NEW PHASE OF OXYGEN AT HIGH-PRESSURE AND LOW-TEMPERATURE

We have extended the phase diagram of oxygen to 34 GPa at 20 K with Raman spectroscopy on samples in diamond anvil cells. At 25.0 GPa and 20 K, a new phase, which we term zeta-O2, appears. In going from epsilon-O2 to zeta-O2, the two low-frequency intermolecular vibrational modes split abruptly by about 10 cm-1, while the vibron remains a single peak. The transition appears to be first order.