Nonlinear Carbon Dioxide at High Pressures and Temperatures

A nonlinear molecular carbon dioxide phase IV was discovered by laser heating $\mathrm{CO}{}_2-\mathrm{III}\left(\mathrm{Cmca}\right)$ between 12 and 30 GPa, followed by quenching to 300 K. The Raman spectrum of quenched $\mathrm{CO}{}_2-\mathrm{IV}$ exhibits a triplet bending mode ${\nu }_2\left(\mathrm{O}=\mathrm{C}=\mathrm{O}\right)$ near $650\mathrm{cm}{}^{-1\mathrm{}}$, suggesting a broken inversion symmetry because of bending. The $650\mathrm{cm}{}^{-1\mathrm{}}$ bending modes soften with increasing pressure, indicating an enhanced intermolecular interaction among neighboring bent $\mathrm{CO}{}_2$ molecules. At 80 GPa, the low-frequency vibron collapses into high-frequency phonons, and $\mathrm{CO}{}_2-\mathrm{IV}$ becomes an extended amorphous solid.