We have conducted a series of melting experiments in the Fe-C system at pressures up to 25 GPa in the temperature range of 1473-2073 K. The results define the phase relations at several pressures, including the eutectic temperature and composition as a function of pressure, carbon partitioning between solid iron and liquid, and change of melting relations involving iron carbides. In order to interpolate and extrapolate the phase relations over a wide pressure and temperature range, we have established a comprehensive thermodynamic model in the Fe-C binary system. The calculated phase diagrams at pressures of 5, 10, and 20 GPa reproduce the experimental data, including the solubility of carbon in solid iron and the effect of pressure on the eutectic temperature and composition. The formation of Fe7C3 at pressures above 5 GPa is correctly modeled and the change of phase relations in the Fe-C system between 5 and 10 GPa is captured in the model. The model provides predictions of the phase relations at 136 GPa and 330 GPa, based on existing knowledge of the thermochemistry of the system at lower pressure. The calculated phase relations can be used to understand the role of carbon during inner core crystallization, predicting carbon distribution between the inner and outer cores and mineralogy of the solid inner core. (C) 2014 Elsevier B.V. All rights reserved.

Based on a shell model potential obtained from first principles calculations, we performed molecular dynamics simulations to investigate the electromechanical response of a ferroelectric perovskite under finite temperature and electric field. We characterize the switching paths by which a homogeneous polarization reorientation process would take place in the prototypical ferroelectric PbTiO(3). We observe the hysteresis loop and butterfly electric-strain curve and obtain finite temperature piezoelectric coefficients in good agreement with experiments. (C) 2011 American Institute of Physics. [doi:10.1063/1.3646377]

A new monoclinic variation of Mg2C3 was synthesized from the elements under high-pressure (HP), high-temperature (HT) conditions. Formation of the new compound, which can be recovered to ambient conditions, was observed in situ using X-ray diffraction with synchrotron radiation. The structural solution was achieved by utilizing accurate theoretical results obtained from ab initio evolutionary structure prediction algorithm USPEX. Like the previously known orthorhombic Pnnm structure (alpha-Mg2C3), the new monoclinic C2/m structure (beta-Mg2C3) contains linear C-3(4-) chains that are isoelectronic with CO2. Unlike alpha-Mg2C3, which contains alternating layers of C-3(4-) chains oriented in opposite directions, all C-3(4-) chains within beta-Mg2C3 are nearly aligned along the crystallographic c-axis. Hydrolysis of beta-Mg2C3 yields C3H4, as detected by mass spectrometry, while Raman and NMR measurements show clear C=C stretching near 1200 cm(-1) and C-13 resonances confirming the presence of the rare allylenide anion.

The oxygen fugacity (fO(2)) at which carbonate-bearing melts are reduced to either graphite or diamond in synthetic eclogite compositions has been measured in multi-anvil experiments performed at pressures between 3 and 7 GPa and temperatures between 800 and 1,300 degrees C using iron iridium and iron platinum alloys as sliding redox sensors. The determined oxygen fugacities buffered by the coexistence of elemental carbon and carbonate-bearing melt are approximately 1 log unit below thermodynamic calculations for a similar redox buffering equilibrium involving only solid phases. The measured oxygen fugacities normalized to the fayalite magnetite quartz oxygen buffer decrease with temperature from similar to-0.8 to similar to-1.7 log units at 3 GPa, most likely as a result of increasing dilution of the carbonate liquid with silicate. The normalized 102 values also decrease with pressure and show a similar decrease with temperature at 6 GPa from similar to-1.5 log units at 1,100 degrees C to similar to-2.4 log units at 1,300 degrees C. In contrast to previous arguments, the stability field of the carbonate-bearing melt extends to lower oxygen fugacity in eclogite rocks than in peridotite rocks, which implies a wider range of conditions over which carbon remains mobile in natural eclogites. The raised prevalence of diamonds in eclogites compared to peridotites may, therefore, reflect more effective scavenging of carbon by melts in these rocks. The ferric iron contents of monomineralic layers of clinopyroxene and garnet contained in the same experiments were also measured using Mossbauer spectroscopy. A preliminary model was derived for determining the fO(2) of eclogitic rocks from the compositions of garnet and clinopyroxene, including the Fe3+/Sigma Fe ratio of garnet, using the equilibrium,

Silicon is ubiquitous in contemporary technology. The most stable form of silicon at ambient conditions takes on the structure of diamond (cF8, d-Si) and is an indirect bandgap semiconductor, which prevents it from being considered as a next-generation platform for semiconductor technologies. Here, we report the formation of a new orthorhombic allotrope of silicon, Si24, using a novel two-step synthesis methodology. First, a Na4Si24 precursor was synthesized at high pressure; second, sodium was removed from the precursor by a thermal 'degassing' process. The Cmcm structure of Si24, which has 24 Si atoms per unit cell (oC24), contains open channels along the crystallographic a-axis that are formed from six- and eight-membered sp(3) silicon rings. This new allotrope possesses a quasidirect bandgap near 1.3 eV. Our combined experimental/theoretical study expands the known allotropy for element fourteen and the unique high-pressure precursor synthesis methodology demonstrates the potential for new materials with desirable properties.

