Recent studies reveal a pressure induced transition from a paramagnetic tetragonal phase (T) to a collapsed tetragonal phase (CT) in CaFe2As2, which was found to be superconducting with pressure at low temperature. We have investigated the effects of electron correlation and a local fluctuating moment in both tetragonal and collapsed tetragonal phases of the paramagnetic CaFe2As2 using self-consistent DFT-DMFT with continuous time quantum Monte Carlo as the impurity solver. From the computed optical conductivity, we find a gain in the optical kinetic energy due to the loss in Hund's rule coupling energy in the CT phase. We find that the transition from T to CT turns CaFe2As2 from a bad metal into a good metal. Computed mass enhancement and local moments also show a significant decrease in the CT phase, which confirms the suppression of the electron correlation in the CT phase of CaFe2As2.

Understanding carbon speciation in Earth materials is important to unravel the geochemical evolution of the Earth's atmosphere, the composition of mantle partial melts, and the overall distribution of carbon in the deep mantle. In an effort to provide the systematic protocols to characterize carbon-bearing fluid inclusions and other carbon-bearing species using high-resolution C-13 solid-state NMR, one of the element-specific probes of local structure around carbon, we explore the atomic configurations around the carbon species formed during the reaction between C-13-enriched amorphous carbon and MgSiO3 enstatite synthesized at 1.5 GPa and 1400 degrees C using C-13 MAS NMR spectroscopy and Raman spectroscopy. The Raman spectra for the fluid inclusion show the presence of multiple molecular species (e.g., CO2, CO, CH4, H2O, and H-2) and reveal heterogeneous distribution of these species within the inclusion. C-13 MAS NMR results show that the sharp peak at 125.2 ppm is dominant. While the peak could be assigned to either molecular CO2 in the fluid phase or fourfold-coordinated carbon (C-[4]), the peak is likely due to fluid CO2, as revealed by Raman analyses of micrometer-sized fluid inclusions in the sample. The peaks at 161.2, 170.9, and 173.3 ppm in the C-13 NMR spectrum correspond to the carbonate ions (CO32-) and additional small peak at 184.5 ppm can be attributed to carbon monoxide. Based on the established relationship between C-13 abundance and peak intensity in the C-13 MAS NMR, the estimated C-13 amounts of CO2, CO32-, and CO species are much larger than those estimated from carbon solubility in the crystals, thus, indicating that those carbon species are from external phases. The C-13 NMR spectrum for amorphous carbon showed a peak shift from similar to 130 to similar to 95 ppm after compression, thereby suggesting that the amorphous carbon underwent permanent pressure-induced densification, characterized by the transition from sp(2) to sp(3) hybridization and/or pressure-induced changes in sp(2) carbon topology. While direct probing of carbon species in the crystalline lattice using NMR is challenging, the current results and method can be utilized to provide quantitative analysis of carbon-species in the fluid-inclusions in silicates, which is essential for understanding the deep carbon cycle and volcanic processes.

We have performed experiments to investigate the solubility and metal-silicate partitioning of gold as a function of metal sulphur content (XS), silicate melt polymerization (NBO/T) and pressure (P). These experiments show that Au becomes less siderophile both with increasing pressure and as the metal phase becomes more sulphur-rich. For the studied range of compositions, melt polymerization has no effect on the solubility of Au. The reduction in the siderophile tendency of gold with increasing metal sulphur content is greater than expected on the basis of activity-composition relationships in the metal phase. This suggests a significant role for complexing between Au and S in the silicate melt. Our new experimental results are combined with literature data to yield a parameterisation for the exchange coefficient of Au (Kd(Au)(Met/Sil)) as a function of P, T and X-S:

We predict a candidate high-temperature, high-pressure structure of FeSiO3 with space-group symmetry Cmmm by applying an evolutionary algorithm within density functional theory (DFT)+U that we call post-perovskite II (PPv-II). An exhaustive search found no other competitive candidate structures with ABO(3) composition. We compared the x-ray diffraction pattern of FeSiO3 PPv-II with experimental results of the recently reported "H phase" of (Fe,Mg)SiO3. The intensities and positions of two main x-ray diffraction peaks of PPv-II FeSiO3 compare well with those of the H phase. We also calculated the static equation of state, the enthalpy, and the bulk modulus of the PPv-II phase and compared it with those of the perovskite (Pv) and post-perovskite (PPv) phases of FeSiO3. According to the static DFT+U computations, the PPv-II phase of FeSiO3 is less stable than the Pv and PPv phases under lower mantle pressure conditions at T = 0 K and has a higher volume. PPv-II may be entropically stabilized, and may be a stable phase in Earth's lower mantle, coexisting with -PbO2 (columbite-structure) silica and perovskite, or with magnesiowustite and/or ferropericlase, depending on the bulk composition.

