High-temperature IR absorption spectra of solid ${\mathrm{C}\mathrm{O}}_2$ in phases II and IV were measured in a resistive heated diamond anvil cell up to 30 GPa. The spectral structures of the bending mode, observed in high quality thin crystalline samples, and of the IR lattice phonons, measured for the first time between 80 and 640 K, are discussed using group theory arguments. According to this analysis the claimed bent molecular geometry of ${\mathrm{C}\mathrm{O}}_2$ in phase IV can be unambiguously ruled out. Furthermore, the structures of both phases II and IV have been identified, among those so far proposed, as orthorhombic.

• Received 29 April 2004

DOI:https://doi.org/10.1103/PhysRevLett.93.205503