First-principles Studies on the Structures and Properties of Glasses and Melts under Extreme Conditions
Date
2022-10-28
Authors
Journal Title
Journal ISSN
Volume Title
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ORCID
0000-0002-6555-9441
Type
Thesis
Degree Level
Doctoral
Abstract
The objective of this thesis is to study the bonding, electronic properties and chemical reactions of glasses and melts under high pressure. The work is mainly based on first- principles molecular dynamics (FPMD) simulations. State-of-the-art first-principles computational methods are employed in the further analyses of the MD trajectories to obtain the electronic properties. This thesis is composed of four projects and it is divided as follows.
The first project investigates the reaction of CaCO3 melts and H2 at pressure-temperature conditions similar to the Earth’s lower mantle and the core-mantle boundary via first-principles molecular dynamics (FPMD) simulations. Two models with different H2/CO3 2– ratios are studied under different pressure-temperature conditions. A variety of chemical reactions are observed. It is found that H dissociates readily and reacts with free CO3 2–, forming various transient chemical species and water molecules. Further reactions of these reactive species serve as intermediates to form C-C and C-O connections. The unreacted bulk carbonates are linked via polymeric-cornered shared CO4 tetrahedra. At 110 GPa and 4087 K, “diamondoids” with tetrahedral C4 moieties are found. This may be the precursor for diamond formation. The theoretical results support recent reports on the observation of tetrahedral CO4 in high-pressure carbonate glasses and suggest a plausible explanation of ice-VII inclusion in the deep-Earth diamonds.
The second project explores the bonding of B2O3 glass up to 350 GPa. The main concern is whether a higher order of B-O bonds can be formed at high pressure. Experiments on the B2O3 glass performed up to 125 GPa have suggested that the coordination numbers of B higher than 4 were presented. This proposal is puzzling since there are no low-lying d orbitals in the second-row elements, i.e., B and O. In this thesis, B K-edge X-ray absorption spectra (XAS) are calculated via the core-hole methods and the Bethe-Salpeter equation (BSE) method. Chemical shifts from nuclear magnetic resonance (NMR), Bader’s quantum theory of atoms in molecules (QTAIM), and electron localization function (ELF) are calculated to investigate whether extra B-O bonds can be formed in the B2O3 glass at high pressure. It is found that 5- and 6-coordinated B is formed under high pressure, but not all the close interactions of B and O are actual covalent bonds. Instead, the BO5 or BO6 clusters consist of 4 short B-O covalent bonds forming weaker B-O interactions with the other O atoms. In a parallel study, structure prediction of crystalline B2O3 is performed at similar pressures to the glass. No 6-coordinated B is found in the predicted structures.
The third project explores the bonding of amorphous SiO2 up to 198 GPa to investigate whether OSi4 quadclusters can be formed in SiO2 glass, as was claimed in a recent experiment. O K-edge X-ray Raman scattering (XRS) spectra are calculated. Various electronic structures and QTAIM analyses are performed. It is found that the OSi4 quadclusters do exist, but the four O-Si are not 4 equivalent covalent O-Si bonds. Instead, OSi4 quadclusters consist of 3 short O-Si bonds and 1 long O-Si with weaker interaction.
The final project starts with investigating the equation of states (EOS) of MgSiO3 glasses at high pressure via FPMD simulations. It is found that at low pressures (0 and 5 GPa), the calculated density of MgSiO3 glass well-reproduced the experimental density values. However, at higher pressures, the density from calculation is underestimated compared with the experiment, partly due to using the generalized gradient approximation (GGA). Despite the deviation in density, the structures of the MgSiO3 glass from calculation are in good agreement with experiments and other calculations. Inspired by the sudden perovskite to post-perovskite transform in the MgSiO3 crystal above 125 GPa and 2000 K, this thesis also studies whether there is a similar sudden change in the local structure of MgSiO3 melts. Analysis of the electronic structure of MgSiO3 melts reveals a semi-metallic/metallic property at high temperature, even at 0 pressure.
Description
Keywords
First-principles, High Pressure, Molecular Dynamics, Glasses and Melts
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Physics and Engineering Physics
Program
Physics