies and structural properties at the GGA and hybrid (B3LYP, HSE06) levels of theory are very similar. Our results for isolated Li and N dopants agree with previous work in that they are not suitable candidates for shallow n-type donors. Here, we investigate the effects of addition of N atoms as a co-dopant with Li, which vary with the Li:N ratio and whether Li is interstitial or substitutional. Reaction energies, geometries, bonding and densities of states (DOS) are examined. Nitrogen stabilises substitutional rather than interstitial lithium. Atom-projected DOS show that the chief contributions to the defect states are not from orbitals on the dopant atoms themselves but from states associated with neighbouring carbon atoms. Only the tetrahedral 1:4 cluster, LiCN4, with substitutional Li is likely to behave as a shallow donor. We propose a non-molecular synthetic route for preparation of the LiCN4-doped material via prior generation of the N4V centre at temperatures at which Li is mobile but LiCN4 clusters are not.

We present inelastic neutron scattering measurements of Y3Al5O12. The measured neutron-weighted phonon density-of-states and optical phonon frequencies are compared with the results from atomistic calculations within the Quasi-Harmonic Approximation (QHA) over a wide temperature range, using both first principles density functional theory calculations and also an extended empirical interatomic-potential shell model scheme. The phonon-related thermodynamic and elastic properties calculated within the QHA are in excellent agreement with the available experimental data.

DFT calculations of the Li substitutional defect in diamond based on the B3LYP functional and a 64-atom supercell indicate that (i) the quartet (Sz = 3/2) state is lower in energy than the doublet (Sz = 1/2) state by 0.07 eV (810 K) for fully relaxed static structures and by 0.09 eV (1045 K) with the inclusion of zero-point vibrations, (ii) the effective charges at the Li and four neighbouring C sites are similar in the two spin states, but there are substantial differences in the corresponding spin distributions, and (iii) there are unprecedented differences in the Raman spectra of the two spin states, in terms of both frequency distributions and intensities, that can most reasonably be attributed to strong spin-phonon coupling, in view of the very similar charge distributions in the two states. These differences are an order of magnitude greater than those reported previously for any bulk transition metal or rare-earth compound. The basis sets and functional used in these calculations predict many of the relevant constants (a0, c11, c44) of diamond mostly to within 1% of the experimental values, most notably the TO(X) Raman frequency and the phonon density of states. Comparisons with the calculated Raman spectra of the quintet (Sz = 2) and singlet (Sz = 0) spin states of the neutral vacancy defect, which have similar spin distributions at the four neighbouring C atoms (Cn) to the vacancy site as those at the corresponding Cn sites in the quartet and singlet states of the Li defect, show that the differences in the two Raman spectra of the latter defect are closely related to those in the former.

Using density functional theory electronic structure calculations, the equation of state, thermodynamic and elastic properties, and sound wave velocities of Fe3S at pressures up to 250 GPa have been determined. Fe3S is found to be ferromagnetic at ambient conditions but becomes non-magnetic at pressures above 50 GPa. This magnetic transition changes the c/a ratio leading to more isotropic compressibility, and discontinuities in elastic constants and isotropic sound velocities. Thermal expansion, heat capacity, and Grüneisen parameters are calculated at high pressures and elevated temperatures using the quasiharmonic approximation. We estimate Fe-Fe and Fe-S force constants, which vary with Fe environment, as well as the 56Fe/54Fe equilibrium reduced partition function in Fe3S and compare these results with recently reported experimental values. Finally, our calculations under the conditions of the Earth’s inner core allow us to estimate a S content of 2.7 wt% S, assuming the only components of the inner core are Fe and Fe3S, a linear variation of elastic properties between end-members Fe and Fe3S, and that Fe3S is kinetically stable. Possible consequences for the core-mantle boundary of Mars are also discussed.

The energy landscape of the fast-ion conductor Bi 4 V 2 O 11 is studied using density functional theory. There are a large number of energy minima, dominated by low-lying thermally accessible configurations in which there are equal numbers of oxygen vacancies in each vanadium–oxygen layer, a range of vanadium coordinations and a large variation in Bi–O and V–O distances. By dividing local minima in the energy landscape into sets of configurations, we then examine diffusion in each different layer using ab initio molecular dynamics. These simulations show that the diffusion mechanism mainly takes place in the 〈110〉 directions in the vanadium layers, involving the cooperative motion of the oxide ions between the O(2) and O(3) sites in these layers, but not O(1) in the Bi–O layers, in agreement with experiment. O(1) vacancies in the Bi–O layers are readily filled by the migration of oxygens from the V–O layers. The calculated ionic conductivity is in reasonable agreement with the experiment. We compare ion conduction in δ-Bi 4 V 2 O 11 with that in δ-Bi 2 O 3 . This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.

Links between dynamical Frenkel defects and collective diffusion of fluorides in

Links between dynamical Frenkel defects and collective diffusion of fluorides in β -PbF 2 are explored using Born–Oppenheimer molecular dynamics. The calculated self-diffusion coefficient and ionic conductivity are 3.2 × 10 −5 cm 2 s −1 and 2.4 Ω −1 cm −1 at 1000 K in excellent agreement with pulsed field gradient and conductivity measurements. The calculated ratio of the tracer-diffusion coefficient and the conductivity-diffusion coefficient (the Haven ratio) is slightly less than unity (about 0.85), which in previous work has been interpreted as providing evidence against collective ‘multi-ion’ diffusion. By contrast, our molecular dynamics simulations show that fluoride diffusion is highly collective. Analysis of different mechanisms shows a preference for direct collinear ‘kick-out’ chains where a fluoride enters an occupied tetrahedral hole/cavity and pushes the resident fluoride out of its cavity. Jumps into an occupied cavity leave behind a vacancy, thereby forming dynamic Frenkel defects which trigger a chain of migrating fluorides assisted by local relaxations of the lead ions to accommodate these chains. The calculated lifetime of the Frenkel defects and the collective chains is approximately 1 ps in good agreement with that found from neutron diffraction. This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.

