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.

Motivated to explore the formation of novel extended carbon-nitrogen solids via well-defined molecular precursor pathways, we studied the chemical reactivity of highly pure phosphorous tricyanide, P(CN)(3), under conditions of high pressure at room temperature. Raman and infrared (IR) spectroscopic measurements reveal a series of phase transformations below 10 GPa, and several low-frequency vibrational modes are reported for the first time. Synchrotron powder X-ray diffraction measurements taken during compression show that molecular P(CN)(3) is highly compressible, with a bulk modulus of 10.0 +/- 0.3 GPa, and polymerizes into an amorphous solid above similar to 10.0 GPa. Raman and IR spectra, together with first-principles molecular-dynamics simulations, show that the amorphization transition is associated with polymerization of the cyanide groups into CN bonds with predominantly sp(2) character, similar to known carbon nitrides, resulting in a novel phosphorous carbon nitride (PCN) polymeric phase, which is recoverable to ambient pressure. (C) 2015 AIP Publishing LLC.

Aluminous bridgmanite (Al-Bm) is the dominant phase in the Earth's lower mantle. In this study, the Mossbauer spectra of an Al-Bm sample Mg0.868Fe0.087Si0.944Al0.101O2.994 were recorded from 65 to 300 K at 1 bar. The temperature dependence of the center shift was fitted by the Debye model and yielded the Debye temperatures of 305 3 K for Fe2+ and 361 22 K for Fe3+. These values are lower than those of Al-free bridgmanite by 17 and 24%, respectively, indicating that the presence of Fe and Al increases the average Fe-O bond length and weakens the bond strength. At 300 K, the calculated recoil-free fractions of Fe2+ (0.637 +/- 0.006) and Fe3+ (0.72 +/- 0.02) are similar and therefore the molar fractions of Fe2+ and Fe3+ are nearly the same as the area fractions of the corresponding Mossbauer doublets. At 900 K, the calculated recoil-free fractions of Fe3+ is 46% higher than that of Fe2+, implying that the molar fraction of Fe3+ is only 41% for a measured spectral area fraction of 50%, and that the area fractions of iron sites may change with temperature without any changes in the valence state or spin state of iron. We infer that Fe3+ accounts for 46 +/- 2% of the iron in the Al-Bm and it enters the A site along with Al3+ in the B site through the coupled-substitution mechanism. An Fe2+ component with large quadrupole splitting (similar to 4.0 mm/s) was observed at cryogenic conditions and interpreted as a high-spin distorted iron site.

We use molecular dynamics with a first-principles-based shell model potential to study the electrocaloric effect (ECE) in lithium niobate, LiNbO3, and find a giant electrocaloric effect along a line passing through the ferroelectric transition. With an applied electric field, a line of maximum ECE passes through the zero field ferroelectric transition, continuing along a Widom line at high temperatures with increasing fields, and along the instability that leads to homogeneous ferroelectric switching below T-c with an applied field antiparallel to the spontaneous polarization. This line is defined as the minimum in the inverse capacitance under an applied electric field. We investigate the effects of pressure, temperature and an applied electric field on the ECE. The behavior we observe in LiNbO3 should generally apply to ferroelectrics; we therefore suggest that the operating temperature for refrigeration and energy scavenging applications should be above the ferroelectric transition region to obtain a large electrocaloric response. The relationship between T-c, the Widom line, and homogeneous switching should be universal among ferroelectrics, relaxors, multiferroics, and the same behavior should be found under applied magnetic fields in ferromagnets.

The structure of a liquid Fe-3.5wt% C alloy is examined for up to 7.2GPa via multiangle energy-dispersive X-ray diffraction using a Paris-Edinburgh type large-volume press. X-ray diffraction data show clear changes in the pressure-dependent peak positions of structure factor and reduced pair distribution function at 5GPa. These results suggest that the liquid Fe-3.5wt% C alloys change structurally at approximately 5GPa. This finding serves as a microscopic explanation for the alloy's previously observed density change at the same pressure. The pressure dependencies of the nearest and second neighbor distances of the liquid Fe-3.5wt% C alloy are similar to those of liquid Fe which exhibits a structural change near the bcc-fcc-liquid triple point (5.2GPa and 1991K). Similarities between Fe-3.5wt% C and Fe suggest that a density change also occurs in liquid Fe and that this structural change extends to other Fe-light element alloys.