Geometry and quantum delocalization of interstitial oxygen in silicon

The problem of the geometry of interstitial oxygen in silicon is settled by proper consideration of the quantum delocalization of the oxygen atom around the bond-center position. The calculated infrared absorption spectrum accounts for the 517 and 1136 ${\mathrm{cm}}^{\mathrm{-}1}$ bands in their position, character, and isotope shifts. The asymmetric line shape of the 517 ${\mathrm{cm}}^{\mathrm{-}1}$ peak is also well reproduced. In addition a non-infrared-active symmetric-stretching mode is found at 596 ${\mathrm{cm}}^{\mathrm{-}1}$. First-principles calculations are presented supporting the nontrivial quantum delocalization of the oxygen atom.