Session P25: Focus Session: Simulation of Matter at Extreme Conditions - Static Pressure

A new metastable phase of silicon in the \textit{Ibam} structure

In a study aimed at finding new useful forms of silicon, we use an \textit{ab initio} random structure searching (AIRSS) method to identify a new phase of silicon in the \textit{Ibam} structure. The \textit{Ibam} phase is found to be semimetallic within density functional theory with a small band overlap, and it is expected that quasiparticle corrections using the GW approximation would yield a semiconducting state with a small band gap. Calculation of the lattice dynamics reveals that the structure is locally stable. Enthalpy-pressure relations are calculated for the \textit{Ibam} structure as well as all other known Si structures, including the previously predicted phases st12 and bct. These results indicate that \textit{Ibam} silicon is metastable over the pressure range considered. Calculated coexistence pressures of the other known phase transitions are in good agreement with experimental observation.   

Ultra-incompressible Three Dimensional Long-range Ordered Amorphous Carbon Clusters

Here we report the synthesis of a long range ordered material constructed from units of amorphous carbon clusters and solvent molecular. This material has super-incompressibility which can make indents on diamonds. It was synthesized by crushing the fullerenes cages at high pressure. Using high pressure x-ray diffraction and Raman spectroscopy, we observed that the fullerenes cages collapse as the pressure higher but the sample remains in crystalline phase even at 60 GPa. The high pressure phase is ultraincompressible, quenchable and much denser than the starting material. The discovery of the existence of such a unique phase should lead to a great deal of interest for design and synthesis of materials with this characteristic.   

Anharmonicity and bonding electrons in silicon under high pressures

Electron density distributions have been measured for silicon at high pressures by single crystal diffraction using a diamond anvil cell. An abrupt change in charge density distribution is observed at 10.1 GPa, a pressure close to a phase transition from diamond structure to beta-tin structure at 12.5 GPa. Our results show a strong anharmonicity effect in silicon in a pressure range of 2.5 GPa before the phase transition to beta-tin.   

Formation of superconducting platinum hydride under pressure: an ab initio approach

Noble metals such as Pt, Au, or Re are commonly used for electrodes and gaskets in diamond anvil cells for high-pressure research because they are expected to rarely undergo structural transformation and possess simple equation of states. Specifically Pt has been used widely for high-pressure experiments and has been considered to resist hydride formation under pressure. Pressure-induced reactions of metals with hydrogen are in fact quite likely because hydrogen atoms can occupy interstitial positions in the metal lattice, which can lead to unexpected effects in experiments. In our study, PRL 107 117002 (2011), we investigated crystal structures using {\it ab initio} random structure searching (AIRSS) and predicted the formation of platinum mono-hydride above 22 GPa and superconductivity T$_{c}$ was estimated to be 10 -- 25 K above around 80 GPa. Furthermore, we showed that the formation of fcc noble metal hydrides under pressure is common and examined the possibility of superconductivity in these materials.   

New ultrahigh pressure phases of H2O ice predicted using an adaptive genetic algorithm

We propose three new phases of H$_{2}$O under ultrahigh pressure. Our structural search was performed using an adaptive genetic algorithm which allows an extensive exploration of crystal structure at density functional theory(DFT) accuracy. The new sequence of pressure-induced transitions beyond ice X at 0 K should be ice X $\to$ Pbcm $\to$ Pbca $\to$ Pmc2$_{1}$ $\to$ P2$_{1} \to$ P2$_{1}$/c phases. Across the Pmc2$_{1}$-P2$_{1}$ transition, the coordination number of oxygen increases from 4 to 5 with a significant increase of density. All stable crystalline phases have nonmetallic band structures up to 7 TPa.   

