Structure and ESR properties of self-trapped holes in pure silica from first-principles density functional calculations

The electronic structure and spectral properties of self-trapped holes (STH's) in $\mathrm{Si}{\mathrm{O}}_{2}$ are revisited using pure and hybrid density functional theory (DFT) approaches and cluster models. The structure and electron spin resonance (ESR) parameters [hyperfine coupling constants (hfcc's) and $g$ factors] have been determined for the two classical variants of STH's: ${\mathrm{STH}}_{1}$, consisting of a hole trapped at the $2p$ nonbonding orbital of an O atom bridging two Si atoms, and ${\mathrm{STH}}_{2}$, a metastable defect where the hole is delocalized over the $2p$ orbitals of two bridging O atoms. The computed ESR parameters fully support the experimental assignments based on the analysis of the ESR spectra. However, a nearly quantitative agreement between measured and computed hfcc's and $g$ factors is found only with a hybrid functional where 50% of the Hartree-Fock exchange is mixed in with the DFT exchange. Standard hybrid functionals, such as B3LYP, or pure DFT functionals, such as BLYP, fail completely in describing the two defects. According to these results, ${\mathrm{STH}}_{2}$ can form only in correspondence of specific O-Si-O angles of about 80\ifmmode^\circ\else\textdegree\fi{}--90\ifmmode^\circ\else\textdegree\fi{}, which classifies the defect center as a molecular polaron.