Investigation of hardness in tetrahedrally bonded nonmolecular CO₂ solids by density-functional theory

Stability and compressibility of several nonmolecular (polymeric) CO₂ solids in structures analogous to those of SiO₂ have been investigated with ab initio density-functional theory. Contrary to the recent experimental reports of a "superhard" high-pressure tridymite form of CO₂, we find that metastable tetrahedrally bonded CO₂ 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 CO₂ P2₁2₁2₁ tridymite. Finally, none of the calculated x-ray spectra of the fully relaxed structures of CO₂ polymorphs obtained from theory agrees with the experiments. The significant discrepancy between experiments and density-functional theory suggests that further studies on nonmolecular CO₂ solids are necessary, and that the assumptions that density-functional theory can describe these materials correctly, or that the framework of the new nonmolecular CO₂ solids contains only CO₄ tetrahedra, must be re-examined.