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In this study we investigate, using all-atom molecular-dynamics computer simulations, the in-plane diffusion of a doxorubicin drug molecule in a thin film of water confined between two silica surfaces. We find that the molecule diffuses along the channel in the manner of a Gaussian diffusion process, but with parameters that vary according to its varying transversal position. Our analysis identifies that four Gaussians, each describing particle motion in a given transversal region, are needed to adequately describe the data. Each of these processes by itself evolves with time at a rate slower than that associated with classical Brownian motion due to a predominance of anticorrelated displacements. Long adsorption events lead to ageing, a property observed when the diffusion is intermittently hindered for periods of time with an average duration which is theoretically infinite. This study presents a simple system in which many interesting features of anomalous diffusion can be explored. It exposes the complexity of diffusion in nanoconfinement and highlights the need to develop new understanding.

Interface diffusion along a metal/ceramic interface present in numerous energy and electronic devices can critically affect their performance and stability. Hole formation in a polycrystalline Ni film on an α-Al2O3 substrate coupled with a continuum diffusion analysis demonstrates that Ni diffusion along the Ni/α-Al2O3 interface is surprisingly fast. Ab initio calculations demonstrate that both Ni vacancy formation and migration energies at the coherent Ni/α-Al2O3 interface are much smaller than in bulk Ni, suggesting that the activation energy for diffusion along coherent Ni/α-Al2O3 interfaces is comparable to that along (incoherent/high angle) grain boundaries. Based on these results, we develop a simple model for diffusion along metal/ceramic interfaces, apply it to a wide range of metal/ceramic systems and validate it with several ab initio calculations. These results suggest that fast metal diffusion along metal/ceramic interfaces should be common, but is not universal., Little is known about diffusion along metal/ceramic interfaces even though it controls the physical behavior and lifetimes of many devices (including batteries, microelectronics, and jet engines). Here, the authors show that diffusion along a nickel/sapphire interface is abnormally fast due to nickel vacancies and generalise their findings to a wide-range of metal/ceramic systems.

We calculate the stability of the MAX-phase materials ${\mathrm{Zr}}_{n+1}{\mathrm{AlC}}_{n}$ and ${\mathrm{Ti}}_{n+1}{\mathrm{AlC}}_{n}$ in the context of the M-A-X ternary phase diagrams and competing binary and ternary compounds, as a function of temperature, by applying density functional theory (DFT) within the quasiharmonic approximation. By examining the convex hull of free energy we find that the Zr-based MAX phases are thermodynamically unstable at room temperature and below with respect to decomposition to carbide and intermetallics, although with increasing temperature the ${\mathrm{Zr}}_{3}{\mathrm{AlC}}_{2}$ phase becomes stable. On the other hand, the pure ${\mathrm{Ti}}_{2}\mathrm{AlC}$ phase is thermodynamically stable at room temperature, consistent with the success in its synthesis.

Vacancies in iron are hydrogen traps, important in the understanding of hydrogen embrittlement of steel. We present a grand canonical approach to computing the trap occupancy as a function of both temperature and hydrogen concentration from practically zero to super-saturation. Our method couples a purpose-made machine-learned H-Fe potential, which enables rapid sampling with near density functional theory accuracy, with a statistical mechanical calculation of the trap occu- pancy using the technique of nested sampling. In contrast to the conventional assumption (based on Oriani theory) that at industrially relevant hydrogen concentrations and ambient conditions vacancy traps are are fully occupied, we find that vacancy traps are less than fully occupied under these conditions, necessitating a reevaluation of how we think about “mobile hydrogen” in iron and steel.

Enhanced ionic conductivity observed in a heteroepitaxial multilayer of yttria-stabilised zirconia and (YSZ) and strontium titanate (STO) has variously been attributed to lattice dilation or a disordered oxygen sublattice, leading to high interfacial mobility of anions, as compared to those of the constituent bulk oxides. We seek to understand the mechanism of ionic motion in such heterostructures by first simulating the atomic structure assuming coherent interfaces. After investigating possible low-energy interface structures using a genetic algorithm, we perform molecular dynamics simulations on these structures to examine the anionic diffusivity in the system. We find that the extreme biaxial tensile strain in the YSZ layer, as imposed between layers of STO, induces phases that differ from fluorite. The lowest energy structure is an unknown phase, which we refer to as quasi-cubic and whose cation sublattice resembles an extension of the perovskite; this structure does not lead to enhanced ionic conductivity, in contradiction to some reports in the literature.

