We study the effects of vibrations in the Pd₃ system using first-principles pseudopotential calculations. We find that upon disordering from the DO₂₂ phase, the decreases by 0.07kB. We explain our results in terms of atomic relaxations and size effects. / Singapore-MIT Alliance (SMA)

In recent years, palladium (Pd) has been extensively studied for a possible alternative for Pt that has been most commonly used as a catalyst in fuel cells. However, Pd shows lower activity than Pt towards the cathodic oxygen reduction reaction (ORR) and also exhibits poor tolerance toward carbon monoxide (CO) poisoning occurring in the anode process. To improve its performance, alloying Pd with other transition metals has been suggested as one of promising solutions as the Pd-based alloys have been found to boost the ORR activity and yield significant improvement in the CO tolerance. However, a detailed understanding of the alloying effects is still lacking, despite its importance in designing and developing new and more cost effective fuel cell catalysts. This is in large part due to the difficulty of direct characterization. Alternatively, computational approaches based on quantum mechanics have emerged as a powerful and flexible means to unravel the complex alloying effects in multimetallic catalysts; such first principles-based computational studies have provided many invaluable insights into the mechanisms of catalytic reactions occurring on the alloy surfaces. Using first-principles density-functional theory calculations, we have examined the surface reactivity of Pd-based bimetallic catalysts with the aim of better understanding the alloying effects in association with atomic arrangement, facet, local strain, ligand interaction, and effective atomic coordination number at the surface. More specifically, this thesis work has focused on examining the following topics: Role of Pd ensembles in selective H₂O₂ formation on AuPd alloys; Effect of local strain and low-coordination number at the surface on the performance of Pd monomer in selective H₂O₂ formation; Different facet effects on the activity of Pd ensembles towards ORR; Structure of ternary Pd-Ir-Co alloys and its reactivity towards ORR; Pd ensembles effects on CO oxidation on CO-precovered Pd ensembles; Role of ligand and ensembles in determining CO chemisorptions on AuPd and AuPt. Our first principles-based theoretical investigation of bimetallic alloys offers some insights into the rational design and development of alloyed catalysts. / text

Hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) is one of the most abundant materials in mammal bone. It crystallizes in an aqueous environment within spaces between tropocollagen protein chains. However, despite its abundance and possible usefulness in the medical field this complex physical system remains poorly understood to date. We present a theoretical study of the energetics of hydroxyapatite, its electronic, mechanical and thermodynamic properties. Our mechanical and thermodynamic properties from first principles are in excellent agreement with the rare available experimental data. The monoclinic and hexagonal phases are lowest in energy. A comparison of the phonon dispersions of these two phases reveals that a phase transition occurs due to a difference in vibrational free energy. The transition is of order-disorder type. Our calculated phase transition temperature is 680 K, in decent agreement with the experimentally determined 470 K. An alternative theoretical model yields 882 K. The phase transition is mediated by OH libration modes. We also report for the first time on a peculiarity in the phonon spectrum of hexagonal and monoclinic HA. When studying the Lyddane-Sachs-Teller shifts in the spectrum close to the [Gamma]-point we identify two vibration modes showing a systematically increasing Lyddane-Sachs-Teller shift in frequency with decreasing dielectric constant. In experiment, the dielectric constant varies between 5 and 20 depending on the Ca/P ratio in the sample. The frequency shifts in the affected modes are as large as 20 cm⁻¹ as one spans the range of the dielectric constant. Thus, a simple spectroscopic analysis of a sample of bone may determine the quality of the sample in a physiological sense. We also identify the chemically stable low energy surface configurations as function of the OH, PO₄ and Ca concentration. In the experimentally relevant OH-rich regime we find only two surfaces competing for lowest energy. The surface most stable over almost the entire OH-rich regime is OH-terminated, and is currently being investigated in the presence of water and atomic substitutions on the HA surface. / text

