Stephen C. Parker

The role of oxygen vacancies on ceria surfaces in the oxidation of carbon monoxide

Computer simulation techniques are used to model the surfaces of ceria, paying special attention to the effects of surface structures and energetics on catalytic activity. Three surfaces of CeO2 have been investigated for both their structure and relative stability. The results show that the three low energy surfaces are the (111), (110) and (310) with the first of these three being the most stable. These surfaces will dominate the morphology of the material. Defects including oxygen vacancies and reduced cerium ions are found to be more stable at the surfaces than in the bulk of the crystal. Finally, we show that the ready formation of oxygen vacancies on the (110) and (310) surfaces of CeO2 significantly promotes the oxidation of carbon monoxide.

Atomistic simulation of dislocations, surfaces and interfaces in MgO

A new simulation code for modelling extended defects e.g. linear (dislocations) and planar (surfaces and grain boundaries) at the atomistic level is introduced. One of the key components is the ability to calculate the Coulombic potential of a solid with one-dimensional periodicity. This approach has been applied to screw dislocations in MgO and we have evaluated the structure (including core size) and stability of the 〈100〉 and 1/2〈110〉 screw dislocations. The 1/2〈110〉 dislocation, which has the shortest Burgers vector, was found to be more stable, as predicted by elasticity theory, although the simulations show that elasticity theory underestimates the energy difference.In addition, it has been shown that by using this new computer simulation code METADISE, following the approach of Tasker, the structure and energetics of surfaces and interfaces can be calculated. This method has been applied to modelling micro-faceting and it was found that micro-facetted {110} and {111} surfaces of MgO are the most stable forms of these surfaces. The formation energy of tilt grain boundaries in MgO ({h10} and {h20}) as a function of misorientation angle was also investigated and it was found that for the {h10} series the formation energy is proportional to the interfacial bond density while no such pattern can be found for the {h20} series.

Computer modelling of fluids polymers and solids

1. An Introduction to Computer Modelling of Condensed Matter.- 2. Towards Realistic Model Intermolecular Potentials.- 3. Molecular Dynamics.- 4. Monte Carlo Simulations.- 5. Non-Equilibrium Statistical Mechanics and Molecular Dynamics Computations.- 6. The Path-Integral Simulation of Quantum Systems.- 7. The Method of Constraints: Application to a Simple N-Alkane Model.- 8. Molecular Dynamics of Chain Molecules.- 9. Computer Modelling of Oxide Surfaces and Interfaces.- 10. Hardware Issues in Molecular Dynamics Algorithm Design.- 11. Parallel Computers and the Simulation of Solids and Liquids.- 12. Molecular Simulations of Protein Structure, Dynamics and Thermodynamics.- 13. Simulation of Plastic Crystals.- 14. Molecular Dynamics Simulations of Aqueous Systems.- 15. Computer Simulation of Inorganic Materials.- 16. Computer Modelling of the Structure and Thermodynamic Properties of Silicate Minerals.- Appendix: Computer Simulation Exercises.

Modelling of the thermal dependence of structural and elastic properties of calcite, CaCO3

A computational method, based on the quasiharmonic approximation, has been computer-coded to calculate the temperature dependence of elastic constants and structural features of crystals. The model is applied to calcite, CaCO3; an interatomic potential based on a C-O Morse function and Ca-O and O-O Borntype interactions, including a shell model for O, has been used. Equilibrations in the range 300–800 K reproduce the experimental unit-cell edges and bond lengths within 1%. The simulated thermal expansion coefficients are 22.3 (//c) and 2.6 (⊥ c), against 25.5 and-3.7×10−6K−1 experimental values, respectively. The thermal coefficients of elastic constants tend to be underestimated; for the bulk modulus, -2.3 against-3.7×10−4K−1 is obtained.

Atomistic simulation of the effect of molecular adsorption of water on the surface structure and energies of calcite surfaces

Atomistic simulation techniques using potentials verified against the structure of ikaite, have been employed to study the molecular adsorption of water onto the stepped and planar calcite {104} surfaces as well as the {0001}, {100}, {101} and {110} surfaces. It was found that physisorption of water is energetically favourable on all surfaces although the {104} planes remain the most stable surfaces, and the stepped planes are found to be good models for growth steps on the experimental {104} surface. Experimentally observed 1×1 surface symmetry of the {104} surface is confirmed. On the partially hydrated surface, notionally equivalent carbonate groups are shown to relax differently as inferred by experiment. The hydrated {101} surface shows bulk ordering with rotated carbonate groups in the surface layers in agreement with experimental findings.