We propose an approach to calculate the anharmonic part of the volumetric-strain and temperature-dependent free energy of a crystal. The method strikes an effective balance between accuracy and computational efficiency, showing a $\ifmmode\times\else\texttimes\fi{}10$ speedup on comparable free energy approaches at the level of density functional theory, with average errors less than 1 meV/atom. As a demonstration we make predictions on the thermodynamics of substoichiometric ${\mathrm{ZrC}}_{x}$, including vacancy concentration and heat capacity.

The electronic (band) structure of polycrystalline Al2O3, in particular the density of near-band edge grain-boundary localized states, plays a significant role in a host of high-temperature phenomena, including sintering, high-temperature creep, oxygen permeability in dense “dry” Al2O3 ceramics, and Al2O3 scale formation on Al2O3 scale-forming alloys. All these phenomena involve creation or annihilation of charged point defects (vacancies and/or interstitials) at grain boundaries and interfaces, and must of necessity involve electrons and holes. Thus, the density of states associated with grain boundaries in Al2O3 assume great importance, and has been calculated using DFT for both nominally undoped and Y-doped Σ7 bi-crystal boundaries. These quantum mechanical calculations must be taken into account when considering why Y2O3 segregation to Al2O3 grain boundaries is so effective in enhancing high-temperature creep resistance of polycrystalline Al2O3, and in understanding the reactive element effect in Al2O3 scale-forming alloys. Finally, a case will be made that grain-boundary diffusion is mediated by the migration of a class of grain-boundary ledge defects called disconnections, which are characterized by a step height h and a Burgers vector b.

We report molecular dynamics simulations and their analysis for a mixed tilt and twist grain boundary vicinal to the $\mathrm{\ensuremath{\Sigma}}7$ symmetric tilt boundary of the type ${1\phantom{\rule{0.16em}{0ex}}2\phantom{\rule{0.16em}{0ex}}3}$ in aluminum. When minimized in energy at $0\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, a grain boundary of this type exhibits nanofacets that contain kinks. We observe that at higher temperatures of migration simulations, given extended annealing times, it is energetically favorable for these nanofacets to coalesce into a large terrace-facet structure. Therefore, we initiate the simulations from such a structure and study as a function of applied driving force and temperature how the boundary migrates. We find the migration of a faceted boundary can be described in terms of the flow of steps. The migration is dominated at lower driving force by the collective motion of the steps incorporated in the facet, and at higher driving forces by the step detachment from the terrace-facet junction and propagation of steps across the terraces. The velocity of steps on terraces is faster than their velocity when incorporated in the facet, and very much faster than the velocity of the facet profile itself, which is almost stationary. A simple kinetic Monte Carlo model matches the broad kinematic features revealed by the molecular dynamics. Since the mechanisms seem likely to be very general on kinked grain-boundary planes, the step-flow description is a promising approach to more quantitative modeling of general grain boundaries.

Under the sponsorship of the U.S. National Science Foundation, a workshop on emerging research opportunities in ceramic and glass science was held in September 2016. Reported here are proceedings of the workshop. The report details eight challenges identified through workshop discussions: Ceramic processing: Programmable design and assembly; The defect genome: Understanding, characterizing, and predicting defects across time and length scales; Functionalizing defects for unprecedented properties; Ceramic flatlands: Defining structure-property relations in free-standing, supported, and confined two-dimensional ceramics; Ceramics in the extreme: Discovery and design strategies; Ceramics in the extreme: Behavior of multimaterial systems; Understanding and exploiting glasses and melts under extreme conditions; and Rational design of functional glasses guided by predictive modeling. It is anticipated that these challenges, once met, will promote basic understanding and ultimately enable advancements within multiple sectors, including energy, environment, manufacturing, security, and health care., National Science Foundation (U.S.) (Award DMR-1619666)

A model for ionic and electronic grain boundary transport through thin films, scales or membranes with columnar grain structure is introduced. The grain structure is idealized as a lattice of identical hexagonal cells – a honeycomb pattern. Reactions with the environment constitute the boundary conditions and drive the transport between the surfaces. Time-dependent simulations solving the Poisson equation self-consistently with the Nernst-Planck flux equations for the mobile species are performed. In the resulting Poisson-Nernst-Planck system of equations, the electrostatic potential is obtained from the Poisson equation in its integral form by summation. The model is used to interpret alumina membrane oxygen permeation experiments, in which different oxygen gas pressures are applied at opposite membrane surfaces and the resulting flux of oxygen molecules through the membrane is measured. Simulation results involving four mobile species, charged aluminum and oxygen vacancies, electrons, and holes, provide a complete description of the measurements and insight into the microscopic processes underpinning the oxygen permeation of the membrane. Most notably, the hypothesized transition between p-type and n-type ionic conductivity of the alumina grain boundaries as a function of the applied oxygen gas pressure is observed in the simulations. The range of validity of a simple analytic model for the oxygen permeation rate, similar to the Wagner theory of metal oxidation, is quantified by comparison to the numeric simulations. The three-dimensional model we develop here is readily adaptable to problems such as transport in a solid state electrode, or corrosion scale growth.