In the field of solar water splitting, searching for and modifying bulk compositions has been the conventional approach to enhancing visible-light activity. In this work, manipulation of heterointerfaces in ZnS-GaP multilayer films is demonstrated as a successful alternative approach to achieving visible-light-active photoelectrodes. The photocurrent measured under visible light increases with increasing number of interfaces for ZnS-GaP multilayer films withthe same total thickness, indicating it to be a predominantly interface-driven effect. The activity extends to long wavelengths (650 nm), much longer than expected for pure ZnS, and also longer than previously reported for GaP. Density functional theory (DFT) calculations of ZnS-GaP multilayers predict the presence of electronic states associated with atoms at the interfaces between ZnS and GaP that are different from those found within the layers away from the interfaces; these states, formed due to unique bonding environments found at theinterfaces, lead to a lowering of the band gap and hence the observed visible-light activity.The presence of these electronic states attributed to the interfaces is confirmed by depthresolved X-ray photoelectron spectroscopy. Thus, we show that interface engineering is a promising route for overcoming common deficiencies of individual bulk materials caused by both wide band gaps and indirect band gaps, and hence enhancing visible-light absorption and photoelectrochemical performance

We compare computational methods for determining the force between carbon atoms as a function of bond length, in order to establish which ones are capable of accurately simulating carbon-carbon bonds breaking due to applied mechanical strain in nanomaterials. Results from Tight-binding, density-functional theory and molecular mechanics potentials are compared to Moller-Plesset perturbation theory and complete-active-space self-consistent-field method through application to bond breaking in small molecules. Of the two molecular mechanics and three tight-binding parameter sets chosen only DFTB3 gives results which are broadly similar to those from the first-principles methods; the others fail to give physically meaningful variation of the forces with internuclear separation. This method and the molecular mechanics potentials are then applied to a periodic carbon nanoribbon under tensile strain. The molecular mechanics methods fail even qualitatively to reproduce the single catastrophic failure shortly after the peak stress indicated by DFTB3. This shows the importance of the electronic behaviour for the carbon-carbon interatomic forces relevant to the determination of the mechanical strength of materials at atomic-length scales.

Inorganic phosphors based on monoclinic Y2WO6 doped with Sm3+ ions were prepared via conventional solid-state reactions at high temperature. A total of six samples were obtained with different Sm3+ concentrations (0–9%). The purity of the as-prepared phases was checked by powder X-ray diffraction (PXRD). The excitation, emission, and time-resolved emission spectra were examined in detail. The experimental decay curves were fitted to the Inokuti–Hirayama model, showing that the Sm3+ ions form clusters at all compositions. Periodic hybrid density functional theory calculations were also carried out on the undoped material and on 144-atom supercells of stoichiometry Y1.9375Sm0.0625WO6 and Y1.875Sm0.125WO6. The different coordination environments at the Y1, Y2 and Y3 sites are analysed in detail. The calculated structure and band gap of Y2WO6 are in good agreement with experiment with one potentially important discrepancy in a Y3-O bond length. The thermodynamically favoured substitution sites for Sm, Y2 and Y3, in the supercell are not those observed under the preparation conditions used here, since the experimental Rietveld analysis suggests occupation of Y1 at all concentrations. Analogous calculations for the Eu-doped system highlight marked differences between Eu and Sm despite their similar ionic sizes. The calculateddensities of states show the position of the 4f levels in the band gap depend on thesites occupied by the dopants and thus we expect marked differences in the luminescencespectra, opening up possibilities for tuning device performance.

Heavy-tailed distributions have been observed for various financial risks and papers have observed that these heavy-tailed distributions are power law distributions. The breakdown of a power law distribution is also seen as an indicator of a tipping point being reached and a system then moves from stability through instability to a new equilibrium. In this paper, we analyse the distribution of operational risk losses in US banks, credit defaults in US corporates and market risk events in the US during the global financial crisis (GFC). We conclude that market risk and credit risk do not follow a power law distribution, and even though operational risk follows a power law distribution, there is a better distribution fit for operational risk. We also conclude that whilst there is evidence that credit defaults and market risks did reach a tipping point, operational risk losses did not. We conclude that the government intervention in the banking system during the GFC was a possible cause of banks avoiding a tipping point.

The world economy has been shown to be a complex adaptive system with the consequence that companies within the global economy are constantly needing to react to influences from the activities of other companies with which they are interconnected and external influences. This paper uses a methodology developed for complex adaptive systems to analyse the characteristics of multiple credit default events in the United States, Europe and Asia over the period 1990–2010 to establish the significance of factors driving credit defaults. The analysis indicates that factors controllable by companies are more significant in the United States and Europe than uncontrollable events, but the reverse occurs in Asia.

