Structure of Carbon Dioxide Phase IV: Breakdown of the Intermediate Bonding State Scenario

The existence of ``intermediate bonding states'' in solid CO₂, separating the low-pressure molecular phases from the high-pressure polymeric forms, has been the matter of a long-standing debate. Here we determine the structure of CO₂-IV using x-ray diffraction of single crystals grown inside a diamond anvil cell at 11.7 GPa and 830 K. It is rhombohedral, space group R3̄c, and is composed of individual, linear CO₂ molecules with bond lengths of 1.155(2) {} at 15 GPa. This shows that CO₂ remains a purely molecular solid in this P-T range, and thus invalidates the intermediate bonding state scenario. First-principles calculations confirm the stability of the proposed structure and match very well observations, including the Raman and IR spectra. Furthermore, these results evidence a striking similarity between the high-pressure polymorphs of solid CO₂ and N₂.