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Article
Scaling of thermodynamic mixing properties in garnet solid solutions
The volumes and enthalpies of mixing, ΔV Mix and ΔH Mix, of binary solid-solution aluminosilicate garnets have been studied by computer simulation. The use of “average atoms” to simulate solid solution was found...
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Article
Rigid unit modes and dynamic disorder: SiO2 cristobalite and quartz
The high temperature (β) phases of SiO2 cristobalite and quartz are studied by performing molecular dynamics simulations using a model which allows easy analysis of tetrahedral motions. The dynamic nature of the...
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Chapter
New Vistas for First-Principles Simulation
We review recent progress in the first-principles treatment of condensed matter, with particular attention to the work of the UKCP consortium on the Cray T3D at EPCC. It is shown that HPC facilities are allowi...
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Article
Negative thermal expansion in beta-quartz
Computer modelling and theoretical analysis are used to explain the nearly zero and slightly negative coefficients of thermal expansion in β-quartz well above the α-β phase transition temperature. Quartz was sele...
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Article
Determination of the origin and magnitude of Al/Si ordering enthalpy in framework aluminosilicates from ab initio calculations
Ab initio total energy calculations based on a new optimised oxygen pseudopotential has been used to determine the enthalpy of disorder for the exchange of Al and Si in tetrahedral coordination in simple deriv...
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Article
A computational study of Al/Si ordering in cordierite
The ordering of Al and Si in Mg cordierite Mg2Al4Si5O18 is considered using computer simulation. First the enthalpy of interaction J ij between sites is derived by computer mod...
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Article
Rigid unit modes in the high-temperature phase of SiO2 tridymite: calculations and electron diffraction
Calculations of the rigid unit mode (RUM) spectrum of the high-temperature phase of SiO2 tridymite are used to explain the patterns of diffuse scattering seen in transmission electron microscopy experiments. Thes...
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Article
Temperature and Strain Rate Effects in Grain Boundary Sliding
We have performed total energy density functional theory calculations to investigate the sliding process at the Σ = 5 (001) twist grain boundary in germanium. The accurate quantum mechanical description of the...
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Article
Ab Initio Investigation of Grain Boundary Sliding
We are using total energy pseudopotential calculations to carry out an extensive investigation into grain boundary sliding in a number of different systems, in order to understand, at microscopic level, the fu...
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Article
Ab initio calculations on (OH)4 defects in α-quartz
Ab initio total energy calculations based on a new optimized oxygen psuedopotential have been used to study the structures and relative energies of α-quartz, a partly (OH)4 substituted version of the α-quartz str...
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Article
The energetics of interaction between oxygen vacancies in sillimanite: A model for the mullite structure
A microscopic model is introduced to discuss the modulated structure of mullite. The oxygen vacancies of this aluminosilicate are known to play a central role. In particular, a single vacancy strongly orders i...
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Article
Atomic ordering around the oxygen vacancies in sillimanite
Computer simulation is used to investigate the short range ordering around an isolated oxygen vacancy in sillimanite. The static lattice energy with the use of empirical potentials is calculated, for different...
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Article
Kinetic rate laws as derived from order parameter theory V: Computer simulations of ordering processes using a soft ising model
The kinetic and equilibrium behaviour of order/disorder systems and related processes are simulated using an Ising spin model in which the coupling between spins occurs via local strain in a harmonic lattice. ...
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Chapter and Conference Paper
The Microscopic Understanding of Modulated Structures and Polytypes
Incommensurate structures and polytypes show a variety of phenomena. These result from the interplay of various forces in solids, sometimes cooperation and sometimes competition between them. A general underst...
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Article
The structure of the incommensurate modulated phase of NaNO2
The incommensurate phase of NaNO2 is known to contain a transverse displacement modulation in addition to the modulated ferroelectric ordering, but the phase and sign of the displacement was left undetermined by ...
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Chapter and Conference Paper
The Physical Mechanisms Leading to Incommensurate Phases
The purpose of this paper is to review the various physical origins of incommensurate (IC) phases, some of which have only become apparent in recent years. We shall discuss two of them in detail in Sections 3 ...
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Chapter
New Light on Magnetic Contributions to the Thermal Expansion of Transition Metal Magnets
It is now generally accepted that some of the transition metal magnets support locally orientated moments whilst retaining an otherwise itinerant nature. We show that in such a system there are two opposing co...
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Article
The origin of charge density waves in the 2H-polytypes of the group-V layer compounds
The group-V metallic layer compounds exhibit superconductivity at low temperatures and all (except NbS2) undergo phase changes due to the formation of CDW at certain temperatures. In order to understand this theo...
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Chapter and Conference Paper
Calculation of Band Structures and Fermi Surfaces
In recent years, several quite detailed band structure calculations have been performed for both ordinary and transition metals. Some of the results will be reviewed in the following section. These calculation...
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Article
The unrestricted Hartree-Fock method
The unrestricted Hartree-Fock method is important for discussing several effects, including the spin density produced in the core of an atom by exchange with an unfilled valence electron shell. The method allo...
