A. P. Sutton

Long-range Finnis–Sinclair potentials

Abstract Finnis–Sinclair potentials are developed for computer simulations in which van der Waals type interactions between well separated atomic clusters are as important as the description of metallic bonding at short range. The potentials always favour f.c.c. and h.c.p. structures over the b.c.c. structure. They display convenient scaling properties for both length and energy, and a number of properties of the perfect crystal may be derived analytically.

Gas permeation in silicon-oxide/polymer (SiOx/PET) barrier films: role of the oxide lattice, nano-defects and macro-defects

We propose a model for permeation in oxide coated gas barrier films. The model accounts for diffusion through the amorphous oxide lattice, nano-defects within the lattice, and macro-defects. The presence of nano-defects indicate the oxide layer is more similar to a nano-porous solid (such as zeolite) than silica glass with respect to permeation properties. This explains why the permeability of oxide coated polymers is much greater, and the activation energy of permeation much lower, than values expected for polymers coated with glass. We have used the model to interpret permeability and activation energies measured for the inert gases (He, Ne and Ar) in evaporated SiOx films of varying thickness (13-70 nm) coated on a polymer substrate. Atomic force and scanning electron microscopy were used to study the structure of the oxide layer. Although no defects could be detected by microscopy, the permeation data indicate that macro-defects (>1 nm), nano-defects (0.3-0.4 nm) and the lattice interstices (<0.3 nm) all contribute to the total permeation

The tight-binding bond model

The authors present a tight-binding model of cohesion and interatomic forces which exploits the variational principle of density functional theory. The binding energy of a solid is expressed as a sum of four terms, each of which has a clear physical meaning. The first two terms are the covalent bond and promotion energies, which are found by solving the electronic Hamiltonian to obtain the density matrix. The remaining two terms describe changes in the total electrostatic and exchange-correlation energies on forming the solid from isolated atoms. The variational principle allows these two terms to be expressed as functionals of a superposition of frozen atomic charge densities. The authors show that they may then be approximated by a sum of pair potentials. The importance of self-consistency in tight-binding models is discussed with particular attention to the evaluation of the bulk modulus and certain interatomic force constants by frozen-phonon calculations. It is shown that serious errors may arise in non-self-consistent models due to the violation of charge conservation and the neglect of variations in the potential caused by charge flow. The authors advocate local charge neutrality as the simplest approximation to self-consistency which overcomes these problems. This assumption leads to a remarkably simple expression for the force on an atom due to its neighbours, which is both physically transparent and computationally efficient. These concepts are illustrated for three-dimensional solids by calculations of covalent bond energies in BCC, FCC and HCP transition metals using a canonical d-band model. Model one-dimensional calculations are also presented which illustrate the computation of covalent bond energies and interatomic forces at surfaces and interfaces, and the importance of local charge neutrality in the model.

Effect of Mott-Hubbard correlations on the electronic structure and structural stability of uranium dioxide

Abstract The influence of Mott-Hubbard electron-electron correlations on the electronic structure and structural stability of uranium dioxide (UO2) has been analysed using the local spin-density approximation (LSDA) + U approach. We have found that the inclusion of a term describing the Hubbard on-site repulsion between 5f electrons results in a dramatic improvement in the description of the equilibrium electronic and magnetic structure of UO2 for which conventional LSDA calculations incorrectly predict a non-magnetic metallic ground state. We have found that the presence of electron-electron correlations in the 5f band modifies the character of chemical bonding in the material, leading to a Heitler-London type of hybridization between the 5f orbitals and giving rise to a larger value of the equilibrium lattice constant in better agreement with experimental observations.

A computer simulation study of 〈001〉 and 〈111〉 tilt boundaries: the multiplicity of structures

Abstract The sources of the multiplicity of metastable structures of grain boundaries are discussed using a recently developed structural unit model of grain boundary structure. Two sets of computer calculations of symmetrical tilt boundaries are presented and it is shown that the calculated multiplicity of metastable structures is readily predicted by the structural unit model. This multiplicity is likely to be very extensive in the case of general grain boundaries. The implications of the structural multiplicity of grain boundaries for various grain boundary phenomena as well as for interpretation of “direct observations” of grain boundary structure are discussed.

The bcc-to-9R martensitic transformation of Cu precipitates and the relaxation process of elastic strains in an Fe-Cu alloy

Abstract High-resolution electron microscopy experiments have been performed to explore the bcc—9R transformation and the subsequent elastic relaxation of Cu precipitates in an Fe—Cu alloy aged at 550°C. It was found that both electron irradiation (at an electron energy of 400 kV) and thermal annealing caused rotation of the close-packed (009)9R planes in twinned 9R Cu precipitates. For 400 kV electron irradiation, such rotations were observed in precipitates smaller than about 12nm in diameter. For specimens cooled from the ageing temperature of 550°C to a given temperature up to −60°C, and then annealed at 400°C, the rotation of (009)9R planes was found to occur only in precipitates above a size which depended on the temperature to which the specimen had been cooled. This critical size ranged from about 9 nm for specimens cooled to 400°C, to 4 nm for specimens cooled to −60°C. It is argued that these critical sizes are indicative of the sizes at which coherent bcc precipitates transform martensitically at different temperatures. At the ageing temperature of 550°C, the transformation to 9R takes place when precipitates reach a size of about 12nm. The number of twin segments in the transformed 9R precipitates is determined by the transformation, depending on the precipitate size. The annealing-induced plane rotations are shown to be connected with the diffusional relaxation of elastic strains, which are created upon the martensitic transformation. From the precipitate size and annealing time dependence of the rotations, it is concluded that the elastic strains relax by atomic diffusion along the interfaces between the Fe matrix and Cu precipitates. The activation energy for the interfacial diffusion is evaluated to be 1.7eV.

On the stability of a tip and flat at very small separations

It is shown that at sufficiently small separations, ∼1–2 A, the tip and flat surfaces in the scanning tunneling microscope or atomic force microscope (AFM) will jump together, irrespective of apparatus construction. Both continuum and atomistic calculations are presented. We discuss the consequences of the resulting forbidden separations for the behavior of the vacuum barrier as it is quenched, and for the resolution of the AFM.

Hydrolysis of the amorphous silica surface. II. Calculation of activation barriers and mechanisms

Using a previously derived model of the dry, amorphous, hydrophilic SiO2 surface, the reactivity of generic defect sites on the surface with respect to water, and the local network rearrangement that accompanies hydrolysis at these sites, is investigated using cluster models. Ab initio methods are used to calculate reaction barriers and reaction pathways. Consequences of the various types of hydrolysis product found are discussed with reference to potential sites for polymer chemisorption on the hydrolyzed, amorphous SiO2 surface.

A genetic algorithm for predicting the structures of interfaces in multicomponent systems

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).

Temperature-dependent interatomic forces

Abstract A method of optimizing the thermally averaged structure of a defect in a crystal at a finite temperature is presented. The basic idea is to augment the interatomic forces that apply at OK with temperature-dependent contributions that arise from the minimization of the free energy at a finite temperature. The structure and thermodynamic properties of the defect can then be obtained with a molecular statics program. The free energy is expressed within the harmonic approximation, but since the interatomic potential at O K is anharmonic, there is an additional temperature-dependent force arising from the vibrational free energy. The same term gives rise to the thermal expansion of the perfect crystal. A second-moment approximation is applied to the phonon local density of states. Projections of thermodynamic functions onto individual atomic sites are then expressed analytically. When the vibrational free energy is differentiated, the resulting force is N-body in nature even for a pairwise interatomic...