Structure of liquid carbon dioxide at pressures up to 10 GPa

International audienceThe short-range structure of liquid carbon dioxide is investigated at pressures ($P$) up to 10GPa and temperatures($T$) from 300 to 709 K by means of x-ray diffraction experiments in a diamond anvil cell (DAC) and classicalmolecular dynamics (MD) simulations. The molecular x-ray structure factor could be measured up to 90 nm$^{−1}$thanks to the use of a multichannel collimator which filters out the large x-ray scattered signal from the diamondanvils. The experimental data show that the short-range structure of fluid CO$_2$ is anisotropic and continuouslychanges from a low density to a high density form. The MD simulations are used to extract a detailed threedimensionalanalysis of the short-range structure over the same P-T range as the experiment. This reveals thatupon compression, a fraction of the molecules in the first-neighbor shell change orientation from the (distorted) Tshape to the slipped parallel configuration, accounting for the observed structural changes. The local arrangementis found similar to that of the $Pa$$\bar 3$ solid at low density and to that of the $Cmca$ crystal at high density. Thecomparison with other simple quadrupolar liquids, either diatomic (I$_2$) or triatomic (CS$_2$), suggests that thisstructural evolution with density is a general one for these systems