© 2017 The Economic Society of Australia The World Economic Forum (WEF) recognises in their recent global risk reports the need to better understand the dynamics of the global economic system, and in particular the interconnectedness in the system in order to better appreciate the possibility of both single and multiple risk events occurring. This paper applies a cladistics analysis technique to identify the commonality of identified characteristics across a sample set of the 2014 WEF global risks (WEF 2014) as a basis for a new understanding of the linkages and potential evolution of the WEF risks. This approach shows strong potential to provide deeper insights into the nature and interdependency of global risks and in particular how this allows for more efficient management of these risks.

This paper explores the characteristics of 2,141 operational risk events amongst European (EU) and US banks over the period 2008–2014. We have analysed the operational risk events using a method originating in biology for the study of interrelatedness of characteristics in a complex adaptive system. The methodology, called cladistics, provides insights into the relationships between characteristics of operational risk events in banks that is not available from the traditional statistical analysis. We have used cladistics to explore if there are consistent patterns of operational risk characteristics across banks in single and different geographic zones. One significant pattern emerged which indicates there are key, stable characteristics across both geographic zones and across banks in each zone. The results identify the characteristics that could then be managed by the banks to reduce operational risk losses. We also have analysed separately the characteristics of operational risk events for “big” banks and extreme events and these results indicate that big banks and small banks have similar key operational risk characteristics, but the characteristics of extreme operational risk events are different to those for the non-extreme events.

H2 physisorption within curved carbon nanomaterials for potential fuel storage on board vehicles is studied using dispersion-corrected Density Functional Theory. Full Cn (n = 20, 60, 180, 540, 960) fullerenes were considered along with single-walled carbon nanotubes ((3,3), (5,5), (9,9)) and graphene to investigate the effects of curvature, confinement, platinum and non-metal (B, N, O) dopants, Cfullerene–H2 and H2H2 distances, H2 orientation on Cfullerene–H2 interactions. The study mainly focuses on H2 stored within the fullerene with some investigation into external H2. A significant attractive Cfullerene–H2 interaction energy of −28 kJ/mol is observed for H2 in curved carbon nanomaterials where H2 molecules are located ca. 2.9 Å from carbon atoms in a highly confined system. Dopants have the potential to increase the favourability of Cfullerene–H2 interactions when multiple H2 molecules are present by affecting the orientation of H2 molecules within the carbon nanomaterial. This paper presents analysis of several carbon nanosystems and then proposes possible materials for H2 storage on board vehicles.

Our desire to deliver increased functionality while setting tighter operational and regulative boundaries has fueled a recent influx of highly coupled systems. Nonetheless, our current capacity to successfully deliver them is still in its infancy. Understanding how such Designed systems are structured, along with how they compare to their naturally Evolved counterparts, can play an important role in bettering our capacity to do so. Based on this premise, the structural patterns underlying a wide range of seemingly unrelated systems are uncovered using tools from network science. By doing so, structural patterns emerge and are subsequently used to highlight both similarities and differences between the two classes of systems. With a focus on the Designed class, and assuming that increased structural variety fuels design uncertainty, it is shown that their adherence to statistical normality i.e., expected vs. encountered patterns and statistical correlations between combinations of such patterns is rather limited. Insight of this sort has both theoretical context agnostic approaches are increasingly relevant within the domain of Systems Engineering, yet are rarely used and practical transferability of knowledge implications.

Hybrid Monte Carlo (HMC) simulations are used to study the growth of Gd-rich domains in Gd doped CeO2, and we probe the conductivity of the resulting and other configurations by molecular dynamics. Previous work has been restricted to the dilute defect limit, assumptions of particular cluster formation, and neglect of all temperature effects. Our methods suffer none of these restrictions. Even at low concentrations Gd3+ segregates into domains. We have examined the local environment of the Gd3+ ions using radial distribution functions and Steinhardt order parameters. The observed structure is consistent with the formation of cubic C-type (Gd2O3) domains, rather than the monoclinic B-type or pyrochlore clusters which have been suggested previously. In addition, previous detailed pair distribution function analysis of the solid solution has indicated different local cation environments from those from a Rietveld analysis-overall our results support the former analysis rather than the latter. At the elevated temperatures (1000 K) of the simulations there is no particular preference for vacancy and dopant cations to be located at second neighbour sites, an issue long discussed for this and similar systems. Both calculated and experimental conductivities show a similar variation with composition, passing through a maximum with increasing Gd concentration. The conductivities of the configurations generated in the hybrid Monte Carlo simulations are lower than those of configurations generated independently in which the Gd ions are distributed at random. The HMC thermally generated Gd nano-domains capture oxygen vacancies, reduce oxygen vacancy mobility and block diffusion paths.

We show how a genetic algorithm (GA) generates efficiently the energy landscape of the equimolar calcite–magnesite (CaCO3—MgCO3) solid solution. Starting from a random configuration of cations and a supercell containing 480 atoms,the lowest energy form of ordered dolomite was found in all runs, in 94% of which it was located with less than 20,000 fitness evaluations. Practical implementation and operation of the GA are discussed in detail. The method can also generate both low-lying and high-lying excited states. Detailed analysis of the energy-minimised structures of the different configurations reveals that low energies are associated with reduction of strain associated with rotation of the carbonate groups, a mechanism possible only when a carbonate layer lies between a layer of just Ca and a layer of just Mg. Such strain relief is not possible in the equimolar MgO–CaO solid solution despite the similarity of the crystal structures of these binary oxides to calcite–magnesite, and therefore, the enthalpy of mixing is very high. Implications for thermodynamic configurational averaging over the minima in the energy landscape are briefly considered. Overall, the genetic algorithm is shown to be a powerful tool in probing non-ideality in solid solutions and revealing the ordering patterns that give rise to such behaviour.

