Melting curve and fluid equation of state of carbon dioxide at high pressure and high temperature

Abstract

The melting curve and fluid equation of state of carbon dioxide have been determined under high pressure in a resistively-heated diamond anvil cell. The melting line was determined from room temperature up to 11.1 ± 0.1 GPa and 800 ± 5 K by visual observation of the solid-fluid equilibrium and in-situ measurements of pressure and temperature. Raman spectroscopy was used to identify the solid phase in equilibrium with the melt, showing that solid I is the stable phase along the melting curve in the probed range. Interferometric and Brillouin scattering experiments were conducted to determine the refractive index and sound velocity of the fluid phase. A dispersion of the sound velocity between ultrasonic and Brillouin frequencies is evidenced and could be reproduced by postulating the presence of a thermal relaxation process. The Brillouin sound velocities were then transformed to thermodynamic values in order to calculate the equation of state of fluid CO2 . An analytic formulation of the density with respect to pressure and temperature is proposed, suitable in the P − T range 0.1-8 GPa and 300-700 K and accurate within 2%. Our results show that the fluid above 500 K is less compressible than predicted from various phenomenological models.

Structure of liquid carbon dioxide at pressures up to 10 GPa

“…3(c), the ZD model predicts lower densities than the EOS of Ref. [9], but the difference remains within 3% below 10 GPa, i.e., close to experimental uncertainties. We also compared the predicted site-site radial distribution functions (SS-RDF) g CC (r), g CO (r), and g OO (r) to those obtained using first-principles calculations [41,42] at the density of 19.17 molecules/nm 3 (1.4 g/cc), corresponding to a pressure of 0.47 GPa at 300 K. The close agreement between the two sets of SS-RDF (see Supplementary Fig.…”

“…For density we use the equation of state of Ref. [9], which agrees very well with present experiments (see below), in order to smooth out the fluctuations due to experimental uncertainties. Both Q m,1 and Q m,2 appear to vary linearly with ρ 0 over the probed density range.…”

“…3(c) and compared to the experimental equation of state (EOS) of Ref. [9]. The latter is based on sound velocity measurements up to 8 GPa and 700 K, and its uncertainty was estimated to be 2%.…”

“…Notably, the phase diagram of CO 2 at conditions of high pressure and temperature has been the subject of many investigations [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], the main results of which are summarized in Fig. 1.…”

Thermophysical properties of liquid carbon dioxide under shock compressions: Quantum molecular dynamic simulations

“…Solid phase diagram of carbon dioxide up to 80 GPa and molecular-nonmolecular transition have already been probed through diamond-anvil cells and confocal micro-Raman spectroscopy [8][9][10][11]. Solid specimens were reported to be stable until the formation of acarbonia (an amorphous extended phase), which can only be revealed around 50 GPa at elevated temperature or above 65 GPa at ambient temperature [11].…”