The technique of plasmonic ELISA is utilised here to detect the HIV-1 protein gp120 with the ultralow limit of detection of 8 × 10(-20) M (10(-17) g mL(-1)) in an independent laboratory. It was corroborated that changes in the concentration of hydrogen peroxide as small as 0.05 μM could lead to nanoparticle solutions of completely different tonality.

In order to elucidate the role of this element in superalloy metallurgy, the binding energy of Re-Re pairs and the stability of small Re clusters in the nickel face-centred cubic (fcc) lattice is investigated using ab initio density functional theory. It is shown that the formation of Re-Re nearest neighbour pairs is energetically unfavourable, and that this repulsive energy is dramatically reduced as soon as the solute atoms move further apart from one another. Furthermore, small nearest neighbour and second neighbour Re clusters are found to be unstable. The calculations are repeated for W and Ta, which lie beside Re in the periodic table; the results are essentially the same, except that some Ta-Ta higher order pairs have a positive binding energy, consistent with the Ni-Ta binary phase diagram exhibiting several ordered intermetallics. The predictions show that Re clusters are unstable in fcc Ni and it is unlikely that clustering has a role in improving creep and fatigue properties (the rhenium-effect) in Ni-based superalloys. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Multilayered oxide heteroepitaxial systems, including that of a 1-nm-thick Y2O3-stabilised ZrO2 (YSZ) sandwiched between layers of SrTiO3 (STO) [1], have been a subject of much interest lately due to their significantly enhanced ionic conductivities as compared to the bulk materials. We aim to provide the foundation for understanding this increase in conductivity by considering the atomic configurations at the interfaces of such systems, specifically a ZrO2/STO multilayer system. Possible stable lattice structures of pure ZrO2 in the system are explored using a genetic algorithm in which the interatomic interactions are modelled by simple pair potentials. The energies of several of the more stable of these structures are then evaluated more accurately within density functional theory (DFT). We find that the fluorite ZrO2 phase is unstable as a coherently strained epitaxial layer in the multilayer system. Instead, anatase-, columbite-, rutile-, and pyrite-like ZrO2 epitaxies are found to be more stable, with the anatase-like epitaxy being the most stable structure over a wide range of chemical potential of the components. We also find a high energy metastable structure resembling the tetragonal fluorite structure which is predicted by DFT to be stabilised by SrO-terminated STO but not by TiO2-terminated STO.

Recent years have seen great advances in our ability to predict crystal structures from first principles. However, previous algorithms have focused on the prediction of bulk crystal structures, where the global minimum is the target. Here, we present a general atomistic approach to simulate in multicomponent systems the structures and free energies of grain boundaries and heterophase interfaces with fixed stoichiometric and non-stoichiometric compositions. The approach combines a new genetic algorithm using empirical interatomic potentials to explore the configurational phase space of boundaries, and thereafter refining structures and free energies with first-principles electronic structure methods. We introduce a structural order parameter to bias the genetic algorithm search away from the global minimum (which would be bulk crystal), while not favouring any particular structure types, unless they lower the energy. We demonstrate the power and efficiency of the algorithm by considering non-stoichiometric grain boundaries in a ternary oxide, SrTiO(3).

The defect formation energies of all possible charge states of point defects in TiO2, including titanium interstitials, titanium vacancies and oxygen vacancies, are calculated in the phase space of temperature, oxygen partial pressure and Fermi level by combining density functional theory (DFT) and thermodynamic calculations. The point defect phase diagram illustrates that fully charged defects dominate in most regimes. The calculations not only give reasonable defect formation energies compared with prior experimental measurements, but also predict n-type TiO2 at high T and low P O 2 , and p-type TiO2 at low T and high P O 2 , which agrees well with experimental data. In addition, we evaluate methods for correcting the effects of artificial electrostatic interactions caused by periodic boundary conditions in the DFT calculations, including the electrostatic potential alignment correction (ΔV correction) and the Makov–Payne correction.

