Nanocasting at high pressure has been recently proposed as a novel strategy for the synthesis of periodic mesoporous materials with crystalline walls. In this study we present results on the synthesis of mesostructured stishovite from mesostructured FDU-12/carbon composite precursor using the multi-anvil press. Results from quenched experiments performed at a pressure of 14 GPa indicate that a minimum temperature of 500 degrees C is needed to crystallize stishovite from the amorphous silica precursor with a preserved mesostructure. Transmission electron microscopy combined with small angle X-ray scattering measurements confirmed the mesostructure of synthetic stishovite having carbon-filled pores with a diameter of similar to 19 nm similar to the pore size of the FDU-12 precursor. Calcination of the stishovite/carbon composite at 450 degrees C in air at ambient condition leads to amorphization of the stishovite. Our results show that mesostructure materials can be synthesized at very high pressures without loss or critical modification of the mesostructure. (C) 2014 Elsevier Inc. All rights reserved.

Our first-principles calculations show that both the compressional and shear waves of epsilon-Fe become elastically isotropic under the Earth's inner core conditions, with the variation in sound velocities along different angles from the c axis within 1%. We computed the thermoelasticity at high pressures and temperatures from quasiharmonic linear response linear-muffin-tin-orbital calculations in the generalized-gradient approximation. The calculated anisotropic shape and magnitude at ambient temperature agree well with previous first-principles predictions, and the anisotropic effects show strong temperature dependences. This implies that other mechanisms, rather than the preferential alignment of the epsilon-Fe crystal along the Earth's rotation axis, account for the seismic P-wave travel time anomalies. Either the inner core is not epsilon-Fe, and/or the seismologically observed anisotropy is caused by inhomogeneity, i.e., multiple phases. Citation: Sha, X., and R. E. Cohen (2010), Elastic isotropy of epsilon-Fe under Earth's core conditions, Geophys. Res. Lett., 37, L10302, doi:10.1029/2009GL042224.

Hydrogen sulfide (H2S) and hydrogen (H-2) crystallize into a 'guest-host' structure at 3.5 GPa and, at the initial formation pressure, the rotationally disordered component molecules exhibit weak van der Waals-type interactions. With increasing pressure, hydrogen bonding develops and strengthens between neighboring H2S molecules, reflected in a pronounced drop in S-H vibrational stretching frequency and also observed in first-principles calculations. At 17 GPa, an ordering process occurs where H2S molecules orient themselves to maximize hydrogen bonding and H-2 molecules simultaneously occupy a chemically distinct lattice site. Intermolecular forces in the H2S + H-2 system may be tuned with pressure from the weak hydrogen-bonding limit to the ordered hydrogen-bonding regime, resulting in a novel clathrate structure stabilized by cooperative interactions.

Silica (SiO2) is an abundant component of the Earth whose crystalline polymorphs play key roles in its structure and dynamics. First principle density functional theory (DFT) methods have often been used to accurately predict properties of silicates, but fundamental failures occur. Such failures occur even in silica, the simplest silicate, and understanding pure silica is a prerequisite to understanding the rocky part of the Earth. Here, we study silica with quantum Monte Carlo (QMC), which until now was not computationally possible for such complex materials, and find that QMC overcomes the failures of DFT. QMC is a benchmark method that does not rely on density functionals but rather explicitly treats the electrons and their interactions via a stochastic solution of Schrodinger's equation. Using ground-state QMC plus phonons within the quasiharmonic approximation of density functional perturbation theory, we obtain the thermal pressure and equations of state of silica phases up to Earth's core-mantle boundary. Our results provide the best constrained equations of state and phase boundaries available for silica. QMC indicates a transition to the dense alpha-PbO2 structure above the core-insulating D '' layer, but the absence of a seismic signature suggests the transition does not contribute significantly to global seismic discontinuities in the lower mantle. However, the transition could still provide seismic signals from deeply subducted oceanic crust. We also find an accurate shear elastic constant for stishovite and its geophysically important softening with pressure.

Fe3P is a candidate component in planetary cores. We have investigated high-pressure behavior of Fe3P by first-principles calculations and synchrotron X-ray diffraction. Theoretical calculations reveal a magnetic collapse at 40-65 GPa, accompanied by a structural transition. The possible high-pressure polymorph is either a distorted cementite structure (Pnma) or a P4/mnc structure. By combining synchrotron X-ray diffraction and laser-heating diamond anvil cell techniques, we have collected in situ diffraction patterns of Fe3P up to 64 GPa and 1650 K. The high-pressure phase transition from I (4) over bar to P4/mnc structure predicted by the first-principles calculations was confirmed. Discontinuous variations of lattice constants and thermal expansion coefficients with pressure were observed around 17 and 40 GPa, indicating a possible magnetic transition developed in this range, which are in agreement with the calculated results. (c) 2014 Elsevier B.V. All rights reserved.

We investigate the contributions of finite-temperature magnetic fluctuations to the thermodynamic properties of bcc Fe as functions of pressure. First, we apply a tight-binding total-energy model parameterized to first-principles linearized augmented plane-wave computations to examine various ferromagnetic, anti-ferromagnetic, and noncollinear spin spiral states at zero temperature. The tight-binding data are fit to a generalized Heisenberg Hamiltonian to describe the magnetic energy functional based on local moments. We then use Monte Carlo simulations to compute the magnetic susceptibility, the Curie temperature, heat capacity, and magnetic free energy. Including the finite-temperature magnetism improves the agreement with experiment for the calculated thermal expansion coefficients.

The efficiency of heat transfer by conduction in the Earth's core controls the dynamics of convection and limits the power available for the geodynamo. We have measured the room temperature electrical resistivity of iron and iron-silicon alloy to 60 GPa and present a new model of the resistivity at high pressures and temperatures relevant to the Earth's core. The model is compared with available shock wave data and theoretical studies. For a power law and linear temperature dependence of electrical resistivity, the calculated thermal conductivity at the core-mantle boundary is similar to 67-145W/m/K for pure Fe and similar to 41-60 W/m/K for Fe-9wt % Si. Impurities in the core have a strong effect on the transport properties of iron that could significantly impact core thermal models. The models describing the data indicate higher thermal conductivity at core pressure than previously suggested, requiring additional energy sources in the past to operate the geodynamo.

An initial observation of the formation of WH under pressure from W gaskets surrounding hydrogen in diamond anvil cells led to a theoretical study of tungsten hydride phases. At P = 1 atm no stoichiometry is found to be stable with respect to separation into the elements, but as the pressure is raised WHn (n = 1-6, 8) stoichiometries are metastable or stable. WH and WH4 are calculated to be stable at P > 15 GPa, WH2 becomes stable at P > 100 GPa and WH6 at P > 150 GPa. In agreement with experiment, the structure computed for WH is anti-NiAs. WH2 shares with WH a hexagonal arrangement of tungsten atoms, with hydrogen atoms occupying octahedral and tetrahedral holes. For WH4 the W atoms are in a distorted fcc arrangement. As the number of hydrogens rises, the coordination of W by H increases correspondingly, leading to a twelve-coordinated W in WH6. In WH8 H-2 units also develop. All of the hydrides considered should be metallic at high pressure, though the Fermi levels of WH4 and WH6 lie in a deep pseudogap. Prodded by these theoretical studies, experiments were then undertaken to seek phases other than WH, exploring a variety of experimental conditions that would favor further reaction. Though a better preparation and characterization of WH resulted, no higher hydrides have as yet been found.