  We propose a di-interstitial model for the P6 center commonly observed in ion
implanted silicon. The di-interstitial structure and transition paths between
different defect orientations can explain the thermally activated transition of
the P6 center from low-temperature C1h to room-temperature D2d symmetry. The
activation energy for the defect reorientation determined by ab initio
calculations is 0.5 eV in agreement with the experiment. Our di-interstitial
model establishes a link between point defects and extended defects,
di-interstitials providing the nuclei for the growth.
