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 CO2 have been investigated to 60 GPa. CO2-1 (Pa3) transforms to CO2-III (Cmca) at 12 GPa with almost no change of density. Although CO2-III persists in Cmca to at least 60 GPa at ambient temperature, it transforms when heated to 1800 K above 40 GPa to tridymite (P212121) CO2-V with 15.3% volume change. Each carbon atom of CO2-V is tetrahedrally bonded to four oxygen atoms. CO2-V is likely superhard with low compressibility B0 = 365 GPa, similar to cubic BN.