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^{1,2,3,4,5,6,7,8,9}. 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 simulations⁹. 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 dynamics⁹, 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.