ASC Image

Atomistic Simulation Centre (ASC)

Modelling Matter at the Atomic Scale

School of Mathematics and Physics, Queen's University Belfast

QUB Image
Home | Research Themes | Computer Packages Members | Opportunities | Publications | Preprints | Contact | Internal

Recent Publications


Professors
Jorge Kohanoff

Readers
Tchavdar Todorov

Lecturers
Daniel Dundas
Myrta Gruening
Meilan Huang
Ian Lane
Lorenzo Stella
Gareth Tribello

Research Fellows
Malachy Montgomery
Asako Terasawa
Carles Triguero de Prades

Postgraduate Students
Brian Cunningham
Alejandro de la Calle
Michael Ferguson
Emmet McBride
Ryan McMillan
Valerio Rizzi
Terence Sheppard
Dawn Thompson
Abigail Wardlow

| 2014 | 2013 | 2012 | 2011 | 2010 | 2009 | 2008 | 2007 | more >

  1. Title: An ignition key for atomic-scale engines

    Author(s): Dundas D., Cunningham B., Buchanan C., Terasawa A., Anthony T Paxton A.T., Todorov T.N.

    Journal of Physics: Condensed Matter, 24, pp. 402203-1-402203-6 (2012)

    doi: 10.1088/0953-8984/24/40/402203

    Abstract

    A current-carrying resonant nanoscale device, simulated by non-adiabatic molecular dynamics, exhibits sharp activation of non-conservative current-induced forces with bias. The result, above the critical bias, is generalized rotational atomic motion with a large gain in kinetic energy. The activation exploits sharp features in the electronic structure, and constitutes, in effect, an ignition key for atomic-scale motors. A controlling factor for the effect is the non-equilibrium dynamical response matrix for small-amplitude atomic motion under current. This matrix can be found from the steady-state electronic structure by a simpler static calculation, providing a way to detect the likely appearance, or otherwise, of non-conservative dynamics, in advance of real-time modelling.

  2. Title: Is the pinning of ordinary dislocations in γ-TiAl intrinsic or extrinsic in nature? A combined atomistic and kinetic Monte Carlo approach.

    Author(s): Katzarov I.H., Paxton A.T.

    Acta Materialia, 59, No. 3, pp. 1281-1290 (2011)

    doi: 10.1016/j.actamat.2010.10.060

    Abstract Full Text

    We address the question of the observed pinning of 1/2 <110] ordinary screw dislocations in γ-TiAl which leads to the characteristic trailing of dipoles in the microstructure. While it has been proposed that these may be variously intrinsic or extrinsic in nature, we are able to rule out the former mechanism. We do this by means of very large scale, three dimensional atomistic simulations using the quantum mechanical bond order potential. We find that the kink-pair formation energy is large: 6eV, while the single kink migration energy is conversely very small: 0.13eV. Using these, and other atomistically derived data, we make kinetic Monte Carlo simulations at realistic time and length scales to simulate dislocation mobility as a function of stress and temperature. In the temperature range of the stress anomaly in γ-TiAl, we determine whether one or several of the pinning and unzipping processes associated with generation of jogs are observed during our simulations. We conclude that the pinning of ordinary dislocations and anomalous mechanical behaviour in γ-TiAl must be attributed to a combination of extrinsic obstacles and extensive cross-slip in a crystal containing impurities.

  3. Title: A tight binding model for water

    Author(s): Paxton A.T., Kohanoff J.J.

    Journal of Chemical Physics, 134, No. 4, Art. No. 044130 (2011)

    doi: 10.1063/1.3523983

    Abstract Full Text

    We demonstrate for the first time a tight binding model for water incorporating polarizable oxygen atoms. A novel aspect is that we adopt a "ground up" approach in that properties of the monomer and dimer only are fitted. Subsequently we make predictions of the structure and properties of hexamer clusters, ice-XI and liquid water. A particular feature, missing in current tight binding and semiempirical hamiltonians, is that we reproduce the almost two-fold increase in molecular dipole moment as clusters are built up towards the limit of bulk liquid. We concentrate on properties of liquid water, particularly dielectric constant and self diffusion coefficient, which are very well rendered in comparison with experiment. Finally we comment on the question of the contrasting densities of water and ice which is central to an understanding of the subtleties of the hydrogen bond.

