Alexander F. Goncharov

Unexpected Stable Stoichiometries of Sodium Chlorides

Salt to Squeeze Simple table salt, NaCl, is the only known stable phase of Na and Cl at ambient conditions. Previous attempts to understand its structure and chemical properties under pressure and at high temperatures revealed phase and bonding transitions, while keeping the balance of one Na to one Cl. Using crystal structure prediction algorithms, Zhang et al. (p. 1502; see the Perspective by Ibáñez Insa) show that other compounds—including Na3Cl, Na2Cl, Na3Cl2, NaCl3, and NaCl7 are as stable as NaCl across a range of pressures. Several phases in the Na-Cl system are stable at high pressures and temperatures. [Also see Perspective by Ibáñez Insa] Sodium chloride (NaCl), or rocksalt, is well characterized at ambient pressure. As a result of the large electronegativity difference between Na and Cl atoms, it has highly ionic chemical bonding (with 1:1 stoichiometry dictated by charge balance) and B1-type crystal structure. By combining theoretical predictions and diamond anvil cell experiments, we found that new materials with different stoichiometries emerge at high pressures. Compounds such as Na3Cl, Na2Cl, Na3Cl2, NaCl3, and NaCl7 are theoretically stable and have unusual bonding and electronic properties. To test this prediction, we synthesized cubic and orthorhombic NaCl3 and two-dimensional metallic tetragonal Na3Cl. These experiments establish that compounds violating chemical intuition can be thermodynamically stable even in simple systems at nonambient conditions.

Direct measurement of thermal conductivity in solid iron at planetary core conditions.

The conduction of heat through minerals and melts at extreme pressures and temperatures is of central importance to the evolution and dynamics of planets. In the cooling Earths core, the thermal conductivity of iron alloys defines the adiabatic heat flux and therefore the thermal and compositional energy available to support the production of Earths magnetic field via dynamo action. Attempts to describe thermal transport in Earths core have been problematic, with predictions of high thermal conductivity at odds with traditional geophysical models and direct evidence for a primordial magnetic field in the rock record. Measurements of core heat transport are needed to resolve this difference. Here we present direct measurements of the thermal conductivity of solid iron at pressure and temperature conditions relevant to the cores of Mercury-sized to Earth-sized planets, using a dynamically laser-heated diamond-anvil cell. Our measurements place the thermal conductivity of Earths core near the low end of previous estimates, at 18-44 watts per metre per kelvin. The result is in agreement with palaeomagnetic measurements indicating that Earths geodynamo has persisted since the beginning of Earths history, and allows for a solid inner core as old as the dynamo.

Hydrogen sulfide at high pressure: Change in stoichiometry

Hydrogen sulfide

Opacity and conductivity measurements in noble gases at conditions of planetary and stellar interiors

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Optical Properties of Fluid Hydrogen at the Transition to a Conducting State.

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

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Structural and chemical properties of the nitrogen-rich energetic material triaminoguanidinium 1-methyl-5-nitriminotetrazolate under pressure.

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

Stable magnesium peroxide at high pressure

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Melting and dissociation of ammonia at high pressure and high temperature

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Hydrogen at extreme pressures (Review Article)

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