Crystal structure of bent carbon dioxide phase IV

The crystal structure of carbon dioxide phase IV (CO₂-IV) has been characterized based on powder angle-resolved x-ray diffraction data obtained in situ at 300--750 K and 11--50 GPa. The results confirm that in this high-pressure polymorph O=C=O molecules are nonlinear. Rietveld analyses of the x-ray data yield two plausible structures: P4₁2₁2 (α-SiO₂ cristobalite) and Pbcn (α-PbO₂, post-stishovite). Carbon dioxide molecules are bent slightly more in the Pbcn phase (<C-O-C=160^∘) than in the P4₁2₁2 (171^∘{}). For both structures, the intramolecular C-O bond is increased to 1.50(±{}0.1) {} at the distance of a C-O single bond, whereas the intermolecular C{⋯}O distance is reduced to around 2.1(±{}0.2) {}. The bending and elongation of the molecular units suggest that phase IV is an intermediate state between the molecular and nonmolecular extended phases and that the molecular-to-nonmolecular transformation in carbon dioxide occurs gradually via intermediate phases IV and II. Transition pathways among all five CO₂ phases (I--V) are also discussed in the context of symmetry arguments and transition mechanisms. Considerations of plausible mechanisms of the transitions among phases I, II, IV, and V favor the Pbcn interpretation of the crystal structure of phase IV.