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 $11.1\pm0.1$ GPa and $800\pm5$ 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 CO$\_2$. 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.