The main goal of this research was to apply first-principles electronic structure calculations to investigate atomic motions in several condensed materials. This thesis consists of five separate but related topics that are classified into two main categories: structure of materials under pressure and Li ion dynamics in lithium battery materials. The atomic structure of liquid gallium was investigated in order to resolve a controversy about an anomalous structural feature observed in the x-ray and neutron scattering patterns. We explored the pressure effect when modifying the liquid structure close to the solid-liquid melting line. The atomic trajectories obtained from first-principles molecular dynamics (FPMD) calculations were examined. The results clarified the local structure of liquid gallium and explained the origin of a peculiar feature observed in the measured static structure factor. We also studied the structure of a recently discovered phase-IV of solid hydrogen over a broad pressure range near room temperature. The results revealed novel structural dynamics of hydrogen under extreme pressure. Unprecedented large amplitude fluxional atomic dynamics were observed. The results helped to elucidate the complex vibrational spectra of this highly-compressed solid. The atomic dynamics of Li ions in cathode, anode, and electrolyte materials - the three main components of a lithium ion battery - were also studied. On LiFePO4, a promising cathode material, we found that in addition to the commonly accepted one-dimensional diffusion along the Li channels in the crystal structure, a second but less obvious multi-step Li migration through the formation of Li-Fe antisites was identified. This discovery confirms the two-dimensional Li diffusion model reported in several Li conductivity measurements and illustrates the importance of the distribution of intrinsic defects in the enhancement of Li transport ability. The possibility of using type-II clathrate Si136 as an anode material was investigated. It was found that lithiated Si-clathrates are intrinsic metals and their crystal structures are very stable. Calculations revealed the charge and discharge voltages are very low and almost independent of the Li concentrations, an ideal property for an anode material. Significantly, migration pathways for Li ions diffusing through the cavities of the clathrate structures were found to be rather complex. Finally, the feasibility of a family of Li3PS4 crystalline and nanoporous cluster phases were studied for application as solid electrolytes. It was found that the ionic conductivity in the nanocluster is much higher than in crystalline phases. It is anticipated that the knowledge gained in the study of battery materials will assist in future design of new materials with improved battery charge and discharge performance.

The Fröhlich coupling describes the interaction between electrons and infrared-active vibrations at long wavelength in polar semiconductors and insulators, and may result in the formation of polaronic quasiparticles. Polarons are electrons dressed by a phonon cloud, which can strongly affect the electronic properties of the crystal. Despite their ubiquitous role in a broad range of technologies, first-principles investigations of the electron-phonon interaction in polar materials are scarce. In this thesis we develop a general formalism for calculating the electron-phonon matrix element in polar semiconductors and insulators from first principles, which represents a generalization of the Fröhlich model and can be used to compute the polar electron-phonon coupling as a straightforward post-processing operation. We apply this procedure to explore an important material for photovoltaics, the hybrid lead halide perovskite CH₃NH₃PbI₃. In this case we show that the temperature dependence of emission line broadening is dominated by Fröhlich coupling. Our method is formulated in conjunction with an ab initio interpolation technique based on maximally localized Wannier functions, which allows to describe all forms of electron-phonon coupling on the same footing. We demonstrate the validity of this approach on the prototypical examples GaN and SrTiO₃. Focusing on anatase TiO₂, a transition metal oxide of wide technological interest, we establish quantitatively the effect of including the ab initio Fröhlich coupling in the calculation of electron lifetimes. The rest of the thesis is devoted to exploring the quasiparticle properties in doped oxides. In particular, we investigate angle-resolved photoemission spectra from first principles in doped anatase TiO₂ by proposing a novel framework that combines our ab initio matrix elements, including the dynamical screening arising from the added carriers, and the cumulant expansion approach. We compare our results with experimental data, and show that the transition from a polaronic to a Fermi liquid regime with increasing doping concentration originates from nonadiabatic polar electron-phonon coupling. We further validate this mechanism by calculating angle-resolved photoemission spectra in the ferromagnetic semiconductor EuO.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第16816号 / 工博第3537号 / 新制||工||1535(附属図書館) / 29491 / 京都大学大学院工学研究科社会基盤工学専攻 / (主査)教授 松岡 俊文, 教授 垣内 隆, 准教授 西山 哲 / 学位規則第4条第1項該当

