Crystal Structure of Carbon Dioxide at High Pressure: “Superhard” Polymeric Carbon Dioxide

The crystal structures of two molecular phases (I and III) and a polymeric phase (V) of ${\mathrm{CO}}_2$ have been investigated to 60 GPa. ${\mathrm{CO}}_2-\mathrm{I}$ $\left(\mathrm{Pa}3\mathrm{}\right)$ transforms to ${\mathrm{CO}}_2-\mathrm{III}$ $\left(\mathrm{Cmca}\right)$ at 12 GPa with almost no change of density. Although ${\mathrm{CO}}_2-\mathrm{III}$ persists in $\mathrm{Cmca}$ to at least 60 GPa at ambient temperature, it transforms when heated to 1800 K above 40 GPa to tridymite $\left({P2\mathrm{}}_1{2}_1{2}_1\right)$ ${\mathrm{CO}}_2-\mathrm{V}$ with $15.3\%$ volume change. Each carbon atom of ${\mathrm{CO}}_2-\mathrm{V}$ is tetrahedrally bonded to four oxygen atoms. ${\mathrm{CO}}_2-\mathrm{V}$ is likely superhard with low compressibility $B_0=365\mathrm{GPa}$, similar to cubic BN.