Quantum Monte Carlo applied to Solids under Pressure

Diffusion quantum Monte Carlo (DMC) has been applied to solids under pressure in several different contexts a high degree of success.\footnote{J. Kolorenc and L. Mitas. Rep. Prog. Phys. 74 026502 (2011)} All of these calculations must address three errors present in DMC calculations of solids: the fixed node approximation, the pseudopotential approximation and the finite size approximation. Due to the varying approximations to address these errors, these calculations suffer from an uncertainty that is almost comparable to that introduced by the choice of functional in density functional theory (DFT). In this presentation, we present lattice constants and bulk moduli of more than fifteen solids under compression performed with a consistent approach to these three approximations. These results help establish the general accuracy that may be expected from DMC calculations of solids under pressure and also provide a reference from which improvements to DMC methods may be judged. \\[4pt] Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.   

First-principles simulations on bonding pathways of chemical transformations under hydrostatic compression

High pressure as a thermodynamic parameter provides a strong structural constraint to lead chemical transformations with selective ways. Thus, chemical transformations under pressure can create novel materials which may not be accessible by covalent synthesis. However, bonding evolution toward high pressure chemical transformations can be a complex process and may happen over widely different pressures. To understand bonding evolution pathways of high pressure chemical transformations, first-principles simulations were performed following hydrostatic compression enthalpy minimization paths to obtain experimentally and theoretically established phase transitions of carbon. The results showed that the chemical transformations from hydrostatic compression carbon to single-bonded phases were characterized by a sudden decrease in principal stress components, indicating the onset of chemical transformation. On this basis, a number of hydrostatic compression chemical transformations from molecular precursors to novel materials were predicted, such as hydrocarbon graphane, a hydrogenated carbon nitride sheet, and carbon nitrides. All predicted hydrostatic compression transformations are featured as a sudden change in principal stress components, representing chemical bonding destruction and formation reactions with a cell volume collapse.   

Dielectric constant of water under deep Earth pressures and temperature conditions

The knowledge of the dielectric constant of water as a function of pressure (P) and temperature (T) plays a critical role in understanding the chemistry of aqueous systems, and in particular of fluids in the Earth mantle, where water is stored in hydrous minerals. By using first-principles molecular dynamics, we have computed the dielectric constant of water at T = 1000 K, between 1 and 10 GPa, under conditions of the Earth upper mantle. We present a detailed comparison of our results with available experimental data and empirical models, and we discuss how the liquid dielectric constant is affected by the changes in the hydrogen-bond network and molecular dipole moment observed upon compression.   

Hydrogen-Helium Mixtures at High Pressures

We extend our previous work on hydrogen-helium mixtures (Morales, M. A., et. al. PNAS 106, 1324 (2009).) to lower pressures and lower temperatures, across the molecular dissociation regime in hydrogen, to the low pressure molecular liquid. Using density functional theory-based molecular dynamics together with thermodynamic integration techniques, we calculate the Gibbs free energy of the dense liquid as a function of pressure, temperature, and composition. Our work focuses on the mixing properties of the liquid, the optical properties including conductivity and reflectivity, and the creation of accurate mixing models for thermodynamic properties, including pressure and entropy. The resulting models will provide the basis for accurate first-principles equations of state for planetary modeling. Prepared by LLNL under Contract DE-AC52-07NA27344.   

Pressure makes mercury a transition metal: a first-principles study of HgF4 solid phases

Mercury is considered as a post-transition metal, because its d shell is filled and does not involve in forming chemical bonds. Yet, because the large relativistic effect pushes up the outmost d level, there is a high expectation that Hg can be stabilized in a higher oxidation state. The HgF4 molecule has been predicted by calculations, and an evidence of such molecule is shown by IR absorption recently. However, there is neither computation nor experiment report on possible high oxidation state of Hg in solid. By using first-principles density functional theory and a structure-searching method, we studied the structural change of a solid system of Hg and F under pressures from 0 to 300 GPa. We found that at lower pressure, the stable structure consists of HgF2 and F2 molecules. At about 25 GPa, the system undergoes a structural change and forms HgF4 planar molecules featuring d8 configuration. The calculations show that the d orbitals of Hg involve in chemical bonding, which is the signature of a transition metal.   

