Investigation of hardness in tetrahedrally bonded nonmolecularCO2solids by density-functional theory

Stability and compressibility of several nonmolecular (polymeric) ${\mathrm{CO}}_2$ solids in structures analogous to those of ${\mathrm{SiO}}_2$ have been investigated with ab initio density-functional theory. Contrary to the recent experimental reports of a “superhard” high-pressure tridymite form of ${\mathrm{CO}}_2,$ we find that metastable tetrahedrally bonded ${\mathrm{CO}}_2$ polymorphs, such as tridymite, cristobalite, and quartz, are relatively compressible, with bulk moduli K of only 1/2 to 1/3 of the reported experimental value. In addition, theory finds that the experimentally reported lattice parameters are not stable for ${\mathrm{CO}}_2$ ${P2\mathrm{}}_1{2}_1{2}_1$ tridymite. Finally, none of the calculated x-ray spectra of the fully relaxed structures of ${\mathrm{CO}}_2$ polymorphs obtained from theory agrees with the experiments. The significant discrepancy between experiments and density-functional theory suggests that further studies on nonmolecular ${\mathrm{CO}}_2$ solids are necessary, and that the assumptions that density-functional theory can describe these materials correctly, or that the framework of the new nonmolecular ${\mathrm{CO}}_2$ solids contains only ${\mathrm{CO}}_4$ tetrahedra, must be re-examined.