First-Principles Study on Structural Properties of GeO2_2 and SiO2_2 under Compression and Expansion Pressure

The detailed analysis of the structural variations of three GeO$_2$ and SiO$_2$ polymorphs ($\alpha$-quartz, $\alpha$-cristobalite, and rutile) under compression and expansion pressure is reported. First-principles total-energy calculations reveal that the rutile structure is the most stable phase among the phases of GeO$_2$, while SiO$_2$ preferentially forms quartz. GeO$_4$ tetrahedras of quartz and cristobalite GeO$_2$ phases at the equilibrium volume are more significantly distorted than those of SiO$_2$. Moreover, in the case of quartz GeO$_2$ and cristobalite GeO$_2$, all O-Ge-O bond angles vary when the volume of the GeO$_2$ bulk changes from the equilibrium point, which causes further deformation of tetrahedra. In contrast, the tilt angle formed by Si-O-Si in SiO$_2$ markedly changes. This flexibility of the O-Ge-O bonds reduces the stress at the Ge/GeO$_2$ interface due to the lattice-constant mismatch and results in the low defective interface observed in the experiments [Matsubara \textit{et al.}: Appl. Phys. Lett. \textbf{93} (2008) 032104; Hosoi \textit{et al.}: Appl. Phys. Lett. \textbf{94} (2009) 202112].Comment: 15 pages, 5 figures and 2 table

First principles calculation of vibrational Raman spectra in large systems: signature of small rings in crystalline SiO2

We present an approach for the efficient calculation of vibrational Raman intensities in periodic systems within density functional theory. The Raman intensities are computed from the second order derivative of the electronic density matrix with respect to a uniform electric field. In contrast to previous approaches, the computational effort required by our method for the evaluation of the intensities is negligible compared to that required for the calculation of vibrational frequencies. As a first application, we study the signature of 3- and 4-membered rings in the the Raman spectra of several polymorphs of SiO2, including a zeolite having 102 atoms per unit cell.Comment: 4 pages, 2 figures, revtex4 Minor corrections; accepted in Phys. Rev. Let

The Effect of Transfer Printing on Pentacene Thin-Film Crystal Structure

The thermal deposition and transfer Printing method had been used to produce pentacene thin-films on SiO2/Si and plastic substrates (PMMA and PVP), respectively. X-ray diffraction patterns of pentacene thin films showed reflections associated with highly ordered polycrystalline films and a coexistence of two polymorph phases classified by their d-spacing, d(001): 14.4 and 15.4 A.The dependence of the c-axis correlation length and the phase fraction on the film thickness and printing temperature were measured. A transition from the 15.4 A phase towards 14.4 A phase was also observed with increasing film thickness. An increase in the c-axis correlation length of approximately 12% ~16% was observed for Pn films transfer printed onto a PMMA coated PET substrate at 100~120 C as compared to as-grown Pn films on SiO2/Si substrates. The transfer printing method is shown to be an attractive for the fabrication of pentacene thin-film transistors on flexible substrates partly because of the resulting improvement in the quality of the pentacene film.Comment: 5 pages, 5 figure

X-선 Raman 산란 스펙트럼 양자계산을 이용한 지구내부에서의 압력 증가에 의한 결정질과 비결정질 SiO2 및 MgSiO3 물질의 국소전자구조 변화에 대한 연구

