We report the catalyst-free synthesis of monolithic mesoporous nanopolycrystalline diamond from periodic mesoporous carbon at pressures between 15 and 21 GPa and a temperature of 1300 degrees C. We investigated the pressure-dependence of the porosity with 3-dimensional electron tomography. We have observed that surface areas increase from 56 to 90, 138 m(2) g(-1), and porosities increase from 10, 24, to 33% for materials produced at 15, 18, and 21 GPa, respectively. The increased porosity at higher pressure may be due to the earlier onset of the nucleation of diamond at higher pressure. (C) 2018 Published by Elsevier Ltd on behalf of Acta Materialia Inc.

We compute the thermal conductivity and electrical resistivity of solid hcp Fe to pressures and temperatures of Earth's core. We find significant contributions from electron-electron scattering, usually neglected at high temperatures in transition metals. Our calculations show a quasilinear relation between the electrical resistivity and temperature for hcp Fe at extreme high pressures. We obtain thermal and electrical conductivities that are consistent with experiments considering reasonable error. The predicted thermal conductivity is reduced from previous estimates that neglect electron-electron scattering. Our estimated thermal conductivity for the outer core is 77 +/- 10 Wm(-1) K-1 and is consistent with a geodynamo driven by thermal convection.

Subduction is a key process for linking the carbon cycle between the Earth's surface and its interior. Knowing the carbonation and decarbonation processes in the subduction zone is essential for understanding the global deep carbon cycle. In particular, the potential role of hydrocarbon fluids in subduction zones is not well understood and has long been debated. Here we report graphite and light hydrocarbon-bearing inclusions in the carbonated eclogite from the Southwest (S.W.) Tianshan subduction zone, which is estimated to have originated at a depth of at least 80 kilometers. The formation of graphite and light hydrocarbon likely results from the reduction of carbonate under low oxygen fugacity (similar to FMQ - 2.5 log units). To better understand the origin of light hydrocarbons, we also investigated the reaction between iron-bearing carbonate and water under conditions relevant to subduction zone environments using large-volume high-pressure apparatus. Our high-pressure experiments provide additional constraints on the formation of abiotic hydrocarbons and graphite/diamond from carbonate-water reduction. In the experimental products, the speciation and concentration of the light hydrocarbons including methane (CH4), ethane (C2H6), and propane (C3H8) were unambiguously determined using gas chromatograph techniques. The formation of these hydrocarbons is accompanied by the formation of graphite and oxidized iron in the form of magnetite (Fe3O4). We observed the identical mineral assemblage (iron-bearing dolomite, magnetite, and graphite) associated with the formation of the hydrocarbons in both naturally carbonated eclogite and the experimental run products, pointing toward the same formation mechanism. The reduction of the carbonates under low oxygen fugacity is, thus, an important mechanism in forming abiotic hydrocarbons and graphite/diamond in the subduction zone settings. (C) 2018 Elsevier Ltd. All rights reserved.

Silicon clathrate, an important allotrope of silicon, has attractive opto-electronic properties for energy applications. However, it remains an experimental challenge to synthesize electrically undoped, intrinsic clathrate. Here we show, through high-throughput computer modeling, that unconventional silicon phases spontaneously nucleate from liquid silicon in the presence of noble gases under high pressure and high temperature. In particular, our results show that a medium-sized noble gas, for example, argon, can trigger the nucleation and growth of inert-gas silicon clathrate, whereas a small noble gas such as helium is able to induce the formation of an unconventional, inclusion-type compound Si2He. The formation of both silicon phases can be attributed to the same thermodynamic and kinetic rationale that explains the crystallization of clathrate hydrate, an isostructural analog. Our findings, along with the gained molecular insights, thus strongly suggest a viable experimental synthesis route for these silicon phases using noble gases at high pressure.

The solidus for the mantle of Mars is an important geophysical parameter in modeling the thermal history and evolution of the planet. This study provides solidus data for a simplified model Martian mantle composition from the midmantle (8 GPa) to the core-mantle boundary (25 GPa) using multianvil experiments. Combining this work with previous studies, the solidus for the entire mantle of Mars is constrained to within 70 degrees C. The major mineral assemblages and phase transitions observed are consistent with those predicted for an iron-rich Martian mantle. The solidus for the Martian composition falls above the solidus of MORB and below that of terrestrial peridotite, providing direct melting data for geophysical modeling of the Martian interior. Our solidus also predicts that a Martian mantle plume should produce melt at depths over a range of 200-350 km in the mantle.

Crystalline polar metallocenes are potentially useful active materials as piezoelectrics, ferroelectrics, and multiferroics. Within density functional theory (DFT), we computed structural properties, energy differences for various phases, molecular configurations, and magnetic states, computed polarizations for different polar crystal structures, and computed dipole moments for the constituent molecules with a Wannier function analysis. Of the systems studied, Mn-2(C9H9N)(2) is the most promising as a multiferroic material, since the ground state is both polar and ferromagnetic. We found that the predicted crystalline polarizations are 30-40% higher than the values that would be obtained from the dipole moments of the isolated constituent molecules, due to the local effects of the self-consistent internal electric field, indicating high polarizabilities.

Hydrothermal experiments aiming at the crystal growth of stishovite near ambient pressure and temperature were performed in conventional autoclave systems using 1 M (molar) NaOH, 0.8 M Na2CO3, and pure water as a mineralizing agent. It was found that the hydrothermal metastability of stishovite and coesite is very different from the thermal metastability in all mineralizing agents and that because of this fact crystals could not be grown. While stishovite and coesite are thermally metastable up to 500 and >1000 degrees C, respectively, their hydrothermal metastability is below 150 and 200 degrees C, respectively. The thermally induced conversion of stishovite and coesite leads to amorphous products, whereas the hydrothermally induced conversion leads to crystalline quartz. Both stishovite and coesite are minerals occurring in nature where they can be exposed to hydrothermal conditions. The low hydrothermal stability of these phases may be an important factor to explain the rarity of these minerals in nature.