Biradical states of oxygen-vacancy defects in α-quartz: centers $E_{2}^{\prime \prime }$ and $E_{4}^{\prime \prime }$

Abstract

Several new radiation defects with total electron spin S = 1 occurring in electron-irradiated, synthetic α-quartz have been observed by using electron paramagnetic resonance spectroscopy. These defects are considered to be biradicals, i.e., pairs of S = 1/2 species. The concentration of these centers depends on the condition of the fast-electron irradiation. They have different decay behaviors that allow measurements of any individual species especially when it predominates over the others. The primary spin Hamiltonian parameter matrices g ₁, g ₂, D have now been determined for two similar defects, which herein are labeled $E_{2}^{\prime \prime }$ and $E_{4}^{\prime \prime }$. Inter-electron distances estimated by using the magnetic dipole model, suggest that the structures of centers $E_{2}^{\prime \prime }$ and $E_{4}^{\prime \prime }$ both involve the unpaired electrons each located in orbitals of two silicon atoms next to a common oxygen vacancy but which have slightly different Si–Si distances at 0.90 and 0.79 nm, respectively. This model is consistent with previous DFT calculations of the triplet configurations with local energetic minima. Observed decay behaviors suggest a transformation of centers $E_{2,4}^{\prime \prime }$ to the analogous $E_{1}^{\prime \prime }$ center. These triplet centers in quartz provide new insights into the structures of analogous defects in amorphous silica.