  4. Title: Nonconservative current-induced forces: A physical interpretation

    Author(s): Todorov T.N., Dundas D., Paxton A.T., Horsfield A.P.

    Beilstein Journal of Nanotechnology, 2, pp. 727-733 (2011)

    doi: 10.3762/bjnano.2.79

    Abstract Full Text

    We give a physical interpretation of the recently demonstrated nonconservative nature of interatomic forces in current-carrying nanostructures. We start from the analytical expression for the curl of these forces, and evaluate it for a point defect in a current-carrying system. We obtain a general definition of the capacity of electrical current flow to exert a nonconservative force, and thus do net work around closed paths, by a formal noninvasive test procedure. Second, we show that the gain in atomic kinetic energy over time, generated by nonconservative current-induced forces, is equivalent to the uncompensated stimulated emission of directional phonons. This connection with electron–phonon interactions quantifies explicitly the intuitive notion that nonconservative forces work by angular momentum transfer.

  5. Title: Microscopic Origin of Channeled Flow in Lamellar Titanium Aluminide

    Author(s): Katzarov I.H., Paxton A.T.

    Physical Review Letters, 104, No. 22, Art. No. 225502 (2010)

    doi: 10.1103/PhysRevLett.104.225502

    Abstract Full Text

    We employ a quantum mechanical bond order potential in an atomistic simulation of channeled flow. We show that the original hypothesis that this is achieved by a cooperative deployment of slip and twinning is correct, first because a twin is able to “protect” a 60° ordinary dislocation from becoming sessile, and second because the two processes are found to be activated by Peierls stresses of similar magnitude. In addition we show an explicit demonstration of the lateral growth of a twin, again at a similar level of stress. Thus these simultaneous processes are shown to be capable of channeling deformation into the observed state of plane strain in so-called “A”-oriented mechanical testing of titanium aluminide superalloy.

  6. Title: Electronic structure and total energy of interstitial hydrogen in iron: Tight binding models

    Author(s): Paxton A.T., Elsässer C.

    Physical Review B, 82, No. 23, Art. No. 23512 (2010)

    doi: 10.1103/PhysRevB.82.235125

    Abstract Full Text

    An application of the tight binding approximation is presented for the description of electronic structure and interatomic force in magnetic iron, both pure and containing hydrogen impurities. We assess the simple canonical d-band description in comparison to a non orthogonal model including s and d bands. The transferability of our models is tested against known properties including the segregation energies of hydrogen to vacancies and to surfaces of iron. In many cases agreement is remarkably good, opening up the way to quantum mechanical atomistic simulation of the effects of hydrogen on mechanical properties.

  7. Title: Ring currents in azulene

    Author(s): Paxton A.T., Todorov T.N., Elena A.M.

    Chemical Physics Letters, 483, No. 1-3, pp. 154-158 (2009)

    doi: 10.1016/j.cplett.2009.10.041

    Abstract Full Text

    We propose a self consistent polarisable ion tight binding theory for the study of push–pull processes in aromatic molecules. We find that the method quantitatively reproduces ab initio calculations of dipole moments and polarisability. We apply the scheme in a simulation which solves the time dependent Schrödinger equation to follow the relaxation of azulene from the second excited to the ground states. We observe rather spectacular oscillating ring currents which we explain in terms of interference between the HOMO and LUMO states.

  8. Title: Atomistic studies of <101] screw dislocation core structures and glide in γ-TiAl

    Author(s): Katzarov I.H., Paxton A.T.

