The crystal structure of methane B at 8 GPa-An alpha-Mn arrangement of molecules

From a combination of powder and single-crystal synchrotron x-ray diffraction data we have determined the carbon substructure of phase B of methane at a pressure of ∼8 GPa. We find this substructure to be cubic with space group I4 ¯ 3m I4¯3m and 58 molecules in the unit cell. The unit cell has a lattice parameter a = 11.911(1) Å at 8.3(2) GPa, which is a factor of √2 larger than had previously been proposed by Umemoto et al. [J. Phys.: Condens. Matter14, 10675 (2002)]. The substructure as now solved is not related to any close-packed arrangement, contrary to previous proposals. Surprisingly, the arrangement of the carbon atoms is isostructural with that of α-manganese at ambient conditions. © 2014, AIP Publishing LLC

Crossover between liquid-like and gas-like behaviour in CH4 at 400 K

We report experimental evidence for a crossover between a liquid-like state and a gas-like state in fluid methane (CH4). This crossover is observed in all of our experiments, up to 397 K temperature; 2.1 times the critical temperature of methane. The crossover has been characterized with both Raman spectroscopy and X-ray diffraction in a number of separate experiments, and confirmed to be reversible. We associate this crossover with the Frenkel line - a recently hypothesized crossover in dynamic properties of fluids extending to arbitrarily high pressure and temperature, dividing the phase diagram into separate regions where the fluid possesses liquid-like and gas-like properties. On the liquid-like side the Raman-active vibration increases in frequency linearly as pressure is increased, as expected due to the repulsive interaction between adjacent molecules. On the gas-like side this competes with the attractive Van der Waal’s potential leading the vibration frequency to decrease as pressure is increased

Solar System Physics for Exoplanet Research

Over the past three decades, we have witnessed one of the great revolutions in our understanding of the cosmos-the dawn of the Exoplanet Era. Where once we knew of just one planetary system (the solar system), we now know of thousands, with new systems being announced on a weekly basis. Of the thousands of planetary systems we have found to date, however, there is only one that we can study up-close and personal-the solar system. In this review, we describe our current understanding of the solar system for the exoplanetary science community-with a focus on the processes thought to have shaped the system we see today. In section one, we introduce the solar system as a single well studied example of the many planetary systems now observed. In section two, we describe the solar system's small body populations as we know them today-from the two hundred and five known planetary satellites to the various populations of small bodies that serve as a reminder of the system's formation and early evolution. In section three, we consider our current knowledge of the solar system's planets, as physical bodies. In section four we discuss the research that has been carried out into the solar system's formation and evolution, with a focus on the information gleaned as a result of detailed studies of the system's small body populations. In section five, we discuss our current knowledge of planetary systems beyond our own-both in terms of the planets they host, and in terms of the debris that we observe orbiting their host stars. As we learn ever more about the diversity and ubiquity of other planetary systems, our solar system will remain the key touchstone that facilitates our understanding and modeling of those newly found systems, and we finish section five with a discussion of the future surveys that will further expand that knowledge

Women in physics in Australia 2017

The focus on women in physics in Australia has increased substantially since the last report at the International Conference of Women in Physics (ICWIP) in 2014. This is evident in many aspects; however, the number of women studying physics has remained static at levels that are far too low. In addition, many different organizations and groups are actively working on combating stereotype problems in culture as well as supporting women in their professional career path. The outlook for women in physics in Australia is now very promising

Rapid Formation of Clathrate Hydrate From Liquid Ethane and Water Ice on Titan

bstract Liquid ethane is present in the lakes and seas observed on Titan's surface by the Cassini-Huygens mission. While interaction between liquid hydrocarbons and water ice is expected to result in the formation of clathrate hydrates, such reaction (and its kinetics) has not yet been demonstrated for cryogenic liquids under relevant planetary conditions. In this paper, we report the first experimental evidence for rapid formation of clathrates upon direct contact of liquid ethane with water ice at 1 bar using micro-Raman spectroscopy. Kinetics experiments conducted in the temperature range 150?173 K yield an activation energy of 14.8 ± 2.2 kJ/mol, which is suggestive of a diffusion-controlled mechanism for clathrate formation. This implies that a clathrate reservoir can form within seasonal time scales on Titan if liquid ethane comes into contact with a pre-existing icy bedrock, which may hold important implications for the structure and dynamics of Titan's crust and its global evolution.With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737

Rapid Formation of Clathrate Hydrate From Liquid Ethane and Water Ice on Titan

bstract Liquid ethane is present in the lakes and seas observed on Titan's surface by the Cassini-Huygens mission. While interaction between liquid hydrocarbons and water ice is expected to result in the formation of clathrate hydrates, such reaction (and its kinetics) has not yet been demonstrated for cryogenic liquids under relevant planetary conditions. In this paper, we report the first experimental evidence for rapid formation of clathrates upon direct contact of liquid ethane with water ice at 1 bar using micro-Raman spectroscopy. Kinetics experiments conducted in the temperature range 150?173 K yield an activation energy of 14.8 ± 2.2 kJ/mol, which is suggestive of a diffusion-controlled mechanism for clathrate formation. This implies that a clathrate reservoir can form within seasonal time scales on Titan if liquid ethane comes into contact with a pre-existing icy bedrock, which may hold important implications for the structure and dynamics of Titan's crust and its global evolution.With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737

Squeezing electrons out of 6s2 lone-pairs in perovskite-type oxides

Having identified a set of conditions that predispose a solid-state ionic compound to a pressure-induced valence transition, we investigated a series of Bi(III) perovskite oxides. We found such a transition below 10 GPa in every case, including one that we synthesised for the first time (double perovskite-type Ba2BiOsO6)

The Acetylene-Ammonia Co-crystal on Titan

Titan, the largest moon of Saturn, likely supports a rich organic mineralogy that we are only beginning to understand. Photochemistry in the upper atmosphere generates a complex array of organic molecules from the simple precursors N₂ and CH₄. These organics continue to react and combine, forming aerosol layers and ultimately depositing on the surface. Organics are transported via pluvial (rain) and fluvial (rivers/flooding) processes into the methane-based hydrocarbon lakes, where evaporation of volatile liquids can create evaporite deposits of remnant dissolved molecules. Within such deposits, chemical and physical processes may be occurring even at low temperatures. We have demonstrated in previous work that benzene and ethane rapidly form a unique and stable co-crystal at Titan surface temperatures (90–95 K), akin to a salt on Earth, where the weak van der Waals interactions in the benzene-ethane co-crystal are analogous to the ionic bonds in a salt. Here, we report the formation of a second co-crystal between acetylene and ammonia, which forms even more quickly and is stable through anticipated conditions of Titan “rain” events of liquid methane, ethane, and propane. Such co-crystals represent an exciting new class of possible minerals for Titan’s surface and may be responsible for processes such as selective sequestration and storage of species as well as having new properties for construction and erosive resistance of geological materials