Understanding the effect of carbon on the density of hcp (hexagonal-close-packed) Fe-C alloys is essential for modeling the carbon content in the Earth's inner core. Previous studies have focused on the equations of state of iron carbides that may not be applicable to the solid inner core that may incorporate carbon as dissolved carbon in metallic iron. Carbon substitution in hcp-Fe and its effect on the density have never been experimentally studied. We investigated the compression behavior of Fe-C alloys with 0.31 and 1.37 wt % carbon, along with pure iron as a reference, by in-situ X-ray diffraction measurements up to 135 GPa for pure Fe, and 87 GPa for Fe-0.31C and 109 GPa for Fe-1.37C. The results show that the incorporation of carbon in hcp-Fe leads to the expansion of the lattice, contrary to the known effect in body-centered cubic (bcc)-Fe, suggesting a change in the substitution mechanism or local environment. The data on axial compressibility suggest that increasing carbon content could enhance seismic anisotropy in the Earth's inner core. The new thermoelastic parameters allow us to develop a thermoelastic model to estimate the carbon content in the inner core when carbon is incorporated as dissolved carbon hcp-Fe. The required carbon contents to explain the density deficit of Earth's inner core are 1.30 and 0.43 wt % at inner core boundary temperatures of 5000 K and 7000 K, respectively.

The 1:1 acetylene-benzene cocrystal, C2H2 center dot C6H6, was synthesized under pressure in a diamond anvil cell (DAC) and its evolution under pressure was studied with single-crystal X-ray diffraction and Raman spectroscopy. C2H2 center dot C6H6 is stable up to 30 GPa, nearly 10x the observed polymerization pressure for molecular acetylene to polyacetylene. Upon mild heating at 30 GPa, the cocrystal was observed to undergo an irreversible transition to a mixture of amorphous hydrocarbon and a crystalline phase with similar diffraction to i-carbon, a nanodiamond polymorph currently tacking a definitive structure. Characterization of this i-carbon-like phase suggests that it remains hydrogenated and may help explain previous observations of nanodiamond polymorphs. Potential reaction pathways in C2H2 center dot C6H6 are discussed and compared with other theoretical extended hydrocarbons that may be obtained through crystal engineering. The cocrystallization of benzene with other more inert gases may provide a novel pathway to selectively control the rich chemistry of these materials.

Molecular dynamics simulations predict a giant electrocaloric effect at the ferroelectric-antiferroelectric phase boundary in PZT (PbTiO3-PbZrO3). These large-scale simulations also give insights into the atomistic mechanisms of the electrocaloric effect in Pb(ZrxTi1-x)O-3. We predict a positive electrocaloric effect in ferroelectric PZT, but antiferroelectric PZT exhibits a negative-to-positive crossover with the increasing temperature or electric field. At the antiferroelectric-to-ferroelectric phase boundary, we find complex domain patterns. We demonstrate that the origin of the giant electrocaloric change of temperature is related to the easy structural response of the dipolar system to external stimuli in the transition region.

The balance of carbon flux in subduction zones is critical to the deep carbon cycle. Carbonate-bearing lithologies are the major carbon carriers transported from Earths surface into its interior at subduction zones. Recently, a number of studies have showed that carbon can be released from the subducting slab through metamorphic decarbonation and dissolution into C-H-O fluids. However, the evolution of the released C-H-O fluids during subduction-zone metamorphism is ambiguous and poorly explored. In this study, we found graphite-rich eclogite veins (VE) in the carbonated eclogites from the Southwestern (SM.) Tianshan subduction zone. The observed graphite with high crystallinity and graphite-bearing fluid inclusions indicate the fluid-deposited origin. Phase equilibrium modelling for the host carbonated-eclogite (HE) in a closed system indicates that it has experienced a retrograde P-T path involving decompression with heating from 26.5 kbar at 487 degrees C to 20.6 kbar at 565 degrees C. The calculation showed that about 0.92-2.03 wt% of CO2 (0.25-0.55 g C per 100 g rock) could be released from the carbonated eclogite during its exhumation process, which is enough to provide the carbon source for graphite precipitation in the VE. Combined with petrological and isotopic results, we suggest that the graphites in the VE were precipitated from carbon-bearing fluids derived from the carbonated eclogites during exhumation metamorphism. The overall redox reaction is: FeO (in silicate, Grt, Omp or Gln) + FeS + (H2O + CO2) (released from Lws and Dol) -> Fe2O3 (in Ep, Andradite or Hematite) + C (graphite) + SO42- + HCO3 + CO32-. Mass balance calculation indicates that carbonates could also be re-precipitated in the VE during fluid-rock interaction, in addition to the graphite precipitation. The finding of fluid-deposited graphite in the carbonated eclogites provides new insights into the fate of carbonic fluids formed in the subducted oceanic crust. We suggest that carbonic fluids formed in the carbonated eclogites by decarbonation or carbonate dissolution may also precipitate abiotic graphite or carbonates under favorable conditions during their migration in addition to the commonly recognized transportation to the mantle wedge.

Crystalline diiodoacetylene (C2I2) was synthesized and then studied under high-pressure conditions using synchrotron X-ray diffraction, Raman/infrared spectroscopies, and first-principles calculations. At similar to 0.3 GPa, the starting tetragonal (P4(2)/n) phase, which is stabilized by donor-acceptor interactions, transforms into a new orthorhombic structure (Cmca) that is more densely packed and analogous to the low-temperature phase of acetylene. Above approximately 4 GPa, compressed C2I2 molecules in the Cmca structure begin to polymerize to form a predominantly sp(2) amorphous carbon network that maintains a significant fraction of C-I bonds. Transport measurements reveal that the polymeric material is electrically conducting. The magnitude of the electrical conductivity is similar to Br-doped polyacetylene and undoped trans-polyacetylene at 8 GPa and 1 atm, respectively. Elemental analyses performed on recovered samples show that the iodine concentration varies with specific processing conditions. Optimization of the pressure-induced polymerization pathway could allow for enhanced electrical properties to be realized, in addition to postpolymerization functionalization using the weak C-I bonds.

