Session Q25: Focus Session: Simulation of Matter at Extreme Conditions - Phase Transitions

Metallization of FeO at High Temperatures and Pressures: DFT-DMFT Computations and Comparisons with Experiments

DFT+Dynamical Mean Field Theory (DMFT) was applied to FeO as a function of pressure and temperature. We use an LAPW basis set, and the lattice terms are evaluated using the WIEN2K LAPW code. The impurity model is solved using continuous time quantum Monte Carlo (CTQMC). Temperature enters explicitly, so we made special efforts to understand high temperature behavior. The computations are fully self-consistent, including the impurity levels and crystal field splitting, and the total energy is evaluated using the full potential and charge density of the lattice plus impurity models. We find with increasing pressure in paramagnetic FeO in a cubic lattice and U=8 eV a high-spin low-spin transition, with a wide transition region between characterized by intermediate occupancies of the t2g and eg states between. We find that at 300K cubic FeO remains insulating to a factor of two compression (over 600 GPa), except for a small region of high spin metal. However, at high temperatures (e.g. 2000K) a metallic state is found. We find excellent agreement with recent high temperature high pressure experiments (Ohta et al.). We are now studying the antiferromagnetic ordering and effects of lattice strain.   

Finite-temperature solid phases and melting of denser lithium

There has been a lot of recent interest in lithium at high pressure, in particular, in relation to deviations from simple metallic behavior, non-intuitive structural changes, and its anomalous melting curve. Most of the theoretical studies have been limited to 0 K static lattices and liquid properties with classical ions. In this talk, we will present results for the stability of lithium up to 250 GPa and finite temperature, as well as its melting curve. Comparison with experimental measurements and the significance of quantum ion dynamics for the observed properties will be discussed.   

Study on the release process of $\gamma $ and $\alpha $ phase transition in cerium material

Cerium has lots of phase transition in high pressure and temperature. A volume change of about 15{\%} occurs when Cerium is subject to high pressure ($\sim $0.7GPa) and $\gamma \to \alpha $ phase change takes place. The phase transition and constitutive model of Cerium can be respectively obtained by calculating the experiment results and taking account of the multi-phase equation of state (EOS). The calculated results indicate that in loading condition the phase transition pressure of Cerium is higher than quasi-static compression. The calculated results indicate that the phase transition under release is difficultly described because the $\alpha \to \gamma $ phase reversal is great influenced by plastic flow. Based on multi-phase equation of state, constitutive model and non-equilibrium phase transition equation, introducing quasi-elastic unloading rule simulated the phase transition under release. The calculated result is according with the experiment.   

Stability of dense liquid carbon dioxide

We have used first-principles molecular dynamics to identify a transition from molecular liquid CO2 to a new polymeric liquid phase under compression. The phase transition is first-order and unlike other such transitions, is not accompanied by metallization. The region near the liquid-liquid-solid triple point is particularly interesting as it coincides with pressure-temperature conditions inside the Earth's mantle. We have characterized the stability of CO2 under these conditions; contrary to previous studies, our calculations show that CO2 does not phase separate into carbon and oxygen. Comparisons with and alternative interpretations of previous measurements will be presented. Routes for experimental detection of our predictions will also be discussed.   

On the stability of body-centered cubic Fe at Earth's core conditions

Elucidation of Earth's core content and structure is extremely important for understanding Earth's behavior influencing human life, from geodynamics to earthquakes. Though cosmochemical and geochemical studies strongly suggest that solid Fe is the main constituent of the inner core, its exact content and crystal structure are still a matter of debate. The recent experiments reported controversial results of phase stability. Dubrovinsky \textit{et al.} showed stabilization of the body centered cubic (bcc) phase of Fe$_{ }$alloyed with 10 at. {\%} of Ni at pressures above 225 GPa and temperatures over 3400 K [1]. Tateno \textit{et al. }on pure Fe at up to 377 GPa and 5700 K, no bcc phase was observed [2]. We offer a resolution of this contradiction based on finite temperature first-principles molecular dynamics calculations of elastic properties of both bcc Fe and Fe$_{90}$Ni$_{10}$ alloy at high-temperature high-pressure conditions. We indicate the stability range for the bcc phase of high-pressure high-temperature Fe and show how experimental conditions may cause diverse phase stabilization. REFERENCES [1] L. S. Dubrovinsky \textit{et al.} Body-centered cubic iron-nickel alloy in Eath's core. \textit{Science} \textbf{316}, 1880-1883 (2007). [2] S. Tateno,K. Hirose, Y. Ohishi, Y. Tatsumi. The structure of iron in Earth's inner core. \textit{Science} \textbf{330}, 359-361 (2010).   

Thermoelastic Properties of Olivine and its High Pressure Polymorphs at High Pressures and Temperatures: A First-Principles Study

We combine density functional theory (DFT) within the local density approximation (LDA), the quasiharmonic approximation (QHA), and a model of vibrational density of states (VDoS) to calculate aggregate elastic moduli and sound velocities of olivine ($\alpha-$phase) (Fe$_x$,Mg$_{1-x}$)$_2$SiO$_4$, and its high pressure polymorphs, wadsleyite ($\beta-$phase) and ringwoodite ($\gamma-$phase), the most abundant minerals of the Earth's upper mantle (UM) and transition zone (TZ). Comparison of results with high-pressure and room-temperature data and ambient-pressure and high-temperature data shows very good agreement. Using our findings, we investigate the discontinuities in elastic moduli and velocities associated with the $\alpha$ to $\beta$ and $\beta$ to $\gamma$ transformations at pressures and temperatures relevant to seismic discontinuities near 410 km and 520 km depth. This information offers clearly defined reference values to advance understanding of the role that chemical composition and temperature play in these mantle boundary layers.   

