Radiation-induced defects in quartz. III. Single-crystal EPR, ENDOR and ESEEM study of a peroxy radical

摘要

The X- and W-band single-crystal electron paramagnetic resonance spectra of an electron-irradiated natural quartz permit quantitative analysis of a Si-29 hyperfine structure (A 12.6 MHz) and an Al-27 hyperfine structure (A a parts per thousand currency sign 0.8 MHz) for a previously reported hole-like center. The Si-29 hyperfine structure arises from interaction with two equivalent Si atoms and is characterized by the direction of the unique A axis close to a Si-O bond direction. The Al-27 hyperfine structure, confirmed by pulsed electron nuclear double resonance and electron spin echo envelope modulation spectra, is characterized by the unique A axis approximately along a twofold symmetry axis. These Si-29 and Al-27 hyperfine data, together with published theoretical results on peroxy radicals in SiO2 as well as our own density functional theory (DFT) calculations on model peroxy centers, suggest this hole-like center to have the unpaired spin on a pair of oxygen atoms linked to two symmetrically equivalent Si atoms and a substitutional Al3+ ion across the c-axis channel, a first peroxy radical in quartz. The nuclear quadrupole matrix P also suggests that the Al3+ ion corresponds closely to the diamagnetic precursor to the [AlO4](0) center.