A computational approach that integrates ab initio electronic structure and thermodynamic calculations is used to determine point defect stability in rutile TiO 2 over a range of temperatures, oxygen partial pressures and stoichiometries. Both donors (titanium interstitials and oxygen vacancies) and acceptors (titanium vacancies) are predicted to have shallow defect transition levels in the electronic-structure calculations. The resulting defect formation energies for all possible charge states are then used in thermodynamic calculations to predict the influence of temperature and oxygen partial pressure on the relative stabilities of the point defects. Their ordering is found to be the same as temperature increases and oxygen partial pressure decreases: titanium vacancy → oxygen vacancy → titanium interstitial. The charges on these defects, however, are quite sensitive to the Fermi level. Finally, the combined formation energies of point defect complexes, including Schottky, Frenkel and anti-Frenkel defects, are predicted to limit the further formation of point defects.

Computer simulation of the core structure and glide of ordinary screw dislocations in single-phase L10 TiAl and in two lamellae forming a twin γ/γ-interface has been performed using recently constructed Bond-Order Potentials (BOPs). BOPs represent a semi-empirical, numerically efficient scheme that works within the orthogonal tight-binding approximation and is able to capture the directionality of bonding. We have studied dislocation glide in perfect L10 TiAl and along a twin interface, transmission of an ordinary screw dislocation between lamellae, and the core structure, mobility and detachment of an interfacial screw dislocation from a twin boundary under applied shear stresses in directions parallel and perpendicular to a (111) plane. Our results show that the glide of ordinary straight screw dislocations under applied stresses in L10 TiAl is characterized by zigzag movement on two conjugated {111} planes. The non-planar core of the screw dislocation is distorted asymmetrically when the elastic centre...

Applying simulated annealing with a classical potential followed by screening of low-energy structures with density functional theory, we examined the atomic and electronic structures of the Σ 7 4 5 ¯ 1 0 [ 0 0 0 1 ] and Σ 7 2 3 ¯ 1 0 [ 0 0 0 1 ] symmetric tilt grain boundaries in α -Al2O3. The lowest energy Σ 7 4 5 ¯ 1 0 [ 0 0 0 1 ] boundary exhibits a pronounced pattern of alternating columns of exclusively four- or fivefold coordinated Al atoms, with a grain boundary energy of 1.84 Jm−2. For the Σ 7 2 3 ¯ 1 0 [ 0 0 0 1 ] boundary, numerous structures were found with energy just below 2.11 Jm−2. Furthermore, by analysing the full set of candidate structures generated by simulated annealing for the two grain boundaries, we find that the number of fivefold coordinated Al atoms tends to increase with grain boundary energy, which we can also correlate with the behaviour of the electronic density of states. On the other hand, we find no systematic trend with energy that might be expected for other quantities, notably the excess volume of the interface. We compare simulated high-resolution transmission electron microscope (HRTEM) images of the lowest energy calculated structures with experimental images. The disparate structural and electronic features of these two boundaries suggest reasons for their very different oxygen diffusion coefficients that have been observed experimentally.

Combining concepts of semiconductor physics and corrosion science, we develop a novel approach that allows us to perform ab initio calculations under controlled potentiostat conditions for electrochemical systems. The proposed approach can be straightforwardly applied in standard density functional theory codes. To demonstrate the performance and the opportunities opened by this approach, we study the chemical reactions that take place during initial corrosion at the water-Mg interface under anodic polarization. Based on this insight, we derive an atomistic model that explains the origin of the anodic hydrogen evolution.

We have studied the (110) surface of NiAl, an ordered alloy of B2 structure, using a plane-wave pseudopotential method. The clean surface and several oxidized surfaces were investigated, with oxygen coverages up to 1.5 ML (1 ML = 1 O-atom per surface metal atom). In order to compare the energies of the oxidized structures, which comprise different numbers of metal and oxygen atoms, one has to take account of the chemical potentials of the Ni, Al and O. To this end, for each system, we have applied simple analytic models to study their surface energy as a function of temperature, alloy stoichiometry (assumed to be near 50–50) and oxygen partial pressure. The calculations predict how, at oxygen pressures just above the threshold for decomposing NiAl, the clean surface should be coated by an alumina layer, with the consequent depletion of Ni near the surface. Varying stoichiometry has the relatively minor effect of shifting the crossovers in oxygen pressure at which different oxidized surfaces become stable.

In an earlier report we explored structural correlations at a liquid-solid interface with molecular dynamics simulations of a model aluminium system using the Ercolessi-Adams potential and up to 4320 atoms. Substrate atoms were pinned to their equilibrium fcc crystalline positions while liquid atoms were free to move. A direct correlation between the amount of ordering in the liquid phase and the underlying substrate orientation was found. In the present paper we extend this study to the case of a fixed bcc substrate in contact with liquid aluminium. We find surprisingly similar results for the density profiles of both (100) and (110) substrates. However, there is a far greater in-plane ordering in the (100) than for the (110) system. For the (100) substrates we observe adsorption of liquid atoms into the terminating plane of the bcc (100) substrate, effectively transforming the bcc (100) plane into an fcc (100) plane.

