PHASE STUDY OF SOLID CO2 TO 20 GPA BY INFRARED-ABSORPTION SPECTROSCOPY

A discrete change in the vibron frequency was observed in association with the transition from the cubic Pa 3 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 gamma-N2 was a more likely structure as the high-pressure phase of solid CO2.