EPR and IR spectral properties of hydrogen-associated bulk and surface defects inSiO2:Ab initiocalculations

Abstract

Electron paramagnetic resonance and IR spectral properties of hydrogen-associated centers in silica have been studied by means of first-principles Hartree-Fock and density-functional-theory calculations. The geometric and electronic structure of the paramagnetic centers ${(\mathrm{\ensuremath{\equiv}}\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{O})}_{3}{\mathrm{Si}}^{\ifmmode\bullet\else\textbullet\fi{}},$ ${(\mathrm{\ensuremath{\equiv}}\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{O})}_{2}{\mathrm{Si}}^{\mathrm{\ifmmode\bullet\else\textbullet\fi{}}}(\mathrm{OH})$ and ${(\mathrm{\ensuremath{\equiv}}\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{O})}_{2}{\mathrm{Si}}^{\mathrm{\ifmmode\bullet\else\textbullet\fi{}}}(\mathrm{H}),$ and of the corresponding diamagnetic centers formally derived from the addition of one H atom, ${(\mathrm{\ensuremath{\equiv}}\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{O})}_{3}\mathrm{Si}(\mathrm{H}),$ ${(\mathrm{\ensuremath{\equiv}}\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{O})}_{2}\mathrm{Si}(\mathrm{OH})(\mathrm{H}),$ and ${(\mathrm{\ensuremath{\equiv}}\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{O})}_{2}\mathrm{Si}(\mathrm{H})(\mathrm{H}),$ have been determined using cluster models. A substantial agreement is found between the computed hyperfine splittings and those measured for bulk paramagnetic centers in high-purity amorphous silica. The different experimental hyperfine splittings obtained for ${(\mathrm{\ensuremath{\equiv}}\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{O})}_{3}{\mathrm{Si}}^{\ifmmode\bullet\else\textbullet\fi{}}$ centers at the surface of mechanically or thermally activated silica are explained in terms of small structural differences from the bulk counterparts. An almost quantitative agreement with the experiment is found also for the vibrational properties of the Si-H groups of the silylhydride centers. The calculations allow a firm assignment of the spectral features to specific point defects in silica.