The energy landscapes of ultra-thin nanofilms of ZnO and ZnS are examined in detail using periodic hybrid density functional calculations. We predict new staggered graphitic forms, which, are stable only for the thinnest films and are of particular interest as the electronic structure shows a spontaneous symmetry breaking across the film and consequently a marked decrease in band gap with thickness. The relative energies of the various forms, their structural and electronic properties and their variation with film thickness are discussed. Possible kinetic pathways for transitions from the graphitic forms are examined by explicit evaluation of transition state energies.For polar surfaces, such as (0001) würtzite and (111) zinc blende, many different mechanisms operate to remove or reduce the surface dipole depending on the number of layers in the nanofilm. The polar ZnS nanofilms, but not the polar ZnO analogues or any non-polar film, are predicted to spontaneously become non-stoichiometric by loss of zinc atoms from the surface.The behaviour of adsorbed water on the ultra-thin films is also examined. There is no dissociation on any ZnS film. For ZnO, dissociation into OH- and H+ takes place not only on (101¯0) würtzite, but also on (110) zinc blende. This result that does not appear to have been reported previously and deserves future experimental study.While we concentrate on ZnO and ZnS, similar energy landscapes are expected for any oxide or sulphide which adopts the würtzite or zinc blende structure in the bulkThe energy landscapes of ultra-thin nanofilms of ZnO and ZnS are examined in detail using periodic hybrid density functional calculations. We predict new staggered graphitic forms, which, are stable only for the thinnest films and are of particular interest as the electronic structure shows a spontaneous symmetry breaking across the film and consequently a marked decrease in band gap with thickness. The relative energies of the various forms, their structural and electronic properties and their variation with film thickness are discussed. Possible kinetic pathways for transitions from the graphitic forms are examined by explicit evaluation of transition state energies.For polar surfaces, such as (0001) würtzite and (111) zinc blende, many different mechanisms operate to remove or reduce the surface dipole depending on the number of layers in the nanofilm. The polar ZnS nanofilms, but not the polar ZnO analogues or any non-polar film, are predicted to spontaneously become non-stoichiometric by loss of zinc atoms from the surface.The behaviour of adsorbed water on the ultra-thin films is also examined. There is no dissociation on any ZnS film. For ZnO, dissociation into OH- and H+ takes place not only on (101¯0) würtzite, but also on (110) zinc blende. This result that does not appear to have been reported previously and deserves future experimental study.While we concentrate on ZnO and ZnS, similar energy landscapes are expected for any oxide or sulphide which adopts the würtzite or zinc blende structure in the bulk

Density functional theory calculations were performed to model the adsorption of up to 1 monolayer (ML) of aluminium on the bare and O-terminated (1 0 0) diamond surface. Large adsorption energies of up to -6.36 eV per atom are observed for the Al-adsorbed O-terminated diamond surface. Most adsorption sites give a negative electron affinity (NEA), with the largest NEAs -1.47 eV on the bare surface (1 ML coverage) and -1.36 eV on the O-terminated surface (0.25 ML coverage). The associated adsorption energies per Al atom for these sites are -4.11 eV and -5.24 eV, respectively. Thus, with suitably controlled coverage, Al on diamond shows promise as a thermally-stable surface for electron emission applications.

Complexity is often quoted as an independent variable that challenges the utility of traditional project management tools and techniques. A large body of work has been devoted in exposing its numerous aspects, yet means for quantitatively assessing it have been scarce. Part of the challenge lies in the absence of hard evidence supporting the hypothesis that projects can be considered as complex systems, where techniques for measuring such complexity are better established. In response, this study uses empirical activity networks to account for the technological aspect of five projects. By doing so, the contribution of this study is twofold. First, a procedure for the quantitative assessment of an aspect of project complexity is presented; namely, structural complexity. Second, results of the analysis are used to highlight qualitatively similar behavior with a well-known complex system, the Internet. As such, it suggests a transition from the current, metaphorical view of projects being complex systems to a literal one.From a practical point of view, this study uses readily captured and widely used data, enabling practitioners to evaluate the structural complexity of their projects to explore system pathologies and, hence, improve the decision-making process around project bidding, resource allocation, and risk management.

Modern society heavily relies on strongly connected, socio-technical systems. As a result, distinct risks threatening the operation of individual systems can no longer be treated in isolation. Consequently, risk experts are actively seeking for ways to relax the risk independence assumption that undermines typical risk management models. Prominent work has advocated the use of risk networks as a way forward. Yet, the inevitable biases introduced during the generation of these survey-based risk networks limit our ability to examine their topology, and in turn challenge the utility of the very notion of a risk network. To alleviate these concerns, we proposed an alternative methodology for generating weighted risk networks. We subsequently applied this methodology to an empirical dataset of financial data. This paper reports our findings on the study of the topology of the resulting risk network. We observed a modular topology, and reasoned on its use as a robust risk classification framework. Using these modules, we highlight a tendency of specialization during the risk identification process, with some firms being solely focused on a subset of the available risk classes. Finally, we considered the independent and systemic impact of some risks and attributed possible mismatches to their emerging nature., Comment: 21 pages, 7 Figures, 4 tables, To appear in Journal of Network Theory in Finance

We calculate the tensile and shear moduli of a series of boron nitride nanotubes and their piezoelectric response to applied loads. We compare in detail results from a simple molecular mechanics (MM) potential, the universal force field, with those from the atomistic finite element method (AFEM) using both Euler-Bernoulli and Timoshenko beam formulations. The MM energy minimisations are much more successful than those using the AFEM, and we analyse the failure of the latter approach both qualitatively and quantitatively.

