Mark Wilson

The high-temperature superionic behaviour of Ag2S

Powder neutron diffraction and molecular dynamics (MD) simulations have been used to investigate the structural behaviour of silver sulfide, Ag2S, at elevated temperatures. Above ~450 K Ag2S adopts the β phase in which the S2- possess a body-centred cubic arrangement. Analysis of the neutron diffraction is in good agreement with the previously proposed structural model in which the Ag+ predominantly reside within the tetrahedral interstices. At ~865 K Ag2S transforms to the α phase in which the anion sublattice adopts a face-centred cubic arrangement. Structural refinements of this phase indicate that the cations are distributed predominantly in the tetrahedral cavities but with a significant fraction in the octahedral holes. MD simulations, using established potentials for this compound, confirm the stability of the two high-temperature superionic phases and show good agreement with the measured Ag+ distribution within the unit cell.

The formation of low-dimensional ionic crystallites in carbon nanotubes

Computer simulation models were constructed and applied to understand the low-dimensional crystallites formed in carbon nanotubes when filled with liquid KI. By using a much simplified model, the precise atomistic region of the specific distortions observed experimentally were determined. As a result, the same model was applied to understanding the actual atomistic mechanism by which these tubes fill. It was shown that the narrow pore radius of the nanotube, coupled with the ionic ordering inherent in the filling material, are critical in the formation of the low-dimension crystallites observed experimentally.

Polyamorphism in aluminate liquids

McMillan, P. F., Wilson, M., Wilding, M. C. (2003). Polyamorphism in aluminate liquids. Journal of Physics: Condensed Matter, 15 (36), 6105-6121 RAE2008

Structure and phase stability of novel ‘twisted’ crystal structures in carbon nanotubes

Abstract The stability of novel, low dimensional, crystal structures formed by a simple alkali halide (KI) in single-walled carbon nanotubes are investigated using a simple computer simulation model. ‘Twisted’ crystals, not clearly related to the bulk structure, and found to form dynamically in specific tube diameters, are investigated. A phase diagram for the confined structures as a function of the nanotube pore size is calculated. Specific ranges of nanotube diameters are shown to favour these unique structures over structures related directly to those adopted by the bulk alkali halide. The high resolution transmission electron microscopy pattern for an example stable twisted crystal is simulated.

The construction and application of a fully flexible computer simulation model for lithium oxide

An aspherical ion model (AIM) is constructed for lithium oxide, Li2O. The model incorporates both many-body polarization and short-range ion distortion effects. A procedure for extracting the required model parameters by fitting to results from a series of electronic structure calculations is described. The model is tested with respect to both static and dynamic properties. The experimentally observed Cauchy violation in the elastic constants and phonon frequencies are well reproduced as is the onset temperature for superionic behaviour in the Li+ sublattice. The system is shown to display a peak in the heat capacity as a function of temperature. The correlated and uncorrelated ion dynamics are studied and the origin of the respective solid- and liquid-state Haven ratios is rationalized.

The formation of low-dimensional metal trihalide crystals in carbon nanotubes

Molecular dynamics computer simulation models are employed to study the direct filling of single-walled carbon nanotubes (which vary in diameter) with an archetypal metal trihalide, LaCl3. The use of relatively simple potential models allows the investigation of details of both the atomistic filling mechanism and the thermodynamic factors controlling the formation. The resulting low-dimensional crystallites are analysed with respect to bulk crystal structures and compared to experimental high-resolution transmission-electron-microscopy images by simulation of equivalent micrographs from one of the obtained potential models, resulting in excellent agreement between the simulated and experimental images.

Metastable phase transitions and structural transformations in solid-state materials at high pressure

We use a combination of diamond anvil cell techniques and large volume (multi-anvil press, piston cylinder) devices to study the synthesis, structure and properties of new materials under high pressure conditions. The work often involves the study of structural and phase transformations occurring in the metastable regime, as we explore the phase space determined as a function of the pressure, temperature and chemical composition. The experimental studies are combined with first principles calculations and molecular dynamics simulations, as we determine the structures and properties of new phases and the nature of the transformations between them. Problems currently under investigation include structural studies of transition metal and main group nitrides, oxides and oxynitrides at high pressure, exploration of new solid-state compounds that are formed within the C-N-O system, polyamorphic low- to high-density transitions among amorphous semiconductors such as a-Si, and transformations into metastable forms of the element that occur when its “expanded” clathrate polymorph is compressed.

Ionic diffusion within the α* and β phases of Ag3SI

The ionic diffusion mechanism of mobile ions within an underlying body-centred cubic (bcc) sublattice of immobile counterions is discussed. In particular, the case of equal numbers of two ionic species forming long-range ordered and disordered bcc arrays is considered, since these form the basis of the cubic perovskite and α-AgI-type crystal structures, respectively. Their structural behaviour, and its influence on the dynamic ionic disorder which characterizes superionic conduction, is illustrated for the case of Ag(+) diffusion within the β and α(*) phases of Ag(3)SI. The calculated behaviour obtained by molecular dynamics (MD) computer simulations is validated with reference to published neutron diffraction and ionic conductivity measurements of Ag(3)SI, and used to examine the preferred diffusion pathways. The relevance of these findings for the anion conduction mechanisms within perovskite structured compounds is briefly discussed.

Formation of, and ion-transport in, low-dimensional crystallites in carbon nanotubes

The formation of low-dimensional crystal structures, obtained by filling carbon nanotubes from the molten salts, is considered for three stoichiometries (the MX, MX2 and MX3). For the MX stoichiometry, general classes of inorganic nanotube (INT) are predicted to exist whose morphology depends both on the low-energy (bulk) crystal structure and the encasing carbon nanotube diameter. These INTs are generally found to have no direct bulk analogues. For both the MX2 and MX3 stoichiometries crystal structures are predicted which either have a direct bulk analogue, or whose structure can be considered as a distortion of a bulk fragment. In both of these stoichiometries unusual (high anion coordination) crystallites are predicted. For the MX stoichiometry the ion transport mechanism is investigated and discussed, whilst for the MX3 the vibrational densities of states are analysed with respect to both the pure liquid and idealized crystallites.

A transferable representation of the induced multipoles in ionic crystals

Electronic structure calculations of the induced dipole and quadrupole moments on a fluoride ion at low symmetry sites in a model crystalline environments are described. The results are used to characterize the short range contributions to the induced multipoles on the F− ion that arise from overlap between the wavefunctions of the ions. These are represented by general functions of the positions of the surrounding ions, suitable for use in a computationally tractable simulation model. The present calculations are designed to test the transferability to other classes of materials of this representation which, to date, largely has been deduced from calculations on distorted alkali halide crystals. First, the induced multipoles in mixed crystals of the alkali halides are considered, in order to check that they are predicted reliably by combining the representations deduced for the corresponding pure materials. Second, polarization effects in the alkaline earth fluorides MgF2, and CaF2, in a locally distorted fluorite crystal structure are examined. It is found that the material-specific parameters in the representation are related through simple functions of the ionic radii. This relationship holds between MgF2, and CaF2, in the fluorite structure, but also extends to the alkali fluorides previously studied. In order to illustrate the significance of polarization effects, the polarization model for CaF2 is combined with an ab initio pair potential for the repulsion and dispersion interactions and used in a molecular dynamics simulation. Inclusion of the polarization term improves greatly the calculated phonon frequencies in the crystal.