    Philosophical Magazine, 89, No. 21, pp. 1731-1750 (2009)

    doi: 10.1080/14786430903037281

    Abstract Full Text

    The core structure of superdislocations in L10 TiAl was investigated with a view to clarifying their dissociation abilities and the mechanisms by which they may become sessile by self-locking. A detailed knowledge of the fine structure of dislocations is essential in analysing the origin of the various deformation features. Atomistic simulation of the core structure and glide of the screw superdislocation was carried out using a bond order potential for γ-TiAl. The core structure of the screw superdislocation was examined, starting with initial unrelaxed configurations corresponding to various dislocation dissociations discussed in the literature. The superdislocation was found to possess in the screw orientation either planar (glissile) or non-planar (sessile) core structures. The response of the core configurations to externally applied shear stress was studied. Some implications were considered of the dissociated configurations and their response to externally applied stress on dislocation dynamics, including the issue of dislocation decomposition, the mechanism of locking and the orientation dependence of the dislocation substructure observed in single-phase γ-TiAl. An unexpectedly rich and complex set of candidate core structures, both planar and non-planar, was found, the cores of which may transform under applied stress with consequent violation of Schmid's law.

  9. Title: Atomistic studies of interactions between the dominant lattice dislocations and γ/γ-lamellar boundaries in lamellar γ-TiAl

    Author(s): Katzarov I.H., Paxton A.T.

    Acta Materialia, 57, No. 11, pp. 3349-3366 (2009)

    doi: 10.1016/j.actamat.2009.03.042

    Abstract Full Text

    This paper reports on atomistic simulations of the interactions between the dominant lattice dislocations in γ-TiAl (ordinary screw 1/2<101] and <101] superdislocations) with all three kinds of γ/γ-lamellar boundaries in polysynthetically twinned (PST) TiAl. The purpose of this study is to clarify the early stage of lamellar boundary controlled plastic deformation in PST TiAl. The interatomic inter- actions in our simulations are described by a bond order potential for L10-TiAl which provides a proper quantum mechanical description of the bonding. We are interested in the dislocation core geometries that the lattice produces in proximity to lamellar boundaries and the way in which these cores are affected by the elastic and atomistic effects of dislocation-lamellar boundary interaction. We study the way in which the interfaces affect the activation of ordinary dislocation and superdislocation slip inside the γ-lamellae and transfer of plastic deformation across lamellar boundaries. We find three new phenomena in the atomic-scale plasticity of PST TiAl, particularly due to elastic and atomic mismatch associated with the 60° and 120° γ/γ-interfaces: (i) two new roles of the γ/γ-interfaces, i.e. decomposition of superdislocations within 120° and 60° interfaces and subsequent detachment of a single ordinary dislocation and (ii) blocking of ordin- ary dislocations by 60° and 120° interfaces resulting in the emission of a twinning dislocation.

  10. Title: An introduction to the tight binding approximation---implementation by diagonalisation

    Author(s): Paxton A.T.

    Winter School: Multiscale Simulation Methods in Molecular Sciences, Forschungszentrum Jülich (MAR 2009)

    Abstract Full Text

  11. Title: Magnetic tight binding and the iron-chromium enthalpy anomaly

    Author(s): Paxton A.T., Finnis M.W.

    Physical Review B, 77, No. 2, Art. No. 024428 (2008)

    doi: 10.1103/PhysRevB.77.024428

    Abstract Full Text

    We describe a self consistent magnetic tight-binding theory based in an expansion of the Hohenberg-Kohn density functional to second order, about a non spin polarised reference density. We show how a first order expansion about a density having a trial input magnetic moment leads to a fixed moment model. We employ a simple set of tight-binding parameters that accurately describes electronic structure and energetics, and show these to be transferable between first row transition metals and their alloys. We make a number of calculations of the electronic structure of dilute Cr impurities in Fe which we compare with results using the local spin density approximation. The fixed moment model provides a powerful means for interpreting complex magnetic configurations in alloys; using this approach we are able to advance a simple and readily understood explanation for the observed anomaly in the enthalpy of mixing.

