Amorphous silica-like carbon dioxide

A new form of CO 2 High pressure modifies the interatomic and intermolecular interactions in condensed matter, profoundly altering the physical and chemical properties of materials. This is dramatically demonstrated in a newly discovered form of carbon dioxide, dubbed a-carbonia. This nonmolecular amorphous carbon dioxide is a high pressure modification of the CO 2 molecular solid. It is a glassy material, homologous to amorphous silica (SiO 2 ) and germania (GeO 2 ). The discovery could initiate new research areas in the solid-state chemistry of light elements. 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 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 . The amorphous forms of silica (a-SiO 2 ) and germania (a-GeO 2 ) are well known at ambient conditions; however, the amorphous, non-molecular form of CO 2 has so far been described only as a result of first-principles simulations 9 . Here we report the synthesis of an amorphous, silica-like form of carbon dioxide, a-CO 2 , which we call ‘a-carbonia’. The compression of the molecular phase III of CO 2 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-SiO 2 and a-GeO 2 , as well as with the structure factor calculated for the a-CO 2 sample obtained by first-principles molecular dynamics 9 , shows that a-CO 2 is structurally homologous to the other group IV dioxide glasses. We therefore conclude that the class of archetypal network-forming disordered systems, including a-SiO 2 , a-GeO 2 and water, must be extended to include a-CO 2 .