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

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

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

Valence instability across magnetostructural transition in USb2_2

We have performed pressure dependent X-ray diffraction and resonant X-ray emission spectroscopy experiments on USb$_2$ to further characterize the AFM-FM transition occurring near 8 GPa. We have found the magnetic transition coincides with a tetragonal to orthorhombic transition resulting in a 17% volume collapse as well as a transient $\textit{f}$-occupation enhancement. Compared to UAs$_2$ and UAsS, USb$_2$ shows a reduced bulk modulus and transition pressure and an increased volume collapse at the structural transition. Except for an enhancement across the transition region, the $\textit{f}$-occupancy decreases steadily from 1.96 to 1.75.Comment: 9 pages, 10 figure

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

Synthesis of Non-molecular Nitrogen Phases at Mbar Pressures by Direct Laser-heating

Direct laser heating of molecular N2 to above 1400 K at 120-130 GPa results in the formation of a reddish amorphous phase and a transparent crystalline solid above 2000 K. Raman and x-ray data confirm that the transparent phase is cubic-gauche nitrogen (cg-N), while the reddish color of the amorphous phase might indicate the presence of N=N dish bonds. The quenched amorphous phase is stable down to at least 70GPa, analogous to cg-N, and could be a new non-molecular phase or an extension of the already known {eta}-phase. A chemo-physical phase diagram is presented which emphasizes the difference between pressure- and temperature-induced transitions from molecular to non-molecular solids, as found in other low Z systems

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

Phase transition kinetics of superionic H 2 O ice phases revealed by Megahertz X-ray free-electron laser-heating experiments

H2O transforms to two forms of superionic (SI) ice at high pressures and temperatures, which contain highly mobile protons within a solid oxygen sublattice. Yet the stability field of both phases remains debated. Here, we present the results of an ultrafast X-ray heating study utilizing MHz pulse trains produced by the European X-ray Free Electron Laser to create high temperature states of H2O, which were probed using X-ray diffraction during dynamic cooling. We confirm an isostructural transition during heating in the 26-69 GPa range, consistent with the formation of SI-bcc. In contrast to prior work, SI-fcc was observed exclusively above ~50 GPa, despite evidence of melting at lower pressures. The absence of SI-fcc in lower pressure runs is attributed to short heating timescales and the pressure-temperature path induced by the pump-probe heating scheme in which H2O was heated above its melting temperature before the observation of quenched crystalline states, based on the earlier theoretical prediction that SI-bcc nucleates more readily from the fluid than SI-fcc. Our results may have implications for the stability of SI phases in ice-rich planets, for example during dynamic freezing, where the preferential crystallization of SI-bcc may result in distinct physical properties across mantle ice layers