title: Electronic structure of carbon dioxide under pressure and insights into the molecular-to-nonmolecular transition.
abstract: Knowledge of the high-pressure behavior of carbon dioxide (CO2), an important planetary material found in Venus, Earth, and Mars, is vital to the study of the evolution and dynamics of the planetary interiors as well as to the fundamental understanding of the C-O bonding and interaction between the molecules. Recent studies have revealed a number of crystalline polymorphs (CO2-I to -VII) and an amorphous phase under high pressure-temperature conditions. Nevertheless, the reported phase stability field and transition pressures at room temperature are poorly defined, especially for the amorphous phase. Here we shed light on the successive pressure-induced local structural changes and the molecular-to-nonmolecular transition of CO2 at room temperature by performing an in situ study of the local electronic structure using X-ray Raman scattering, aided by first-principle exciton calculations. We show that the transition from CO2-I to CO2-III was initiated at around 7.4 GPa, and completed at about 17 GPa. The present study also shows that at ~37 GPa, molecular CO2 starts to polymerize to an extended structure with fourfold coordinated carbon and minor CO3 and CO-like species. The observed pressure is more than 10 GPa below previously reported. The disappearance of the minority species at 63(± 3) GPa suggests that a previously unknown phase transition within the nonmolecular phase of CO2 has occurred.
authors.affiliation: Departments of Earth Sciences, and Physics and Astronomy, University of Western Ontario, London, ON, Canada N6A 5B7.
authors.forename: Sean R
authors.initials: SR
authors.lastname: Shieh
authors.forename: Ignace
authors.initials: I
authors.lastname: Jarrige
authors.forename: Min
authors.initials: M
authors.lastname: Wu
authors.forename: Nozomu
authors.initials: N
authors.lastname: Hiraoka
authors.forename: John S
authors.initials: JS
authors.lastname: Tse
authors.forename: Zhongying
authors.initials: Z
authors.lastname: Mi
authors.forename: Linada
authors.initials: L
authors.lastname: Kaci
authors.forename: Jian-Zhong
authors.initials: JZ
authors.lastname: Jiang
authors.forename: Yong Q
authors.initials: YQ
authors.lastname: Cai
mesh_terms: D002245:Carbon Dioxide / Q000737:chemistry*; D008968:Molecular Conformation; D011312:Pressure; D013059:Spectrum Analysis, Raman; D056928:X-Ray Absorption Spectroscopy
chemical_list: D002245:Carbon Dioxide
keywords: diamond anvil cell; inelastic x-ray scattering; mineral physics