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 P4₂/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) {}, and four are at a collapsed intermolecular distance, 2.377(2) {}, at the apices of highly distorted octahedral. Strong intermolecular association results in a large splitting of the symmetric stretching ν₁ mode and in a high bulk modulus 131.5 GPa. At 11 GPa, CO₂-II is about 6.8% denser than CO₂-III (Cmca).