학위논문(박사)--서울대학교 대학원 :자연과학대학 지구환경과학부,2017. 8. 이성근.The potential presence of high-density SiO2- and MgSiO3-rich silicate melts has been suggested to be one of the origins of the ultralow velocity zones (ULVZ) at the lowermost mantle region. The pressure dependences of the transverse acoustic wave velocities of SiO2 and MgSiO3 glasses have been explored up to ~207 GPa to understand the elastic properties of SiO2- and MgSiO3-rich silicate melts in Earths interior. They have been suggested to be correlated with the coordination transition of Si atoms upon compression. However, because the elastic properties of amorphous oxides are determined from the short-range structures and associated electronic structures, the correlation between the densification processes of SiO2 and MgSiO3 glasses and associated changes in the electronic structures, as well as their elastic properties, should be discussed more precisely. The in situ high-pressure x-ray Raman scattering (XRS) experiments can probe the element-specific electronic bonding structures and associated local atomic structures around the target element of Earth materials at high pressures. Thus, it has been used to explore the structural changes around O atoms of SiO2 and MgSiO3 glasses from upon compression from the pressure-induced bonding transitions. Despite the efforts in the previous studies, establishing the direct correlation between the structural changes around O atoms and the evolution in O K-edge XRS features of SiO2 and MgSiO3 glasses has been experimentally challenging because of the intrinsic structural disorder in the amorphous oxides. Recent advances in the ab initio calculations have provided the opportunity to explore the electronic structures and XRS features of Earth materials at extremely high pressure that cannot be easily achieved in the current in situ high-pressure XRS experiments. Here, the l-resolved partial density of states (PDOS) and O K-edge XRS features of the SiO2 and MgSiO3 high-pressure polymorphs and the high-density noncrystalline MgSiO3 melts in a pressure range from ~0 to ~131 GPa were systematically calculated using the ab initio calculations. The pressure-induced changes in O K-edge XRS features of SiO2 and MgSiO3 high-pressure polymorphs, including an emergence of double-peak-like features of the SiO2 high-pressure polymorphs, are revealed to be correlated with the enhanced proximity between neighboring O atoms. In addition, the significant changes in O K-edge XRS features of MgSiO3 melts upon compression seem to be correlated with the decreases in interatomic distances around O atoms, primarily the O-O distances. Therefore, the pressure-induced changes in O K-edge XRS spectra of amorphous SiO2 and MgSiO3 glasses, from the in situ high-pressure XRS experiments, might be indicative of the decreases in interatomic distances around O atoms, particularly the O-O distances, rather than the coordination transition of Si and O atoms. These results suggest that the electronic structures of the crystalline and noncrystalline SiO2 and MgSiO3 phases at high pressures are strongly affected by changes in the O-O distances upon compression. Because changes in the elastic properties of amorphous oxides are induced from the short-range structural changes and associated changes in their electronic structures, the pressure dependences of transverse wave acoustic wave velocities of SiO2 and MgSiO3 glasses should be explained with changes in the nearest neighboring O-O distances rather than changes in the Si coordination environment. In further, I expect that the current study can be applied to future studies of the pressure-induced bonding transition of a wide range of crystalline and noncrystalline oxides at extremely high pressures.Chapter 1. Introduction 1 Chapter 2. Summary of this study 11 Chapter 3. Pressure-induced changes in local electronic structures of SiO2 and MgSiO3 polymorphs: Insights from ab initio calculations of O K-edge energy-loss near-edge structure spectroscopy 19 Abstract 20 Introduction 22 Calculations 28 Silica and Mg-silicate polymorphs 28 Electronic structure calculations 28 Calculation of PDOS and ELNES spectra 29 Results and discussion 31 Calculated O K-edge ELNES spectra for SiO2 and MgSiO3 polymorphs 31 Site-resolved O K-edge features of SiO2 and MgSiO3 polymorphs 35 Pressure-induced changes in O K-edge XRS spectra of MgSiO3 glasses 36 Appendix 39 Table and Figures 40 Chapter 4. Atomistic origins of pressure-induced changes in the O K-edge x-ray Raman scattering features of MgSiO3 high-pressure polymorphs: Insights from ab initio calculations 44 Abstract 45 Introduction 46 Calculations 52 Crystal structures 52 Calculating electronic structures