Melting and other phase transformations of oxygen from 120 to 650 K

Abstract

Direct observation and Raman spectra of molecular and lattice vibrations have been used to extend the melting curve of O2 to more than 650 K and to determine many of the solid-solid phase boundaries from 119 to 650 K. The Raman spectra of the lattice vibrations between 3 and 8 GPa at temperatures below 200 K are suggestive of a new phase, phi-O2, that may be stable between the fields of US -O2 and delta-O2. Four triple points have been located: fluid-US -epsilon at 645 +/- 10 K and 16.3 +/- 0.7 GPa; US -delta-epsilon at 368 +/- 5 K and 11.5 +/- 0.3 GPa; fluid-el-US at 283 +/- 4 K and 5.0 +/- 0.3 GPa; and US -phi-delta at 200 +/- 15 K and 6.3 +/- 0.8 GPa. Above 645 K, large crystals of epsilon-O2 suitable for X-ray diffraction studies can be grown from the fluid.