Rigid intertetrahedron angular interaction of nonmolecular carbon dioxide solids

We have theoretically investigated several polymeric (nonmolecular) CO₂ solids analogous to various phases of SiO₂. Our calculations reveal that CO₂ differs dramatically from SiO₂ because of its rigid intertetrahedron angular interaction. Because of this, we find that a β-cristobalitelike phase is energetically the favored polymeric CO₂ phase rather than quartz, which is preferred in SiO₂. The calculated Raman spectrum of β-cristobalite CO₂ shows a dominant A₁ peak at about 780 cm^-1 at 40 GPa which shifts down to about 640 cm^-1 at 1 GPa. This is consistent with the recent high-pressure experiments of Iota et al. [Science 283, 150 (1999)].