Crystal structure of pseudo-six-fold carbon dioxide phase II at high pressures and temperatures

The crystal structure of CO2-II is determined to be tetragonal P4(2) /mnm with z=2, with evidence of some tetragonal-to-orthorhombic (Pnnm) disorder in the ab plane. In this structure, carbon atoms are pseudo-sixfold-coordinated by oxygens; two oxygens are at an elongated intramolecular C=O bond distance, 1.331(3) Angstrom, and four are at a collapsed intermolecular distance, 2.377(2) Angstrom, at the apices of highly distorted octahedral. Strong intermolecular association results in a large splitting of the symmetric stretching nu(1) mode and in a high bulk modulus 131.5 GPa. At 11 GPa, CO2-II is about 6.8% denser than CO2-III (Cmca).