Preparing amorphous phases of carbon with mostly sp(3) bonding in bulk is challenging, but macroscopic samples that are nearly pure sp(3) are synthesized here by heating fullerenes at high pressure.

Si-24 is a new, open-framework silicon allotrope that is metastable at ambient conditions. Unlike diamond cubic silicon, which is an indirect-gap semiconductor, Si-24 has a quasidirect gap near 1.4 eV, presenting new opportunities for optoelectronic and solar energy conversion devices. Previous studies indicate that Na can diffuse from micron-sized grains of a high-pressure Na4Si24 precursor to create Si-24 powders at ambient conditions. Remarkably, we demonstrate here that Na remains highly mobile within large (similar to 100 mu m) Na4Si24 single crystals. Na readily diffuses out of Na4Si24 crystals under vacuum with gentle heating (10(-4) mbar at 125 degrees C) and can be further reacted with iodine to produce large Si-24 crystals that are 99.9985 at% silicon, as measured by wavelength-dispersive x-ray spectroscopy. Si-24 crystals display a sharp, direct optical absorption edge at 1.51(1) eV with an absorption coefficient near the band edge that is demonstrably greater than diamond cubic silicon. Temperature-dependent electrical transport measurements confirm the removal of Na from metallic Na(4)Si(24)to render single-crystalline semiconducting samples of Si-24. These optical and electrical measurements provide insights into key parameters such as the electron donor impurity level from residual Na, reduced electron mass, and electron relaxation time. Effective Na removal on bulk length scales and the high absorption coefficient of single-crystal Si-24 indicate promise for use of this material in bulk and thin film forms with potential applications in optoelectronic technologies.

We explore the element redistribution at mid-ocean ridges (MOR) using a numerical model to evaluate the role of decompression melting of the mantle in Earth's geochemical cycle, with focus on the formation of the depleted mantle component. Our model uses a trace element mass balance based on an internally consistent thermodynamic-petrologic computation to explain the composition of MOR basalt (MORB) and residual peridotite. Model results for MORB-like basalts from 3.5 to 0 Ga indicate a high mantle potential temperature (Tp) of 1650-1500 degrees C during 3.5-1.5 Ga before decreasing gradually to similar to 1300 degrees C today. The source mantle composition changed from primitive (PM) to depleted as Tp decreased, but this source mantle is variable with an early depleted reservoir (EDR) mantle periodically present. We examine a twostage Sr-Nd-Hf-Pb isotopic evolution of mantle residues from melting of PM or EDR at MORs. At high-Tp (3.5-1.5 Ga), the MOR process formed extremely depleted DMM. This coincided with formation of the majority of the continental crust, the subcontinental lithospheric mantle, and the enriched mantle components formed at subduction zones and now found in OIB. During cooler mantle conditions (1.5-0 Ga), the MOR process formed most of the modern ocean basin DMM. Changes in the mode of mantle convection from vigorous deep mantle recharge before similar to 1.5 Ga to less vigorous afterward is suggested to explain the thermochemical mantle evolution.

A high-magnetic field once existed in the early history of the Moon, suggesting the core once possessed a thermally driven dynamo. The thermal conductivity of core materials is a significant parameter of the dynamo. The lunar core composition is thought to be iron or iron-alloyed with some light elements (e.g., S, P, Si, and C), but its transport properties remain uncertain. We measured the electrical resistivity of iron and Fe-3 wt%P alloys at 5 GPa and high temperatures. Apart from the quasi four-point technique, the four-probe van der Pauw technique was also employed to measure the resistivity of pure iron. Adding similar to 3 wt% phosphorus to iron slightly increases the resistivity at 5 GPa and 1000-1500 K due to the impurity effect. The resistivity of Fe-3 wt%P alloys increases at the onset of melting. Via the Wiedemann-Franz law, the thermal conductivity at the lunar core-mantle boundary (CMB) is estimated to be 28.6-34.2 Wm(-1)K(-1) for a light-element free core and 31.5 +/- 1.9 Wm(-1)K(-1) for a phosphorus-bearing (similar to 3 wt% P) core. Therefore, small amounts of phosphorus in the lunar core slightly impact its thermal conductivity. The estimated conductive heat flow across the lunar CMB varies from 4.5 to 5.7 GW, and the adiabatic heat flux varies from 3.3 to 4.2 mW/m(2), depending on the core's composition (Fe or Fe-3 wt%P). Integrating our results with previous lunar core evolution models, we suggest that a thermally driven dynamo persisted until 3.63-3.88 Ga ago.