Ab-initio parametrization of a fully polarizable and dissociable force field for water

A novel all-atom, dissociative, and polarizable force field for water is presented. The force field is parameterized based on forces, stresses and energies obtained form ab-initio calculations of liquid water at ambient conditions. The accuracy of the force field is tested by calculating structural and dynamical properties of liquid water and the energetics of small water clusters. The transferability of the force field to dissociated states is studied by considering the solvation a proton and the ionization of water at extreme conditions of pressure and temperature. In the case of the solvated proton the force field properly describes the presence of both Eigen and Zundel configurations. In the case of the pressure-induced ice VIII / ice X transition and the temperature-induced transition to a superionic phase, the force field is found to describe accurately the proton symmetrization and the melting of the proton sublattice,   

Spin states and hyperfine interactions of iron incorporated in MgSiO$_3$ post-perovskite

Using density functional theory + Hubbard $U$ (DFT+$U$) calculations, we investigate the spin states and nuclear hyperfine interactions of iron incorporated in magnesium silicate (MgSiO$_3$) post-perovskite (Ppv), a major mineral phase in the Earth's D'' layer, where the pressure ranges from about 120 to 135 GPa. In this pressure range, ferrous iron (Fe$^{2+}$) substituting for magnesium at the dodecahedral (A) site remains in the high-spin (HS) state; intermediate-spin (IS) and low-spin (LS) states are highly unfavorable. As to ferric iron (Fe$^{3+}$), which substitutes magnesium at the A site and silicon at the octahedral (B) site to form (Mg,Fe)(Si,Fe)O$_3$ Ppv, we find the combination of HS Fe$^{3+}$ at the A site and LS Fe$^{3+}$ at the B site the most favorable. Neither A-site nor B-site Fe$^{3+}$ undergoes a spin-state crossover in the D'' pressure range. The computed iron quadrupole splittings are consistent with those observed in M{\"o}ssbauer spectra. The effects of Fe$^{2+}$ and Fe$^{3+}$ on the equation of state of Ppv are found nearly identical, expanding the unit cell volume while barely affecting the bulk modulus.   

Effects of aluminum on spin-state crossover of iron in the Earth's lower mantle

Using density functional theory + Hubbard $U$ (DFT+$U$) calculations, we investigate how aluminum affects the spin-state crossover of iron in MgSiO$_3$ perovskite and post-perovskite, the major mineral phases in and at the bottom of the Earth's lower mantle. We find that aluminum does not change the response of iron spin state to the increasing pressure, namely, only the ferric iron (Fe$^{3+}$) residing the octahedral (B) site undergoes a crossover from high-spin to low-spin state, same as aluminum-free iron-bearing MgSiO$_3$. The presence of aluminum, however, does affect the population of B-site ferric iron significantly -- the majority of Fe$^{3+}$ reside the dodecahedral (A) site at lower pressures, and the population of B-site Fe$^{3+}$ increases with pressure at higher pressure range. Therefore, in the Earth's lower mantle, the amount of B-site Fe$^{3+}$ and the degree of elastic anomalies (and thus the possible seismic anomalies) associated with spin-state crossover is directly affected by the concentration and configuration of aluminum.   

Phase stability of mixtures at extreme conditions: implications for the interior structure of the Outer Planets

The unusual magnetic fields of the planets Uranus and Neptune represent important observables for constraining and developing deep interior models. Models suggests that the non-dipolar and non-axial magnetic fields of these planets originate from a thin convective and conducting shell of material around a stratified fluid core. We present a computational study of the physical properties of a fluid compositionally similar to what is expected in the interior of Uranus and Neptune. Our diffusivity and conductivity results suggest that the core cannot be well mixed if it is to generate non-axisymmetric magnetic fields. The simulations highlight the importance of chemistry on the properties of this complex mixture, including the possible formation of carbon and nitrogen clusters. We present results concerning the overall phase stability of the mixture under conditions relevant to the planetary interiors.