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A Single-Crystal Epr Study of Radiation-Induced Defects

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A Single-Crystal Epr Study of Radiation-Induced Defects A SINGLE-CRYSTAL EPR STUDY OF RADIATION-INDUCED DEFECTS IN SELECTED SILICATES A Thesis Submitted to the College of Graduate Studies and Research In Partial Fulfillment of the Requirements For the Degree of Doctor of Philosophy In the Department of Geological Sciences University of Saskatchewan Saskatoon By Mao Mao Copyright Mao Mao, October, 2012. All rights reserved. Permission to Use In presenting this thesis in partial fulfilment of the requirements for a Doctor of Philosophy degree from the University of Saskatchewan, I agree that the Libraries of this University may make it freely available for inspection. I further agree that permission for copying of this thesis in any manner, in whole or in part, for scholarly purposes may be granted by the professor or professors who supervised my thesis work or, in their absence, by the Head of the Department or the Dean of the College in which my thesis work was done. It is understood that any copying or publication or use of this thesis or parts thereof for financial gain shall not be allowed without my written permission. It is also understood that due recognition shall be given to me and to the University of Saskatchewan in any scholarly use which may be made of any material in my thesis. Requests for permission to copy or to make other use of material in this thesis in whole or part should be addressed to: Head of the Department of Geological Sciences 114 Science Place University of Saskatchewan Saskatoon, Saskatchewan S7N5E2, Canada i Abstract This thesis presents a series of single-crystal electron paramagnetic resonance (EPR) studies on radiation-induced defects in selected silicate minerals, including apophyllites, prehnite, and hemimorphite, not only providing new insights to mechanisms of radiation-induced damage in minerals but also having direct relevance to remediation of heavy metalloid contamination and nuclear waste disposal. The NH 2 free radical, which is one of the most studied triatomic molecules and is widely used as spin labels in biophysical and biomedical research, in fluorapophyllite has been observed and characterized by single-crystal EPR spectra. Fourier-transform infrared (FTIR) spectrum supports electron-microprobe analyses that fluorapophyllite on the cavity walls in a phonolite (North Bohemia, Czech + Republic) contains ammonium NH 4 . The spin-Hamiltonian parameters of the NH 2 free radical show that this molecule is oriented parallel to (and rapidly rotated about) the crystallographic c axis. The NH 2 free radical in fluorapophyllite, most likely + formed from radiolysis of the NH 4 ion, remains stable after annealing at 300°C, but is bleached at 340°C. This is the first report of the NH 2 free radical in a mineral lattice. An O - center and its biradicals in hydroxylapophyllite have been investigated by use of single-crystal and powder EPR spectroscopy at 290 and 90 K and three-pulse electron spin echo envelope modulation (ESEEM) spectroscopy at 25 K. The spin-Hamiltonian parameters show that the O - center (σ-type) represents an unpaired electron in the 2 pz orbital of the hydroxyl oxygen atom. A series of weak ii satellite peaks accompanying the main lines have been attributed to four geometrically distinct pairs of neighboring O - centers ( i.e ., biradicals). These biradicals have a point-dipole character and further support the O - model and its location. The O - center in hydroxylapophyllite is most likely produced by natural radiation and can be enhanced by gamma-ray irradiation. It is bleached at 300°C but can be restored readily by gamma-ray irradiation. An Al-O- center in gamma-ray-irradiated prehnite has been investigated by single-crystal EPR spectroscopy at 298 and 160 K. The spin-Hamiltonian parameters g and A(27 Al) at 298 K show that the hole traps on an apical hydroxyl oxygen of the octahedral O4Al(OH) 2 group, after removal of the proton. Pulsed electron nuclear double resonance (ENDOR) spectra measured at 25 K further confirm the structural model of the Al-O- center. Isothermal and isochronal annealing experiments show that the Al-O- center exhibits second-order decay kinetics. The Al-O- center is bleached after annealing at 375°C but can be restored by gamma-ray irradiation. These results from the Al-O- center in prehnite provide support for and new insights into Clozel et al . (1995)’s VI Al −O−−VI Al model for the B center in kaolinite. 