Kirigami, the ancient Japanese art of cutting and folding paper at the macroscale, has been recently applied to produce nanostructures with unusual deformation mechanisms. In this work we analyse the mechanical properties and deformation mechanisms leading to remarkable stretching and bilinear stiffness in armchair and zigzag hexagonal boron nitride (h-BN) Kirigami nanosheets using classical molecular dynamics simulations. We identify three geometric parameters that govern the mechanics and ductility of Kirigami h-BN. Enhancements in tensile strains up to 3–5 times higher than those of pristine h-BN can be obtained. The variations of stiffness, ultimate strength and strain with the parameters defining the Kirigami defect patterns can be used to tune the mechanical properties of h-BN and other nano 2D structures, potentially expanding their applications in bio-compatible strain-engineered nanodevices and nanoelectronics.

The complex nature of organizational culture challenges our ability to infers its underlying dynamics from observational studies. Recent computational studies have adopted a distinct different view, where plausible mechanisms are proposed to describe a wide range of social phenomena, including the onset and evolution of organizational culture. In this spirit, this work introduces an empirically-grounded, agent-based model which relaxes a set of assumptions that describes past work - (a) omittance of an individual's strive for achieving cognitive coherence, (b) limited integration of important contextual factors - by utilizing networks of beliefs and incorporating social rank into the dynamics. As a result, we illustrate that: (i) an organization may appear to be increasingly coherent in terms of organizational culture, yet be composed of individuals with reduced levels of coherence, (ii) the components of social conformity - peer-pressure and social rank - are influential at different aggregation levels., 20 pages, 8 figures

We propose a methodology applied to complex systems to analyze operational risk events in banks, with the objective of determining an understanding of the key characteristics and their relationships in initiating operational risk losses. We applied our methodology to operational risk losses in Australian banks over the period 2010-14. The analysis identified that there are a small number of characteristics that are common to many operational risk events, and these "level 1" characteristics are stable across time, which implies operational risk losses could be controlled by managing these characteristics. The methodology adds value to the existing analysis by identifying the main characteristics of operational risk events in a rigorous manner.

ZnO and ZnS are both good photocatalysts, but their band gaps are too wide for efficient use under sunlight. In this work, solid solutions formed by the addition of a small amount of AlN or GaN to ZnO and AlP or GaP to ZnS are studied with first-principles calculations. Energies of mixing are low in all cases and addition of a small amount of solute can significantly reduce the band gap. Results for the different solid solutions are compared.

Ionic conductivities in the solid oxide fuel cell (SOFC) electrolytes yttria-stabilised zirconia (YSZ), calcia-stabilised zirconia (CSZ), gadolinium-doped ceria (GDC) and samarium-doped ceria (SDC) and the cathode material lanthanum strontium cobalt oxide (LSCO) are directly calculated using DL_AKMC, an adaptive kinetic Monte Carlo (aKMC) program which assumes limited a priori knowledge of the kinetics of systems. The materials were simulated over several milliseconds and over the range of experimentally most relevant temperatures and dopant concentrations (2–18 mol% for doped zirconia, 5–25 mol% for doped ceria and 5–80 mol% for LSCO). Ionic conductivities of the electrolytes at 1000 K are in good agreement with the observed values: CSZ in the range 3 × 10−3 to 1 × 10−2 S cm−1 depending on dopant concentration, YSZ 4 × 10−3 to 3 × 10−2 S cm−1, GDC 1 × 10−2 to 5 × 10−2 S cm−1, SDC 1 × 10−2 to 7 × 10−2 S cm−1. LSCO is predicted to have an ionic conductivity of the order of 10−2 to 10−1 S cm−1 depending on Sr content. Average activation energies over all migration processes are 0.4–0.5 eV for the stabilised zirconias and 0.2–0.3 eV for the doped cerias and 0.3 eV for LSCO, in agreement with experiment. aKMC provides a distinct advantage over traditional KMC methods, in which one has to provide a list of system state transitions. Here, all of the state transitions are dynamically generated, leading to a more accurate simulation of the kinetics as the system evolves.

Recent computational work has shown that light metals adsorbed onto the oxygenated diamond (1 0 0) surface have the potential to give diamond a temperature-stable negative electron affinity (NEA). Here, we use density functional theory to study three of these metals, lithium, magnesium and aluminium, on the (1 1 1) surface. We show that all three of these metals adsorbed onto the ketone O-terminated diamond surface can possess a large NEA and adsorption energies above that of H-termination at monolayer (ML) or sub-ML coverages. Adsorption onto the ether O-terminated surface gives similarly large NEAs but lower adsorption energies. These results are promising for the development of novel NEA surfaces such as those required for thermionic devices.