Using density functional perturbation theory, we computed the phonon frequencies and Raman and IR activities of hafnia polymorphs (P4(2)nmc, Pca2(1), Pmn2(1), Pbca OI, brookite, and baddeleyite) for phase identification. We investigated the evolution of Raman and IR activities with respect to epitaxial strain and provide plots of frequency differences as a function of strain for experimental calibration and identification of the strain state of the sample. We found Raman signatures of different hafnia polymorphs: omega ( A(1g) ) = 300 cm(-1) for P4(2)nmc, omega (A( 1)) = 343 cm(-1) for Pca2(1), omega ( B-2) = 693 cm(-1) for Pmn2(1), omega (A( g)) = 513 cm(-1) for Pbca (OI), omega (A(g)) = 384 cm(-1) for brookite, and omega (A(g)) = 496 cm(-1) for baddeleyite. We also identified the Raman B-1g mode, an anti-phase vibration of dipole moments [omega (B-1g) = 758 cm(-1) for OI and omega ( B-1g ) = 784 cm(-1) for brookite], as the Raman signature of antipolar Pbca structures. We calculated a large splitting between the longitudinal optical and transverse optical modes [delta omega(LO) - TO ( A(1)(z)) = 255 cm(-1) in Pca2(1) and delta omega( LO) (- TO) ( A 1 ) = 263 cm(-1) in Pmn2(1)] to the same order as those observed in perovskite ferroelectrics and related them to the anomalously large Born effective charges of Hf atoms [ Z * ( Hf ) = 5.54]. Published under an exclusive license by AIP Publishing.

We report a carbon-boron clathrate with composition 2 La@B6C6 (LaB3C3). Like recently reported SrB3C3,([1]) single-crystal X-ray diffraction and computational modelling indicate that the isostructural La member crystallizes in the cubic bipartite sodalite structure (Type-VII clathrate) with La atoms encapsulated within truncated octahedral cages composed of alternating carbon and boron atoms. The covalent nature of the B-C bonding results in a hard, incompressible framework, and owing to the balanced electron count, La3+[B3C3](3-) exhibits markedly improved pressure stability and is a semiconductor with an indirect band gap predicted near 1.3 eV. A variety of different guest atoms may potentially be substituted within Type-VII clathrate cages, presenting opportunities for a large family of boron-stabilized, carbon-based clathrates with ranging physical properties.

On the basis of the van der Pauw method, we developed a new technique for measuring the electrical resistivity of metals in a cubic multi-anvil high-pressure apparatus. Four electrode wires were introduced into the sample chamber and in contact with the pre-pressed metal disk on the periphery. The sample temperature was measured with a NiCr-NiSi (K-type) thermocouple, which was separated from the sample by a thin hexagonal boron nitride layer. The electrodes and thermocouple were electrically insulated from each other and from the heater by an alumina tube as well. Their leads were in connection with cables through the gap between the tungsten carbide anvils. We performed experiments to determine the temperature dependence of electrical resistivity of pure iron at 3 and 5 GPa. The experiments produce reproducible measurements and the results provide an independent check on electrical resistivity data produced by other methods. The new technique provides reliable electrical resistivity measurements of metallic alloys and compounds at high pressure and temperature.

A strong correlation exists between the average slip rate by short-term slow slip events (SSEs) and changes in the slab geometry in Cascadia and Nankai. The generation of short-term SSEs is generally assumed to be related to the presence of fluids and we investigate the hypothesis that fluids released by metamorphic dehydration reactions migrate in 3-D due to complex slab geometry. The associated along-arc focusing of fluid flux is likely to cause higher average slip rate in certain patches. To test this hypothesis, we investigate how fluid migration is modified by along-strike changes in slab geometry. We use a numerical model of two-phase flow in subduction zones. In this model fluids migrate subparallel to the slab surface due to the anisotropic permeability inside a serpentinite layer just above the slab. In 3-D, we find that fluids migrate in the maximum-dip direction of the slab, rather than subparallel to the plate motion. As a result fluid paths concentrate with increasing porosity where the slab has a convex shape (and diverge with decreasing porosity where it has a concave shape). These results suggest that regions with a high average slip rate by short-term SSEs in Cascadia and Nankai can be explained by 3-D focusing of fluid migration. We predict a defocusing of fluids below the Kii Channel, Nankai, which may be the reason for the observed small slip by short-term SSEs in this location.

We report the synthesis of bulk, highly oriented, crystalline 4H hexagonal silicon (4H-Si), through a metastable phase transformation upon heating the single-crystalline Si-24 allotrope. Remarkably, the resulting 4H-Si crystallites exhibit an orientation relationship with the Si-24 crystals, indicating a structural relationship between the two phases. Optical absorption measurements reveal that 4H-Si exhibits an indirect band gap near 1.2 eV, in agreement with first principles calculations. The metastable crystalline transition pathway provides a novel route to access bulk crystalline 4H-Si in contrast to previous transformation paths that yield only nanocrystalline-disordered materials.

Knowledge of the sound velocity of core materials is essential to explain the observed anomalously low shear wave velocity (V-S) and high Poisson's ratio (sigma) in the solid inner core. To date, neither V-S nor sigma of Fe and Fe-Si alloy have been measured under core conditions. Here, we present V-S and sigma derived from direct measurements of the compressional wave velocity, bulk sound velocity, and density of Fe and Fe-8.6 wt%Si up to similar to 230 GPa and similar to 5400 K. The new data show that neither the effect of temperature nor incorporation of Si would be sufficient to explain the observed low V-S and high sigma of the inner core. A possible solution would add carbon (C) into the solid inner core that could further decrease V-S and increase sigma. However, the physical property-based Fe-Si-C core models seemingly conflict with the partitioning behavior of Si and C between liquid and solid Fe.