  12. Title: Boron in copper: A perfect misfit in the bulk and cohesion enhancer at a grain boundary

    Author(s): Lozovoi A.Y., Paxton A.T.

    Physical Review B, 77, No. 16, Art. No. 165413 (2008)

    doi: 10.1103/PhysRevB.77.165413

    Abstract Full Text

    Using first principles electronic structure methods, we calculate the effects of boron impurities in bulk copper and at surfaces and grain boundaries. We find that boron segregation to the Sigma5(310)[001] grain boundary should strengthen the boundary up to 1.5 ML coverage (15.24 at./nm2). The maximal effect is observed at 0.5 ML and corresponds to boron atoms filling exclusively grain boundary interstices. In copper bulk, B causes significant distortion both in interstitial and regular lattice sites, for which boron atoms are either too big or too small. The distortion is compensated to a large extent when the interstitial and substitutional boron combine together to form a strongly bound dumbbell. Our prediction is that bound boron impurities should appear in a sizable proportion if not dominate in most experimental conditions. A large discrepancy between calculated heats of solution and experimental terminal solubility of B in Cu is found, indicating either a significant failure of the density functional approach or, more likely, strongly overestimated solubility limits in the existing B-Cu phase diagram.

  13. Title: Atomistic study of ordinary 1/2 < 110] screw dislocations in single-phase and lamellar gamma-TiAl

    Author(s): Katzarov I.H., Cawkwell M.J., Paxton A.T., Finnis M.W.

    Philosophical Magazine, 87, No. 12, pp. 1795-1809 (2007)

    doi: 10.1080/14786430601080252

    Abstract Full Text

    Computer simulation of the core structure and glide of ordinary 1/2 [110] screw dislocations in single-phase L1(0) TiAl and in two lamellae forming a twin gamma/gamma-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 1/2 [110] 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 1/2 [110] straight screw dislocations under applied stresses in L1(0) TiAl is characterized by zigzag movement on two conjugated {111} planes. The non-planar core of the 1/2 (110] screw dislocation is distorted asymmetrically when the elastic centre of the dislocation is close to a twin gamma/gamma-interface and the dislocation moves on one of the (111) planes, depending on the magnitude of the corresponding Schmid factor. Ordinary dislocations become ordinary interfacial dislocations when they reach the interface. With increasing applied stress they can glide into the adjacent lamella, leaving no remnant interfacial dislocation.

  14. Title: Structural and chemical embrittlement of grain boundaries by impurities: A general theory and first-principles calculations for copper

    Author(s): Lozovoi A.Y., Paxton A.T., Finnis M.W.

    Physical Review B, 74, No. 15, Art. No. 155416 (2006)

    doi: 10.1103/physRevB.74.155416

    Abstract Full Text

    First-principles calculations of the Sigma 5(310)[001] symmetric tilt grain boundary in Cu with Bi, Na, and Ag substitutional impurities provide evidence that in the phenomenon of Bi embrittlement of Cu grain boundaries electronic effects do not play a major role; on the contrary, the embrittlement is mostly a structural or "size" effect. Na is predicted to be nearly as good an embrittler as Bi, whereas Ag does not embrittle the boundary in agreement with experiment. While we reject the prevailing view that "electronic" effects (i.e., charge transfer) are responsible for embrittlement, we do not exclude the role of chemistry. However, numerical results show a striking equivalence between the alkali metal Na and the semimetal Bi, small differences being accounted for by their contrasting "size" and "softness" (defined here). In order to separate structural and chemical effects unambiguously if not uniquely, we model the embrittlement process by taking the system of grain boundary and free surfaces through a sequence of precisely defined gedanken processes; each of these representing a putative mechanism. We thereby identify three mechanisms of embrittlement by substitutional impurities, two of which survive in the case of embrittlement or cohesion enhancement by interstitials. Two of the three are purely structural and the third contains both structural and chemical elements that by their very nature cannot be further unraveled. We are able to take the systems we study through each of these stages by explicit computer simulations and assess the contribution of each to the net reduction in intergranular cohesion. The conclusion we reach is that embrittlement by both Bi and Na is almost exclusively structural in origin; that is, the embrittlement is a size effect.

