Paramagnetic centres associated with bonding defects in v-SiO2

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Abstract
Paramagnetic centres are produced in vitreous (v)-SiO2 by the trapping of photogenerated electrons and holes at the intrinsic bonding defect centres associated with over- and under-coordinated oxygen atoms, C3 + and C1- respectively. Capture of a hole by the C1- converts that centre to C1 0, the oxygen hole centre (OHC), whilst the capture of an electron by the C3 + centre converts it to a C3 0 which reconstructs forming the E′ centre, a threefold coordinated neutral silicon atom T3 0. The C3 +, C1- bonding defects occur as near neighbours in the network structure, so-called intimate valence-alternation pairs or I.V.A.P.s, so that an I.V.A.P. centre may trap either a hole or an electron, or both carriers. The unusual annealing behaviour of the OHC E.S.R. signal is shown to be related to centres which initially capture both an electron and a hole, and then convert from a spin-inactive configuration (T3 +, Cl −) to the spin-active configuration (T3 +, C1 0) through the thermal release of one electron.