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Structure, Electronic Structure and Electronic Spectra of Simple Materials at High Pressure



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Under extreme conditions such as temperature and pressure, the chemical bonding, electronic structures and properties of materials undergo significant changes that leads to the discovery of new and unusual chemical species not obtainable at ambient conditions. Hence, chemical bonding plays a significant role in the description of systems in physics, solid state chemistry, material science etc. This makes the study and immense understanding of the structure and chemical bonding of solids significant and constitutes one of the main objectives of this thesis. The second part of this thesis employed state-of-the-art ab initio molecular dynamics simulation to reconstruct the phase transition in elemental Cs. Also, the Bethe-Salpeter Equation (BSE) was used to calculate the X-ray Absorption Spectra (XAS) and Non-Resonant Inelastic X-ray Scattering (NRIXS) spectra of crystalline ice Ih and compressed water. In the first project, the structure and bonding analysis of K2Ag and K3Ag intermetallics were studied at 4.0GPa and 6.4GPa respectively by employing all available bonding analysis methods. Analysis of the K2Ag reveal the K atom transfers electrons to the Ag atom and forms K-K, K-Ag and Ag-Ag closed shell interactions with the K-Ag being the strongest bond interaction present in the compound. Contrary to the K2Ag, topological analysis of the K3Ag yielded no Ag-Ag bond interaction. This is due to the very large bond length of the first nearest neighbour Ag-Ag interaction. All the plane wave and localized basis set dependent bond analysis methods employed gave consistent results. However, the projected density of state (PDOS) computed using the localized basis set method implemented in the LOBSTER code should always be checked against the PDOS calculated using a plane wave method before validating the crystal orbital overlap population (COOP) and crystal orbital Hamiltonian population (COHP) results from the LOBSTER code. In summary, the results from this study show that, all the bonding analysis techniques should be carefully applied when treating high pressure systems, due to the extensive modification of the electron density on application of pressure. Hence, a naive localized description is not appropriate and may lead to erroneous interpretation. The second project focused on the analysis of bonding in the three phases of Na-Au intermetallics following the benchmark established in the first project. Analysis of the phase I Na2Au structure at 0.83GPa revealed the presence of non-nuclear maximum (NNM) in the structure commonly known as electrides. The obtained NNMs were found to form off the Na atoms in agreement with the experimental maximum entropy method (MEM) analysis. The experimental structure of the Phase II Na3Au intermetallics was found to have either a trigonal Cu3As or hexagonal Cu3P-type structure. The two structures could not be distinguished from experiment and DFT equations of state. However, through topological analysis of both structures, only the tetragonal structure does satisfy the Morse sum and is thus said to be the accurate phase II structure as it is topologically stable. Further analysis of the topologically stable phase II structure at 2GPa and the phase III Na3Au at 51.7GPa yielded no NNMs. This implies the Na-Au intermetallics are stabilized by decreased localization of electrons at the interstitial sites at high pressure, contrary to elemental alkali metals that show increased localization of interstitial electrons at high pressure. Finally, Bader's quantum theory of atoms in molecule (QTAIM) revealed all the bond interactions present in the structures are closed shell interactions. The third project reconstructs the phase transition paths of elemental Cs around the complex Cs-III in other to define the transition mechanism. In addition, topological properties of the Cs-II, Cs-III and Cs-IV structures were examined and the result show electrides in the three phases. The molecular dynamics results reveal the transition in the Cs-III to Cs-IV and Cs-II to Cs-III transformations are typical crystalline solid-solid transitions with no evidence of melting in the transition states. In addition, the transformation mechanism observed in the Cs-III to Cs-IV is not martensitic ( i.e a transformation that occurs through a diffusionless cooperative motion of all the atoms in a transformation region) rather it occurs through nucleation and growth. The Cs-II to Cs-III transformation on the other hand was found to occur through a cooperative motion of all the atoms in the super cell. Also, the results suggest existence of a very large activation barrier for the reverse transformation to Cs-II from a backward (i.e Cs-III to Cs-II) transition. In the final project, BSE method was employed to calculate the XAS and NRIXS of crystalline ice Ih and compressed water at different momentum transfer values. Theoretical spectra computed using snapshots from the PICMD simulation performed here yield results in good agreement with experiment for both water and ice Ih. Further analysis of the trajectories revealed the water maintain approximate tetrahedral coordination and not dramatically different from crystalline ice. In addition, the results show dense water form interpenetrating hydrogen bonds by compressing the second nearest neighbour water molecules into the first coordination shell similar to the behaviour of high density ice.



Electronic Structure, X-ray Absorption Spectra, Chemical Bonding, Band Structure, Molecular Dynamics



Doctor of Philosophy (Ph.D.)


Physics and Engineering Physics




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