Single-Crystal EPR Study of an O---Al Center in Prehnite: Implications for Radiation- Induced Defects in Clay Minerals
Radiation-induced defects (RIDs) in clay minerals (e.g., kaolinite, dickite, illite, and montmorillonite) have received considerable interests, because they are sensitive dosimeters for determining and monitoring the migration of radionuclides in the Earth's surface environments and have direct relevance to long-term nuclear waste disposal. These RIDs in clay minerals have been proposed to represent various O- centers with the unpaired electron in the Π nonbonding orbitals and linked to Si and/or Al. However, all previous studies of RIDs in clay minerals were based on powder electron paramagnetic resonance (EPR) spectra. Consequently, there remain significant uncertainties about the proposed structural models. Prehnite consists of 'puckered' layers of SiO4 and AlO4 tetrahedra and, therefore, is a close structural analogue of the clay minerals. Single-crystal EPR spectra of natural prehnite (Jeffrey mine, Asbestos, Quebec) revealed a rhombic O- center [g1=2.0487(1), g2=2.0234(1), and g3=2.0022(1)], which is characterized by a well-resolved 27Al (I=5/2 and natural isotope abundance = 100%) hyperfine structure [A∥ /ge βe= 0.61(1) mT and A⊥ /ge βe= 0.74(1) mT] arising from interaction with a single neighboring Al nucleus. Spin-Hamiltonian parameters show that this O- center (Π-type) originates from an OH group (i.e., removal of the proton and trapping of a hole on the oxygen atom during irradiation) coordinated to Al at the octahedral site in prehnite. Further characterization of this O- center by use of pulsed electron spin echo envelope modulation (ESEEM) spectroscopy is currently underway. Structural details about this O- center in prehnite provide insights into analogous RIDs in clay minerals.
EPR and ESEEM studies of radiation-induced defects in apophyllites
Radiation-induced defects in a suite of natural apophyllites have been investigated by single-crystal and powder electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM) spectroscopy. Single-crystal and powder EPR spectra of a gamma-ray-irradiated fluorapophyllite specimen (Czech Republic) revealed the presence of an NH2 radical and an axial O- center (g∥=2.00219 and g⊥=2.04442 at 90K and g∥=2.00309 and g⊥=2.04419 at 290K, and the unpaired electron in the Pz orbital). This O- center is also present in a hydroxylapophyllite specimen without any artificial irradiation. The single-crystal EPR spectra of this O- center in the hydroxylapophyllite specimen are accompanied by a series of weak satellite peaks, which have been identified to arise from 5 geometrically distinct pairs of neighboring O- centers (i.e., biradicals). Spin- Hamiltonian parameters of these biradicals provide compelling evidence for the origin of this O- center from the OH group (i.e., removal of the proton and trapping of a hole on the oxygen atom during irradiation) in apophyllites. 29Si, 1H and 39K superhyperfine constants determined from the ESEEM spectra provide additional information about the nature and local structural environment of the O- center in apophyllites.