  15. Title: Stability of Sr adatom model structures for SrTiO3(001) surface reconstructions

    Author(s): Liborio L.M., Sanchez C.G., Paxton A.T., Finnis M.W.

    Journal Of Physics-Condensed Matter, 17, No. 23, pp. L223-L230 (2005)

    doi: 10.1088/0953-8984/17/23/L01

    Abstract Full Text

    We report results of first-principles calculations on the thermodynamic stability of different Sr adatom structures that have been proposed to explain some of the observed reconstructions of the (001) surface of strontium titanate (Kubo and Nozoye 2003 Surf Sci. 542 177). From surface free energy calculations, a phase diagram is constructed indicating the range of conditions over which each structure is most stable. These results are compared with Kubo and Nozoye's experimental observations. It is concluded that low Sr adatom coverage structures can only be explained if the surface is far from equilibrium. Intermediate coverage structures are stable only if the surface is in or very nearly in equilibrium with the strontium oxide.

  16. Title: Theory of the near K-edge structure in electron energy loss spectroscopy

    Author(s): Paxton A.T.

    Journal Of Electron Spectroscopy And Related Phenomena, 143, No. 2-3, pp. 51-64 (2005)

    doi: 10.1016/j.elspec.2004.05.007

    Abstract Full Text

    Arguments are given that lead to a formalism for calculating near K-edge structure in electron energy loss spectroscopy (EELS). This is essentially a one electron picture, while many body effects may be introduced at different levels, such as the local density approximation to density functional theory or the GW approximation to the electron self-energy. Calculations are made within the all electron LMTO scheme in crystals with complex atomic and electronic structures, and these are compared with experiment. (c) 2004 Elsevier B.V. All rights reserved.

  17. Title: Bismuth embrittlement of copper is an atomic size effect

    Author(s): Schweinfest R., Paxton A.T., Finnis M.W.

    Nature, 432, No. 7020, pp. 1008-1011 (DEC 23 2004)

    doi: 10.1038/nature03198

    Abstract Full Text

    Embrittlement by the segregation of impurity elements to grain boundaries is one of a small number of phenomena that can lead to metallurgical failure by fast fracture(1). Here we settle a question that has been debated for over a hundred years(2): how can minute traces of bismuth in copper cause this ductile metal to fail in a brittle manner? Three hypotheses for Bi embrittlement of Cu exist: two assign an electronic effect to either a strengthening(3) or weakening(4) of bonds, the third postulates a simple atomic size effect(5). Here we report first principles quantum mechanical calculations that allow us to reject the electronic hypotheses, while supporting a size effect. We show that upon segregation to the grain boundary, the large Bi atoms weaken the interatomic bonding by pushing apart the Cu atoms at the interface. The resolution of the mechanism underlying grain boundary weakening should be relevant for all cases of embrittlement by oversize impurities.

  18. Title: On the solvation of L-aspartic acid

    Author(s): Paxton A.T., Harper J.B.

    Molecular Physics, 102, No. 9-10, pp. 953-958 (2004)

    doi: 10.1080/00268970410001711913

    Abstract Full Text

    We use molecular statics and dynamics to study the stability of L-aspartic acid both in vacuo and solvated by polar and non-polar molecules using density functional theory in the generalized gradient approximation. We find that structures stable in vacuo are unstable in aqueous solution and vice versa. From our simulations we are able to come to some conclusions about the mechanism of stabilisation of zwitterions by polar protic solvents, water and methanol.

