Phase study of solidCO2to 20 GPa by infrared-absorption spectroscopy

A discrete change in the vibron frequency was observed in association with the transition from the cubic Pa3 structure at about 12 GPa and ambient temperature. The pressure dependence of the vibron frequencies measured for the high-pressure phase up to 20 GPa was not in agreement with those calculated previously for a presumed high-pressure phase of an orthorhombic Cmca structure. Two Raman-active libron peaks observed in the high-pressure phase indicated that the primitive cell probably contained two molecules, instead of four as in the Cmca structure. These aspects of the vibron and libron spectra suggested that a tetragonal structure isomorphic with γ-${\mathrm{N}}_2$ was a more likely structure as the high-pressure phase of solid ${\mathrm{CO}}_2$.