Structure and compressibility of the high-pressure molecular phase II of carbon dioxide

The structure and equation of state of the crystalline molecular phase II of carbon dioxide have been investigated at room temperature from 15.5 to 57.5 GPa using synchrotron x-ray diffraction methods. The present value is similar to that of the free molecule and is in very good agreement with predictions based on density functional theory. The compressibility of CO sub(2)-II determined here also disagrees with the previous study: our value for the zero-pressure bulk modulus, B sub(0) = 8.5 (3) GPa [with B' sub(0) = ([partialdifferential] B/[partialdifferential] P) sub(0) = 6.29], is times smaller. Carrying out density functional theory calculations of the elastic tensor and stress-strain relation, we interpret this as due to the softness of the crystal against deviatoric stress in the [110] and symmetry-related directions. Unlike the other dioxides of the group-14 elements, there is however no mechanical or dynamical instability of the P4 sub(2)/mnm structure in CO sub(2) up to 57.5 GPa at 295 K, and therefore no symmetry lowering to P sub(nnm).