Valentin Iota
Lawrence Livermore National Laboratory, Livermore, California 94551, USA
e-mail: iota1@llnl.gov

       We have recently discovered a new stishovite-like phase of Carbon Dioxide (CO₂ -VI), by compressing CO₂ above 50GPa and 550K [1].  Once formed at high temperatures the new CO₂ phase remains stable upon quenching to 300K over a wide range of pressures  (15-100GPa). Raman and X-Ray diffraction experiments suggest a 6-fold average coordinated structure (P4₂/mnm symmetry), in which each carbon atom is enclosed in an octahedral cage formed by six neighboring oxygen atoms.
       At ambient conditions, carbon dioxide (CO₂) is a prototypical molecular system, with strong covalent O=C=O molecular bonds and relatively weak quadrupolar interactions between molecules. At high pressures and temperatures, CO₂ transforms to a series of solid polymorphs with differing crystal structures, intermolecular interactions and chemical bonding [2-4]. In particular, two fully covalent (extended) solid phases have been reported above 40GPa, with characteristics analogous to SiO₂ polymorphs. First, CO₂-V (above 40GPa and 1500K), consists of a network of corner sharing CO₄ tetrahedra is structurally similar to SiO₂ tridymite [4]. Another extended-solid amorphous phase (a-carbonia), similar to amorphous silica, has been reported at room temperature above 40GPa [5]. The correspondence between CO₂ and SiO₂ phases would argue for the existence of a six-fold coordinated CO₂ solid – analogous to SiO₂ stishovite. However, the strong covalence in C-O bonds and the rigidity of sp³- bond angles have been presumed to impede the formation of six-fold coordinated carbon units. Thus, total-energy calculations predict that six-fold CO₂ would stabilize only at ultra-high pressures above 400GPa [5].
       The current discovery of six-fold coordinated CO₂ at much lower pressures argues for the need to improvement the current theoretical models, and should stimulate further attempts to understand the stability of CO₆ structures within the standard sp³ carbon configuration. 
       Finally, considering the rich abundance of carbon, oxygen and silicon in the Earth’s mantle, the high-density forms of four- and six-fold carbon dioxide may offer new concepts in geo- and mineral-chemistry. For example, CO₂ could exist in the Earth mantle as four- and six-fold covalent solids and within alloys or solid solutions with SiO₂ and/or other minerals. The structural similarities between CO₂ and SiO2 polymorphs would presumably enhance their mutual solubility and chemical reactivity at the pressure-temperature conditions of the Earth's mantle. The structural instability of 6-fold CO₂ below 10-15 GPa at low pressures and its enhanced ionic character upon decompression, would help account for the carbonate minerals originating from the Earth’s interior as well as for the high-temperature origin of carbonates in Martian Meteorites. 

1.  Six-fold Coordinated Carbon Dioxide VI, Valentin Iota, C-S. Yoo, Z. Jenei, J. Park-Klepeis. H. Cynn and W. Evans; Nature Materials, 6, 34 (2007). PDF file
2.  Phase Diagram of Carbon Dioxide: Evidence for an Associated Phase; Valentin Iota and C-S. Yoo, Phys. Rev. Lett. 86, 5922 (2001). PDF file
3.  Crystal structure of carbon dioxide at high pressure: “superhard polymeric carbon dioxide”; C.S. Yoo, H. Cynn, F. Gygi, G. Galli, V. Iota, M. Nicol and C. Mailhiot, Phys. Rev. Lett. 83, 5527 (1999).
4.  Quartz-like CO₂: an optically nonlinear extended solid at high pressures and temperatures; Valentin Iota, C. S. Yoo, H. Cynn, Science 283, 1510 (1999). PDF file
5.  Amorphous silica-like carbon dioxide, M. Santoro, F. A. Gorelli, R. Bini, G. Ruocco, S. Scandolo and W. A. Crichton; Nature 441, 857-860 (2006)