The bulk structure and the nonpolar, stoichiometric (110) surface of stishovite, rutile structure type SiO₂, has been studied using a first principles, density functional method. The geometric and electronic structure, including the density of states, charge density, and electron localization function for both the bulk and the surface have been examined. The electron pair properties of both bulk and surface-layer atoms were found to be similar to molecular analogs. The analogs allowed for the description of surface electronic structure using simple molecular models. The adsorption of hydrogen fluoride was studied on the (110) surface. The geometry optimized and electronic structure have been found for various initial geometries. Relaxed structures of certain initial geometries give dissociated hydrogen fluoride upon geometry optimization. / Master of Science

This thesis presents ab-initio quantum mechanical calculations at the density functional theory (DFT) level on defect centers hosted by crystalline systems of geologic importance (i.e. fluorite, quartz, stishovite). The research brings new, complementary data to the current understanding of defect structures in minerals and explores the advantages of a theoretical approach in the field of mineral spectroscopy. This present research presents the first ab-initio calculations of the O23- type defects in crystalline solids. New data on the electronic properties and structural characteristics of O23--Y3+ defect in fluorite-type structures (CaF2 and SrF2) were obtained at the DFT level. These results confirm the stability and the molecular character of the O23--Y3+ center, revealing a spin density that is equally distributed between the two oxygen atoms. Our results report an O-O bond distance of 2.47 Å in CaF2 and 2.57 Å in SrF2. The calculated 17O and 19F hyperfine constants for of the O23--Y3+ center are in good agreement with their corresponding experimental values reported by previous electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) studies, while discrepancies are notable for the 89Y hyperfine constants and are probably attributable to an inadequate basis set for Y.<p> The present study provides a more complete picture of the coupled Al-M substitution for Si in quartz, while investigating the characteristics and electronic properties of the diamagnetic [AlO4/M+]0 (where M = H, Li, Na and K) defects. The diamagnetic [AlO4/M+(a<)]0 defects with M = H, Li and Na have been shown to be more stable than their [AlO4/M+(a>)]0 structural analogues (where a> and a< denote the location of the charge compensating ion on the long-bond and short-bond side, respectively), correctly predicting the common occurrence of paramagnetic [AlO4/M+(a>)]+ centers. The present study confirms previous suggestions that incorporation of the [AlO4/M+]0 defects results in significant structural relaxations that extend at least to the nearest Si atoms. The [AlO4/K+]0 defects have been investigated for the first time and are shown to be stable in quartz. The results of this study have implications for the uptake of Al in quartz.<p> The present research evaluates the structural models of [AlO4/Li] paramagnetic defects in α-quartz. The results confirm the previous experimental findings and propose an additional paramagnetic defect [AlO4/Li+(csmall)]+, with the unpaired electron located on a short-bonded O atom and the Li compensator just off the edge of the small channel. Accordingly we suggest that three distinct Al-Li paramagnetic defects can be can be found in quartz, two of them having the hole located on a short-bonded O and one trapping the hole on a long-bonded O atom. However the structural similarities with the [AlO4/Li+(a>)]+ defect would require detection and measurement of the 17O hyperfine structure for an unequivocal EPR identification.<p> The present work also reports on first-principles quantum-mechanical calculations on the previously proposed [O23--Al3+] defect in stishovite. Our results show that the unpaired spin is 85% localised on one of the six oxygen atoms at an AlO6 octahedron, while the calculated 27Al hyperfine constants are similar to those determined by EPR experiments. Accordingly we propose the Al center to represent an [AlO6]0 defect, and hole hoping among equivalent oxygen atoms is responsible for its detection only at cryogenic temperatures. Theoretical calculations also show that diamagnetic precursors [AlO6/H+]0, [AlO6/Li+]0 and [AlO6/Na+]0 are stable in stishovite. The calculated OH bond distance and orientation are in excellent agreement with those inferred from FTIR spectra and previous theoretical calculations. The calculated [AlO6/Li+]0 and [AlO6/Na+]0 defects suggest that monovalent cations such as Li+ and Na+ are potentially important in accommodating Al in stishovite in the lower mantle.

