Structural phase transitions of solid oxygen at low temperature and high pressure

The phase transition of solid oxygen has been investigated at low temperatures of 19, 180, and 240 K and at high pressures of up to 12 GPa using monochromatic synchrotron x-ray-diffraction techniques, and the phase relation among alpha-, beta-, delta-, and epsilonO(2) has been revealed, based on the structural data. The monoclinic alpha -O-2 phase at 19 K, with increasing pressure, directly transformed to the epsilon -O-2 phase at 7.6 GPa. The alpha-delta phase boundary was located on a higher-temperature region than 240 K and the alpha -O-2 phase was stable over a wide pressure and temperature range. The b/a ratio of the lattice constants at 19 K exhibited a monotonic increase with pressure and reached 0.708 at 7.6 GPa, while it abruptly decreased to 0.690 by the alpha-epsilon transition. From the anisotropy of compressibility, the existence of a collinear antiferromagnetic order in the alpha -O-2 phase was confirmed at pressure up to 7.2 GPa. The equation of state of the alpha -O-2 phase at 19 K has also been determined.