Stability of dense liquid carbon dioxide

Abstract

We present ab initio calculations of the phase diagram of liquid CO 2 and its melting curve over a wide range of pressure and temperature conditions, including those relevant to the Earth. Several distinct liquid phases are predicted up to 200 GPa and 10,000 K based on their structural and electronic characteristics. We provide evidence for a first-order liquid-liquid phase transition with a critical point near 48 GPa and 3,200 K that intersects the mantle geotherm; a liquid-liquid-solid triple point is predicted near 45 GPa and 1,850 K. Unlike known first-order transitions between thermodynamically stable liquids, the coexistence of molecular and polymeric CO 2 phases predicted here is not accompanied by metallization. The absence of an electrical anomaly would be unique among known liquid-liquid transitions. Furthermore, the previously suggested phase separation of CO 2 into its constituent elements at lower mantle conditions is examined by evaluating their Gibbs free energies. We find that liquid CO 2 does not decompose into carbon and oxygen up to at least 200 GPa and 10,000 K.high pressure | density functional theory | first principles molecular dynamics | polymerization A t ambient conditions, the sp-valent second-row elements C, N, and O form simple volatile molecules characterized by double and triple bonds. These materials often undergo dramatic transformations at high pressures into extended single-bonded covalent phases with novel optical, energetic, and mechanical properties (1, 2). The polymerization of solid carbon dioxide has been studied extensively as a prototype for the evolution of a chemical bond under compression (2-7). CO 2 also plays a fundamental role in the physics and chemistry of the Earth interior and its climate (8)(9)(10)(11)(12)(13)(14). However, the thermodynamic, chemical, and physical properties of CO 2 at the high temperature (above 2,000 K) and pressure conditions relevant to planetary interiors remain largely unknown.A critical factor for the Earth's climate is the concentration of CO 2 in the atmosphere, which is controlled by a complicated dynamical cycle involving terrestrial reservoirs and fluxes (8). The vast majority of CO 2 is stored in the mantle primarily in the form of Ca and Mg carbonates (8-13). Experimental (11) and theoretical (13) works suggest that CO 2 is produced at high pressure (P) and temperature (T) during decarbonating reactions with silica in subducted basalts and is subsequently released into the ocean and atmosphere during volcanic activity (8). Moreover, reactions between silica and free CO 2 may also take place under such conditions, leading to the formation of silicon carbonates (15). Whether free CO 2 is stable or decomposes into oxygen and diamond in the mantle is currently unclear (11,12,16,17). Therefore, understanding the stability of CO 2 is a major challenge in establishing the more general issue of terrestrial cycles of C and CO 2 . Furthermore, the presence of CO 2 fluid is believed to be responsible for partial melting and rheological we...

Ab initio simulations of liquid carbon monoxide at high pressure

“…The trajectory for each of these simulations was used to determine the atomic coordination of C atoms. Here we use the terminology employed in previous studies [23,24] to define the carbon coordination as the number of oxygen and carbon atoms within a specified radial cut-off distance of a given carbon atom. The cut-off distance used for coordination determination was the first minimum of the C O pair correlation function, see for example Boates et al [23].…”

“…All simulations were performed with periodic boundary conditions and temperatures were kept constant by a Nosé-Hoover thermostat [21,22] are converged with 32 molecules (96 atoms) in the simulation cell [23,24]. Hence we used a 3 × 3 × 3 body centered cubic supercell of 54 CO molecules (108 atoms) to mitigate finite size effects.…”

“…Molecular dynamics studies of N 2 [28], H 2 [29], and CO 2 [24] found that a plateau in the P-V equation of state for those systems corresponded to a first order liquid-liquid phase transition (LLPT). Boates et al determined that the CO 2 first-order LLPT corresponded to a rapid decrease in the 2-coordination of C, increase in C 3-coordination, and the formation of stable CO 3 molecules [24]. Beyond the LLPT at much higher temperatures and pressures, carbon gradually transition from 3-coordination to short-lived 4-coordination species.…”

Stability and properties of liquid CO2 at high pressure and high temperature: Implications for electrical conductivities in Earth's lower mantle

“…A potentially important prediction from that study is that the conductivity of the dissociated liquid is similar to a metal. In a subsequent theoretical study by the same group the liquid-liquid phase transition was revised to a slightly higher pressure and lower temperature near 48 GPa and 3200 K [Boates et al, 2012], but no metallization of the liquid was found. Gibbs free-energy calculations further suggest that it is energetically unfavorable for molecular CO 2 to dissociate into free C and O 2 .…”

“…Thus, the study of materials as a function of pressure and temperature almost always lead to the formation of new materials. Carbon dioxide (CO 2 ), one of the most important planetary materials, has been extensively studied in many perspectives not only related to Earth and planetary sciences but also in climate change and materials research [Kelemen and Matter, 2008;Lal, 2009;Litasov et al, 2011;Boates et al, 2011Boates et al, , 2012Yoo et al, 2011;Yoo, 2013;Teweldebrhan et al, 2013;Ague and Nicolescu, 2014]. CO 2 is a molecular gas at ambient conditions.…”