ELECTRON PARAMAGNETIC RESONANCE (EPR) SPECTROSCOPIC INVESTIGATION OF DEFECT CENTERS IN SELECTED BORATES AND BOROSILICATES
This thesis presents the results of a single-crystal electron paramagnetic resonance (EPR) spectroscopic investigation of defect centers in selected borates and borosilicates (i.e., datolite, danburite, and jeremejevite). The research brings new complementary data to the current understanding of defect structures in minerals, which are not only important to Earth Sciences but also directly relevant to environmental applications (e.g., nuclear waste disposal) and materials science. Single-crystal EPR spectra of a gamma-ray-irradiated datolite from Bergen Hill, New Jersey, USA, reveal the presence of a boron-oxygen hole center (BOHC). Spin-Hamiltonian parameters obtained from single-crystal EPR spectra and radiation-dose-dependence experiments allow us to confirm the BOHC center in datolite as the [BO4]0 type, involving hole trapping on the hydroxyl oxygen atom after the removal of the hydrogen atom: via a reaction O3BOH --> O3BO• + H0, where • denotes the unpaired electron. Density functional theory (DFT) calculations support the proposed structural model, and the calculated 11B hyperfine coupling constants are in excellent agreement with the experimental results. Also, isochronal and isothermal annealing experiments provide information about the thermal stability and decay kinetics of the [BO4]0 center in datolite. The confirmation of the [BO4]0 center and its formation from the O3BOH precursor in datolite are compared with other BOHCs in minerals and are discussed with relevance to the implications for not only understanding of BOHCs in alkali borosilicate glasses but also their applications to nuclear waste disposal. A combined study by use of synchrotron X-ray absorption spectroscopy (XAS), single-crystal EPR and pulse electron spin echo envelope modulation (ESEEM) spectroscopy provides compelling evidence for lattice-bound arsenic in danburite from Charcas, San Luis Potosi, Mexico. Arsenic K-edge X-ray absorption near-edge (XANES) spectra show that the dominant oxidation state is +3, and modeling of the extended X-ray absorption fine structure (EXAFS) spectra suggests that As3+ mainly occupies the Si site. Detailed single-crystal EPR spectra, measured before and after gamma-ray irradiation, reveal three arsenic-associated paramagnetic electron centers (I, II and III). Centers I and II are varieties of the [AsO2]2 radicals, formed from electron trapping on a substitutional As3+ ion at the Si site. This model is also supported by the 11B superhyperfine structures determined by ESEEM spectra at 80 K. Center III is the [AsO3]2 radical, originated from electron trapping on a [AsO4]3¬ group after removal of the O4 atom during gamma-ray irradiation. Therefore, arsenic in danburite is present in both the +3 and +5 oxidation states and preferentially occupies the Si site. Single-crystal EPR spectra of jeremejevite from Cape Cross, Namibia, reveal an S = 1/2 hole center characterized by a hyperfine structure arising from interaction with two equivalent 27Al nuclei. Our results suggest that this aluminum-associated oxygen hole center represents hole trapping on a hydroxyl oxygen atom linked to two equivalent octahedral Al3+ ions, after the removal of the proton (i.e., a VIAl−O−−VIAl center). Periodic ab initio UHF and DFT calculations confirmed the experimental 27Al hyperfine coupling constants and directions, supporting the proposed structural model. Also, isochronal annealing experiments provide information about the thermal stability of the VIAl−O−−VIAl center. These data obtained from the VIAl−O−−VIAl center in jeremejevite provide new insights into analogous defects that have been documented in several other minerals.
DegreeMaster of Science (M.Sc.)
CommitteeAnsdell, Kevin; Merriam, Jim; George, Graham; Sammynaiken, Ramaswami
Copyright DateNovember 2012
ELECTRON PARAMAGNETIC RESONANCE