Experimental and theoretical methods were employed to investigate the ambient-pressure, metastable phase transition pathways for Mg2C, which was recovered after high-pressure synthesis. We demonstrate that at temperatures above 600 K isolated C4- anions within the Mg2C structure polymerize into longer-chain carbon polyanions, resulting in the formation of the alpha-Mg2C3 (Pnnm) structure, which is another local energy minimum for the carbon-magnesium system. Access to the thermodynamic ground state (decomposition into graphite) was achieved at temperatures above similar to 1000 K. These results indicate that recoverable high-pressure materials can serve as useful high-energy precursors for ambient-pressure materials synthesis, and they show a novel mechanism for the formation of carbon chains from methanide structures.

Pressure-induced amorphization (PIA) and thermal-driven recrystallization have been observed in many crystalline materials. However, controllable switching between PIA and a metastable phase has not been described yet, due to the challenge to establish feasible switching methods to control the pressure and temperature precisely. Here, we demonstrate a reversible switching between PIA and thermally-driven recrystallization of VO2(B) nanosheets. Comprehensive in situ experiments are performed to establish the precise conditions of the reversible phase transformations, which are normally hindered but occur with stimuli beyond the energy barrier. Spectral evidence and theoretical calculations reveal the pressure-structure relationship and the role of flexible VOx polyhedra in the structural switching process. Anomalous resistivity evolution and the participation of spin in the reversible phase transition are observed for the first time. Our findings have significant implications for the design of phase switching devices and the exploration of hidden amorphous materials.

We have performed quantum Monte Carlo (QMC) simulations and density functional theory calculations to study the equations of state of MgSiO3 perovskite (Pv, bridgmanite) and post-perovskite (PPv) up to the pressure and temperature conditions of the base of Earth's lower mantle. The ground-state energies were derived using QMC simulations and the temperature-dependent Helmholtz free energies were calculated within the quasiharmonic approximation and density functional perturbation theory. The equations of state for both phases of MgSiO3 agree well with experiments, and better than those from generalized gradient approximation calculations. The Pv-PPv phase boundary calculated from our QMC equations of state is also consistent with experiments, and better than previous local density approximation calculations. We discuss the implications for double crossing of the Pv-PPv boundary in the Earth.

Earth's magnetic field has been thought to arise from thermal convection of molten iron alloy in the outer core, but recent density functional theory calculations have suggested that the conductivity of iron is too high to support thermal convection(1-4), resulting in the investigation of chemically driven convection(5,6). These calculations for resistivity were based on electron-phonon scattering. Here we apply self-consistent density functional theory plus dynamical meanfield theory (DFT+DMFT)(7) to iron and find that at high temperatures electron-electron scattering iscomparable to the electron-phonon scattering, bringing theory into agreement with experiments and solving the transport problem in Earth's core. The conventional thermal dynamo picture is safe. We find that electron-electron scattering of d electrons is important at high temperatures in transition metals, in contrast to textbook analyses since Mott(8,9), and that 4s electron contributions to transport are negligible, in contrast to numerous models used for over fifty years. The DFT+DMFT method should be applicable to other high-temperature systems where electron correlations are important.

The thermal conductivity of the Earth's core can be estimated from its electrical resistivity via the Wiedemann-Franz law. However, previously reported resistivity values are rather scattered, mainly due to the lack of knowledge with regard to resistivity saturation (violations of the Bloch-Gruneisen law and the Matthiessen's rule). Here we conducted high-pressure experiments and first-principles calculations in order to clarify the relationship between the resistivity saturation and the impurity resistivity of substitutional silicon in hexagonal-close-packed (hcp) iron. We measured the electrical resistivity of Fe-Si alloys (iron with 1, 2, 4, 6.5, and 9 wt.% silicon) using four-terminal method in a diamond anvil cell up to 90 GPa at 300 K. We also computed the electronic band structure of substitutionally disordered hcp Fe-Si and Fe-Nialloy systems by means of Korringa-Kohn-Rostoker method with coherent potential approximation (KKR-CPA). The electrical resistivity was then calculated from the Kubo-Greenwood formula. These experimental and theoretical results show excellent agreement with each other, and the first principles results show the saturation behavior at high silicon concentration. We further calculated the resistivity of Fe-Ni-Si ternary alloys and found the violation of the Matthiessen's rule as a consequence of the resistivity saturation. Such resistivity saturation has important implications for core dynamics. The saturation effect places the upper limit of the resistivity, resulting in that the total resistivity value has almost no temperature dependence. As a consequence, the core thermal conductivity has a lower bound and exhibits a linear temperature dependence. We predict the electrical resistivity at the top of the Earth's core to be 1.12 x 10(-6) Omega m, which corresponds to the thermal conductivity of 87.1 Wpm/K. Such high thermal conductivity suggests high isentropic heat flow, leading to young inner core age (<0.85 Gyr old) and high initial core temperature. It also strongly suppresses thermal convection in the core, which results in no convective motion in inner core and possibly thermally stratified layer in the outer core. (C) 2016 Elsevier B.V. All rights reserved.