High pressure cosmochemistry applied to major planetary interiors: Experimental studies

The overall goal of this project is to determine properties of the H-He-C-N-O system, as represented by small molecules composed of these elements, that are needed to constrain theoretical models of the interiors of the major planets. Much of our work now concerns the H2O-NH3 system. This project is the first major effort to measure phase equilibria in binary fluid-solid systems in diamond anvil cells. Vibrational spectroscopy, direct visual observations, and X-ray crystallography of materials confined in externally heated cells are our primary experimental probes. We also are collaborating with the shockwave physics group at Lawrence Livermore Laboratory in studies of the equation of state of a synthetic Uranus fluid and molecular composition of this and other H-C-N-O materials under planetary conditions

Melting curve and phase diagram of vanadium under high-pressure and high-temperature conditions

We report a combined experimental and theoretical study of the melting curve and the structural behavior of vanadium under extreme pressure and temperature. We performed powder x-ray diffraction experiments up to 120 GPa and 4000 K, determining the phase boundary of the bcc-to-rhombohedral transition and melting temperatures at different pressures. Melting temperatures have also been established from the observation of temperature plateaus during laser heating, and the results from the density-functional theory calculations. Results obtained from our experiments and calculations are fully consistent and lead to an accurate determination of the melting curve of vanadium. These results are discussed in comparison with previous studies. The melting temperatures determined in this study are higher than those previously obtained using the speckle method, but also considerably lower than those obtained from shock-wave experiments and linear muffin-tin orbital calculations. Finally, a high-pressure high-temperature equation of state up to 120 GPa and 2800 K has also been determined

A new molecular solid phase of carbon dioxide at high pressure and temperature

We report the discovery of a new high-pressure molecular phase of carbon dioxide. The new polymorph, CO{sub 2} -IV, is formed by heating the high-pressure orthorhombic phase III to temperatures above 1000K at pressures between 12 and 33GPa. Analysis of the Raman spectrum of the new phase suggests a structure lacking inversion symmetry

High Pressure Thermoelasticity of Body-centered Cubic Tantalum

We have investigated the thermoelasticity of body-centered cubic (bcc) tantalum from first principles by using the linearized augmented plane wave (LAPW) and mixed--basis pseudopotential methods for pressures up to 400 GPa and temperatures up to 10000 K. Electronic excitation contributions to the free energy were included from the band structures, and phonon contributions were included using the particle-in-a-cell (PIC) model. The computed elastic constants agree well with available ultrasonic and diamond anvil cell data at low pressures, and shock data at high pressures. The shear modulus $c_{44}$ and the anisotropy change behavior with increasing pressure around 150 GPa because of an electronic topological transition. We find that the main contribution of temperature to the elastic constants is from the thermal expansivity. The PIC model in conjunction with fast self-consistent techniques is shown to be a tractable approach to studying thermoelasticity.Comment: To be appear in Physical Review

Phase Diagram for Ammonia-Water Mixtures at High Pressures: Implications for Icy Satellites

The (NH_3)x(H_20)_(1.x) phase diagram for 0 ≤ X ≤ 0.50 has been reexamined at temperatures from 125 K to 400 K and at pressures to 6.0 GPa using diamond-anvil cells. By electroplating the gasket materials with gold, complicated reactions between sample solutions and gasket materials, which affected earlier studies, have been avoided. Sample pressures were determined using the ruby-luminescence technique, and phase assignments were made using optical characterization. Phase assignments were confirmed by Raman spectroscopy. At room temperature the stable phases observed were fluid, high pressure ices (VI and VII), and ammonia monohydrate, NH_3·H_2O. The Ice VI and Ice VII liquidi at 295 K were extrapolated to intersect at X = 0.26 ± 0.01 and 2.1 GPa. At room temperature, the eutectic for Ice VII and NH_3·H_2O was observed at 3.3 ± 0.2 GPa, and extrapolation of the room temperature liquidus indicates that the cotectic composition is near X= 0.45. Near X= 0.33. the stable phases were high pressure ices (VI, VII, and VIII), NH_3·H_2O, and another phase tentatively identified as ammonia dihydrate, NH_3·2H_2O. At this composition, the Ice VI liquidus and the congruent melting curve of NH_3·2H_2O interesect at 1.8 ± 0.2 GPa and 252 ± 5 K, and the Ice VII liquidus is approximately linear with a slope of 0.016 ± 0.002 GPa K^(-1. To within the uncertainty of the experiment, the Ice VI liquidus continues smoothly from the Ice VII liquidus. The quadruple point among NH_3·H_2O, NH_3·H2O , Ice VI, and fluid is located at 250 ± 5 K and 1.9 ± 0.3 GPa, with the accompanying double cotectic at a composition of X= 0.36 ± O.Ql. The eutectic for NH_3·H_2O and Ice VII is approximately linear with a slope of 0.033 ± 0.003 GPa K^(-1). We have applied these data to the interior of Titan in a manner similar to the analysis of Lunine and Stevenson (1987). The main implication of these results is that Titan is likely to have a thicker NH_3·H_2O ocean than previously suspected, because the stability field of NH_3·2H_2O is smaller than previously supposed. Implications for methane and ammonia volcanism on Titan are briefly discussed. The experimentally observed reactivity between the liquid and iron (for example) may also have implications for planetary and satellite evolution