The MD simulation of the equation of state of MgO: Application as a pressure calibration standard at high temperature and high pressure

Abstract Molecular dynamics (MD) simulation is used to calculate the elastic constants and their temperature and pressure derivatives, and the T-P-V equation of state of MgO. The interionic potential is taken to be the sum of pairwise additive Coulomb, van der Waals attraction, and repulsive interactions. In addition, to account for the observed large Cauchy violation of the elastic constants of MgO, the breathing shell model (BSM) is introduced in MD simulation, in which the repulsive radii of O ions are allowed to deform isotropically under the effects of other ions in the crystal. Quantum correction to the MD pressure is made using the Wigner-Kirkwood expansion of the free energy. Required energy parameters, including oxygen breathing parameters, were derived empirically to reproduce the observed molar volume and elastic constants of MgO, and their measured temperature and pressure derivatives as accurately as possible. The MD simulation with BSM is found to be very successful in reproducing accurately the measured molar volumes and individual elastic constants of MgO over a wide temperature and pressure range. The errors in the simulated molar volumes are within 0.3% over the temperature range between 300 and 3000 K at 0 GPa, and within 0.1% over the pressure range from 0 up to 50 GPa at 300 K. The simulated bulk modulus is found to be correct to within 0.7% between 300 and 1800 K at 0 GPa. Here we present the MD simulated T-P-V equation of state of MgO as an accurate internal pressure calibration standard at high temperatures and high pressures

THE LATTICE-DYNAMICS AND THERMODYNAMICS OF THE MG2SIO4 POLYMORPHS

We use an approach based upon the Born model of solids, in which potential functions represent the interactions between atoms in a structure, to calculate the phonon dispersion of forsterite and the lattice dynamical behaviour of the beta-phase and spinel polymorphs of Mg2SiO4. The potential used (THB1) was derived largely empirically using data from simple binary oxides, and has previously been successfully used to model the infrared and Raman behaviour of forsterite. It includes ‘bond bending’ terms, that model the directionality of the Si-O bond, in addition to the pair-wise additive Coulombic and short range terms. The phonon dispersion relationships of the Mg2SiO4 polymorphs predicted by THB1 were used to calculate the heat capacities, entropies, thermal expansion coefficients and Gruneisen parameters of these phases. The predicted heat capacities and entropies are in outstandingly good agreement with those determined experimentally. The predicted thermodynamic data of these phases were used to construct a phase diagram for this system, which has Clausius-Clapeyron slopes in very close agreement with those found by experiment, but which has predicted transformation pressures that show less close agreement with those inferred from experiment. The overall success, however, that we have in predicting the lattice dynamical and thermodynamic properties of the Mg2SiO4 polymorphs shows that our potential THB1 represents a significant step towards finding the elusive quantitative link between the microscopic or atomistic behaviour of minerals and their macroscopic properties.

Rutile (β-)MnO2 Surfaces and Vacancy Formation for High Electrochemical and Catalytic Performance

MnO2 is a technologically important material for energy storage and catalysis. Recent investigations have demonstrated the success of nanostructuring for improving the performance of rutile MnO2 in Li-ion batteries and supercapacitors and as a catalyst. Motivated by this we have investigated the stability and electronic structure of rutile (β-)MnO2 surfaces using density functional theory. A Wulff construction from relaxed surface energies indicates a rod-like equilibrium morphology that is elongated along the c-axis, and is consistent with the large number of nanowire-type structures that are obtainable experimentally. The (110) surface dominates the crystallite surface area. Moreover, higher index surfaces than considered in previous work, for instance the (211) and (311) surfaces, are also expressed to cap the rod-like morphology. Broken coordinations at the surface result in enhanced magnetic moments at Mn sites that may play a role in catalytic activity. The calculated formation energies of oxygen vacancy defects and Mn reduction at key surfaces indicate facile formation at surfaces expressed in the equilibrium morphology. The formation energies are considerably lower than for comparable structures such as rutile TiO2 and are likely to be important to the high catalytic activity of rutile MnO2.

Computer simulations of the structural and physical properties of the olivine and spinel polymorphs of Mg2SiO4

The aim of the work presented is to develop a computer simulation technique which will predict the structure and physical properties of forsterite and ringwoodite, the major mantle-forming polymorphs of Mg2SiO4. The technique is based upon energy minimization, in which all structural parameters are varied until the configuration with the lowest energy is achieved. The lattice energy and physical properties (e.g. elasticity and dielectric constants) are calculated from interatomic potentials, which generally include electrostatic and short-range terms. We investigate several types of traditional potential models, and present a new type of model which includes partial ionic charges and a Morse potential to describe the effect of covalency on the Si-O bond. This new form of potential model is highly successful, and not only reproduces the zero-pressure structural, elastic and dielectric properties of forsterite and ringwoodite, but also accurately describes their pressure dependence.

Thermal physics of the lead chalcogenides PbS, PbSe, and PbTe from first principles

The lead chalcogenides represent an important family of functional materials, in particular due to the benchmark high-temperature thermoelectric performance of PbTe. A number of recent investigations, experimental and theoretical, have aimed to gather insight into their unique lattice dynamics and electronic structure. However, the majority of first-principles modeling has been performed at fixed temperatures, and there has been no comprehensive and systematic computational study of the effect of temperature on the material properties. We report a comparative lattice-dynamics study of the temperature dependence of the properties of PbS, PbSe, and PbTe, focusing particularly on those relevant to thermoelectric performance, viz. phonon frequencies, lattice thermal conductivity, and electronic band structure. Calculations are performed within the quasiharmonic approximation, with the inclusion of phonon-phonon interactions from many-body perturbation theory, which are used to compute phonon lifetimes and predict the lattice thermal conductivity. The results are critically compared against experimental data and other calculations, and add insight to ongoing research on the PbX compounds in relation to the off-centering of Pb at high temperatures, which is shown to be related to phonon softening. The agreement with experiment suggests that this method could serve as a straightforward, powerful, and generally applicable means of investigating the temperature dependence of material properties from first principles.