Elastic stability of CO2 phase I under high temperature and pressure

Abstract

Carbon dioxide exhibits a richness of high-pressure polymorphs ranging from typical molecular solids to fully extended covalent solids, which, in turn, makes it a very appealing topic of fundamental research in condensed-matter physics and simultaneously provides valuable insights into the routes of developing possibly novel materials with advanced properties. The single-crystal x-ray diffraction (XRD) and Brillouin scattering spectroscopy of $\mathrm{C}{\mathrm{O}}_{2}\ensuremath{-}\mathrm{I}$ were performed under high temperature and pressure. Densities, acoustic velocities, and elastic moduli of $\mathrm{C}{\mathrm{O}}_{2}\ensuremath{-}\mathrm{I}$ were obtained along 300-, 400-, and 580-K isotherms up to the phase-transition boundaries. $\mathrm{C}{\mathrm{O}}_{2}\ensuremath{-}\mathrm{I}$ transforms to phase III and phase IV at room temperature (at 12.19 GPa) and 580 K (at 10.83 GPa), respectively. It was observed that high temperature suppresses pressure-induced stress in single-crystal $\mathrm{C}{\mathrm{O}}_{2}\ensuremath{-}\mathrm{I}$. All elastic constants and thermal elasticity parameters of $\mathrm{C}{\mathrm{O}}_{2}\ensuremath{-}\mathrm{I}$ were obtained and analyzed using finite-strain theory and thermal equation of state modeling. The ${C}_{11}$, ${C}_{12}$, and ${K}_{S}$ increase almost linearly with pressure, while shear moduli ${C}_{44}$ and G exhibit a downward trend with pressure, showing a noticeable reduction at higher temperature. Elastic anisotropy A is practically independent of pressure along each isotherm and increases from 1.75 to 1.9.