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Amorphous silica-like carbon dioxide


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 pressure1,2,3,4,5,6,7,8,9. 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.

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Figure 1: Infrared spectra showing the transformation of the molecular solid, CO 2 -III, into a-CO 2 for two different samples.
Figure 2: Comparison of a-CO 2 IR spectrum with mass-shifted spectra.
Figure 3: Comparison of a-CO 2 Raman spectrum with phase V spectra and mass-shifted spectra.
Figure 4: Static structure factor, S(Q ), of a-CO2.

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We thank F. Sette and M. Mezouar for discussions and the ESRF for provision of beamtime at ID27. S.S. thanks Y. Liang for preparing the molecular dynamics configurations. This work was supported by the European Community and the Ente Cassa di Risparmio di Firenze.

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Correspondence to Mario Santoro or Federico A. Gorelli.

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Supplementary Figures

This file shows the procedure used for the x-ray diffraction data processing. The figure shows the diffracted total intensity I(Q), the background intensity Ib(Q) and the sample diffracted intensity, given by the difference between the two. The legend describes how to obtain the S(Q) from the sample diffracted intensity. (PDF 35 kb)

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Santoro, M., Gorelli, F., Bini, R. et al. Amorphous silica-like carbon dioxide. Nature 441, 857–860 (2006).

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