Observation of structural transformations in metal oxygen.

X-ray diffraction and Raman measurements have been performed on solid oxygen up to, respectively, 115 and 120 GPa. Metallization at 96 GPa is shown to be associated with a continuous displacive structural transformation. At 110 GPa, a new structure is stabilized. The evolution of the vibron mode reflects also the sequence of phase transitions through the continuity of the vibron frequency at 96 GPa, then the broadening of the vibron peak into the background and finally the reappearance of a sharp vibron at 110 GPa with a -2.5% discontinuity in the frequency. The observation of the vibron peak demonstrates that metallic oxygen is molecular up to at least 120 GPa.