4- 2- Two arsenic-centered oxyradicals ([AsO 4] and [AsO 4] ) in gamma-ray irradiated hemimorphite (Mapimi, Durago, Mexico) have been observed and characterized by single-crystal EPR at ~295 K. The spin-Hamiltonian parameters 4- suggest that the [AsO 4] radical is produced from electron trapping by a locally 3- 4- uncompensated [AsO 4] group substituting for the [SiO 4] group. The 2- 29 1 spin-Hamiltonian parameters of the [AsO 4] radical, including its Si and H iii superhyperfine coupling constants, suggest hole trapping on the bridging oxygen linked to a Si 4+ ion. Hydrothermal experiments at 200 °C and ~9.5 MPa show that hemimorphite is able to accommodate up to 2.5 wt% As 2O5. These results demonstrate that hemimorphite is capable of sequestering arsenate in its crystal lattice, so it is a natural sink for attenuating As in supergene non-sulfide Zn deposits and Zn mine tailings. Hemimorphite commonly contains elevated contents of heavy metalloids such as As, Cu, Cd, and Pb. Cation-exchange experiments of hemimorphite with 0.1M CaCl 2 solution at 110°C show that As and Cu are retained, whereas Cd and Pb are readily exchanged. This exchange behavior of Cd and Pb suggests that they may reside in the channel. Single-crystal EPR results at 295 and 120 K show that Cu 2+ resides at the tetrahedral Zn site, not in the channel as previously suggested by the powder EPR study of Gallegos et al . (2009). These results suggest that hemimorphite is potentially useful for the remediation of heavy metalloid contamination. Single-crystal EPR spectra of gamma-ray-irradiated hemimorphite after storage at room temperature for three months reveal a hydroperoxy radical HO 2 with complex proton hyperfine and superhyperfine structures. The single-crystal EPR spectra of this HO 2 radical, measured from 4 K to 275 K, confirm two reversible phase transitions at ~98 K and ~21 K. Spin-Hamiltonian parameters show that the HO 2 radical at 110 K forms from the H 2O molecule in the channel and interacts with two equivalent protons of the nearest hydroxyl groups. The HO 2 radical changes in site symmetry from monoclinic to triclinic across the ~98 K phase transitions and iv confirms dynamic ordering and rotation of its precursor water molecule in the channel at <98 K. The EPR spectra of the HO 2 radical at <21 K and results from density functional theory (DFT) calculations suggest that hemimorphite adopts the monoclinic space group Im with completely ordered O−H systems at low temperature. v Acknowledgements I would have never completed this research without my supervisor Dr. Yuanming Pan. He has taught me the way to do research and pushed me to work hard. I am grateful to him for the time and patience that he has generously spent on my research in these years. I also thank the members of my advisory committee: Dr. Kevin Ansdell, Dr. Jim Merriam, Dr. Ingrid Pickering, Dr. Robin W. Renaut, and Dr. Robert Scott, as well as my external examiner Dr. Mostafa Fayek, for incisive criticisms and helpful suggestions. Special thanks to Dr. Mark J. Nilges for the collaborative work with ESEEM and ENDOR experiments at Illinois EPR Research Center, and Dr. Ramaswami Sammynaiken of Saskatchewan Structural Science Centre for his assistance on the X-band EPR experiments. I am very grateful to Mr. T. Bonli, Dr. J. Fan, Dr. Z. Li, and Mr. B. Novakovski of the Department of Geological Science for their assistance with electron microprobe analysis, inductively-coupled plasma mass spectroscopy analysis, DFT calculations, and sample preparation, respectively. I also want to express my gratitude to my family for their support and encouragements. They have been always proud of my achievement and encouraged me when I faced problems on research. At last, I gratefully thank Natural Science and Engineering Research Council of Canada, Areva Resources Canada Scholarship, and University of Saskatchewan Graduate Teaching Fellowship for financial support to my research. vi TABLE OF CONTENTS PERMISSION TO USE ................................................................................................. i ABSTRACT.................................................................................................................. ii ACKNOWLEDGEMENTS ......................................................................................... vi TABLE OF CONTENTS............................................................................................ vii LIST OF TABLES ...................................................................................................... xii LIST OF FIGURES.................................................................................................... xiv 1.INTRODUCTION AND OBJECTIVES ..................................................................1 1.1 STUDY OVERVIEW ................................................................................................................... 1 1.1.1 Defects.......................................................................................................... 1 1.1.2 The selected silicates...................................................................................
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