Theoretical modeling of defect centers in selected minerals

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. 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 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. The present research evaluates the structural models of [AlO4/Li] paramagnetic defects in &#945;-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. 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