The objective of this thesis is to study the crystal structures and electronic properties of solids at high pressure using state-of-the-art electronic structure computational methods. The thesis is divided into two main sections. The first part is to examine the performance and reliability of several current density functionals in the description of the electronic structures of small band gap materials and strongly correlated systems. The second part is to compare and evaluate two recently proposed first-principles methods for the prediction of stable structures of solids at high pressure. To accomplish the first goal, first-principle electronic structure calculations employing density functional theory (DFT) and several “correlation corrected” functionals calculations were used to investigate the properties of solid AlH3 and EuO at high pressure. The primary reason to study AlH3 is to resolve a discrepancy between previously predicted superconductivity behavior at 110 GPa but was not observed in experimental resistance measurements. The key to resolve the discrepancy is an accurate calculation of the valence and conduction band energies. The results shows that the Fermi surface is modified by the “improved” functionals over the previous calculations using “standard” gradient corrected functional. These changes in the Fermi surface topology removed the possibility of nesting of the electronic bands, therefore, solid AlH3 above 100 GPa is a poor metal instead of a superconductor. In the second system, we have studied EuO with highly localized electrons in the Eu 4f orbitals. A particular interest in this compound is the report of an anomalous isostructural phase transition with a significant volume reduction at 35-40 GPa and the relationship with the electronic state of Eu at high pressure. Using the Hubbard on-site repulsion model (LDA+U), we successfully predicted the insulator  metal transition of EuO at 12 GPa and the trend in the Mössbauer isomer shifts. However, the isostructural transition was not reproduced. The U on-site repulsion to localized Eu 4f orbtials helped to ameliorate some deficiencies of the PBE functional and improved the agreement with experimental observations but not all the properties were correctly reproduced. The second objective of this investigation is to predict energetically stable crystalline structures at high pressure. The reliability and relative efficiency of two recently proposed structure prediction methods, viz, Particle Swarm Optimization (PSO) and the Genetic Algorithm (GA) were critically examined. We applied the techniques to two separate systems. The first system is solid CS2. The motivation is that this compound was recently found to be a superconductor with a critical temperature of 6 K from 60 – 120 GPa. However, no crystalline structure was found by experiment in this pressure range. Our calculations suggest the energetic favorable structures contain segregated regions of carbon and sulfur atoms. The sulfur atoms adopt a planar closed pack arrangement forming 2D square or hexagonal networks and the carbon atoms tend to form hexagonal rings. A global minimum crystalline structure with structural features observed in the amorphous structure was found and shown to be superconductive. In the second case, we studied the possibility on the existence of Xe-halides (XeHn (H=Cl, Br and I, n = 1, 2 and 4)) compounds below 60 GPa. We reported the stability, crystal and electronic structures, vibrational and optical spectra of a number of stoichiometric crystalline polymorphs. We found that only XeCl and XeCl2 form thermodynamically stable compounds at pressure exceeding 60 GPa. A stable cubic fcc structure of XeBr2 was found to be a superconductor with critical temperature of 1.4 K. From these studies, we found both merits and shortcomings with the two structural prediction approaches. In the end, we proposed a hybrid approach to assure the same stable structure is predicted from both computational strategies.

