Nonlinear Carbon Dioxide at High Pressures and Temperatures

A nonlinear molecular carbon dioxide phase IV was discovered by laser heating CO₂-III0ex0ex(Cmca) between 12 and 30 GPa, followed by quenching to 300 K. The Raman spectrum of quenched CO₂-IV exhibits a triplet bending mode ν₂(O=C=O) near 650cm^-1, suggesting a broken inversion symmetry because of bending. The 650cm^-1 bending modes soften with increasing pressure, indicating an enhanced intermolecular interaction among neighboring bent CO₂ molecules. At 80 GPa, the low-frequency vibron collapses into high-frequency phonons, and CO₂-IV becomes an extended amorphous solid.