New transformations of CO(2) at high pressures and temperatures.

CO(2) laser heating of solid CO(2) at pressures between 30 and 80 GPa shows that this compound breaks down to oxygen and diamond along a boundary having a negative P-T slope. This decomposition occurs at temperatures much lower than predicted in theory or inferred from previous experiment. Raman spectroscopy and x-ray diffraction were used as structural probes. At pressures higher than 40 GPa the decomposition is preceded by the formation of a new CO(2) phase (CO(2)-VI). These findings limit the stability of nonmolecular CO(2) phases to moderate temperatures and provide a new topology of the CO(2) phase diagram.