EPR parameters of E′ centers in v−SiO2 from first-principles calculations
Résumé
A first-principles investigation of E′ centers in vitreous silica (v−SiO2) based on calculations of the electron paramagnetic resonance (EPR) parameters is presented. The EPR parameters are obtained by exploiting the gauge including projector augmented wave method as implemented in the quantum-espresso package. First, we analyze the EPR parameters of a large number of Si2 dimers. The g tensor of the Si2 dimers is shown to possess an average rhombic symmetry and larger g principal values with respect to those observed, e.g., for the E′γ center in silica. Furthermore, the g principal values clearly show a linear trend with the Si-Si dimer length. Our results suggest that the Si2 dimers could correspond to an unidentified paramagnetic center, though occasionally the calculated g principal values of the Si2 dimer might be compatible with those found experimentally for the E′δ center. Next, we generate nondimer configurations by a procedure involving structural relaxations in the subsequent positively charged states. In particular, puckered, unpuckered, doubly puckered, and forward-oriented configurations are generated. The distributions of the calculated EPR parameters of the puckered and unpuckered configurations further support the assignment of the E′γ center to an unpaired spin localized at a threefold coordinated silicon dangling bond. Moreover, by analyzing Fermi contacts and g tensors of the puckered and forward-oriented configurations, we suggest the assignment of the E′α center to the latter type of configurations. This work also suggests that the differences in the EPR parameters of E′α and E′γ centers mainly arise from the strained geometry of the silicon dangling bond. In the forward-oriented configurations, one Si-O bond is about 0.2 Å longer than the remaining two, whereas in the silicon dangling bond of the puckered and unpuckered configurations, all three bonds have a length of ∼1.6 Å each.