Computer modelling studies of mackinawite, greigite and cubic FeS.
Doctoral thesis, UCL (University College London).
In this thesis we develop rigorous theoretical models for the simulation of the iron sulfides mackinawite, greigite and cubic FeS using both ab initio and interatomic potential methods. The mineral mackinawite (tetragonal FeS) takes a layered PbO-type structure, with Fe atoms coordinated tetrahedrally to S ligands. We have used GGA+U calculations to show that the inter-layer interaction is very difficult to accurately describe using this form of DFT, and instead a single-layer formulism is developed which allows the modelling of the electronic and magnetic properties of a single layer of mackinawite. These results are used to derive an interatomic potential to investigate the surfaces of this phase, and we use the calculated surface energies to successfully reproduce the observed crystal morphology of mackinawite. The effect of impurity atoms in the interlayer sites is investigated, and it is found that these contribute considerably to the stabilisation of the mackinawite structure. Greigite (Fe3S4) is the iron sulfide analogue of the famous iron oxide magnetite. We use spin-polarised GGA+U calculations to model the magnetic and electronic structure of greigite, and this phase is found to be most accurately described using an applied Ueff value of 1 eV. Further calculations show that a Verwey-type low temperature transition in greigite is energetically unfavourable. Cubic FeS takes the cubic sphalerite structure at room temperature. A low temperature transition to an antiferromagnetic orthorhombic structure has been observed experimentally. GGA+U calculations demonstrate that applying a value for the Hubbard Ueff parameter of 2 eV provides an excellent description of both the lowand high-temperature structures. It is found that the previously derived potential for mackinawite predicts the cubic FeS structure as well as non-spin-polarised GGA. The work described in this thesis has provided a greater understanding of the electronic, magnetic and structural properties of these iron sulfides.
|Title:||Computer modelling studies of mackinawite, greigite and cubic FeS|
|Open access status:||An open access version is available from UCL Discovery|
|UCL classification:||UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Chemistry|
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