Preparation of solid solutions represents an effective means to improve the photocatalytic properties of semiconductor-based materials. Nevertheless, the effects of site-occupancy disorder on the mixing stability and electronic properties of the resulting compounds are difficult to predict and consequently many experimental trials may be required before achieving enhanced photocatalytic activity. Here, we employ first-principles methods based on density functional theory to estimate the mixing free energy and the structural and electronic properties of (GaP)x(ZnS)1-x solid solutions, a representative semiconductor-based optoelectronic material. Our method relies on a multi-configurational supercell approach that takes into account the configurational and vibrational contributions to the free energy. Phase competition among the zinc-blende and wurtzite polymorphs is also considered. We demonstrate overall excellent agreement with the available experimental data: (1) zinc-blende emerges as the energetically most favorable phase, (2) the solid solution energy band gap lies within the 2-3 eV range for all compositions, and (3) the energy band gap of the solid solution is direct for compositions x < 75%. We find that at ambient conditions most (GaP)x(ZnS)1-x solid solutions are slightly unstable against decomposition into GaP- and ZnS-rich regions. Nevertheless, compositions x = 25, 50, and 75% render robust metastable states that owing to their favorable energy band gaps and band levels relative to vacuum are promising hydrogen evolution photocatalysts for water splitting under visible light. The employed theoretical approach provides valuable insights into the physicochemical properties of potential solid-solution photocatalysts and offers useful guides for their experimental realization.

The existence of ultra-flexible low-energy forms of boron oxides (B2O3 and BO) is demonstrated, in particular structures in which B3O3 or B4O2 six-membered rings are linked by single B-O-B bridges. The minima in the energy landscapes are remarkably broad; the variation in the internal energies is very small over a very large range of volumes. Such volume changes may even exceed 200%. This remarkable behavior is attributed predominantly to the pronounced angular flexibility of the B-O-B bridges linking the rings, which is unusual for a covalent bond. At larger volumes, the structures are nanoporous; the pores collapse upon compression with negligible change in energy, making these suitable as guest-host materials. In marked contrast, in other materials where low density frameworks have been reported or predicted, such low-density phases are considerably higher in energy. The flexibility of the structures also offers a resolution of the long-standing controversy reconciling the structure and density of vitreous B2O3.

Atomistic simulation techniques are used to examine the stability of Ruddlesden–Popper (R–P) phases Srn+1n3n+1(n = 1, 2, 3, 4 and ∞). Various sets of empirical pair potentials are employed to determine the formation energies of the R–P phases. Formation energies are also calculated with Density Functional Theory (DFT). The tendency of a given R–P phase to dissociate into a lower order R–P phase plus SrTiO3 perovskite is found to increase with increasing n. The results obtained are compared with experiment and previous computational studies. The stability of intergrowth phases with respect to the pure R–P compounds is examined. In all cases the intergrowths are calculated to be thermodynamically less stable than the pure R–P phase, but the differences are in some cases negligible. Finally, the energy for SrO partial Schottky disorder in strontium titanate is computed taking the formation of R–P phases into account.

We have recently undertaken comprehensive computational studies predicting possible crystal structures of the as yet unknown phosphorus carbide as a function of composition. In this work, we report the synthesis of amorphous phosphorus–carbon films by pulsed laser deposition. The local bonding environments of carbon and phosphorus in the synthesised materials have been analysed by x-ray photoelectron spectroscopy; we have found strong evidence for the formation of direct P–C bonding and hence phosphorus carbide. There is a good agreement between the bonding environments found in this phosphorus carbide material and those predicted in the computational work. In particular, the local bonding environments are consistent with those found in the β - InS - like structures that we predict to be low in energy for phosphorus:carbon ratios between 0.25 and 1.

Noble-gas geochemistry is an important tool for understanding planetary processes from accretion to mantle dynamics and atmospheric formation1,2,3,4. Central to much of the modelling of such processes is the crystal–melt partitioning of noble gases during mantle melting, magma ascent and near-surface degassing5. Geochemists have traditionally considered the ‘inert’ noble gases to be extremely incompatible elements, with almost 100 per cent extraction efficiency from the solid phase during melting processes. Previously published experimental data on partitioning between crystalline silicates and melts has, however, suggested that noble gases approach compatible behaviour, and a significant proportion should therefore remain in the mantle during melt extraction5,6,7,8. Here we present experimental data to show that noble gases are more incompatible than previously demonstrated, but not necessarily to the extent assumed or required by geochemical models. Independent atomistic computer simulations indicate that noble gases can be considered as species of ‘zero charge’ incorporated at crystal lattice sites. Together with the lattice strain model9,10, this provides a theoretical framework with which to model noble-gas geochemistry as a function of residual mantle mineralogy.

Understanding the behaviour of solid solutions over wide ranges of temperature and pressure remains a major challenge to both theory and experiment. Here we report a detailed exchange Monte Carlo study using a classical ionic model of the model perovskite parascandolaite-neighborite (K,Na)MgF3 solid solution and its end-members for temperatures in the range 300-1000 K and pressures from 0-8 GPa. Full account is taken of the local environment of the individual cations, clustering and thermal effects. Properties considered include the crystal structure, phase transitions, the thermodynamics of mixing and the non-ideality of the solid solution. Clustering of the potassium ions is examined via a short-range order parameter. Where experimental data are available for comparison, agreement is very good.

