${\mathrm{CO}}_2$ laser heating of solid ${\mathrm{CO}}_2$ 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 ${\mathrm{CO}}_2$ phase ( ${\mathrm{CO}}_2$-VI). These findings limit the stability of nonmolecular ${\mathrm{CO}}_2$ phases to moderate temperatures and provide a new topology of the ${\mathrm{CO}}_2$ phase diagram.

• Received 11 January 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.075701