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 I: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 49 GPa to-tridymite, (P2(1)2(1)2(1)) 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 B-0 = 365 GPa, similar to cubic BN.