Sergey S. Lobanov

A stable compound of helium and sodium at high pressure

Helium is generally understood to be chemically inert and this is due to its extremely stable closed-shell electronic configuration, zero electron affinity and an unsurpassed ionization potential. It is not known to form thermodynamically stable compounds, except a few inclusion compounds. Here, using the ab initio evolutionary algorithm USPEX and subsequent high-pressure synthesis in a diamond anvil cell, we report the discovery of a thermodynamically stable compound of helium and sodium, Na2He, which has a fluorite-type structure and is stable at pressures >113 GPa. We show that the presence of He atoms causes strong electron localization and makes this material insulating. This phase is an electride, with electron pairs localized in interstices, forming eight-centre two-electron bonds within empty Na8 cubes. We also predict the existence of Na2HeO with a similar structure at pressures above 15 GPa.

Carbon precipitation from heavy hydrocarbon fluid in deep planetary interiors

The phase diagram of the carbon-hydrogen system is of great importance to planetary sciences, as hydrocarbons comprise a significant part of icy giant planets and are involved in reduced carbon-oxygen-hydrogen fluid in the deep Earth. Here we use resistively- and laser-heated diamond anvil cells to measure methane melting and chemical reactivity up to 80 GPa and 2,000 K. We show that methane melts congruently below 40 GPa. Hydrogen and elementary carbon appear at temperatures of >1,200 K, whereas heavier alkanes and unsaturated hydrocarbons (>24 GPa) form in melts of >1,500 K. The phase composition of carbon-hydrogen fluid evolves towards heavy hydrocarbons at pressures and temperatures representative of Earths lower mantle. We argue that reduced mantle fluids precipitate diamond upon re-equilibration to lighter species in the upwelling mantle. Likewise, our findings suggest that geophysical models of Uranus and Neptune require reassessment because chemical reactivity of planetary ices is underestimated.

Hydrogen sulfide at high pressure: Change in stoichiometry

Hydrogen sulfide

Thermal equation of state and thermodynamic properties of molybdenum at high pressures

({\mathrm{H}}_{2}\mathrm{S})

Stable high-pressure phases in the H-S system determined by chemically reacting hydrogen and sulfur

was studied by x-ray synchrotron diffraction and Raman spectroscopy up to 150 GPa at 180--295 K and by quantum-mechanical variable-composition evolutionary simulations. The experiments show that

Optical properties of siderite (FeCO3) across the spin transition: Crossover to iron-rich carbonates in the lower mantle

{\mathrm{H}}_{2}\mathrm{S}

Stable magnesium peroxide at high pressure

becomes unstable with respect to formation of compounds with different structure and composition, including Cccm and a body-centered cubic like (

Stability of numerous novel potassium chlorides at high pressure

R3m

Pressure, stress, and strain distribution in the double-stage diamond anvil cell

or

Backbone NxH compounds at high pressures

Im\ensuremath{-}3m