This thesis presents the results of ab initio based simulation studies of magnetite (Fe₃O₄) and related FeM₂X₄ (thio)spinels with M = Cr, Mn, Fe, Co and Ni and X = O and S. Using density functional theory with long-range dispersion correction and on-site Coulomb interactions (DFT + U – D2), we have investigated a number of properties of these materials. Firstly, we present a study of the inversion degree and its relevance in the electronic structure and magnetic properties of the spin filter candidates FeM₂X₄, which are one of the key devices in spintronic applications. We also analyze the role played by the size of the ions and by the crystal field stabilization effects in determining the equilibrium inversion degree. Secondly, we present the calculations of the elastic constants and other macroscopic mechanical properties by applying elastic strains on the unit cell of Fe₃O₄, which is the main component in different types of catalysts used in myriad of industrial processes. Thirdly, we calculate the geometries and surface free energies of a number of Fe₃O₄ surfaces at different compositions, including the non-dipolar stoichiometric plane, and those with a deficiency or excess of oxygen atoms. We propose a morphology in thermodynamic equilibrium conditions for the nanocrystals of this compound. We also present the simulated scanning tunnelling microscopy images of the different terminations of the surfaces shown on the Fe₃O₄ morphology. Finally, we investigate the initial oxidation stages of the greigite (Fe₃S₄) (001) surface induced by water. Fe₃S₄ is a mineral widely identified in anoxic aquatic environments and certain soils, which can be oxidised by these environments producing and extremely acid solution of sulfur-rich wastewater called acid mine drainage (AMD). We propose a number of mechanisms involving one or two water molecules and one OH group to explain the replacement of one sulfur by one oxygen atom in this mineral. The findings presented in this thesis provides a theoretical insight into various bulk and surface properties of this group of compounds.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:639598 |
| Date | January 2015 |
| Creators | Santos Carballal, D. |
| Publisher | University College London (University of London) |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://discovery.ucl.ac.uk/1463302/ |
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