Fundamental advances in cathode materials are key to lowering the operating temperature of solid oxide fuel cells (SOFCs). Detailed understanding of the structural, electronic and defect formation characteristics are essential for rational design of cathode materials. In this thesis we employ first principles methods to study La(Mn/Co)O3 and LnBaCo2O5+δ (Ln = Pr, Gd; δ = 0.5, 1) as cathode for SOFCs. Specifically, factors affecting the O vacancy formation and migration are investigated. We demonstrate that for LaMnO3 the anisotropy effects often neglected at high operating temperatures become relevant when the temperature is lowered. We show that this fact has consequences for the material properties and can be further enhanced by strain and Sr doping. Tensile strain promotes both the O vacancy formation and migration in pristine and Sr doped LaMnO3, while Sr doping enhances the O vacancy formation but not the migration. The effect of A-site hole doping (Mg2+, Ca2+ or Ba2+) on the electronic and magnetic properties as well as the O vacancy formation and migration in LaCoO3 are studied. All three dopants are found to facilitate O vacancy formation. Substitution of La3+ with Ba2+/Mg2+ yields the lowest O vacancy formation energy for low/intermediate spin Co, implying that not only the structure, but also the spin state of Co is a key parameter. Only for low spin Co the ionic radius is correlated with the O migration barrier. Enhanced migration for intermediate spin Co is ascribed to the availability of additional space at the transition state. For LnBaCo2O5+δ we compare the O vacancy formation in GdBaCo2O5.5 (Pmmm symmetry) and GdBaCo2O6 (P4/mmm symmetry), and the influence of Sr doping. The O vacancy formation energy is demonstrated to be smaller in the already O deficient compound. This relation is maintained under Sr doping. It turns out that Sr doping can be utilized to significantly enhance the O vacancy formation in both compounds. The observed trends are explained on a microscopic level. Furthermore, we consider antisite defects as they may modify the electronic and O migration properties but are rarely studied in double perovskite oxides. It turns out that O vacancy formation is significantly easier in PrBaCo2O5.5 than in GdBaCo2O5.5, the difference in formation energy being hardly modified by antisite defects. Finally, having established that the O vacancy formation energy is significantly lower in PrBaCo2O5.5 than in GdBaCo2O5.5, we study the O Frenkel energy and migration of O ions in PrBa(Co/Fe)2O5.5. The electronic structure and charge redistribution during defect formation are analyzed. We demonstrate that Co↔Fe substitution strongly affects the formation of defects and, consequently, the O migration. The low O Frenkel energy points to a high concentration of O vacancies. The migration of the O ions shows a distinct anisotropy.

Point defects in silicon, vanadium dioxide, and doped ceria are investigated by density functional theory. Defects involving vacancies and interstitial oxygen and carbon in silicon are after formed in outer space and significantly affect device performances. The screened hybrid functional by Heyd-Scuseria-Ernzerhof is used to calculate formation energies, binding energies, and electronic structures of the defective systems because standard density functional theory underestimates the bang gap of silicon. The results indicate for the A-center a −2 charge state. Tin is proposed to be an effective dopant to suppress the formation of A-centers. For the total energy difference between the A- and B-type carbon related G-centers we find close agreement with the experiment. The results indicate that the C-type G-center is more stable than both the A- and B-types. The electronic structures of the monoclinic and rutile phases of vanadium dioxide are also studied using the Heyd-Scuseria-Ernzerhof functional. The ground states of the pure phases obtained by calculations including spin polarization disagree with the experimental observations that the monoclinic phase should not be magnetic, the rutile phase should be metallic, and the monoclinic phase should have a lower total energy than the rutile phase. By tuning the Hartree-Fock fraction α to 10% the agreement with experiments is improved in terms of band gaps and relative energies of the phases. A calculation scheme is proposed to simulate the relationship between the transition temperature of the metal-insulator transition and the dopant concentration in tungsten doped vanadium dioxide. We achieve good agreement with the experimental situation. 18.75% and 25% yttrium, lanthanum, praseodymium, samarium, and gadolinium doped ceria supercells generated by the special quasirandom structure approach are employed to investigate the impact of doping on the O diffusion. The experimental behavior of the conductivity for the different dopants is understood in terms of the calculated lattice constants and the O migration barriers obtained from nudged elastic band calculations.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19684号 / 工博第4139号 / 新制||工||1639(附属図書館) / 32720 / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 北村 隆行, 教授 立花 明知, 教授 鈴木 基史 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20699号 / 工博第4396号 / 新制||工||1683(附属図書館) / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 北村 隆行, 教授 西脇 眞二, 教授 鈴木 基史 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM