title: Amorphous silica-like carbon dioxide.
abstract: Among the group IV elements, only carbon forms stable double bonds with oxygen at ambient conditions. At variance with silica and germania, the non-molecular single-bonded crystalline form of carbon dioxide, phase V, only exists at high pressure. The amorphous forms of silica (a-SiO2) and germania (a-GeO2) are well known at ambient conditions; however, the amorphous, non-molecular form of CO2 has so far been described only as a result of first-principles simulations. Here we report the synthesis of an amorphous, silica-like form of carbon dioxide, a-CO2, which we call 'a-carbonia'. The compression of the molecular phase III of CO2 between 40 and 48 GPa at room temperature initiated the transformation to the non-molecular amorphous phase. Infrared spectra measured at temperatures up to 680 K show the progressive formation of C-O single bonds and the simultaneous disappearance of all molecular signatures. Furthermore, state-of-the-art Raman and synchrotron X-ray diffraction measurements on temperature-quenched samples confirm the amorphous character of the material. Comparison with vibrational and diffraction data for a-SiO2 and a-GeO2, as well as with the structure factor calculated for the a-CO2 sample obtained by first-principles molecular dynamics, shows that a-CO2 is structurally homologous to the other group IV dioxide glasses. We therefore conclude that the class of archetypal network-forming disordered systems, including a-SiO2, a-GeO2 and water, must be extended to include a-CO2.
authors.affiliation: LENS, European Laboratory for Non-linear Spectroscopy and INFM, Via N. Carrara 1, I-50019 Sesto Fiorentino, 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: Giancarlo
authors.initials: G
authors.lastname: Ruocco
authors.forename: Sandro
authors.initials: S
authors.lastname: Scandolo
authors.forename: Wilson A
authors.initials: WA
authors.lastname: Crichton