We present a force field for bulk alumina (Al2O3), which has been parametrized by fitting the energies, forces, and stresses of a large database of reference configurations to those calculated with density functional theory (DFT). We use a functional form that is simpler and computationally more efficient than some existing models of alumina parametrized by a similar technique. Nevertheless, we demonstrate an accuracy of our potential that is comparable to those existing models and to DFT. We present calculations of crystal structures and energies, elastic constants, phonon spectra, thermal expansion, and point defect formation energies.

The electron energy-loss near-edge structure ~ELNES! at the oxygen K-edge has been investigated in a range of yttria-stabilized zirconia ~YSZ! materials. The electronic structure of the three polymorphs of pure ZrO2 and of the doped YSZ structure close to the 33 mol %Y2O3 composition have been calculated using a full-potential linear muffin-tin orbital method ~NFP-LMTO! as well as a pseudopotential based technique. Calculations of the ELNES dipole transition matrix elements in the framework of the NFP-LMTO scheme and inclusion of core hole screening within Slater’s transition state theory enable the ELNES to be computed. Good agreement between the experimental and calculated ELNES is obtained for pure monoclinic ZrO2. The agreement is less good with the ideal tetragonal and cubic structures. This is because the inclusion of defects is essential in the calculation of the YSZ ELNES. If the model used contains ordered defects such as vacancies and metal Y planes, agreement between the calculated and experimental O K-edges is significantly improved. The calculations show how the five different O environments of Zr 2Y2O7 are connected with the features observed in the experimental spectra and demonstrate clearly the power of using ELNES to probe the stabilization mechanism in doped metal oxides.

Gallium impurities affect the atomic processes and material properties of aluminum metal to a high degree. Various ab initio calculations have been performed on a Sigma = 11 (113) symmetric tilt boundary in aluminum with and without some gallium substitutions. A simple interpretation of the results emerges, which can be applied to grain boundaries in general. The calculations relate to the energetics of gallium substitution on various sites, local relaxation effects, vibrational frequencies and a barrier to grain boundary migration. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.

We derive a formula, useful for first-principles calculations, which relates the free energy of an oxide/metal interface to the free energies of surfaces and the work of separation of the interface. We distinguish the latter mechanical quantity from the thermodynamic work of adhesion, and we describe explicitly how both may be calculated. Our formulas for interfacial and surface energies are cast in terms of quantities which can be calculated or looked up in tables, and include as additional parameters the ambient temperature and partial pressure of oxygen ${P}_{{\mathrm{O}}_{2}}.$ From total-energy calculations for the $\mathrm{Nb}(111)/\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ (0001) interface, free Nb and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ surfaces, we obtain firstly numerical estimates of the works of separation, which are independent of ${P}_{{\mathrm{O}}_{2}}.$ We then obtain surface energies, interfacial energies, and the equilibrium work of adhesion as a function of ${P}_{{\mathrm{O}}_{2}}.$

We predict the existence of an isostructural first-order phase transition in high-pressure Al-rich NiAl alloys, between vacancy-rich and antisite-rich phases. A critical point terminates the region of phase coexistence. The driving force for this phase transition is the positive defect mixing enthalpy originating from elastic interaction between vacancies and antisites on the Ni sublattice. We highlight the similarity between the critical behavior of Al-rich alloys and that of binary liquid mixtures. Analogous isostructural phase transitions are expected in other compounds with constitutional defects.

Understanding the growth of Al2O3 scales requires knowledge of the details of the chemical reactions at the scale–gas and scale–metal interfaces, which in turn requires specifying how the creation/annihilation of O and Al vacancies occurs at these interfaces. The availability of the necessary electrons and holes to allow for such creation/annihilation is a crucial aspect of the scaling reaction. The electronic band structure of polycrystalline Al2O3 thus plays a decisive role in scale formation and is considered in detail, including the implications of a density functional theory (DFT) calculation of the band structure of a Σ7 { 4 5 ¯ 1 0 } bicrystal boundary, for which the atomic structure of the boundary was known from an independent DFT energy-minimization calculation and comparisons with an atomic-resolution transmission electron micrograph of the same boundary. DFT calculations of the formation energy of O and Al vacancies in bulk Al2O3 in various charge states as a function of the Fermi energy suggested that electronic conduction in Al2O3 scales most likely involves excitation of both electrons and holes, which are localized on singly charged O vacancies, V O and doubly charged Al vacancies, V Al ″ , respectively. We also consider the variation of the Fermi level across the scale and bending (“tilting”) of the conduction band minimum and valence band maximum due to the electric field developed during the scaling reaction. The band structure calculations suggest a new mechanism for the “reactive element” effect—a consequence of segregation of Y, Hf, etc., to grain boundaries in Al2O3 scales, which results in improved oxidation resistance—namely, that the effect is due to the modification of the near-band edge grain-boundary defect states rather than any blocking of diffusion pathways, as previously postulated. Secondly, Al2O3 scale formation is dominated by grain boundary as opposed to lattice diffusion, and there is unambiguous evidence for both O and Al countercurrent transport in Al2O3 scale-forming alloys. We postulate that such transport is mediated by migration of grain boundary disconnections containing charged jogs, rather than by jumping of isolated point defects in random high-angle grain boundaries.

