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.

The thermal expansion at constant pressure of solid CD4 III is calculated for the low-temperature region where only the rotational tunneling modes are essential and the effect of phonons and librons can be neglected. It is found that in mK region there is a giant peak of the negative thermal expansion. The height of this peak is comparable or even exceeds the thermal expansion of solid N-2, CO, O-2, or CH4 in their triple points. It is shown that like in the case of light methane, the effect of pressure is quite unusual: as evidenced from the pressure dependence of the thermodynamic Gruneisen parameter (which is negative and large in the absolute value), solid CD4 becomes increasingly quantum with rising pressure.

With the exception of lithium, alkali metals do not react with elemental nitrogen either at ambient conditions or at elevated temperatures, requiring the search for alternative synthetic routes to their nitrogen-containing compounds. Here using a controlled decomposition of sodium azide (NaN3) at high pressure conditions, we synthesize two novel compounds, Na-3(N-2)(4) and NaN2, both containing dinitrogen anions. NaN2 synthesized at 4 GPa might be the common intermediate in high-pressure solid-state metathesis reactions, where NaN3 is used as a source of nitrogen, while Na-3(N-2)(4) opens a new class of compounds, where [N-2] units accommodate a noninteger formal charge of 0.75-. This finding can dramatically extend the expected compositions in other group 1 and 2 metal-nitrogen systems. Electronic structure calculations show the metallic character for both compounds.

Alkaline earth metal peroxides are typical examples of ionic compounds containing polyanions. We herein report a stable BaO2 phase at high pressure up to 130 GPa found via a first-principles computational structure search and high-pressure experimental investigations. The identified monoclinic structure (space group C2/m) can be derived by sublattice distortions of Ba atoms and peroxide groups associated with the phonon mode softening of the lower-pressure Cmmm structure. Contrary to the previous expectation of polymerization of the peroxide group at elevated pressure, this phase retains the peroxide group and, interestingly, exhibits an insulating behavior demonstrating an increase of the band gap under compression. Our synchrotron x-ray diffraction (XRD) measurements could not distinguish between Cmmm and C2/m BaO2 definitively because the difference in XRD patterns is very subtle. However, our data do not show any sign of polymerization transition up to 120 GPa. Raman spectra of the O-O peroxide vibration show a small anomaly in frequency at 110 GPa, which is qualitatively like that predicted theoretically due to the Cmmm to C2/m phase transition, thus supporting the predicted transformation.

The ultrafast synthesis of epsilon-Fe3N1+x in a diamond-anvil cell (DAC) from Fe and N-2 under pressure was observed using serial exposures of an X-ray free electron laser (XFEL). When the sample at 5 GPa was irradiated by a pulse train separated by 443 ns, the estimated sample temperature at the delay time was above 1400 K, confirmed by in situ transformation of alpha- to gamma-iron. Ultimately, the Fe and N-2 reacted uniformly throughout the beam path to form Fe3N1.33, as deduced from its established equation of state (EOS). We thus demonstrate that the activation energy provided by intense X-ray exposures in an XFEL can be coupled with the source time structure to enable exploration of the time-dependence of reactions under high-pressure conditions.

Earth's lowermost mantle displays complex geological phenomena that likely result from its heterogeneous physical interaction with the core. Geophysical models of core-mantle interaction rely on the thermal and electrical conductivities of appropriate geomaterials which, however, have never been probed at representative pressure and temperature (P-T) conditions. Here we report on the opacity of single crystalline bridgmanite and ferropericlase and link it to their radiative and electrical conductivities. Our results show that light absorption in the visible spectral range is enhanced upon heating in both minerals but the rate of change in opacity with temperature is a factor of six higher in ferropericlase. As a result, bridgmanite in the lowermost mantle is moderately transparent while ferropericlase is highly opaque. Our measurements support previous indirect estimates of low (< 1 W/m/K) and largely temperature-independent radiative conductivity in the lowermost mantle. This implies that the radiative mechanism has not contributed significantly to cooling the Earth's core throughout the geologic time. Opaque ferropericlase is electrically conducting and mediates strong core-mantle electromagnetic coupling, explaining the intradecadal oscillations in the length of day, low secular geomagnetic variations in Central Pacific, and the preferred paths of geomagnetic pole reversals. (C) 2021 Elsevier B.V. All rights reserved.

We synthesized two C-S-H compounds from a mixture of carbon and sulfur in hydrogen-C : (H2S)(2)H-2 and from sulfur in mixed methane-hydrogen fluids-(CH4)(x)(H2S)((2-x))H-2 at 4 GPa. X-ray synchrotron single-crystal diffraction and Raman spectroscopy have been applied to these samples up to 58 and 143 GPa, respectively. Both samples show a similar Al-2 Cu-type I4/mcm basic symmetry, while the hydrogen subsystem evolves with pressure via variously ordered molecular and extended modifications. The methane-bearing sample lowers symmetry to an orthorhombic Pnma structure after laser heating to 1400 K at 143 GPa. The results suggest that C-S-H compounds are structurally different from a common Im-3m H3S.

Earth's core is composed of iron (Fe) alloyed with light elements, e.g., silicon (Si). Its thermal conductivity critically affects Earth's thermal structure, evolution, and dynamics, as it controls the magnitude of thermal and compositional sources required to sustain a geodynamo over Earth's history. Here we directly measured thermal conductivities of solid Fe and Fe-Si alloys up to 144GPa and 3300K. 15 at% Si alloyed in Fe substantially reduces its conductivity by about 2 folds at 132GPa and 3000K. An outer core with 15 at% Si would have a conductivity of about 20Wm(-1) K-1, lower than pure Fe at similar pressure-temperature conditions. This suggests a lower minimum heat flow, around 3 TW, across the core-mantle boundary than previously expected, and thus less thermal energy needed to operate the geodynamo. Our results provide key constraints on inner core age that could be older than two billion-years. Thermal conductivity of Earth's core affects Earth's thermal structure, evolution and dynamics. Based on thermal conductivity measurements of iron-silicon alloys at high pressure and temperature conditions, the authors here propose Earth's inner core could be older than previously expected.

The synthesis of polynitrogen compounds is of great importance due to their potential as high-energy-density materials (HEDM), but because of the intrinsic instability of these compounds, their synthesis and stabilization is a fundamental challenge. Polymeric nitrogen units which may be stabilized in compounds with metals at high pressure are now restricted to non-branched chains with an average N-N bond order of 1.25, limiting their HEDM performances. Herein, we demonstrate the synthesis of a novel polynitrogen compound TaN5 via a direct reaction between tantalum and nitrogen in a diamond anvil cell at circa 100 GPa. TaN5 is the first example of a material containing branched all-single-bonded nitrogen chains [N-5(5-)](infinity). Apart from that we discover two novel Ta-N compounds: TaN4 with finite N-4(4-) chains and the incommensurately modulated compound TaN2-x, which is recoverable at ambient conditions.