The fate of subducted carbonates in the lower mantle and at the core-mantle boundary was modelled via experiments in the MgCO3-Fe-0 system at 70-150 GPa and 800-2600 K in a laser-heated diamond anvil cell. Using in situ synchrotron X-ray diffraction and ex situ transmission electron microscopy we show that the reduction of Mg-carbonate can be exemplified by: 6MgCO(3) + 19Fe = 8FeO +10(Mg0.6Fe0.4)O + Fe7C3 + 3C. The presented results suggest that the interaction of carbonates with Fe-0 or Fe-0-bearing rocks can produce Fe-carbide and diamond, which can accumulate in the D '' region, depending on its carbon to Fe ratio. Due to the sluggish kinetics of the transformation, diamond can remain metastable at the core-mantle boundary (CMB) unless it is in a direct contact with Fe-metal. In addition, it can be remobilized by redox melting accompanying the generation of mantle plumes. (C) 2019, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V.

The presence of H-2 and H2O in planetary interiors prompts the need for fundamental studies on these compounds under corresponding conditions. Here, we summarize data on H-2 properties In aqueous systems under conditions of high temperature and pressure. We explain how to measure important H-2 fugacities in hydrothermal systems. We present available experimental data and thermodynamic models for H-2 solubility and vapor-liquid partitioning under hydrothermal conditions. In addition, we introduce the fascinating world of H-2-H2O clathrate hydrates under extreme temperatures and pressures. The properties of the H-2-H2O system are well established below the critical point of water (374 degrees C and 22.06 MPa), but far less is known under higher temperatures and pressures, or the effect of salt.

The heat flux across the core-mantle boundary (Q(CMB)) is the key parameter to understand the Earth's thermal history and evolution. Mineralogical constraints of the Q(CMB) require deciphering contributions of the lattice and radiative components to the thermal conductivity at high pressure and temperature in lower mantle phases with depth-dependent composition. Here we determine the radiative conductivity (k(rad)) of a realistic lower mantle (pyrolite) in situusing an ultra-bright light probe and fast time-resolved spectroscopic techniques in laser-heated diamond anvil cells. We find that the mantle opacity increases critically upon heating to similar to 3000 K at 40-135 GPa, resulting in an unexpectedly low radiative conductivity decreasing with depth from similar to 0.8 W/m/K at 1000 km to similar to 0.35 W/m/K at the CMB, the latter being similar to 30 times smaller than the estimated lattice thermal conductivity at such conditions. Thus, radiative heat transport is blocked due to an increased optical absorption in the hot lower mantle resulting in a moderate CMB heat flow of similar to 8.5 TW,on the lower end of previous Q(CMB) estimates based on the mantle and core dynamics. This moderate rate of core cooling implies an inner core age of about 1 Gy and is compatible with both thermally- and compositionally-driven ancient geodynamo. (C) 2020 Elsevier B.V. All rights reserved.

Both the vibrational and electrical transport properties of 2H-TaS2 have been investigated at high pressures and low temperatures. The collapse of the charge-density-wave order at pressures above 7.3 GPa has been verified by Raman scattering, resistivity, and Hall coefficient measurements. For pressures above the critical pressure of 7.3 GPa, the superconducting transition temperature continues to increase and reaches its maximum value at 11.5 GPa, suggesting that it is not a simple competition between the charge-density-wave order and superconductivity. Through the standard resistivity fit in the normal state, the decline of the superconducting transition temperature with increasing pressure up to 47.0 GPa is due to the decrease of interaction strength and the increase of the impurity scattering. These results are very important in understanding the superconducting mechanism of transition-metal dichalcogenides.

The effect of pressure on thermal expansion of solid CH4 is calculated for the low-temperature region where the contributions from phonons and librons can be neglected and only the rotational tunnelling modes are essential. The effect of pressure is shown to increase the magnitude of the peaks of the negative thermal expansion and shifts the positions of the peaks to the low-temperature region, which goes asymptotically to zero temperature with increasing pressure. The Gruneisen thermodynamical parameter for the rotational tunnelling modes is calculated. It is large, negative, and increases in magnitude with rising pressure. Published under license by AIP Publishing.