We investigate the thermal equation of state, bulk modulus, thermal expansion coefficient, and heat capacity of MH-III (CH4 filled-ice Ih), needed for the study of CH4 transport and outgassing for the case of Titan and superTitans. We employ density functional theory and ab initio molecular dynamics simulations in the generalizedgradient approximation with a van der Waals functional. We examine the temperature range 300-500 K and pressures between 2 and 7 GPa. We find that in this P-T range MH-III is less dense than liquid water. There is uncertainty in the normalized moment of inertia (MOI) of Titan; it is estimated to be in the range of 0.33-0.34. If Titan's MOI is 0.34, MH-III is not stable at present in Titan's interior, yielding an easier path for the outgassing of CH4. However, for an MOI of 0.33, MH-III is thermodynamically stable at the bottom of an ice-rock internal layer capable of storing CH4. For rock mass fractions less than or similar to 0.2 upwelling melt is likely hot enough to dissociate MH-III along its path. For super-Titans considering a mixture of MH-III and ice VII, melt is always positively buoyant if the H2O:CH4 mole fraction is >5.5. Our thermal evolution model shows that MH-III may be present today in Titan's core, confined to a thin (approximate to 10 km) outer shell. We find that the heat capacity of MH-III is higher than measured values for pure water ice, larger than heat capacity often adopted for ice-rock mixtures with implications for internal heating.

Observations of the geochemical diversity of mid-oceanic ridge and ocean-island basalts have traditionally been attributed to the existence of large-scale mantle heterogeneity. In particular, the layered convection model has provided an important conceptual basis for discussing the chemical evolution of the Earth. In this model, a long-term boundary is assumed between a well-mixed and depleted upper mantle and a heterogeneous and more primitive lower mantle. The existence of high He-3/He-4 in ocean-island sources has been used to argue for the preservation of a primitive component in the deep mantle. Nevertheless, a primitive deep layer is difficult to reconcile with the abundant lithophile isotopic evidence for recycling of oceanic crust and the lack of preservation of primitive mantle. In addition, the widespread acceptance of geophysical evidence for whole mantle flow has made straightforward application of the layered convection model problematic. Model calculations show that whole mantle convection with present day heat flow and surface velocities is sufficiently vigorous to mix large-scale heterogeneity to an extent that is incompatible with the geochemical observations. Several concepts have been proposed in recent years to resolve the apparent conflicts between the various observational constraints and theoretical interpretations. The suggestions include the presence of deeper layering, preservation of highly viscous blobs, core mantle interaction, and strong temporal variations in mantle dynamics. Although these models generally appear to solve parts of the puzzle, at present no single model is able to account for all of the major observations. The reconciliation of conflicting evidence awaits improvements in observational and experimental techniques integrated with better model testing of hypotheses for the generation and destruction of mantle heterogeneity.

Phosphorus in Martian mantle is believed to be five to ten times more abundant than in Earth's mantle, and the distribution of this essential ingredient for life between different deep reservoirs is critical for understanding the habitability of the red planet. In this study, we investigated the behavior of phosphorus in a Martian magma ocean scenario, and measured the partition coefficient of phosphorus (D-p) between liquid metal and silicate melt within the pressure range of 3-8 GPa, temperatures between 1973 and 2173 K and oxygen fugacity ranging from -1.5 to similar to -2.5 as normalized to the iron-wustite oxygen buffer. Our results show D-p increasing with pressure but decreasing with temperature. A decrease of oxygen fugacity has a negative effect on D-p. The moderately siderophile character of phosphorus indicates that the Martian core might be an important reservoir of phosphorous. Based on our experimental results and phosphorus abundance in Martian mantle and bulk Mars, a minimum pressure of 5.8-10.4 GPa is estimated at the base of Martian magma ocean or during the impact melting if a contribution from the late accretion scenario is taken into account. The shallow Martian magma ocean would avail the preservation of volatiles after the rapid solidification of the planet.

The formation of the Moon is thought to be the result of a giant impact between a Mercury-to-proto-Earth-sized body and the proto-Earth. However, the initial thermal state of the Moon following its accretion is not well constrained by geochemical data. Here, we provide geochemical evidence that supports a high-temperature origin of the Moon by performing high-temperature (1973-2873 K) metal-silicate partitioning experiments, simulating core formation in the newly-formed Moon. Results indicate that the observed lunar mantle depletions of Ni and Co record extreme temperatures (>2600-3700 K depending on assumptions about the composition of the lunar core) during lunar core formation. This temperature range is within range of the modeled silicate evaporation buffer in a synestia-type environment. Our results provide independent geochemical support for a giant-impact origin of the Moon and show that lunar thermal models should start with a fully molten Moon. Our results also provide quantitative constraints on the effects of high-temperature lunar differentiation on the lunar mantle geochemistry of volatile, and potentially siderophile elements Cu, Zn, Ga, Ge, Se, Sn, Cd, In, Te and Pb. At the extreme temperatures recorded by Ni and Co, many of these elements behave insufficiently siderophile to explain their depletions by core formation only, consistent with the inferred volatility related loss of Cr, Cu, Zn, Ga and Sn during the Moon-forming event and/or subsequent magma-ocean degassing. (C) 2020 Elsevier B.V. All rights reserved.