Si-O bond-length modification in pressure-densified amorphous SiO2.

The hyperfine splitting of the oxygen-vacancy center resonance in amorphous ${\mathrm{SiO}}_{2}$ has been measured as a function of density in samples densified under pressure at high temperature. Densification of \ensuremath{\sim}24% is found to produce an increase in the isotropic hyperfine coupling constant of 11.5% which is interpreted as resulting from a reduction in the mean O\char22{}Si\char22{}O tetrahedral bond angle of the three Si\char22{}O backbonds from 108.3\ifmmode^\circ\else\textdegree\fi{} to 107.2\ifmmode^\circ\else\textdegree\fi{}. This reduction corresponds to an increase in mean Si\char22{}O bond length from 1.618 to 1.623 A\r{} and to a reduction in the mean Si\char22{}O\char22{}Si bond angle associated with the defect back bonds and adjacent ${\mathrm{SiO}}_{4}$ tetrahedra of 5.1\ifmmode^\circ\else\textdegree\fi{}. This reduction is in excellent agreement with that estimated from x-ray scattering and nuclear magnetic resonance studies on the densified ${\mathrm{SiO}}_{2}$ network.