Hydrogen is expected to display remarkable properties under extreme pressures and temperatures stemming from its low mass and thus propensity to quantum phenomena. Exploring such phenomena remains very challenging even though there was a tremendous technical progress both in experimental and theoretical techniques since the last comprehensive review (McMahon et al.) was published in 2012. Raman and optical spectroscopy experiments including infrared have been extended to cover a broad range of pressures and temperatures (P-T) probing phase stability and optical properties at these conditions. Novel pulsed laser heating and toroidal diamond anvil techniques together with diamond anvil protecting layers drastically improved the capabilities of static compression methods. The electrical conductivity measurements have been also performed to much higher than previously pressures and extended to low temperatures. The dynamic compression techniques have been dramatically improved recently enabling ramp isentropic compression that allows probing a wide range of P-T thermodynamic pathways. In addition, new theoretical methods have been developed beyond a common DFT theory, which make them predictive and in better agreement with experiments. With the development of new theoretical and experimental tools and sample loading methods, the quest for metallic hydrogen accelerated recently delivering a wealth of new data, which are reviewed here. Published under license by AIP Publishing.

Polymeric nitrogen at 120 GPa-180 GPa is known in two monatomic crystalline cubic gauche (cg-N) and layered polymeric (LP-N) phases and one amorphous modification (eta -N), and all these high-pressure phases attract considerable attention for their potential application as a high energy density material. Here, we investigated the stability of these modifications at high pressures in the laser heated diamond anvil cell upon decompression from 161 GPa. Pure LP-N was synthesized above 152 GPa upon laser heating of eta -N to 2500 K, while cg-N forms below 150 GPa. Upon laser heating at 129 GPa and 123 GPa, the LP-N clearly diminished, indicating that the synthesis of cg-N becomes more favorable in a mixed phase region below 129 GPa. Upon unloading, cg-N and LP-N were metastable to at least 71 GPa at up to 2500 K and at room temperature, respectively. These observations clarified a complicated polymorphism of monatomic nitrogen at high pressures and large hysteretic phenomena related to a transition to nonmolecular nitrogen.

Raman and optical spectroscopy are versatile tools for nondestructive characterization of a wide range of properties of novel materials and minerals in situ at extreme and ambient conditions. These techniques are genuinely complementary to X-ray tools (diffraction and spectroscopy) in the probe energy, momentum transfer, and time scale, making concomitant X-ray and optical probes available for advanced sample analysis. We have built a state-of-the-art, user-friendly integrated Raman and optical spectroscopy system at Sector 13 (GeoSoilEnviroCARS, University of Chicago, IL) of the Advanced Photon Source (APS), Argonne National Laboratory (ANL), where optical probes are available now in combination with high resolution in-situ synchrotron X-ray diffraction and spectroscopy tools (XRD, IXS, XES, NFS, and others) for extensive sample investigation. The integrated optical system enables a variety of techniques including multi-colored (five laser lines: 266, 473, 532, 660, and 946 nm) confocal Raman, fluorescence, and optical spectroscopy from ultraviolet (UV) to near infrared (IR) spectral ranges (266-1600 nm), and Coherent Anti-Stokes Raman spectroscopy (CARS) in combination with near IR double sided laser heating.

X-ray synchrotron diffraction (XRD) measurements of single-crystal and powder molybdenum disulfide MoS2 are performed at pressures (P) up to 78 GPa and temperatures (T) of 20 to 298 K in diamond-anvil cells. The results on single crystals demonstrate a sharp pressure induced isosymmetric phase transition of 2H(c) to 2H(a) modification at 23 GPa at 40 and 300 K. The structure of the high-pressure 2H(a) phase previously inferred theoretically and from powder XRD data is confirmed by our single-crystal XRD data solution, which also definitively determines the atomic potions as a function of pressure. No additional periodicity (commensurate or incommensurate) or distortion is found in the whole P-T range of this study. These results suggest that a previously proposed hypothetic charge-density-wave phase does not host pressure induced superconductivity experimentally found above 90 GPa.

We report a new hydrogen clathrate hydrate synthesized at 1.2 GPa and 298 K documented by single-crystal x-ray diffraction, Raman spectroscopy, and first-principles calculations. The oxygen sublattice of the new clathrate hydrate matches that of ice II, while hydrogen molecules are in the ring cavities, which results in the trigonal R3c or R (3) over barc space group (proton ordered or disordered, respectively) and the composition of (H2O)(6)H-2. Raman spectroscopy and theoretical calculations reveal a hydrogen disordered nature of the new phase C-1', distinct from the well-known ordered C-1 clathrate, to which this new structure transforms upon compression and/or cooling. This new clathrate phase can be viewed as a realization of a disordered ice II, unobserved before, in contrast to all other ordered ice structures.