The work presented in this dissertation focuses on the [FeFe]-hydrogenase active site as inspiration for the design and synthesis of complexes capable of the electrocatalytic generation of molecular hydrogen from protons and electrons. The majority of work discussed uses gas-phase photoelectron spectroscopy (PES) and density functional theory (DFT) to probe and analyze the bonding and electron distribution in potential catalysts. These two techniques are also used to explore the nature of cyanide as a ligand, due to its presence and unknown role in these enzymes. This dissertation begins with the study of (η⁵-C₅H₅)Fe(CO)₂X (FpX) and (η⁵- C₅Me₅)Fe(CO)₂X (Fp*X) complexes where X = H⁻, Cl⁻, and CN⁻ to assess and compare their π-accepting abilities, which is contradicted in the literature. The shifts in ionization energies measured by PES provide a measure of the relative bonding effects. The results indicate cyanide is, overall, a weak π-acceptor, and the σ- and π-donor interactions are important to understanding the chemistry. The molecule [(μ-ortho-C₆H₄S₂)][Fe(CO)₃]₂ was examined, in part due to the delocalized π-orbitals of the C₆H₄S₂ ligand, which could facilitate the redox chemistry necessary for catalysis. Computations show that upon ionization, the complex adopts a semi-bridging carbonyl; termed “rotated structure”. The reorganization energy of this geometry change was determined, which may provide understanding of how the active site in the enzyme enables electron transfer to achieve this catalysis. Next complexes of the form (μ-SCH₂XCH₂S)[Fe(CO)₃]₂, where X=CH₂, O, NH, ᵗBuN, MeN, were explored in order to provide insight to the unknown atom at the central bridging position of the alkyl chain in the [FeFe]-hydrogenase enzyme. The likelihood of a rotated cationic structure is also shown, with reorganization energy values similar to that seen for [(μ-ortho-C₆H₄S₂)][Fe(CO)₃]₂. The final chapter explores the replacement of selenium for sulfur in (μ- X(CH₂)₃X)[Fe(CO)₃]₂ and (μ-X(CH₂)₂CH(CH₃)X)[Fe(CO)₃]₂, where X is either sulfur or selenium. The PES data show destabilization of the selenium complex ionizations compared to the sulfur complexes and a lower reorganization energy was calculated. The computed HOMO-LUMO gap energy for the selenium-based complex is roughly 0.17 eV smaller than for the sulfur analogs, which may indicate a lower reduction potential is needed.

The properties of a new family of carbon structures based on stacked cyclic C6 rings and intercalated cyclic C6 structures: (C6)n and (C6)nMen-1 have been studied theoretically using ab initio DFT (Density Functional Theory). Calculations of the structural, electronic, and vibrational properties of a range of these molecules have been carried out using DFT techniques with the best correspondence to experimental results. The chemical and structural stability of structures based on stacks of cyclic C6 has also been estimated for pure carbon molecules (C6)n and for metal-organic sandwich molecules intercalated with Fe and Ru atoms. These have (C6)nFen-1 and (C6)n Run-1 compositions, respectively These structures are predicted to show a variety of new electronic, vibrational and magnetic properties. Ultra-small diameter tubular molecules are also found to have unique rotational electron states and high atomic orbital pi-sigma hybridization giving rise to a high density of electron states. All phonons in these structures have collinear wave vectors leading to an ultrahigh density of phonon states in dominant modes suggesting that some of these structures may exhibit superconductivity. These properties, as well as a predicted high electron mobility, make these structures promising as components in nanoelectronics. Experiments using femto-second laser pulses for the irradiation of organic liquids suggest that such structures may appear under certain conditions. In particular, a new type of iron carbide has been found in these experiments.

This thesis presents the results of torque magnetometry and resistivity measurements of the electronic structure of novel quantum materials, specifically using the techniques of quantum oscillations and angle-dependent magnetoresistance oscillations. Measurements of the Fermi surfaces of these materials, alongside comparisons to the electronic structure predicted by density functional theory calculations, can elucidate much about the novel physical properties they exhibit and the competing interactions which govern their phase diagrams. The first system studied is the Iron-based superconductor FeSe_1-xS_x, an isoelectronically doped version of a system of much current interest, FeSe. Doping up to x = 0.2 is found to suppress the structural transition in this system entirely, with superconductivity continually present at low temperatures. Shubnikov-de Haas measurements across this range find a small quasi-two dimensional Fermi surface that increases in size and warping continuously with doping, with orbital dependent effective masses that do not change significantly within the orthorhombic phase. The second material studied is the antiferromagnetic intermetallic CeZn₁₁ which, featuring an unpaired 4f electron, is considered a possible candidate for heavy fermion behaviour. De Haas-van Alphen oscillations are seen once the antiferromagnetic phase is suppressed, and comparable frequencies of oscillation are measured in the non-magnetic analogue LaZn11, although with relatively smaller effective masses. GGA+U calculations, once magnetic breakdown is considered, match well the measured frequencies, confirming CeZn₁₁ to be a localised moment system with the 4f electron well below the Fermi level. The final material studied is the transition metal dichalcogenide IrTe₂, which undergoes dimerisation upon cooling into a number of possible charge modulated structures. Low temperature de Haas-van Alphen measurements find multiple domains of a quasi-two dimensional Fermi surface, no longer perpendicular to the lattice planes. Angular-dependent magnetoresistance oscillations observe a similarly tilted quasi-one dimensional Fermi surface, again with many domains present. Together these measurements confirm the unusual dimensionality of the dimerised Fermi surface of IrTe₂.

