Triplet states at an O vacancy in α-quartz

Abstract

The energy landscape of an ␣-quartz O vacancy in the lowest triplet state is investigated. Four local minima are identified and geometries, total energies, and electron paramagnetic resonance ͑EPR͒ parameters are obtained. On the basis of calculated values for the magnetic dipole interaction between the electrons two of the structures are identified with known triplet centers detected by EPR. The effect of Ge substitution is studied, and it is concluded that Ge impurities in silica act as traps for dangling bonds with a trapping energy of ϳ1.5 eV relative to Si ions.

E′Centers inαQuartz in the Absence of Oxygen Vacancies: A First-Principles Molecular-Dynamics Study

“…This kind of defect plays a role both in the degradation of SiO 2 -based devices [1] and in the change of refractive index in glass fibers [2]. Hence, it has attracted particular attention by experimentalists [3][4][5][6][7][8] and theoreticians [9][10][11][12][13][14][15][16][17]. To our surprise, however, a unique identification of the E 0 centers is still missing.…”

Stable states of E″ defects in α-quartz

“…A general consensus has been reached in terms of the large lattice distortion [13][14][15]. Spin Hamiltonian parameters allowed us to suggest that E 1 center is E 1 in neutral state but predicted theoretical distances 4.07 [10] and 4.4 Å [16] between two silicon atoms for such center are at variance with ∼5.6 Å (6.1 Å without considering the spin densities) obtained in pointdipole approximation from D parameter [4]. The principal directions of parameter D z of stable E 6 center and of E 1 center are closely comparable.…”

“…Since D is very close to being axial we interpret, as in the preceding papers [4,5], the principal values with the point magnetic dipole-dipole approximation. Then for two interacting electrons the parameter D is a matrix defined in [10] by…”

“…However we observed a dozen of E centers and half of them are stable at RT and above for years. Yet notwithstanding our experimental and some theoretical [10] works the atomic nature of E centers is still tentative, mainly in the lack of a theoretical base. In the present work using electron paramagnetic resonance (EPR) and optical absorption spectroscopy we investigate the stable E centers in synthetic quartz induced with electron irradiation (3 MeV, a fluence of 10 17 cm −2 ) followed by heating.…”

Growth of paramagnetic defects by gamma rays irradiation in oxygen-deficient silica

“…In this respect, a crucial study would concern the individuation in oxygendeficient silica of a paramagnetic defect in a spin triplet state [4,6]. Some authors have hypothesized [4,6,7] that this defect could be analogous in silica of the E 00 center of quartz, a defect that has been related to oxygen vacancies [8,9]. If so, through the study of the spectroscopic properties of this center, much information could be gained about the structural properties of SiO 2 in its amorphous and crystalline forms.…”