Crystal Structure of Solid Oxygen at High Pressure and Low Temperature

The results of x-ray diffraction experiments on solid oxygen at low temperature and at pressures up to 10 GPa are presented. A careful sample preparation and annealing around 240 K allowed very good diffraction patterns in the orthorhombic delta phase to be obtained. This phase is stable at low temperatures, in contrast to some recent data (Y. Akahama et al., Phys. Rev. B 64, 054105 (2001)), and transforms with decreasing pressure into a monoclinic phase, which is identified as the low-pressure alpha phase. The discontinuous change of the lattice parameters, and the observed metastability of the alpha phase increasing pressure suggest that the transition is of first order.