Identification of paramagnetic centers in irradiated Sn-doped silicon dioxide by first-principles.

We present a first-principles investigation of Sn paramagnetic centers in Sn-doped vitreous silica
based on calculations of the electron paramagnetic resonance (EPR) parameters. The present
investigation provides evidence of an extended analogy between the family of Ge paramagnetic
centers in Ge-doped silica and the family of Sn paramagnetic centers in Sn-doped silica for SnO2
concentrations below phase separation. We infer, also keeping into account the larger spin-orbit
coupling of Sn atoms with respect to Ge atoms, that a peculiar and highly distorted three-fold coordinated Sn center (i.e. the Sn forward-oriented configuration) should give rise to an orthorhombic EPR signal of which we suggest its fingerprint in the EPR spectra recorded by Chiodini et al. 2001 Phys. Rev. B. 64, 073102:1-4. Given its structural analogy with the E'α and Ge(2) centers, we here name it as the "Sn(2) center". Moreover, we show that the
single trapped electron at a SnO4 tetrahedron constitutes a paramagnetic center responsible for
the orthorhombic EPR signal reported in Chiodini et al. 1998 Phys. Rev. B. 58, 9615:1-4, confuting
the early assignment to a distorted variant of the Sn-E' center. We hence relabel the latter orthorhombic
EPR signal as the "Sn(1) center" due to its analogy to the Ge(1) center in Ge-doped silica.

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