This thesis presents ab-initio quantum mechanical calculations at the density functional theory (DFT) level on defect centers hosted by crystalline systems of geologic importance (i.e. fluorite, quartz, stishovite). The research brings new, complementary data to the current understanding of defect structures in minerals and explores the advantages of a theoretical approach in the field of mineral spectroscopy. This present research presents the first ab-initio calculations of the O23- type defects in crystalline solids. New data on the electronic properties and structural characteristics of O23--Y3+ defect in fluorite-type structures (CaF2 and SrF2) were obtained at the DFT level. These results confirm the stability and the molecular character of the O23--Y3+ center, revealing a spin density that is equally distributed between the two oxygen atoms. Our results report an O-O bond distance of 2.47 Å in CaF2 and 2.57 Å in SrF2. The calculated 17O and 19F hyperfine constants for of the O23--Y3+ center are in good agreement with their corresponding experimental values reported by previous electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) studies, while discrepancies are notable for the 89Y hyperfine constants and are probably attributable to an inadequate basis set for Y.<p> The present study provides a more complete picture of the coupled Al-M substitution for Si in quartz, while investigating the characteristics and electronic properties of the diamagnetic [AlO4/M+]0 (where M = H, Li, Na and K) defects. The diamagnetic [AlO4/M+(a<)]0 defects with M = H, Li and Na have been shown to be more stable than their [AlO4/M+(a>)]0 structural analogues (where a> and a< denote the location of the charge compensating ion on the long-bond and short-bond side, respectively), correctly predicting the common occurrence of paramagnetic [AlO4/M+(a>)]+ centers. The present study confirms previous suggestions that incorporation of the [AlO4/M+]0 defects results in significant structural relaxations that extend at least to the nearest Si atoms. The [AlO4/K+]0 defects have been investigated for the first time and are shown to be stable in quartz. The results of this study have implications for the uptake of Al in quartz.<p> The present research evaluates the structural models of [AlO4/Li] paramagnetic defects in α-quartz. The results confirm the previous experimental findings and propose an additional paramagnetic defect [AlO4/Li+(csmall)]+, with the unpaired electron located on a short-bonded O atom and the Li compensator just off the edge of the small channel. Accordingly we suggest that three distinct Al-Li paramagnetic defects can be can be found in quartz, two of them having the hole located on a short-bonded O and one trapping the hole on a long-bonded O atom. However the structural similarities with the [AlO4/Li+(a>)]+ defect would require detection and measurement of the 17O hyperfine structure for an unequivocal EPR identification.<p> The present work also reports on first-principles quantum-mechanical calculations on the previously proposed [O23--Al3+] defect in stishovite. Our results show that the unpaired spin is 85% localised on one of the six oxygen atoms at an AlO6 octahedron, while the calculated 27Al hyperfine constants are similar to those determined by EPR experiments. Accordingly we propose the Al center to represent an [AlO6]0 defect, and hole hoping among equivalent oxygen atoms is responsible for its detection only at cryogenic temperatures. Theoretical calculations also show that diamagnetic precursors [AlO6/H+]0, [AlO6/Li+]0 and [AlO6/Na+]0 are stable in stishovite. The calculated OH bond distance and orientation are in excellent agreement with those inferred from FTIR spectra and previous theoretical calculations. The calculated [AlO6/Li+]0 and [AlO6/Na+]0 defects suggest that monovalent cations such as Li+ and Na+ are potentially important in accommodating Al in stishovite in the lower mantle.

Les propriétés thermodynamiques de quelques minéraux métamorphiques de haute pression sont calculées par modélisation de la densité d'états vibratoires à partir de spectres infrarouge et Raman. Une étude spectroscopique détaillée de la lawsonite (CaAI2(Si207)(OH)H20) conduit à une identification des modes d'étirement des groupements Si207 dans le minéral. La description du spectre vibrationnel ainsi obtenue sert à modéliser l'entropie S et la capacité calorifique Cp du minéral. De la même manière, la position de l'équilibre zoïsite-clinozoïsite (Ca2(AI1- pFep)AIO(Si04)(Si207)), sur lequel les résultats expérimentaux divergent, est estimée à partir des valeurs de S et Cp ainsi modélisées. D'autre part, le rôle sur les fonctions thermodynamiques de la substitution AI/Fe dans la série clinozoïsiteépidote est estimé à partir de l'enregistrement de spectres sur des épidotes de compositions variées. Dans les conditions de haute pression et haute température, les minéraux ne se comportent pas comme des solides harmoniques : c'est notamment le cas des polymorphes de Si02, quartz, coesite et stishovite. L'effet de la pression et la température sur leurs spectres de vibration, mesuré expérimentalement, permet en particulier d'expliquer pourquoi et comment les valeurs de la capacité calorifique s'écartent à haute température de la limite prédite par la théorie harmonique.

The dramatic change in properties of water near its critical point (i.e. T = 374 °C and p = 22.1 MPa, note: 100 MPa = 0.1 GPa = 1 kbar ≈ 1000 atm) has been a subject of numerous studies and also lead to the development of various applications (e.g. in waste destruction, biomass processing, and the synthesis of advanced ceramic materials). However, comparatively little is known about the behavior of water at gigapascal pressures. The present study attempts to explore catalytical properties and reactivity of extreme water with respect to several oxide systems: SiO2, TiO2 and LiAlSiO4. “Extreme water” here is defined as existing at p,T conditions of 0.25–10 GPa and 200–1000 °C, thus considering both supercritical fluid and hot compressed ice. The study shows that extreme water can make high pressure mineral phases accessible at relatively mild T conditions. At the same time, high pressure aqueous environments appear efficient in stabilizing novel metastable structures and may be considered as a general route for synthesizing new materials. The hydrothermal treatment of SiO2 glass at 10 GPa and 300–550 °C yielded an unusual ultrahydrous form of stishovite with up to 3% of structural water. At the same time, the extreme water environment enhanced notably the kinetics of stishovite formation, making it accessible at unprecedentedly low temperatures. Thus, for the SiO2–H2O system water acts as both catalyst and reactant. For TiO2 a hydrothermal high pressure treatment proved to be of high importance for overcoming the kinetical hindrance of the rutile – TiO2-II transformation. 6 GPa and 650 °C were established as the mildest conditions for synthesizing pure TiO2-II phase in less than two hours. The crystallization of LiAlSiO4 glass in an extreme water environment yielded a number of different phases. In the low pressure region (0.25 – 2 GPa) mainly a zeolite (Li-ABW) and a dense anhydrous aluminosilicate (α-eucryptite) were obtained. At pressures above 5 GPa the formation of novel pyroxene-like structures with crystallographic amounts of structural water was observed. The overall conclusion of this study is that extreme water environments show a great potential for catalyzing phase transitions in oxide systems and for stabilizing novel structures via structural water incorporation. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.</p>

No description available.