Session D23: Focus Session: High Pressure III - Earth and Planetary Materials

First principles investigation of the ice VII-VIII (order-disorder) phase boundary

Phase boundaries among the various forms of ice are difficult to determine experimentally because of the large hystereses involved. Theoretical determination is also very challenging. Treatment of disorder in hydrogen sublattice is one of major problems. We present a first-principles study of order-disorder transition between ice VII and VIII. This study involves the complete statistical sampling of configurations generated within a 16 molecules supercell and includes the important effects of vibrational energy on this phase boundary. Since this transition has been well constrained experimentally, it is a good test of our treatment.   

Freezing kinetics in overcompressed water

The transformation of water into ice is among the most common first order phase transitions occurring in nature, but it is far from being an ordinary one. Water has unusual physical properties both as a liquid and as a solid due largely to hydrogen bonding effects, which also play a major role in determining the characteristics of its freezing kinetics. We report high pressure dynamic compression experiments of liquid water along a quasi-adiabatic path leading to the formation of ice VII. We observe dynamic features resembling Van der Waals loops and find that liquid water is compacted to a metastable state close to the ice density before the onset of crystallization. By analyzing the characteristic kinetic time scale involved we estimate the nucleation barrier and conclude that liquid water has been compressed to a high pressure state close to its thermodynamic stability limit.   

The effect of dynamic compression on phase transformation: Solidification of water and crystal growth of ice VI using dynamic diamond anvil cell

The kinetics of phase transformation depends on how driving parameters are applied. Under high pressure, compression rate can give different paths of phase transformation. For this purpose, we have developed a new device, called dynamic diamond anvil cell (d-DAC), which can modulate a given static pressure with various compression rate and type. Using d-DAC, liquid water can be overpressurized up to 75 {\%} in ice VI phase field without crystallization, and after transforms to metastable iceVII phase in the stable ice VI pressure field. Interestingly, when fast sinusoidal compression is applied, the crystal morphology of ice VI surrounded by liquid water dramatically changes to fractal and dendritic shape. In this talk, we will describe the details of crystallization, following a brief description of the technical development of d-DAC.   

Laser-driven shock studies on planetary ices

Planetary ices such as water, methane and ammonia make up the bulk composition of planets such as Uranus and Neptune. Additionally, extra-solar planets recently discovered may also be partially composed of these ices. Due to their shear size, the interiors of these planets are at simultaneous high pressures and temperatures. Using laser-driven shock compression, experiments at these extreme conditions---up to $\sim $10 TPa pressures currently and up to $\sim $100 TPa (1 Gbar) in the near future---is possible and covers the full range of planetary pressures, including ``super-giant'' extra-solar planets. Additionally we can couple the dynamic compression with that of static compression in a diamond-anvil cell in order to decrease the temperatures along the principal Hugoniot. Laser-driven shock compression of water samples pre-compressed to 1 GPa produces high-pressure and high-temperature conditions inducing two significant changes in the optical properties of water: the onset of opacity followed by enhanced reflectivity in the initially transparent water. The onset of reflectivity at infrared wavelengths can be interpreted as a semi-conductor $\leftrightarrow $ electronic conductor transition in water, and is found at pressures above $\sim $130 GPa for single-shocked samples pre-compressed to 1 GPa in contrast to pressures above $\sim $100 GPa for water samples without precompression. Our results indicate that conductivity in the deep interior of ``icy'' giant planets is greater than realized previously because of an additional contribution from electrons.   

High pressure-temperature Raman spectroscopy of H$_{2}$-H$_{2}$O clathrate.

The melting curve of the C2 clathrate H$_{2}$-H$_{2}$O has been determined by \textit{in-situ} Raman spectroscopy measurements in an externally heated diamond anvil cell. We have determined the melting curve to a maximum pressure of 27 GPa. These are the first measurements on the melting line in this clathrate. Depending on the stoichiometry of the starting mixture of H$_{2}$ and H$_{2}$O, we are able to study either a mixture of C2 and H$_{2}$O or C2 and H$_{2}$. In either case, we were able to pinpoint the melting of the clathrate from the measurements of the molecular stretching mode (vibron) in the clathrate. In the case of C2 + Ice VII, we observe the vibron in the clathrate at a frequency higher than in pure H$_{2}$ at the same pressure. We have cross-calibrated the melting temperatures using the Stokes-anti Stokes ratio of the diamond first order and Raman active TO phonon of cubic Boron Nitride. We find that the clathrate melts well above the H$_{2}$ melting at all pressures studied indicating that the stabilization of this clathrate at high pressures is indeed due to interactions between the host and guest molecules.   

Computational analysis of methane occupation within gas hydrates

Gas hydrates are considered a future energy resource that have large quantities of hydrocarbon gases (mostly methane) trapped and stabilized under moderate pressures in continental shelf and permafrost regions. The global estimate of hydrocarbon stored in these ice-like structures far exceeds all fossil fuel reserves. Methane escape from these phases, therefore, is considered a potential global warming contributor. These characteristics make the gas hydrates energy recovery a technological challenge and requires constraining the methane diffusion process within the structure. Here, we present a first principles theoretical investigation into the structure, energetics and dynamics of the `guest' molecule in gas hydrates, with the goal of building a physical model for methane diffusion. In particular, our study focuses on the sI (low pressure) methane hydrate phase by combining isolated cluster calculations and periodic structure calculations and closely guided by high pressure experimental work on methane hydrate. Based on the known crystal structure, we compare binding energies for methane and other gas molecules (e.g. Xe, Ar, CO$_{2}$) to guide the high pressure experiments.   

Theoretical Tools for the Analysis and Prediction of Multi-component Systems at High Pressures and Densities

J. F. Kenney, Gas Resources Corporation, Houston, Texas, U.S.A. To describe or predict theoretically the evolution of a multi-component system at high pressures, one must have a reliable expression for the system's partition function, or its Helmholtz free energy, or its equation of state. Such formalism must possess the following properties: The formalism must be based upon fundamental, first-principles, quantum statistical mechanics argument, and the highest level of rigor available; it cannot be \textit{ad hoc}, or use fitted expressions; the equation of state developed by the formalism must be generate accurately, not only the system's basic pressure-density relationship, but also its multi-phase transition and coexistence lines, and its complex-behavior curves; and it must include also an adequate optimization procedure capable to determine the equilibrium state of the system. Here is described a general formalism that has been used to describe high pressure systems and has resolved the previously-outstanding problems of optical activity in abiological compounds, the anomalous distribution of isomers in petroleum, and the spontaneous generation of the hydrocarbon system.   

Chemical Dissociation of Cyclohexane under Shock Loading

We present a study of the chemical dissociation process in the ringed hydrocarbon cyclohexane under shock loading. Cyclohexane was subjected to shock loading in the pressure range of 12 GPa to 39 GPa. The dissociation was observed using double pass optical absorption spectroscopy. We observed the onset of dissociation as the shock pressure was increased. A strong wavelength dependence was observed in the absorption first beginning at 650 nm and eventually at 400 nm at 39 GPa. The absorption mechanism is is suggestive of Mie scattering of fine carbon particles. The kinetics of the dissociation and the formation of the carbon particles will be discussed.\\\\ This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.