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-I (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 B₀0ex0ex=0ex0ex3650ex0exGPa, similar to cubic BN.