and the O K-edge XRS spectra 53 Results and Discussion 56 Pressure-induced structural changes in MgSiO3 polymorphs 56 Pressure-induced changes in PDOSs of MgSiO3 polymorphs 56 Calculated O K-edge XRS spectra for MgSiO3 polymorphs 58 Pressure-induced changes in O K-edge XRS spectra of MgSiO3 glasses 64 Conclusion 67 Table and Figures 69 Appendix 84 Pressure-induced topological changes in the MgSiO3 bridgmanite 84 Partial DOS of the bridgmanite with varying pressure 85 Calculated O K-edge XRS spectra for post-bridgmanite 87 Chapter 5. Pressure-induced bonding transitions in MgSiO3 melts: Insights from ab initio calculations of oxygen K-edge x-ray Raman scattering spectrum 89 Abstract 90 Introduction 92 Calculations 99 MgSiO3 melt configurations at high pressures 99 Calculating PDOSs and O K-edge XRS spectra 102 Results and Discussion 104 Populations of Si and O species of MgSiO3 melts at high pressures 104 Partial radial distribution functions of MgSiO3 melts at high pressures 105 Topological changes of MgSiO3 melts upon compression 106 Calculated partial density of states for MgSiO3 melts at high pressures 112 Calculated O K-edge XRS spectra for MgSiO3 melts at high pressures 118 Pressure-induced changes in O K-edge XRS spectra of MgSiO3 melts 121 Origins of the pressure-induced changes in the O K-edge XRS spectra of MgSiO3 glasses 126 Implications 132 Conclusion 133 Figures 137 Appendix 145 Pressure estimation for the simulated MgSiO3 melt configurations 145 Populations of Si and O species in the MgSiO3 melts at high pressures 147 Mean square displacements of atoms in the MgSiO3 melts at high pressures 150 Partial radial distribution functions of SiO2 and MgSiO3 melts at ~0 GPa 152 Velocity auto-correlation functions of the MgSiO3 melts at high pressures 154 Band gap correction for the density of states of the MgSiO3 melts 155 Calculated O K-edge XRS spectra of the crystalline MgSiO3 phases 158 Pressure dependence of Si-O coordination 161 Chapter 6. Pressure-induced changes in O K-edge x-ray Raman scattering features of SiO2 high-pressure polymorphs: Implication for high-density SiO2 melt and glass in the Earths deep interior 162 Abstract 163 Introduction 165 Calculations 170 Crystal structures of the SiO2 high-pressure polymorphs 170 Calculating density of states and O K-edge XRS spectra 172 Results and Discussion 174 Structural characteristics of the SiO2 high-pressure polymorphs 174 Calculated l-resolved O PDOSs for the SiO2 high-pressure polymorphs 176 Calculated O K-edge XRS spectra for the SiO2 high-pressure polymorphs 176 The band gap and absorption threshold energies 180 Implications for the densified SiO2 glasses 181 Conclusion 184 Appendix 194 l-resolved PDOSs of the SiO2 high-pressure polymorphs 194 PDOSs and O K-edge XRS features of the coesite and penta-SiO2 structures 196 Pressure dependence of the densities of SiO2 high-pressure polymorphs 197 Spatial distribution of Si-O and O-O pairs 198 Band gap and absorption threshold energies 200 Chapter 7. Pressure-induced changes in Fe L2,3-edge x-ray Raman scattering spectra of Fe-including oxides at high pressures: Insights from ab initio calculations 202 Introduction 204 Calculations 205 Strongly correlated Fe 3d state (Hubbard U parameter) 206 Calculating the density of states 207 Calculating the Fe L2,3-edge XRS features 208 Results and discussion 208 Spin-state transition of Fe atom at high pressures 208 Calculated Fe L2,3-edge XRS features 210 Future study 210 The effects of Fe spin-state-transition to the Fe L2,3-edge XRS features 210 The effects of Hubbard U parameters to the Fe L2,3-edge XRS features 211 The effects of phase transition to the Fe L2,3-edge XRS features 212 Pressure-induced magnetization of FeO wustite 212 Fe L2,3-edge XRS features of noncrystalline Fe-including oxides 212 Figures 214 Chapter 8. Chemical shielding and electric field gradient tensors of Li atoms in lepidolite structures: Insights from ab initio calculations 220 Introduction 221 Calculations 222 Crystal structures of the lepidolite 223 Geometry optimization 224 Calculating NMR chemical shielding and EFG tensors 225 Results and discussion 226 Structural changes in the lepidolite due to the substitution of Li atoms 226 Calculated total energies for the lepidolite structures 228 Calculated Mulliken charges for the lepidolite structures 230 Calculated NMR parameters for the lepidolite structures 231 Conclusion 234 Figures and Tables 236 Appendix 241 Theoretical background 242 Practical approach for solving the unexpected glitch when using WIEN2k 250 Published to Journal of the Mineralogical society of Korea 263 Publication list 265 Conference list 268 References 272 Abstract of Korean 288Docto

