Rigid intertetrahedron angular interaction of nonmolecular carbon dioxide solids

We have theoretically investigated several polymeric (nonmolecular) ${\mathrm{CO}}_2$ solids analogous to various phases of ${\mathrm{SiO}}_2.$ Our calculations reveal that ${\mathrm{CO}}_2$ differs dramatically from ${\mathrm{SiO}}_2$ because of its rigid intertetrahedron angular interaction. Because of this, we find that a $\beta$-cristobalitelike phase is energetically the favored polymeric ${\mathrm{CO}}_2$ phase rather than quartz, which is preferred in ${\mathrm{SiO}}_2.$ The calculated Raman spectrum of $\beta$-cristobalite ${\mathrm{CO}}_2$ shows a dominant $A_1$ peak at about 780 ${\mathrm{cm}}^{-1}$ at 40 GPa which shifts down to about 640 ${\mathrm{cm}}^{-1}$ at 1 GPa. This is consistent with the recent high-pressure experiments of Iota et al. [Science 283, 150 (1999)].