Fate of MgSiO3 post-perovskite at multi-Mbar pressures

The discovery of the post-perovskite (PPV) transition of MgSiO$_{3}$ in 2004 invited a new question: What would be the next phase transition from the PPV phase? The importance of this question has increased, since many terrestrial exoplanets with masses of a few to 10 times Earth's (super-Earth) have been recently discovered. Here we predict the new class of phase transitions of MgSiO$_{3}$ PPV under ultrahigh pressure by first-principles calculations combined with the adaptive genetic algorithm, which is a powerful tool for blind structural searches for systems with the large number of atoms. We discuss implications of these new phase transitions in modeling of interiors of terrestrial exoplanets.   

Raman study of the Verwey transition in Magnetite at high-pressure and low-temperature; effect of Al doping

We report high-pressure low-temperature Raman measurements of the Verwey transition in pure and Al --doped magnetite (Fe$_{3}$O$_{4})$ Al-doped magnetite Fe$_{2.8}$Al$_{0.2}$O$_{4}$ (T$_{V}$=116.5K) displays a nearly linear decrease of the transition temperature with an increase of pressure yielding dP/dT$_{V}$=-0.096$\pm $0.013 GPa/K. In contrast pure magnetite displays a significantly steeper slope of the PT equilibrium line with dP/dT$_{V}$ = -0.18$\pm $0.013 GPa/K. Contrary to earlier high pressure resistivity reports we do not observe quantum critical point behavior at 8 GPa in the pure magnetite. Our data indicates that Al doping leads to a smaller entropy change and larger volume expansion at the transition. The trends displayed by the data are consistent with the mean field model of the transition that assumes charge ordering in magnetite.   

Magnetic and Thermal Fluctuations in Fe and (Fe,Ni) alloys at Earth Core Conditions

Several lines of evidence suggest that the earth's inner core is dominated by an iron rich (Fe,Ni) alloy. In this study we address the influence of magnetic and thermal fluctuations as driving forces for phase transitions in Fe and Fe$_{7}$Ni structures at inner core pressure and temperature conditions. Bcc iron is stable at ambient conditions due to its ferromagnetic nature which highlights the importance of magnetism for structural stability. \textit{Ab-initio} electronic structure calculations are used to study the thermal and magnetic fluctuations in Fe and (Fe,Ni) alloys up to pressures and temperatures expected in the earth's inner core. The variable cell shape molecular-dynamics simulations include the magnetic moment and thermal fluctuations. Our preliminary results show a phase transformation in hcp-Fe$_{7}$Ni alloy that occurs after 2.5 ps, well after equilibration. The correlation of magnetic and thermal fluctuations suggests that the residual magnetism is too weak to induce the observed transition. Instead, large thermal fluctuations at the onset of the transition provide a likely driving force.   

Mechanism of body-centered cubic phase stabilization in Fe and He at high pressure and temperature

We have investigated the stabilization of the body-centered cubic phase in Fe and He at high P and T by means of ab initio and classical molecular dynamics. These phases are dynamically unstable at high P and low T, however, they become dynamically stable at high T. We calculated the phonon density of states for Fe and He phases and observed that the bcc PDOS contains long-wavelength phonon states (absent in the close packed phases) that contribute to the free energy. This observation is consistent with the mechanism of stabilization proposed earlier (P. Loubeyre, J.-P. Hansen, PRB 31, 634 (1985); B. L. Holian et al., JCP 59, 5444 (1973)). Direct ab initio simulations of Fe crystallization and classical co-existence simulations for He indicate that the bcc phase is a submelting phase at high P. Previous calculations of the free energy in the bcc phase have been performed on small samples and could not adequately take the long-wavelength correlated motion into account.   

Computation of free energy of liquids and its application to melting of CO$_2$ and N

A computationally efficient method is proposed to compute the free energy of liquids with accuracy comparable to {\it ab initio} thermodynamic integration. The method has been applied to predict melting curves of CO$_2$ and N over a wide range of pressure using the solid-liquid phase coexistence approach. The calculated melting lines are compared with available experimental data and the crossing of the geotherm and melting line of CO$_2$ is determined.   

Melting behaviour of high pressure Na. An ab initio study

The melting curve of sodium for a pressure range up to 120 GPa has been evaluated by the orbital free ab initio molecular dynamics method. This method uses the electronic density as the basic variable and scales almost linearly with system size which allows to perform simulations with a large number of particles and for long simulation times. For various pressures and temperatures we have calculated some static properties (pair distribution functions, static structure factors and short-range order parameters), dynamic properties (mean square displacement, velocity autocorrelation functions and dynamic structure factors) and transport coefficients (self-diffusion, adiabatic sound velocities and shear viscosities). The calculated melting curve reproduces the main qualitative features found in the experiment.