New E′ centers in neutron-irradiated α -quartz

Abstract

PACS 42.70.Ce -Glasses, quartz PACS 61.72.jd -Defects and impurities in crystals; microstructure Vacancies PACS 61.72.J--Points defects and defect clusters Abstract -Several E -type defects have been revealed in neutron-irradiated natural and synthetic α-quartz by using electron paramagnetic resonance (EPR) spectroscopy. For the known E 2 center the primary spin Hamiltonian parameter matrices g and A( 29 Si) (hyperfine interaction with 29 Si) have been refined and provide compelling evidence for spin trapping on the long-bond Si atom. The EPR spectra of the new E 11 center demonstrate that the super-hyperfine structure arises from the interaction with 27 Al, the first-ever example of Al-associated E centers in crystalline quartz. The matrices g and A( 29 Si) of E 11 and another new center (E 12 ) support the forwardoriented configuration proposed for the E α center in amorphous silica.

Triplet states of oxygen vacancy defects in α -quartz: Center $\bf{\text{E}^{\prime\prime}_{9}}$

“…A twofold decrease in HF coupling follows from spin Hamiltonian for two interacting electrons [22] and was also observed for the E 1 center [13]. The direction of the unique principal g 1 , A 1 principal axes is approximately along the short Si1-O1 bond (table 1) and similar to those for the E 10 center [1]. On the other hand, the unique A 2 direction is nearly along the long Si2-O1 bond.…”

“…The two strong HF values of the triplet center are about half of that of the E 10 doublet center [1], which is simultaneously observed in the sample (fig. 1(b)).…”

“…1(a)). The intensity ratio of 29 Si 3,4,5 line's amplitude to the main line is about 2.5% and is constant at both annealing temperatures of 200 and 210 • C. Here the well-known E 1,4 [6,17] and E 9,10 centers [1,18] are observable too. The HFS of the centers E 1,9,10 with spacings ∼41 mT and the strong HFS of the E 9 center with spacings about 21 mT caused by two slightly nonequivalent 29 Si 1 and 29 Si 2 nuclei (bdeg and acfh lines, respectively) interacting with the trapped spins are demonstrated in fig.…”

Paramagnetic defects in neutron-irradiated α-quartz: Novel Al-associated E’ centers

“…and other SiO 2 -based materials (e.g., glasses, thin films and nanoparticles) and has been the subject of intense experimental and theoretical research for more than 60 years (for recent reviews, see refs. [6][7][8][9]). Single-crystal EPR studies [10][11][12] have determined one strong and two weak 29 Si hyperfine matrices for the classic E 1 center in quartz as well as one strong 73 Ge, two weak 29 Si and three nonequivalent 17 O hyperfine matrices for its Ge analog (denoted as Ge E 1 ).…”

“…Li and Pan [13] noted that the tri-vacancy (i.e., removing one silicon and its two neighboring oxygen atoms) with an Al impurity model not only best reproduces all experimentally determined hyperfine parameters of the E 1 center with the •Si≡O moiety but also explains its close linkage with the [AlO 4 ] 0 center. Interestingly, Brower [16] discovered an Al analog of the E 1 center in vitreous silica after electron irradiation, which is characterized by one strong 27 Al hyperfine interaction (with isotropic and anisotropic values: a = 35 and b = 1.6 mT) in powder EPR spectra. Brower [16] interpreted this defect to represent spin trapping on a central Al atom coordinated to three nearest-neighbor O atoms and used it to suggest a diamagnetic AlO 3 precursor in vitreous silica.…”