Nonlinear carbon dioxide at high pressures and temperatures

A nonlinear molecular carbon dioxide phase IV was discovered by laser heati ng CO2-III (Cmca) between 12 and 30 GPa, followed by quenching to 300 K. Th e Raman spectrum of quenched CO2-IV exhibits a triplet bending mode v(2)(O = C = O) near 650 cm(-1), suggesting a broken inversion symmetry because of bending. The 650 cm(-1) bending modes soften with increasing pressure, ind icating an enhanced intermolecular interaction among neighboring bent CO2 m olecules. At 80 GPa, the low-frequency vibron collapses into high-frequency phonons, and CO2-IV becomes an extended amorphous solid.