Using infrared spectroscopy, we have measured the vibrational spectrum of interstitial oxygen in silicon under hydrostatic pressures as high as 70 kbar at temperatures from 4 to 20 K. The application of pressure transforms the transverse motion of the oxygen from that of a harmonic oscillator to that of a rotor. As the motion becomes more rotational, the splitting between the $l=0\mathrm{}$ and $l=\pm 1$ low-frequency modes decreases. In addition, the splitting between the stretch modes at 1136 and $1128{\mathrm{cm}}^{\mathrm{-}1}$ decreases with increasing pressure and can no longer be resolved for pressures greater than 55 kbar.

• Received 13 May 1997

DOI:https://doi.org/10.1103/PhysRevB.56.9520