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

Stability and compressibility of several nonmolecular (polymeric) COP 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 fur CO2 P2(1)2(1)2(1) 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.