French, Samuel Arthur; (2001) Computer modelling of oxide and molecular superconducting materials. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The techniques for the computational simulation of solids have, in recent years, been developed to the level that they now provide a predictive tool. The increased understanding of interatomic forces in solids coupled with improvement in numerical and computational methods allows the study of solid state properties, including structural, thermodynamic and transport properties. This thesis presents both development and application studies on two topical classes of materials: first organic superconductors, and secondly metal oxides and their surfaces. A detailed computational study of the organic superconducting charge transfer BEDT-TTF (ET) salts has been undertaken in Part I of this thesis. Molecular mechanics calculations have been used to investigate the extended lattice, of a perfect crystal, for various ET salts. The chosen forcefield, ESFF, reproduces known crystal structures very well, although the complexity of the materials of interest limits the ability to predict new structure. As the electronic properties of ET salts depend strongly on the packing of the donor- radical cations, electronic structure calculations have been performed for various salts at the molecular and periodic level using Density Functional Theory (DFT). These calculations have been used to investigate the magnetic ordering effect of the BEDT-TTFFeBr4 salt. By fixing the overall spin-state of the system we compared the relative energies of the various spin states. The intermolecular spin ordering was found to be primarily mediated via the covalent sulphur-sulphur interaction on neighbouring BEDT-TTF cations. We have performed optimisation calculations with periodic boundary conditions for some of the smaller examples of ET salts. Part II of the thesis details the parameterisation of the COMPASS forcefield to include electronic polarisation by implementation of the shell model for metal oxide systems to allow the study of surface adsorption by interatomic potential methods. The capabilities of the interatomic potentials are demonstrated by calculations of the adsorption of various small molecules on surface models. Finally, an investigation of the nucleation of silicate fragments around sodium cations, by calculations using DFT, is reported in Appendix I.

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