1. LENS, European Laboratory for Non-linear Spectroscopy and INFM, Via N. Carrara 1, I-50019 Sesto Fiorentino, Firenze, Italy
2. CRS-SOFT-INFM-CNR, c/o Università di Roma "La Sapienza", I-00185 Roma, Italy
3. Dipartimento di Chimica dell'Università di Firenze, Via della Lastruccia 3, I-50019 Sesto Fiorentino, Firenze, Italy
4. Dipartimento di Fisica, Università di Roma "La Sapienza", I-00185 Roma, Italy
5. The Abdus Salam International Centre for Theoretical Physics (ICTP) and INFM/Democritos National Simulation Center, 34014 Trieste, Italy
6. European Synchrotron Research Facility, BP 220, F38043 Grenoble, France

Correspondence to: Mario Santoro^1,2Federico A. Gorelli^1,2 Correspondence and requests for materials should be addressed to M.S. (Email: santoro@lens.unifi.it) or F.A.G. (Email: gorelli@lens.unifi.it).

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.

1. LENS, European Laboratory for Non-linear Spectroscopy and INFM, Via N. Carrara 1, I-50019 Sesto Fiorentino, Firenze, Italy
2. CRS-SOFT-INFM-CNR, c/o Università di Roma "La Sapienza", I-00185 Roma, Italy
3. Dipartimento di Chimica dell'Università di Firenze, Via della Lastruccia 3, I-50019 Sesto Fiorentino, Firenze, Italy
4. Dipartimento di Fisica, Università di Roma "La Sapienza", I-00185 Roma, Italy
5. The Abdus Salam International Centre for Theoretical Physics (ICTP) and INFM/Democritos National Simulation Center, 34014 Trieste, Italy
6. European Synchrotron Research Facility, BP 220, F38043 Grenoble, France

Correspondence to: Mario Santoro^1,2Federico A. Gorelli^1,2 Correspondence and requests for materials should be addressed to M.S. (Email: santoro@lens.unifi.it) or F.A.G. (Email: gorelli@lens.unifi.it).

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