Single-crystal synchrotron x-ray diffraction, Raman spectroscopy, and first principles calculations have been used to identify the structure of the high-pressure (HP) phase of molecular methane above 20 GPa up to 71 GPa at room temperature. The structure of the HP phase is trigonal R3, which can be represented as a distortion of the cubic phase B, previously documented at 7-15 GPa and confirmed here. The positions of hydrogen atoms in the HP phase have been obtained from first principles calculations, which also demonstrated the stability of this structure above 260 K at 25 GPa. The molecules occupy four different crystallographic sites in phase B and 11 sites in the HP phase, which result in splitting of molecular stretching modes detected in Raman spectroscopy and assigned here based on a good agreement with the Raman spectra calculated from the first principles. Our study points out to a single HP phase unlike up to three HP phases proposed previously based on the Raman spectroscopy results only.

A laser heating system for samples confined in diamond anvil cells paired with in situ X-ray diffraction measurements at the Extreme Conditions Beamline of PETRA III is presented. The system features two independent laser configurations (on-axis and off-axis of the X-ray path) allowing for a broad range of experiments using different designs of diamond anvil cells. The power of the continuous laser source can be modulated for use in various pulsed laser heating or flash heating applications. An example of such an application is illustrated here on the melting curve of iron at megabar pressures. The optical path of the spectroradiometry measurements is simulated with ray-tracing methods in order to assess the level of present aberrations in the system and the results are compared with other systems, that are using simpler lens optics. Based on the ray-tracing the choice of the first achromatic lens and other aspects for accurate temperature measurements are evaluated.

Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one-step synthesis of metal-inorganic frameworks Hf4N20 center dot N2, WN 8 center dot N2, and Os5N28 center dot 3N2 via direct reactions between elements in a diamond anvil cell at pressures exceeding 100 GPa is reported. The porous frameworks (Hf4N20, WN 8, and Os5N28) are built from transition-metal atoms linked either by polymeric polydiazenediyl (polyacetylene-like) nitrogen chains or through dinitrogen units. Triply bound dinitrogen molecules occupy channels of these frameworks. Owing to conjugated polydiazenediyl chains, these compounds exhibit metallic properties. The high-pressure reaction between Hf and N2 also leads to a non-centrosymmetric polynitride Hf2N11 that features double-helix catenapoly[tetraz-1-ene-1,4-diyl] nitrogen chains [-N-N-N=N-](infinity.)

Understanding of recently reported putative close-to-room-temperature superconductivity in C-S-H compounds at 267 GPa demands a reproducible synthesis protocol as well as knowledge of the compounds' structure and composition. We synthesized C-S-H compounds with various carbon compositions at high pressures from elemental carbon C and methane CH4, sulfur S, and molecular hydrogen H-2. Here, we focus on compounds synthesized using methane as these allow a straightforward determination of their structure and composition by combining single-crystal x-ray diffraction and Raman spectroscopy. We applied a two-stage synthesis of [(CH4)(x)(H2S)((1-x))](2)H-2 compounds with various compositions by first reacting sulfur and mixed methane-hydrogen fluids and forming CH4-doped H2S crystals at 0.5-3 GPa and then by growing single crystals of the desired hydrogen-rich compound. Raman spectroscopy applied to this material shows the presence of CH4 molecules incorporated into the lattice and allows the determination of the CH4 content, while single-crystal x-ray diffraction results suggest that the methane molecules substitute H2S molecules. The structural behavior of these compounds is very similar to the previously investigated methane-free crystals demonstrating a transition from Al2Cu type I4/mcm molecular crystal to a modulated molecular structure at 20-30 GPa and back to the same basic I4/mcm structure in an extended modification with greatly modified Raman spectra. This latter phase demonstrates a distortion into a Pnma structure at 132-159 GPa and then transforms into a common I m 3 over bar m H3S phase at higher pressures; however, no structural anomaly is detected near 220 GPa, where a sharp upturn in T-c has been reported.

