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 nm(-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 CO2 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 (3) over bar solid at low density and to that of the Cmca crystal at high density. The comparison with other simple quadrupolar liquids, either diatomic (I-2) or triatomic (CS2), suggests that this structural evolution with density is a general one for these systems.