No description available.

Harmonic vibrational frequencies of several small organic molecules which were used to validate the scaled quantum mechanical (SQM) force field procedure of Pulay et al. were calculated using six popular density functional (DFT) methods and compared with experimental results. The combination of Becke's exchange with either Lee-Yang-Parr (BLYP) or Perdew's correlation functional (BP86) reproduces the observed frequencies satisfactorily with deviations similar to those of the Hartree-Fock SQM methods. Three hybrid DFT methods are found to yield frequencies which were generally higher than the observed fundamental frequencies. When the calculated frequencies were compared with 'experimental' harmonic frequencies however, Becke's three-parameter hybrid method with Lee-Yang-Parr correlation functional is found to be slightly more accurate, especially for C-H stretching modes. The results indicate that BLYP calculation is a very promising approach for understanding the observed spectral features.

The chemistry of fullerenes, which are a class of carbon allotropes that can be prepared by vaporization of graphite in an electric arc in a low pressure atmosphere,1 has become a topic of much experimental and theoretical study over the past 25 years.  Herein we present a series of theoretical studies related to recently discovered or studied endohedral metallofullerenes (EMF) and a theory as to the selective stability of certain isomers of EMFs. Computational treatments of the anions of C80 and C94 are presented and compared in an effort to gain an understanding and predictive model for which isomers of each cage size EMF will be most stable.  A model is proposed in which the pentagons of fullerene anions are seen as charge localization centers that repel one another, making the pyracyclene bonding motif much more unstable for fullerene anions than for fullerene neutral cages. Computational treatments are also presented for two newly discovered EMFs, Y2C2@C92 and Gd2@C79N.  Y2C2@C92 is reported to exhibit a previously undiscovered mode of internal cluster rotation, while Gd2@C79N is shown to have unusual stability for an azofullerene with a large spin quantum number (15/2). Finally, computational techniques are employed to predict the thermodynamic feasibility of a chemical reaction replacing one metal atom in a trimetallic-nitride template (TNT) endohedral metallofullerene with different metal atom.  At least two of these are predicted to be thermodynamically practical. / Ph. D.

We investigate the effect of the substitution of Ni, Ti and Co in MnBi using first principles calculations based on density functional theory (DFT) within the generalized gradient approximation (GGA). We also performed total energy calculations to compare different structures to determine the ground state structures and investigate their magnetic properties. Our calculation shows that the substitution of Ni, Co and Ti lowers the total magnetization of MnBi. We also found that the stable structure of Ni and Ti substitute is to replace Mn atoms in their regular site while the substitute Co is most stable when Co occupies the interstitial site of MnBi unit cell.