We have performed molecular dynamics simulations using a shell model potential developed by fitting first-principles results to describe the behavior of the relaxor-ferroelectric (1 - x)PbMg1/3Nb2/3O3-xPbTiO(3) (PMN-xPT) as a function of concentration and temperature, using site occupancies within the random site model. In our simulations, PMN is cubic at all temperatures and behaves as a polar glass. As a small amount of Ti is added, a weak polar state develops, but structural disorder dominates, and the symmetry is rhombohedral. As more Ti is added the ground state is clearly polar and the system is ferroelectric, but with easy rotation of the polarization direction. In the high Ti content region, the solid solution adopts ferroelectric behavior similar to PT, with tetragonal symmetry. The ground state sequence with increasing Ti content is R-M-B-O-M-C-T. The high-temperature phase is cubic at all compositions. Our simulations give the slopes of the morphotropic phase boundaries, crucial for high-temperature applications. We find that the phase diagram of PMN-xPT can be understood within the random site model.

The physical properties of iron (Fe) at high pressure and high temperature are crucial for understanding the chemical composition, evolution, and dynamics of planetary interiors. Indeed, the inner structures of the telluric planets all share a similar layered nature: a central metallic core composed mostly of iron, surrounded by a silicate mantle, and a thin, chemically differentiated crust. To date, most studies of iron have focused on the hexagonal closed packed (hcp, or epsilon) phase, as epsilon-Fe is likely stable across the pressure and temperature conditions of Earth's core. However, at the more moderate pressures characteristic of the cores of smaller planetary bodies, such as the Moon, Mercury, or Mars, iron takes on a face-centered cubic (fcc, or gamma) structure. Here we present compressional and shear wave sound velocity and density measurements of gamma-Fe at high pressures and high temperatures, which are needed to develop accurate seismic models of planetary interiors. Our results indicate that the seismic velocities proposed for the Moon's inner core by a recent reanalysis of Apollo seismic data are well below those of gamma-Fe. Our dataset thus provides strong constraints to seismic models of the lunar core and cores of small telluric planets. This allows us to propose a direct compositional and velocity model for the Moon's core.

Silicon is ubiquitous in contemporary technology. The most stable form of silicon at ambient conditions takes on the structure of diamond (cF8, d-Si) and is an indirect bandgap semiconductor, which prevents it from being considered as a next-generation platform for semiconductor technologies(1-4). Here, we report the formation of a new orthorhombic allotrope of silicon, Si-24, using a novel two-step synthesis methodology. First, a Na4Si24 precursor was synthesized at high pressure(5); second, sodium was removed from the precursor by a thermal 'degassing' process. The Cmcm structure of Si-24, which has 24 Si atoms per unit cell (oC24), contains open channels along the crystallographic a-axis that are formed from six- and eight-membered sp(3) silicon rings. This new allotrope possesses a quasidirect bandgap near 1.3 eV. Our combined experimental/theoretical study expands the known allotropy for element fourteen and the unique high-pressure precursor synthesis methodology demonstrates the potential for new materials with desirable properties.

Electrical conductivity of FeO was measured up to 141 GPa and 2480 K in a laser-heated diamond-anvil cell. The results show that rock-salt (B1) type structured FeO metallizes at around 70 GPa and 1900 K without any structural phase transition. We computed fully self-consistently the electronic structure and the electrical conductivity of B1 FeO as a function of pressure and temperature, and found that although insulating as expected at ambient condition, B1 FeO metallizes at high temperatures, consistent with experiments. The observed metallization is related to spin crossover.

Recently, we have described a successful synthesis route to obtain mesoporous quartz and its high-pressure polymorph coesite by nanocasting at high pressure using periodic mesostructured precursors, such as SBA-16 and FDU-12/carbon composite as starting materials. Periodic mesoporous high-pressure silica polymorphs are of particular interest as they combine transport properties and physical properties such as hardness that potentially enable the industrial use of these materials. In addition, synthesis of mesoporous crystalline silica phases can allow more detailed geology-related studies such as water/mineral interaction, dissolution/crystallization rate and the surface contribution to the associated thermodynamic stability (free energy and enthalpy) of the various polymorphs and their crossover. Here, we present results of synthesis of mesoporous stishovite from cubic large-pore periodic mesoporous silica LP-FDU-12/C composite as precursor with an fcc lattice. We describe the synthesis procedure using multi-anvil apparatus at 9 GPa (about 90,000 atm) and temperature of 500 A degrees C. The synthetic mesoporous stishovite is, then, characterized by wide and small-angle X-ray diffraction, scanning/transmission electron microscopy and gas adsorption. Results show that this new material is characterized by accessible mesopores with wide pore size distribution, surface area of similar to 45 m(2)/g and volume of pores of similar to 0.15 cm(3)/g. Results from gas adsorption indicate that both porosity and permeability are retained at the high pressures of synthesis but with weak periodic order of the pores.