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 to obtain very good diffraction patterns in the orthorhombic delta-phase. This phase is stable at low temperature, in contrast to some recent data [Y. Akahama et al., Phys. Rev. B64, 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 the first order.
@article{arxiv.cond-mat/0111412,
title = {Crystal structure of solid Oxygen at high pressure and low temperature},
author = {Federico A. Gorelli and Mario Santoro and Lorenzo Ulivi and Michael Hanfland},
journal= {arXiv preprint arXiv:cond-mat/0111412},
year = {2009}
}