The Management Board of the UK Actuarial Profession has identified enterprise risk management (ERM) as an area of growth, particularly within the financial sector. It is an area which offers opportunities for actuaries, working with other disciplines, to move out of their traditional sectors of employment, with the skill set required fitting well with an actuary's training and practical focus.Members of the Profession also highlighted ERM as one of the two main areas where they wanted the Profession to focus their research efforts in the membership survey in 2009. Consequentially the Management Board allocated funds to support research projects in ERM in 2010–2011 and has worked with the ERM Practice Area Committee to identify the topics that they feel most suited to external research where the outputs will have a broad strategic value to the financial services sector.

Trace elements are widely used to unravel magmatic processes and constrain the chemical differentiation of the Earth. Key to their use is understanding the controls on fractionation between solid and liquid, which hinge on the energetics of trace element incorporation into crystals. At equilibrium most trace elements are incompatible in bulk magmatic minerals because of unfavourable incorporation energies. We use computational methods to explore the role that crystal interfaces can play in trace element incorporation in minerals. We demonstrate that differences between bulk and interface incorporation energies can be very large and lead to concentration differences of many orders of magnitude, consistent with experimental evidence for interface enrichment. By emphasising the importance of adsorption/incorporation at interfaces, we account for the competing effects of bulk equilibrium and interface segregation operating during melting and crystallisation. Computational results are presented for divalent cations and Ar in bulk forsterite, at the {010} surface and at the {100}/{010} stepped grain boundary. For all species studied larger than Mn 2 + , segregation to the interface is highly exothermic. We discuss in particular the take-up of noble gases where, in contrast to earlier work, a recent experimental study concluded that argon is highly compatible in mantle minerals. In contrast our calculations indicate bulk Ar solubility is very small and suggests incorporation at mineral interfaces is overwhelmingly favourable.

Mathematical skills beyond that taught at GCSE level (under 16 in UK) are required to pursue a physical sciences degree in the UK. However, many departments are unable to recruit sufficient students who have both the physical science and Mathematics qualification at A-level (post-16). Therefore, students are admitted with GCSE Mathematics and are taught the mathematical skills during the degree course. In this paper we investigate the impact of running a pre-university mathematics summer school for students about to start a physical sciences degree who have GCSE Mathematics as their highest mathematics qualification. The students are tracked through their first two years of a UK chemistry degree. It is shown that they perform significantly better than similarly qualified students in first year physical chemistry and second year theoretical chemistry units. Reasons for these results are presented.

This paper reviews the annual energy and water performance of an aspiring low energy/carbon affordable housing development in southern UK, comprising 25 houses heated by a biomass fueled district heating network. Electrical, heat and water consumption data was collected for all dwellings and more detailed data logging was conducted on a sample of four. The data was analysed to benchmark the overall site performance and compare the individual dwellings. The individual dwellings performed efficiently in terms of electricity, heat and water consumption. Significant variation was identified between maximum and minimum consumers, even when typical performance correlates (i.e. number of occupants, floor area) were accounted for. Water consumption varies by a factor of > 7 and heat and electrical consumption by > 3 which, given the homogeneous design and specification of the buildings, suggested that occupant behaviour was important. Site-wide energy consumption and losses (over and above individual dwelling use) reduced energy and carbon emissions performance by 85% and 70% respectively. Although energy consumption in UK homes can be substantially reduced, the challenge for zero-carbon design is to address issues of total system performance, occupant behaviour and whole-building energy performance.

We examine the structure of the hydrated Ti(iv) complex under both ambient and supercritical conditions using first-principles molecular dynamics. We find that an unanticipated fivefold coordination of Ti(iv) is favoured under ambient conditions, with rapid interconversions between square pyramidal and trigonal bipyramidal structures. At supercritical conditions the Ti coordination increases from five to six, adopting both octahedral and trigonal prismatic geometries. At 1000 K, the magnitude of the increase in the Ti to oxygen coordination number with increasing water density is similar to that of Li-O under comparable conditions. We present a detailed picture of the bonding in the hydrated Ti(iv) complex under both ambient and supercritical conditions. © the Owner Societies.

Substitutional clusters of multiple light element dopants are a promising route to the elusive shallow donor in diamond. To understand the behaviour of co-dopants, this report presents an extensive first principles study of possible clusters of boron and nitrogen. We use periodic hybrid density functional calculations to predict the geometry, stability and electronic excitation energies of a range of clusters containing up to five N and/or B atoms. Excitation energies from hybrid calculations are compared to those from the empirical marker method, and are in good agreement. When a boron-rich or nitrogen-rich cluster consists of 3 - 5 atoms, the minority dopant element - a nitrogen or boron atom respectively - can be in either a central or peripheral position. We find B-rich clusters are most stable when N sits centrally, whereas N-rich clusters are most stable with B in a peripheral position. In the former case, excitation energies mimic those of the single boron acceptor, while the latter produce deep levels in the band-gap. Implications for probable clusters that would arise in high-pressure high-temperature (HPHT) co-doped diamond and their properties are discussed.

The UK Government's Building a Greener Future: Policy Statement (2007) announced that all new homes must be zero carbon from 2016. To date, a number of housing sites around the UK have strived to reduce carbon emissions by following sustainable design principles and utilizing renewable technologies. On paper, these sites exceed regulatory compliance and are regarded as high-performance buildings. However, their actual performance is seldom validated from the perspective of either the design engineer or the occupants. Findings are presented from an on-going post-occupancy evaluation of a UK EcoHomes site with an ‘excellent’ rating (the highest rating of the predecessor to the current standard, the Code for Sustainable Homes). The detailed post-occupancy evaluation investigated the energy performance of the buildings (as well as water consumption) and the comfort and satisfaction of users. A bespoke behavioural survey and interview were developed and implemented to distinguish between and quantify frugal an...

