CRYSTAL-STRUCTURE OF THE HIGH-PRESSURE PHASE OF SOLID CO2

X-ray diffraction study of solid CO2 at room temperature has shown that the powder pattern of the high-pressure phase, which supersedes the low-pressure cubic Pa3 phase at about 10 gigapascals, is consistently interpreted in ferms of an orthorhombic Cmca structure. The orthorhombic cell at 11.8 gigapascals has dimensions of 4.330 +/- 0.015, 4,657 +/- 0.005, 5.963 +/- 0.009 angstroms for its a, b, and c faces, respectively, and a volume of 120.3 +/-0.5 cubic angstroms. Four molecules contained in the unit cell are located at the base-centered positions with their molecular axes inclined at about 52-degrees with respect to the crystallographic c axis. The volume change associated with the Pa3-Cmca transition is close to zero. The structural dimensions obtained for the high-pressure crystalline phase of CO2 are of great importance for a theoretical understanding of the role of intermolecular interactions, including quadrupole-quadrupole interactions, in molecular condensation.