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

Abstract

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. ${\mathrm{CO}}_{2}\ensuremath{-}\mathrm{I}$ $(\mathrm{Pa}3)$ transforms to ${\mathrm{CO}}_{2}\ensuremath{-}\mathrm{III}$ $(\mathrm{Cmca})$ at 12 GPa with almost no change of density. Although ${\mathrm{CO}}_{2}\ensuremath{-}\mathrm{III}$ persists in $\mathrm{Cmca}$ to at least 60 GPa at ambient temperature, it transforms when heated to 1800 K above 40 GPa to tridymite $({P2}_{1}{2}_{1}{2}_{1})$ ${\mathrm{CO}}_{2}\ensuremath{-}\mathrm{V}$ with $15.3%$ volume change. Each carbon atom of ${\mathrm{CO}}_{2}\ensuremath{-}\mathrm{V}$ is tetrahedrally bonded to four oxygen atoms. ${\mathrm{CO}}_{2}\ensuremath{-}\mathrm{V}$ is likely superhard with low compressibility ${B}_{0}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}365\phantom{\rule{0ex}{0ex}}\mathrm{GPa}$, similar to cubic BN.