A simple 1-dimensional Monte Carlo (MC) model has been developed to simulate the chemical vapour deposition (CVD) of a diamond (100) surface. The model considers adsorption, etching/desorption, lattice incorporation, and surface migration along and across the dimer rows. The reaction probabilities are taken from experimental or theoretical literature values for standard CVD diamond conditions. Renucleation events believed to be due to reactive adsorbates, such as C atoms or CN groups, were modelled by creating random surface defects which form a critical nucleus upon which to nucleate a new layer. By assuming that migrating C species that encounter these surface defects add to the growing surface by templating either from the underlying layer or from the surface defect, and with suitable colour coding, the evolution of different grains has been modelled. On increasing the probability of creating surface defects, the number of grains increases while their mean size decreases. This simulates the film morphology changing from that of single crystal diamond to microcrystalline, nanocrystalline and finally ultrananocrystalline diamond. With the formation of such defects which can act as renucleation points turned off, but using random seed crystals, the films develop a columnar structure similar to that observed for heteroepitaxial microcrystalline diamond.

The preferential incorporation of High-Field-Strength Elements (HFSE) in rutile (TiO 2 ), combined with its supposed stability in subduction zone settings, make it an ideal candidate to explain the low HFSE concentrations in subduction-derived magmas. The solubility behaviour of rutile is key to these arguments, but at present experimental and field-based evidence are contradictory. We have used ab initio molecular (meta)dynamics to investigate the coordination environment of Ti(IV) in pure water at 300 and 1000 K and densities ranging from 900–1260 kg m −3 (approximate pressures 0.9–3.6 GPa). In all high temperature simulations, the long-range structure of the solvent indicates a breakdown of the hydrogen bonding network as expected for supercritical water. The five-fold coordination of titanium to water is energetically most favourable in aqueous fluids at room temperature and pressure, separated from four and six-fold configurations by ∼175 and ∼200 kJ mol −1 , respectively. The average first shell Ti–O distance is 2.00 A, in excellent agreement with bond lengths obtained from experiments. At similar densities and 1000 K, titanium is on average six-fold coordinate with water, and shows some degree of water dissociation in the first hydration shell. This coordination environment is remarkably persistent with increasing density from 1021 to 1260 kg m −3 at constant temperature (1000 K). At lower densities, however, (900 kg m −3 at 1000 K), the coordination with first shell water molecules is less than five. The observed coordination changes could promote association of titanium with peralkaline or peraluminous domains in the aqueous fluid, and thereby explain field-and laboratory based evidence of enhanced HFSE concentrations. This study demonstrates that ab initio molecular dynamics has considerable potential to access details of element behaviour in aqueous fluids at geologically relevant conditions that are impossible to examine otherwise. Changes in the solvent structure due to density variations lead to differences in solvent behaviour allowing access to new domains for fluid–solid interaction. Moreover, changes in the solvent structure are strongly linked to the effectiveness of element solvation.

Momentum-space descriptors are used to construct compact multiple linear regression models for the toxicity to Tetrahymena pyriformis of two sets of molecules examined previously (A.G. Saliner, X. Girones, Theochem 727 (2005) 97). The resulting few-descriptor models for 30 aliphatic molecules and for 48 anilines compare favorably with the earlier study of these two sets of molecules. Furthermore, models based on a very small number of trivial feature count descriptors (numbers of atoms of a given type and the number of bonds) are found to be of practically the same statistical quality as those that require quantum mechanical calculations.

Triamantane was isolated from petroleum, per-fluorinated and the vibrational behaviour investigated by experimental and theoretical methods. Close agreement was found between the experimental and calculated Raman spectrum. The surface vibrational modes are shifted to lower wavenumbers relative to hydrogenated triamantane, due to the increased mass of the surface groups. The Raman spectrum of the fluorinated molecule more closely resembles that of pure diamond compared with hydrogenated triamantane. The absence of a peak at 1150 cm 1 for fluorinated triamantane suggests that this peak, often seen in the spectrum of nanocrystalline diamond, cannot be attributed to vibrations of diamond nanocrystals.

Atomistic simulations have been carried out to investigate the mechanisms of noble gas incorporation in minerals using both the traditional two-region approach and the “supercell” method. The traditional two-region approach has been used to calculate defect energies for Ne, Ar, Kr and Xe incorporation in MgO, CaO, diopside and forsterite in the static limit and at one atmosphere pressure. The possibilities of noble gas incorporation via both substitution and interstitial mechanisms are studied. The favored mechanism varies from mineral to mineral and from noble gas to noble gas. In all minerals studied, the variation of the solution energies of noble gas substitution with atomic radius appears approximately parabolic, analogous to those for 1 + , 2 + , 3 + and 4 + trace element incorporation on crystal lattice sites. Noble gas solution energies thus also fall on a curve, similar to those previously observed for cations with different charges, but with much lower curvature. The “supercell” method has been used to investigate the pressure dependence of noble gas incorporation in the same systems. Results indicate a large variation of the solubility of the larger noble gases, Kr and Xe with pressure. In addition, explicit simulation of incorporation at the (0 0 1) surface of MgO shows that the solubility of the heavier noble gases may be considerably enhanced by the presence of interfaces.