Thermal conductivity of glasses above the plateau: first-principles theory and applications

Predicting the thermal conductivity of glasses from first principles has hitherto been a prohibitively complex problem. In fact, past works have highlighted challenges in achieving computational convergence with respect to length and/or time scales using either the established Allen-Feldman or Green-Kubo formulations, endorsing the concept that atomistic models containing thousands of atoms -- thus beyond the capabilities of first-principles calculations -- are needed to describe the thermal conductivity of glasses. In addition, these established formulations either neglect anharmonicity (Allen-Feldman) or miss the Bose-Einstein statistics of atomic vibrations (Green Kubo), thus leaving open the question on the relevance of these effects. Here, we present a first-principles formulation to address the thermal conductivity of glasses above the plateau, which can account comprehensively for the effects of structural disorder, anharmonicity, and quantum Bose-Einstein statistics. The protocol combines the Wigner formulation of thermal transport with convergence-acceleration techniques, and is validated in vitreous silica using both first-principles calculations and a quantum-accurate machine-learned interatomic potential. We show that models of vitreous silica containing less than 200 atoms can already reproduce the thermal conductivity in the macroscopic limit and that anharmonicity negligibly affects heat transport in vitreous silica. We discuss the microscopic quantities that determine the trend of the conductivity at high temperature, highlighting the agreement of the calculations with experiments in the temperature range above the plateau where radiative effects remain negligible (50<T <450 K).Comment: 24 pages, 13 figure

Octahedral conversion of a-SiO2-host matrix by pulsed ion implantation

This is the abstract. The results of measurements of X-ray photoelectron spectra (XPS) of a-SiO2-host material after pulsed implantation with [Mn+] and [Co+, Mn+]-ions as well as DFT-calculations are presented. The low-energy shift is found in XPS Si 2p and O 1s core-levels of single [Mn+] and dual [Co+, Mn+] pulsed ion-implanted a-SiO2 (E = 30 keV, D = 2*10^17 cm^-2) with respect to those of untreated a-SiO2.The similar changes are found in XPS Si 2p and O 1s of stishovite compared to those of quartz. This means that the pulsed ion-implantation induces the local high pressure effect which leads to an appearance of SiO6-structural units in alpha-SiO2 host, forming "stishovite-like" local atomic structure. This process can be described within electronic bonding transition from the four-fold "quartz-like" to six-fold "stishovite-like" high-pressure phase in SiO2 host-matrix. It is found that such octahedral conversion depends on the fluence and starts with doses higher than D = 3*10^16 cm^-2.Comment: 15 pages, 6 figures, 1 table, accepted in phys. stat. solidi (b

Defects and Dopants in CaFeSi2O6: classical and DFT simulations

Calcium (Ca)-bearing minerals are of interest for the design of electrode materials required for rechargeable Ca-ion batteries. Here we use classical simulations to examine defect, dopant and transport properties of CaFeSi2O6. The formation of Ca-iron (Fe) anti-site defects is found to be the lowest energy process (0.42 eV/defect). The Oxygen and Calcium Frenkel energies are 2.87 eV/defect and 4.96 eV/defect respectively suggesting that these defects are not significant especially the Ca Frenkel. Reaction energy for the loss of CaO via CaO Schottky is 2.97 eV/defect suggesting that this process requires moderate temperature. Calculated activation energy of Ca-ion migration in this material is high (&gt;4 eV), inferring very slow ionic conductivity. However, we suggest a strategy to introduce additional Ca2+ ions in the lattice by doping trivalent dopants on the Si site in order to enhance the capacity and ion diffusion and it is calculated that Al3+ is the favourable dopant for this process. Formation of Ca vacancies required for the CaO Schottky can be facilitated by doping of gallium (Ga) on the Fe site. The electronic structures of favourable dopants were calculated using density functional theory (DFT)