Deep Carbon and other systems under pressure

We are studying carbon bearing solids and fluids under pressure, to improve our understanding of the physical and chemical behavior of carbon at extreme conditions, as found in the deep interiors of Earth and other planets.

We are also investigating how tweaking and modifying chemical bonds with temperature and pressure may lead to the discovery of new materials and novel properties. In particular, applying pressure to materials may lead to the synthesis of new structures, recoverable once pressure is released.

CO2 in water-rich fluids at extreme conditions

We used first-principles molecular dynamics simulations to study carbonates and carbon dioxide dissolved in water at pressures (P) and temperatures (T) approximating the conditions of the Earth’s upper mantle. Contrary to popular geochemical models assuming that molecular CO2(aq) is the major carbon species present in water under deep earth conditions, we found that at 11 GPa and 1000 K carbon exists almost entirely in the forms of solvated carbonate (CO3)2- and bicarbonate (HCO3)- ions, and that even carbonic acid (H2CO3(aq)) is more abundant than CO2(aq). Furthermore, our simulations revealed that ion pairing between Na+ and (HCO3)-/(CO3)2- is greatly affected by P-T conditions, decreasing with increasing pressure at 800~1000 K. Our results suggest that in the Earth’s upper mantle, water-rich geo-fluids transport a majority of carbon in the form of rapidly interconverting (CO3)2- and (HCO3)- ions, not solvated CO2(aq) molecules.

Tetrahedrally coordinated carbonates in Earth’s lower mantle

Only a small fraction of our planet’s total carbon budget is found at the surface. In fact, Earth’s mantle is thought to be the largest carbon reservoir. Carbonates, and in particular ferromagnesite ((Mg,Fe)CO3), are likely candidates for deep-Earth carbon storage and therefore play a key role in the deep carbon cycle. We recently reported unequivocal evidence of tetrahedrally coordinated carbon in high pressure carbonates, obtained by combined experimental and theoretical studies. In particular, we identified a unique vibrational signature present only in the high-pressure phase of magnesite, and thus a new carbon-oxygen bond formed under pressure.[DCO link]

The refractive index and electronic gap of water and ice increase with increasing pressure

Experimentally, it is not yet possible to measure absorption processes taking place in water and ice in diamond anvil cells. The band gap of diamond is smaller than that of water and ice, at least up to 30 GPa, and at high temperatures water becomes corrosive. We used ab initio molecular dynamics simulations and electronic structure calculations to show that both the refractive index and the electronic gap of water and ice increase with increasing pressure (up to 30 GPa), contrary to previous assumptions and contrary to the results of simple, widely used models. Subtle electronic effects, related to the nature of inter-band transitions and band edge localization under pressure, are responsible for this apparently anomalous behavior. [DCO link]

Water and carbonates under pressure

Water is a major component of fluids in the Earth’s mantle, where its properties are substantially different from those at ambient conditions. At the pressures and temperatures of the mantle, experiments on aqueous fluids are challenging, and several fundamental properties of water are poorly known; e.g., its dielectric constant has not been measured. Using ab initio molecular dynamics, we computed the dielectric constant of water under the conditions of the Earth’s upper mantle, and we predicted the solubility products of carbonate minerals.

Hydrocarbons under pressure

The thermodynamic and kinetic properties of hydrocarbons at high pressures and temperature are important for understanding carbon reservoirs and fluxes in the Earth. This is one of the of foci of the Deep Carbon Observatory project, funded by Sloan. Our combined ab-initio MD and free energy calculations reveal how higher hydrocarbons may be formed from methane at deep Earth pressures and temperature. This work provides a basis for understanding experiments that demonstrated polymerization of methane to form high hydrocarbons, and earlier methane forming reactions under pressure (in the news).

Melting of ice

We combined first-principles molecular dynamics simulations with a two phase approach to determine the melting temperature of the ice-VII phase in the range of 10–50 GPa. Our computed melting temperatures are consistent with existing diamond anvil cell experiments. We find that for pressures between 10 and 40 GPa, ice melts as a molecular solid. For pressures above 45 GPa, there is a sharp increase in the slope of the melting curve due to the presence of molecular dissociation and proton diffusion in the solid before melting. The onset of significant proton diffusion in ice-VII as a function of increasing temperature is found to be gradual and bears many similarities to that of a type-II superionic solid.

Phase transitions in nitrogen

Two independent experiments have reported the presence of a maximum in the nitrogen melting curve, below 90 GPa, however the position and the interpretation of the origin of such maximum differ. By means of ab initio MD simulations and thermodynamic integration techniques, we have determined the phase diagram of nitrogen in the range between 20 and 100 GPa. We find a maximum in the melting line, related to a transformation in the liquid, from molecular N2 to polymeric nitrogen accompanied by an insulator-to-metal transition.