  19. Title: SrTiO3(001)(2x1) reconstructions: First-principles calculations of surface energy and atomic structure compared with scanning tunneling microscopy images

    Author(s): Johnston K., Castell M.R., Paxton A.T., Finnis M.W.

    Physical Review B, 70, No. 8, Art. No. 085415 (2004)

    doi: 10.1103/PhysRevB.70.085415

    Abstract Full Text

    (1x1) and (2x1) reconstructions of the (001) SrTiO3 surface were studied using the first-principles full-potential linear muffin-tin orbital method. Surface energies were calculated as a function of TiO2 chemical potential, oxygen partial pressure and temperature. The (1x1) unreconstructed surfaces were found to be energetically stable for many of the conditions considered. Under conditions of very low oxygen partial pressure the (2x1) Ti2O3 reconstruction [Martin R. Castell, Surf. Sci. 505, 1 (2002)] is stable. The question as to why STM images of the (1x1) surfaces have not been obtained was addressed by calculating charge densities for each surface. These suggest that the (2x1) reconstructions would be easier to image than the (1x1) surfaces. The possibility that the presence of oxygen vacancies would destabilise the (1x1) surfaces was also investigated. If the (1x1) surfaces are unstable then there exists the further possibility that the (2x1) DL-TiO2 reconstruction [Natasha Erdman Nature (London) 419, 55 (2002)] is stable in a TiO2-rich environment and for p(O2)>10(-18) atm.

  20. Title: Bandstructure approach to near edge structure

    Author(s): Paxton A.T., Craven A.J., Gregg J.M., McComb D.W.

    Journal Of Microscopy-Oxford, 210, No. 1, pp. 35-44 (2003)

    doi: 10.1046/j.1365-2818.2003.01182.x

    Abstract Full Text

    We review the current state of the art in EELS fingerprinting by computer simulation, focusing on the bandstructure approach to the problem. Currently calculations are made using a one electron theory, but we describe in principle the way to go beyond this to include final state effects. We include these effects within the one electron framework using the Slater transition state formula and assess the errors involved. Two examples are then given which illustrate the use of the one electron approximation within density functional theory. Our approach is to combine predicted atomic structure with predicted electronic structure to assist in fingerprinting of complex crystal structures.

  21. Title: A stabilization mechanism of zirconia based on oxygen vacancies only

    Author(s): Fabris S., Paxton A.T., Finnis M.W.

    Acta Materialia, 50, No. 20, pp. 5171-5178 (2002)

    doi: 10.1016/S1359-6454(02)00385-3

    Abstract Full Text

    The microscopic mechanism leading to stabilization of cubic and tetragonal forms of zirconia (ZrO2) 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 (Y2O3) 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 111 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. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

  22. Title: Electron energy-loss near-edge shape as a probe to investigate the stabilization of yttria-stabilized zirconia

    Author(s): Ostanin S., Craven A.J., McComb D.W., Vlachos D., Alavi A., Paxton A.T., Finnis M.W.

    Physical Review B, 65, No. 22, Art. No. 224109 (2002)

    doi: 10.1103/PhysRevB.65.224109

    Abstract Full Text

    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.

  23. Title: Material effects on stress-induced defect generation in trenched silicon-on-insulator structures

    Author(s): Nevin W.A., Somasundram K., McCann P., Magee S., Paxton A.T.

    Journal Of The Electrochemical Society, 148, No. 11, pp. G649-G654 (2001)

    doi: 10.1149/1.1408636

    Abstract Full Text

    We have investigated the influence of the material properties of the silicon device layer on the generation of defects, and in particular slip dislocations, in trenched and refilled fusion-bonded silicon-on-insulator structures. A strong dependence of the ease of slip generation on the type of dopant species was observed, with the samples falling into three basic categories; heavily boron-doped silicon showed ready slip generation, arsenic and antimony-doped material was fairly resistant to slip, while silicon moderately or lightly doped with phosphorous or boron gave intermediate behavior. The observed behavior appears to be controlled by differences in the dislocation generation mechanism rather than by dislocation mobility. The introduction of an implanted buried layer at the bonding interface was found to result in an increase in slip generation in the silicon, again with a variation according to the dopant species. Here, the greatest slip occurred for both boron and antimony-implanted samples. The weakening of the implanted material may be related to the presence of a band of precipitates observed in the silicon near the bonding interface. (C) 2001 The Electrochemical Society.

