Infrared fundamentals and phase transitions inCO2up to 50 GPa

Midinfrared (IR) fundamental modes of ${\mathrm{CO}}_2$ were measured at hydrostatic pressures up to 50 GPa and room temperature. A distinct splitting in the fundamental stretching ${\nu }_3$ and bending ${\nu }_2$ modes marks the structural transition from phase I to III. No other solid-solid phase transitions were observed but the spectrum acquired near 2 GPa suggests that liquid ${\mathrm{CO}}_2$ may be stable over a narrow pressure range. The number of stretching and bending modes observed for the high-pressure polymorph corroborates a Cmca structure. Overtone-combination modes were also observed and their frequencies agree well with previous data. The measured frequencies of overtone-combination modes and those calculated from observations of the fundamentals differ by as much as 70 ${\mathrm{cm}}^{\mathrm{-}1}$, which indicates a significant amount of anharmonicity for crystalline ${\mathrm{CO}}_2$. The phase transition is not easily detected in the overtones because both phases I and III were observed between 9 and 13 GPa, and because the coexistence of two phases with overlapping bands causes the frequency of the overtone to be ‘‘curved’’ near the transition rather than ‘‘kinked,’’ as was seen for the fundamentals. The sluggish kinetics and different sampling scales could explain discrepancies in transition pressure among previous studies and suggestions of structures in addition to Cmca and Pa3.