A simple d-p tight binding model is employed to study the TiC(111) surface. We show that it reproduces the observed titanium terminated TiC(111) surface relaxation and we demonstrate that the observed Ti surface termination is a consequence of the difference in the evaporation rates for carbon and titanium atoms at the surface. The carbon atoms evaporate much faster than the titanium atoms.

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Atom probe tomography (APT) is used to characterise the distributions of rhenium in a binary Ni-Re alloy and the nickel-based single-crystal CMSX-4 superalloy. A purpose-built algorithm is developed to quantify the size distribution of solute clusters, and applied to the APT datasets to critique the hypothesis that rhenium is prone to the formation of clusters in these systems. No evidence is found to indicate that rhenium forms solute clusters above the level expected from random fluctuations. In CMSX-4, enrichment of Re is detected in the matrix phase close to the matrix/precipitate (γ/γ′) phase boundaries. Phase field modelling indicates that this is due to the migration of the γ/γ′ interface during cooling from the temperature of operation. Thus, neither clustering of rhenium nor interface enrichments can be the cause of the enhancement in high temperature mechanical properties conferred by rhenium alloying. © 2009 Acta Materialia Inc.

We address the calculation within density functional theory (DFT) of defect formation energies in alumina, a ceramic oxide often considered an archetype for a wide variety of other similar oxides. We examine the conditions under which calculated defect formation energies, especially those of charged defects, are independent of the principal approximations of the plane-wave DFT formalism, most significant of which is the finite-sized supercell in which the calculation must be performed. We introduce a variation on existing methods of extrapolation to infinite system size to reduce dependence of the result on finite-size errors in the electrostatic and elastic energies of a periodic supercell containing a defect. We also show how the results can be made relatively insensitive to the choice of exchange-correlation functional and pseudopotential by a suitable treatment of the chemical potentials of the atomic species. Our results for formation energies of charged defects are less sensitive than traditional approaches to supercell size and choices of exchange-correlation functional and pseudopotential, and differ notably from previous results.

The technological importance of polycrystalline strontium titanate $({\text{SrTiO}}_{3})$ is directly linked to its interfacial and grain boundary properties, which are at present poorly understood. A complete understanding (including links with experiment) requires information from many length scales, including electronic and atomistic, up to microstructural and macroscopic. In addition, the size and complexity of many general grain boundaries makes first-principles simulations prohibitively expensive. We have tested the ability of a number of interatomic potentials from the literature to accurately describe at least the structures of some simple grain boundaries in ${\text{SrTiO}}_{3}$. The potentials we have tested are of three types: rigid ion model with either fixed formal or partial charges and shell model. We have also performed a detailed density functional theory (DFT) study of the same boundaries and used this data (interface structures and energies) to validate the interatomic potentials. Our conclusion is that none of the potentials can reproduce the energy ordering of the boundaries predicted by the DFT calculations. The boundary structures produced by some of the potentials do however agree reasonably well with the DFT structures. We discuss the implications of our findings for ionic oxide grain boundary research and critically examine the rigid ion and shell model approximations.

The surface structure of magnetite ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ in contact with oxygen and water is investigated using spin-density functional theory plus on-site Coulomb interactions. The present results unravel apparent contradictions in the experimental data regarding the equilibrium stoichiometry of the bare surface termination. Both for 298 and $1200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the equilibrium structure is terminated by $\frac{1}{4}$ monolayer (ML) of iron (Fe) on top of a full oxygen layer, consistent with an earlier low-energy electron diffraction analysis. Nonetheless, the calculated negative slope of the surface energies vs oxygen partial pressure shows that a $\frac{1}{2}$ ML Fe termination would become stable under oxygen-poor conditions at high temperatures, in agreement to interpretation of scanning tunneling microscopy experiments. Initial water adsorption is dissociative and saturates when all Fe sites are occupied by OH groups, while the H atoms bind to surface oxygen. Further, water bridges the OH and H groups resulting in a unique type of H-bonded molecular water with its oxygen forming a hydronium-ion-like structure $\mathrm{O}{\mathrm{H}}_{3}^{+}\text{\ensuremath{-}}\mathrm{O}\mathrm{H}$. This water structure is different from the water dimeric structures found as yet on oxide and metal surfaces for partially dissociated $({\mathrm{H}}_{2}\mathrm{O}\text{\ensuremath{-}}\mathrm{O}\mathrm{H}\text{\ensuremath{-}}\mathrm{H})$ overlayers.