  24. Title: Free energy and molecular dynamics calculations for the cubic-tetragonal phase transition in zirconia

    Author(s): Fabris S., Paxton A.T., Finnis M.W.

    Physical Review B, 63, No. 9, Art. No. 094101 (2001)

    doi: 10.1103/PhysRevB.63.094101

    Abstract Full Text

    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 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 behavior above the transition temperature.

  25. Title: Effect of relaxation on the oxygen K-edge electron energy-loss near-edge structure in yttria-stabilized zirconia

    Author(s): Ostanin S., Craven A.J., McComb D.W., Vlachos D., Alavi A., Finnis M.W., Paxton A.T.

    Physical Review B, 62, No. 22, pp. 14728-14735 (2000)

    doi: 10.1103/PhysRevB.62.14728

    Abstract Full Text

    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,Y,O, 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.

  26. Title: Relative energetics and structural properties of zirconia using a self-consistent tight-binding model

    Author(s): Fabris S., Paxton A.T., Finnis M.W.

    Physical Review B, 61, No. 10, pp. 6617-6630 (2000)

    doi: 10.1103/PhysRevB.61.6617

    Abstract Full Text

    We describe an empirical, self-consistent, orthogonal tight-binding model for zirconia, which allows for the polarizability of the anions at dipole and quadrupole levels and for crystal field splitting of the cation d orbitals, This is achieved by mixing the orbitals of different symmetry on a site with coupling coefficients driven by the Coulomb potentials up to octapole level. The additional forces on atoms due to the self-consistency and polarizabilities are exactly obtained by straightforward electrostatics, by analogy with the Hellmann-Feynman theorem as applied in first-principles calculations. The model correctly orders the zero temperature energies of all zirconia polymorphs. The Zr-O matrix elements of the Hamiltonian, which measure covalency, make a greater contribution than the polarizability to the energy differences between phases. Results for elastic constants of the cubic and tetragonal phases and phonon frequencies of the cubic phase are also presented and compared with some experimental data and first-principles calculations. We suggest that the model will be useful for studying finite temperature effects by means of molecular dynamics.

  27. Title: The near-edge structure in energy-loss spectroscopy: many-electron and magnetic effects in transition metal nitrides and carbides

    Author(s): Paxton A.T., van Schilfgaarde M., MacKenzie M., Craven A.J.

    Journal Of Physics-Condensed Matter, 12, No. 5, pp. 729-750 (2000)

    doi: 10.1088/0953-8984/12/5/319

    Abstract Full Text

    We investigate the ability of the local density approximation (LDA) in density functional theory to predict the near-edge structure in electron energy-loss spectroscopy in the dipole approximation. We include screening of the core hole within the LDA using Slater's transition state theory. We find that anion K-edge threshold energies are systematically overestimated by 4.22 +/- 0.44 eV in twelve transition metal carbides and nitrides in the rock-salt (B1) structure. When we apply this 'universal' many-electron correction to energy-loss spectra calculated within the transition state approximation to LDA, we find quantitative agreement with experiment to within one or two eV for TiC, TiN and VN. We compare our calculations to a simpler approach using a projected Mulliken density which honours the dipole selection rule, in place of the dipole matrix element itself. We find remarkably close agreement between these two approaches. Finally, we show an anomaly in the near-edge structure in CrN to be due to magnetic structure. In particular, we find that the N K edge in fact probes the magnetic moments and alignments of ther sublattice.