The thermal conductivities of materials are extremely important for many practical applications, such as in understanding the thermal balance and history of the Earth, energy conversion of devices and thermal management of electronics. However, measurements of the thermal conductivity of materials under pressure and understanding of associated thermal transport mechanisms remain some of the most difficult challenges and complex topics in high-pressure research. Breakthroughs in high-pressure experimental techniques enable in situ measurements of thermal conductivity at extreme pressure-temperature conditions. This new capability provides not only a unique insight to understand thermal transport mechanisms in materials but also opportunities to realize reversible modulation of materials' thermal properties. In this Review, we discuss recent progresses in characterization techniques developed at high pressures, in the determination of the thermal conductivity of gases, liquids and solids, as well as in establishing the correlated thermal transport mechanisms. In addition, we focus on the applications of high-pressure and high-temperature experimental simulations of materials in the Earth's interior.

Human emissions of CO2 now outpace natural sources by two orders of magnitude. The current concentration of CO2 has not been substantially exceeded in the past 30 million years. Multiple model exercises indicate that consuming all fossil fuels would result in concentrations more than double present levels, even after 10,000 years. The global warming effect of carbon emissions appears within 5-7 years. However, since the effect of present infrastructure over its expected life would only modestly increase CO2 concentrations and global temperature, human choices over its replacement will decisively influence ultimate carbon impacts, both short-term and long-term.

The electronic structure near the Fermi surface determines the electrical properties of the materials, which can be effectively tuned by external pressure. Bi0.5Sb1.5Te3 is a p-type thermoelectric material which holds the record high figure of merit at room temperature. Here it is examined whether the figure of merit of this model system can be further enhanced through some external parameter. With the application of pressure, it is surprisingly found that the power factor of this material exhibits lambda behavior with a high value of 4.8 mW m(-1) K-2 at pressure of 1.8 GPa. Such an enhancement is found to be driven by pressure-induced electronic topological transition, which is revealed by multiple techniques. Together with a low thermal conductivity of about 0.89 W m(-1) K-1 at the same pressure, a figure of merit of 1.6 is achieved at room temperature. The results and findings highlight the electronic topological transition as a new route for improving the thermoelectric properties.

Global carbon emissions continue to acidify the oceans, motivating growing concern for the ability of coral reefs to maintain net positive calcification rates. Efforts to develop robust relationships between coral reef calcification and carbonate parameters such as aragonite saturation state ((arag)) aim to facilitate meaningful predictions of how reef calcification will change in the face of ocean acidification. Here we investigate natural trends in carbonate chemistry of a coral reef flat over diel cycles and relate these trends to benthic carbon fluxes by quantifying net community calcification and net community production. We find that, despite an apparent dependence of calcification on (arag) seen in a simple pairwise relationship, if the dependence of net calcification on net photosynthesis is accounted for, knowing (arag) does not add substantial explanatory value. This suggests that, over short time scales, the control of (arag) on net calcification is weak relative to factors governing net photosynthesis.

As theoretically hypothesized for several decades in group IV transition metals, we have discovered a dynamically stabilized body-centered cubic (bcc) intermediate state in Zr under uniaxial loading at sub-nanosecond timescales. Under ultrafast shock wave compression, rather than the transformation from alpha-Zr to the more disordered hex-3 equilibrium omega-Zr phase, in its place we find the formation of a previously unobserved nonequilibrium bcc metastable intermediate. We probe the compression-induced phase transition pathway in zirconium using time-resolved sub-picosecond x-ray diffraction analysis at the Linac Coherent Light Source. We also present molecular dynamics simulations using a potential derived from first-principles methods which independently predict this intermediate phase under ultrafast shock conditions. In contrast with experiments on longer timescale (> 10 ns) where the phase diagram alone is an adequate predictor of the crystalline structure of a material, our recent study highlights the importance of metastability and time dependence in the kinetics of phase transformations.

The Earth warms both when fossil fuel carbon is oxidized to carbon dioxide and when greenhouse effect of carbon dioxide inhibits longwave radiation from escaping to space. Various important time scales and ratios comparing these two climate forcings have not previously been quantified. For example, the global and time-integrated radiative forcing from burning a fossil fuel exceeds the heat released upon combustion within 2 months. Over the long lifetime of CO2 in the atmosphere, the cumulative CO2-radiative forcing exceeds the amount of energy released upon combustion by a factor >100,000. For a new power plant, the radiative forcing from the accumulation of released CO2 exceeds the direct thermal emissions in less than half a year. Furthermore, we show that the energy released from the combustion of fossil fuels is now about 1.71% of the radiative forcing from CO2 that has accumulated in the atmosphere as a consequence of historical fossil fuel combustion.