Nowadays, studying materials for renewable energy applications are highly de- manded. Hybrid halide perovskites have proven to be promising materials for such technology since their first application in solar cells in 2008, with a power conversion efficiency of 2.7%. Since then, hybrid halide perovskites have proven their superior properties for light-absorbing devices. In this scope, studying the optical properties is ultimately essential. This work investigates the tempera- ture dependence of the optical spectra for formamidinium lead iodide/bromide perovskites (FAPb[IxBr1-x]3 (0 ≤ x ≤ 1) using spectroscopic ellipsometry mea- surements, empirical optical modeling, density functional theory, and molecular dynamics. Five FAPb[IxBr1-x]3 perovskite samples were fabricated by a hybrid processing technique. External Quantum Efficiency measurements reported an energy bandgap range between 1.58 eV and 1.77 eV for the resulted samples. Next, multi-angle spectroscopic ellipsometry measurements were applied with a temperature-controlled stage, allowing the variance of temperature from 25 ◦C to 75 ◦C. The results show a blue shift in the optical spectra at elevated tempera- tures. We then conducted a temperature-dependent empirical model that predicts the optical spectra for the sample of study at higher temperatures using input data of the spectra at room temperature. The model reports low mean squared errors which are less than ≈ 2 around the bandgap, and further development can be applied for better utilization. First-principles investigations were conducted on four FAPb[IxBr1-x]3 per- ovskite unit cells. Structural optimization was applied with assuming fixed angles of the lattice. Atomic configuration was chosen to achieve minimal ground state energies. Ab initio molecular dynamics simulations were applied to each opti- mized structures at target temperatures of 300 K and 350 K using Berendsen thermostat. The simulation time was 4ps with 1fs time step, and the electronic energy bandgap was calculated at each step using PBE functional. The simula- tions reported a rotational motion for the FA molecule that showed to be faster at 350 K, along with higher mean energy bandgap compared to the reported value at 300 K. The optical spectra were extracted using a snapshot from the resulted structures. Similar to the spectroscopic ellipsometry measurements, a temperature induced blue shift was reported. Overall, this work detects and predicts the temperature-dependent optical spectra and confirms the role of the atomic thermal motion. With further devel- opment, higher accuracy can be achieved along with broadening the materials of study for photovoltaic and optoelectronic applications.

Understanding the phase behaviour of colloidal systems is relevant to designing new colloid-based nanostructured materials. One common platform for studying the colloidal system is the model of hard spheres. Over the last few decades, different hard-sphere models have been developed. We study the phase behaviour of three hard-sphere models: the lattice gas model, the local density approximation model, and the white bear version of the fundamental measure theory, with short-range attractive and long-range repulsive (SALR) interactions. The competition between the attraction and repulsion results in the formation of clusters composed of many particles, whereas the spatial arrangement of these clusters leads to the formation of long-range ordered phases. Phase diagrams containing the commonly observed body-center-cubic (BCC) and hexagonally close-packed (HCP) phases, as well as the novel Frank-Kasper $\sigma$ and A15 phases, have been constructed using the density functional theory applied to hard spheres with SALR interactions. Similar phase transition sequences have been predicted for the three hard-sphere models, implying a universality of the observed phase behaviour for hard spheres interacting with SALR potentials. However, the details of the phase diagrams could vary significantly. The results obtained from our study shed light on understanding the emergence of complex phases from simple systems. / Thesis / Master of Science (MSc) / Soft condensed matter physics, a sub-field of condensed matter physics, primarily concerns the investigation of physical properties of pliable, deformable materials such as plastics, gels, and colloidal suspensions. One particularly intriguing feature of these soft materials is their ability to self-assembly, leading to the spontaneous formation of ordered structures, including but not limited to body-centered cubic and face-centered cubic phases. In particular, a group of complex spherical phases, known as the Frank-Kasper phases, has been identified in various soft matter systems, encompassing polymeric blends, colloidal suspensions, and more. Notably, in colloidal systems, when nanoparticles are grafted with polymer chains, the Frank-Kasper phases could become stable. However, the emergence of these complex phases from the diverse soft matter systems have not been fully understood. In this thesis, we employ the classical density functional theory based on three different hard-sphere models to probe the formation of the Frank-Kasper phases in colloidal systems. Our results provide insights into the formation mechanism of the Frank-Kasper phases in a simple system and demonstrate the universality of different hard-sphere models.