Density functional theory calculations have been used to study relaxed interstitial configurations in FeCr alloys. The ionic and electronic ground states of 69 interstitial structures have been determined. Interstitials were placed in alloys with up to 14 at. % Cr. Cr atoms were either monatomically dispersed or clustered together within a periodically repeated supercell consisting of 4x4x4 cubes of bcc unit cells. The distance between the interstitials and Cr atoms was varied within the supercells. It is shown that Cr atoms beyond third-nearest-neighbor distance from the interstitial can still have an interaction with it of up to 0.9 eV. The multibody nature of the Cr-Cr interactions causes the Cr-interstitial interaction to be strongly concentration dependent. The Cr-Cr interaction in defect-free alloys is also dependent on the overall Cr concentration. The effective Cr-Cr repulsion is weaker in alloys than in an environment of pure Fe. Apart from the Cr concentration, the Cr-interstitial interaction also depends on the dispersion level of Cr atoms beyond third-nearest-neighbor distance from the interstitial. The formation energy differences between dumbbell interstitials with different orientations are independent of the Cr concentration. We show that the long-range influence of Cr atoms on the interstitial is not due to the interstitial strain field protruding into Cr-rich parts of the supercells. The Fermi-level and band energies were found not to be the sole governing parameter in determining the formation energies. Implications for the construction of empirical potentials are discussed.

TiO2 has been intensively studied as a wide band-gap transition metal oxide partially due to the multi-valence nature of its cation. Here, density-functional theory calculations within the supercell approximation are carried out to determine the preferred charge state of point defects in rutile TiO2. The first component of this work is to investigate the dependence of the defect formation energies on supercell size and the electrostatic Makov-Payne correction. The results show that the Makov-Payne correction improves the convergence of defect formation energies as a function of supercell size for positively charged titanium interstitials and negatively charged titanium vacancies. However, in the case of positively charged oxygen vacancies, applying the Makov-Payne correction gives the wrong sign for the defect formation energy correction. This is attributed to the shallow nature of the transition levels for this defect in TiO2. Finally, we combine the calculated defect formation energies with thermodynamic data to evaluate the influence of temperature on the relative stabilities of point defects. The results indicate that when the Makov- Payne correction is applied, a stable charge transition occurs for titanium interstitials. In addition, as the temperature increases, the dominant point defect in TiO2 changes from oxygen vacancies to titanium interstitials.

Density-functional theory calculations, within the plane-wave-ultrasoft pseudopotential framework, were performed in the ⟨110⟩ projection for MgO and for the coherent {111} $\mathrm{Ag}\ensuremath{-}\mathrm{Mg}\mathrm{O}$ polar interface. First-principles calculations were incorporated in high-resolution transmission electron microscopy (HRTEM) simulations by converting the charge density into electron scattering factors to examine the influence of charge transfer, charge redistribution at the interface, and ionicity on the dynamical electron scattering and on calculated HRTEM images. It is concluded that the ionicity of oxides and the charge redistribution at interfaces play a significant role in HRTEM image simulations. In particular, the calculations show that at oxygen-terminated {111} $\mathrm{Ag}\mathrm{Mg}\mathrm{O}$ interfaces the first oxygen layer at the interface is much brighter than that in calculations with neutral atoms, in agreement with experimental observations.

s131d and s231d reconstructions of the (001) SrTiO3 surface were studied using the first-principles fullpotential linear muffin-tin orbital method. Surface energies were calculated as a function of TiO2 chemical potential, oxygen partial pressure and temperature. The s131d unreconstructed surfaces were found to be energetically stable for many of the conditions considered. Under conditions of very low oxygen partial pressure the s231d Ti2O3 reconstruction [Martin R. Castell, Surf. Sci. 505, 1 (2002)] is stable. The question as to why STM images of the s131d surfaces have not been obtained was addressed by calculating charge densities for each surface. These suggest that the s231d reconstructions would be easier to image than the s131d surfaces. The possibility that the presence of oxygen vacancies would destabilise the s131d surfaces was also investigated. If the s131d surfaces are unstable then there exists the further possibility that the s2 31d DL-TiO2 reconstruction [Natasha Erdman et al. Nature (London) 419, 55 (2002)] is stable in a TiO2-rich environment and for pO2 .10−18 atm.

