Structure of liquid carbon dioxide at pressures up to 10 GPa

The short-range structure of liquid carbon dioxide is investigated at pressures $(P)$ up to 10 GPa and temperatures $(T)$ from 300 to 709 K by means of x-ray diffraction experiments in a diamond anvil cell (DAC) and classical molecular dynamics (MD) simulations. The molecular x-ray structure factor could be measured up to $90{\mathrm{nm}}^{\ensuremath{-}1}$ thanks to the use of a multichannel collimator which filters out the large x-ray scattered signal from the diamond anvils. The experimental data show that the short-range structure of fluid ${\mathrm{CO}}₂$ is anisotropic and continuously changes from a low density to a high density form. The MD simulations are used to extract a detailed three-dimensional analysis of the short-range structure over the same P-T range as the experiment. This reveals that upon compression, a fraction of the molecules in the first-neighbor shell change orientation from the (distorted) T shape to the slipped parallel configuration, accounting for the observed structural changes. The local arrangement is found similar to that of the $Pa\overline{3}$ solid at low density and to that of the $Cmca$ crystal at high density. The comparison with other simple quadrupolar liquids, either diatomic $({\mathrm{I}}₂)$ or triatomic $({\mathrm{CS}}₂)$, suggests that this structural evolution with density is a general one for these systems.