Transformation and structure of silicatelike CO2-V

We report the evidence of two different polymorphs for polymeric CO${}_{2}$-V in tridymitelike (V${}_{\mathrm{TD}}$ in $P$2${}_{1}$2${}_{1}$2${}_{1}$) and \ensuremath{\beta}-cristobalitelike (V${}_{\mathrm{CR}}$ in $I$-42$d$) structures. The V${}_{\mathrm{TD}}$ phase is produced by laser-heating phase III (Cmca) above 40 GPa, whereas the V${}_{\mathrm{CR}}$ phase by laser-heating highly compressed phase II ($P$4${}_{2}$/mnm---iso-space group to stishovite) and IV ($P$4${}_{1}$2${}_{1}$2---iso-space group to \ensuremath{\alpha}-cristobalite) above 35 GPa. The density of the V${}_{\mathrm{CR}}$ (3.988 g/cm${}^{3}$) is \ensuremath{\sim}12$%$ larger than that of the V${}_{\mathrm{TD}}$ (3.559 g/cm${}^{3}$) at 50 GPa, while the density difference reduces to \ensuremath{\sim}4$%$ at ambient pressure. This results in a substantially smaller bulk modulus (${B}_{o}$ $=$ 127 GPa, $B$\ensuremath{'} $=$ 5.6) for the V${}_{\mathrm{CR}}$ phase than that of the V${}_{\mathrm{TD}}$ (${B}_{o}$ $=$ 270 GPa, $B$\ensuremath{'} $=$ 1.9). The smaller density of the V${}_{\mathrm{TD}}$ is due to the open structure of corner shared CO${}_{4}$ tetrahedra and a great level of distortion in C-O-C angles resulting in a highly buckled layer structure; yet, the structural relationship gives rise to the specific transition to occur depending on the initial phase, either displacively from phase IV to phase V${}_{\mathrm{CR}}$ or diffusively from phase III to phase V${}_{\mathrm{TD}}$. The results also provide new constraints for the phase/chemical transformation diagram of carbon dioxide.