Structure of liquid carbon dioxide at pressures up to 10 GPa

Abstract

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 nmthanks 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 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 threedimensional 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 P a3 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 (CS 2 ), suggests that this structural evolution with density is a general one for these systems.

Data-Driven Many-Body Models for Molecular Fluids: CO2/H2O Mixtures as a Case Study

“…[16][17][18][19][20][21][22][23] At the molecular level, vibrational spectroscopy is used to characterize structure and dynamics of CO 2 clusters and clathrates, liquid and supercritical CO 2 as well as the properties of CO 2 in mixtures with water, small alcohols and hydrocarbons, and in ionic liquids. [24][25][26][27][28][29][30][31][32][33][34] Recently, X-ray diffraction has been used to determine the local structure of liquid CO 2 at pressures up to 10 GPa and temperatures from 300 to 709 K. 35 From a theoretical standpoint, electronic structure calculations and molecular dynamics (MD) simulations have been used to model the energetics as well as the structural, thermodynamic, and dynamical properties of CO 2 , in both single-and multi-component systems. Most of the early molecular models adopted relatively simple functional forms parameterized to reproduce vapor/liquid equilibrium properties.…”

CO2 packing polymorphism under pressure: Mechanism and thermodynamics of the I-III polymorphic transition

“…Despite its simple molecular structure, CO 2 has a rather complex solid-state phase diagram 24 (partially reported in Figure 1 (a)). Indeed, at high temperature and pressure, seven different crystal structures have been detected so far, among which many are still debated [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] . The first form detected was molecular phase I, also called dry ice, crystallised directly from the melt; phase III followed, obtained through the compression of dry ice; the discovery of a polymeric structure, classified as phase V, attracted more interest to the study of this system, resulting in the identification of two more phases, II and IV.…”

Structural and Chemical Modifications of Carbon Dioxide on Transport to the Deep Earth

“…The melting line was determined by visual observation, fluid-solid (phase I) coexistence in the 300-800 K temperature range, and pressures up to 11 GPa (Giordano et al, 2006). In almost the same P-T range, the structure of liquid CO 2 was recently investigated by both synchrotron XRD and classical molecular dynamics simulations showing an anisotropic short-range structure (Datchi et al, 2016). This is essentially related to a change in the orientation of some molecules in the first shell from the characteristic quadrupole-driven T-shape arrangement to a slipped parallel configuration.…”