The phase diagrams of MgSiO3 and MgO are studied from first-principles theory for pressures and temperatures up to 600 GPa and 20,000 K. Through the evaluation of finite-temperature Gibbs free energies, using density-functional theory within the generalized gradient approximation as well as with hybrid exchange-correlation functionals, we find evidence for a vast pressure-temperature regime where molten MgSiO3 decomposes into liquid SiO2 and solid MgO, with a volume change of approximately 1.2%. The demixing transition is driven by the crystallization of MgO--the reaction only occurs below the high-pressure MgO melting curve. The predicted transition pressure at 10,000 K is in close proximity to an anomaly reported in recent laser-driven shock experiments of MgSiO3. We also present new results for the high-pressure melting curve of MgO and its B1-B2 solid phase transition, with a triple point at 364 GPa and 12,000 K.

We report results of first-principles molecular dynamics simulations, which predict a first-order phase transition from molecular to polymeric liquid nitrogen. The liquid-liquid phase boundary is near 88 GPa along the 2000 K isotherm and has a critical point between 4000 and 5000 K and 50 to 75 GPa. At higher temperatures, molecular nitrogen undergoes temperature-driven dissociation to an atomic liquid. The nature of the liquid-liquid transition and the structure of the new polymeric phase are characterized, and ways to experimentally confirm our findings are proposed.

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...

We report on the use of first-principles molecular dynamics calculations to examine properties of liquid carbon dioxide in the pressure-temperature range of 0-1 TPa and 200-100 000 K. The computed equations of state points are used to predict a series of shock Hugoniots with initial starting conditions that are relevant to existing and ongoing shock-wave experiments. A comparison with published measurements up to 70 GPa shows excellent agreement. We find that the liquid undergoes a gradual phase transition along the Hugoniot and have characterized this transition based on changes in bonding and structural properties as well as the conductivity and reflectivity of the fluid.