Résumé
Non molecular CO2 has been a leading subject of study in high pressure physics and chemistry for the past decade. It gives rise to a novel carbon chemistry and has implications for planetary science. The phase diagram of CO2 includes several non molecular phases above 30 GPa. Among these materials CO2-V is the first discovered. CO2-V appears to be similar to some extent to silica and is relatively hard, but its structure is controversial. Simulation studies show that the thermodynamic phase corresponding to CO2-V should be a beta-cristobalite related structure (I-42d) with tetrahedral carbon coordination similar to silicon in SiO2, whereas the structure reported experimentally appears to be highly metastable. We investigated CO2-V obtained from molecular CO2 at 40-50 GPa and T>1500 K by synchrotron X-ray diffraction, optical spectroscopy, and computer simulations (Santoro et al., PNAS 2012). The structure refined by the Rietveld method is a partially collapsed variant of beta-cristobalite SiO2, space group I-42d, in which the CO4 tetrahedra are tilted by 38.4 degrees about the c-axis. The existence of CO4 tetrahedra (average O-C-O angle of 109.5 degrees ) is demonstrated. This significantly adds to the knowledge of the chemistry of carbon, which now forms mineral phases similar to SiO2 and has potential implications for the Earth interior.