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 δ 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 α phase. The discontinuous change of the lattice parameters, and the observed metastability of the α phase increasing pressure suggest that the transition is of first order.