The electron energy-loss near-edge structure ~ELNES! at the O K edge has been studied in yttria-stabilized zirconia ~YSZ!. The electronic structure of YSZ for compositions between 3 and 15 mol % Y2O3 has been computed using a pseudopotential-based technique to calculate the local relaxations near the O vacancies. The results showed phase transition from the tetragonal to cubic YSZ at 10 mol % of Y2O3, reproducing experimental observations. Using the relaxed defect geometry, calculation of the ELNES was carried out using the full-potential linear muffin-tin orbital method. The results show very good agreement with the experimental O K-edge signal, demonstrating the power of using ELNES to probe the stabilization mechanism in doped metal oxides.

The microscopic mechanism leading to stabilization of cubic and tetragonal forms of zirconia (ZrO$_2$) is analyzed by means of a self-consistent tight-binding model. Using this model, energies and structures of zirconia containing different vacancy concentrations are calculated, equivalent in concentration to the charge compensating vacancies associated with dissolved yttria (Y$_2$O$_3$) in the tetragonal and cubic phase fields (3.2 and 14.4% mol respectively). The model is shown to predict the large relaxations around an oxygen vacancy, and the clustering of vacancies along the $$ directions, in good agreement with experiments and first principles calculations. The vacancies alone are shown to explain the stabilization of cubic zirconia, and the mechanism is analyzed., 19 pages, 6 figures. To be published in J. Am. Ceram. Soc

The high-temperature cubic-tetragonal phase transition of pure stoichiometric zirconia is studied by molecular dynamics (MD) simulations and within the framework of the Landau theory of phase transformations. The interatomic forces are calculated using an empirical, self-consistent, orthogonal tight-binding (SC-TB) model, which includes atomic polarizabilities up to the quadrupolar level. A first set of standard MD calculations shows that, on increasing temperature, one particular vibrational frequency softens. The temperature evolution of the free energy surfaces around the phase transition is then studied with a second set of calculations. These combine the thermodynamic integration technique with constrained MD simulations. The results seem to support the thesis of a second-order phase transition but with unusual, very anharmonic behaviour above the transition temperature.

The electron energy-loss near-edge structure (ELNES) and x-ray absorption near-edge structure (XANES) at the oxygen K-edge has been investigated in a range of yttria-stabilised zirconia (YSZ) materials. Analysis of near-edge structure reveals that both the crystallographic phase and the metal fraction of yttrium present can be determined directly from the oxygen K-edge data. Simulation of the ELNES/XANES was achieved using a pseudopotential based method to obtain the relaxed atomic coordinates combined with full-potential LMTO method to calculate the electronic structure.

Selective oxidation of the surface of an ordered alloy requires redistribution of the atomic species in the vicinity of the surface. This process can be understood in terms of the formation and movements of point defects in the compound. On the basis of ab initio density-functional calculation we found both the creation of exchange defects near the NiAl surface and segregation of Ni vacancies to the top layer to be extremely favorable in the presence of oxygen. Scenarios for the initial oxidation of NiAl are suggested which demonstrate the appearance of an additional energy barrier on the Ni-rich side compared to the Al-rich side. The expulsion of Ni from the oxide layer as it forms is the driving force for its stability.

Tetragonal zirconia, (t′–ZrO2) a ferroelastic material, readily forms domains with domain boundaries on {011}. For example, by compressing a single crystal along [100] the formation and movement of such domain walls has been demonstrated experimentally. We have made atomistic simulations of a domain wall with a self-consistent tight-binding model which cor- rectly reproduces both the high temperature tetragonal to cubic phase transition exhibited by zirconia, and its low temperature monoclinic phase. We analyse the results of our simulation, in particular the width of the domain wall, in terms of a Landau–Ginzburg theory in which the order parameter measures the degree of tetragonality of the lattice

We investigate the effect of elevated pressures on the point defect thermodynamics in NiAl alloys. A particular motivation for this study is due to the expected elimination of structural vacancies on the Al-rich side at high pressure. We employ the density functional theory to compute point defect energies as a function of pressure, which are in turn used as input to the Wagner-Schottky model. We find that at about 200 kbar a change in the constitutional defect from V Ni to Al Ni does take place. The extension of the Wagner-Schottky model by introducing elastic interactions between defects leads to the prediction of a qualitatively new phenomenon in the system, namely the appearance of an isostructural phase transition terminated at a critical point. Similar behaviour is expected in some other ordered off-stoichiometric compounds.