High Pressure Materials Research using Advanced Third-Generation Synchrotron X-ray

The recent discoveries of nonmolecular phases of simple molecular solids [1,2] demonstrate the proof-of-the-principles for producing exotic phases by application of high pressure. Modern advances in theoretical and computational methodologies now make possible to explain or even predict novel structures and properties in a relatively wide range of length scales on the basis of thermodynamic stability [3]. Equally important in materials research is the recent developments in advanced x-ray and laser diagnostics that enable in-situ observations at the formidable pressure-temperature conditions [4]. Having benefited by all these developments, we discuss the first principle of the pressure-induced chemistry, 'Mbar Chemistry', with a few examples that may have important implications in materials research

Thermal Equation of State of Tantalum

We have investigated the thermal equation of state of tantalum from first principles using the Linearized Augmented Plane Wave (LAPW) and pseudopotential methods for pressures up to 300 GPa and temperatures up to 10000 K. The equation of state at zero temperature was computed using LAPW. For finite temperatures, mixed basis pseudopotential computations were performed for 54 atom supercells. The vibrational contributions were obtained by computing the partition function using the particle in a cell model, and the the finite temperature electronic free energy was obtained from the LAPW band structures. We discuss the behavior of thermal equation of state parameters such as the Gr\"uneisen parameter $\gamma$, $q$, the thermal expansivity $\alpha$, the Anderson-Gr\"uneisen parameter $\delta_T$ as functions of pressure and temperature. The calculated Hugoniot shows excellent agreement with shock-wave experiments. An electronic topological transition was found at approximately 200 GPa

Equation of state and high-pressure/high-temperature phase diagram of magnesium

The phase diagram of magnesium has been investigated to 211 GPa at 300 K, and to 105 GPa at 4500 K, by using a combination of x-ray diffraction and resistive and laser heating. The ambient pressure hcp structure is found to start transforming to the bcc structure at ∼45 GPa, with a large region of phase-coexistence that becomes smaller at higher temperatures. The bcc phase is stable to the highest pressures reached. The hcp-bcc phase boundary has been studied on both compression and decompression, and its slope is found to be negative and steeper than calculations have previously predicted. The laser-heating studies extend the melting curve of magnesium to 105 GPa and suggest that, at the highest pressures, the melting temperature increases more rapidly with pressure than previously reported. Finally, we observe some evidence of a new phase in the region of 10 GPa and 1200 K, where previous studies have reported a double-hexagonal-close-packed (dhcp) phase. However, the additional diffraction peaks we observe cannot be accounted for by the dhcp phase alone

Ultrahard carbon film from epitaxial two-layer graphene

Atomically thin graphene exhibits fascinating mechanical properties, although its hardness and transverse stiffness are inferior to those of diamond. To date, there hasn't been any practical demonstration of the transformation of multi-layer graphene into diamond-like ultra-hard structures. Here we show that at room temperature and after nano-indentation, two-layer graphene on SiC(0001) exhibits a transverse stiffness and hardness comparable to diamond, resisting to perforation with a diamond indenter, and showing a reversible drop in electrical conductivity upon indentation. Density functional theory calculations suggest that upon compression, the two-layer graphene film transforms into a diamond-like film, producing both elastic deformations and sp2-to-sp3 chemical changes. Experiments and calculations show that this reversible phase change is not observed for a single buffer layer on SiC or graphene films thicker than 3 to 5 layers. Indeed, calculations show that whereas in two-layer graphene layer-stacking configuration controls the conformation of the diamond-like film, in a multilayer film it hinders the phase transformation.Comment: Published online on Nature Nanotechnology on December 18, 201