title: Partially collapsed cristobalite structure in the non molecular phase V in CO2.
abstract: Non molecular CO(2) has been an important subject of study in high pressure physics and chemistry for the past decade opening up a unique area of carbon chemistry. The phase diagram of CO(2) includes several non molecular phases above 30 GPa. Among these, the first discovered was CO(2)-V which appeared silica-like. Theoretical studies suggested that the structure of CO(2)-V is related to that of β-cristobalite with tetrahedral carbon coordination similar to silicon in SiO(2), but reported experimental structural studies have been controversial. We have investigated CO(2)-V obtained from molecular CO(2) at 40-50 GPa and T > 1500 K using synchrotron X-ray diffraction, optical spectroscopy, and computer simulations. The structure refined by the Rietveld method is a partially collapsed variant of SiO(2) β-cristobalite, space group I42d, in which the CO(4) tetrahedra are tilted by 38.4° about the c-axis. The existence of CO(4) tetrahedra (average O-C-O angle of 109.5°) is thus confirmed. The results add to the knowledge of carbon chemistry with mineral phases similar to SiO(2) and potential implications for Earth and planetary interiors.
authors.affiliation: Istituto di Fisica Applicata Nello Carrara del Consiglio Nazionale delle Ricerche (IFAC-CNR), Sesto Fiorentino, 50019 Firenze, Italy. santoro@lens.unifi.it
authors.forename: Mario
authors.initials: M
authors.lastname: Santoro
authors.forename: Federico A
authors.initials: FA
authors.lastname: Gorelli
authors.forename: Roberto
authors.initials: R
authors.lastname: Bini
authors.forename: Julien
authors.initials: J
authors.lastname: Haines
authors.forename: Olivier
authors.initials: O
authors.lastname: Cambon
authors.forename: Claire
authors.initials: C
authors.lastname: Levelut
authors.forename: Javier A
authors.initials: JA
authors.lastname: Montoya
authors.forename: Sandro
authors.initials: S
authors.lastname: Scandolo