9 Search Results

We have performed a combined experimental and theoretical study of ethane and methane at high pressures of up to 120 GPa at 300 K using x-ray diffraction and Raman spectroscopies and the USPEX ab initio evolutionary structural search algorithm, respectively. For ethane, we have determined the crystallization point, for room temperature, at 2.7 GPa and also the low pressure crystal structure (phase A). This crystal structure is orientationally disordered (plastic phase) and deviates from the known crystal structures for ethane at low temperatures. Moreover, a pressure induced phase transition has been identified, for the first time, at 13.6 GPa tomore » a monoclinic phase B, the structure of which is solved based on good agreement with the experimental results and theoretical predictions. For methane, our x-ray diffraction measurements are in agreement with the previously reported high-pressure structures and equation of state (EOS). Finally, we have determined the EOSs of ethane and methane, which provides a solid basis for the discussion of their relative stability at high pressures.« less

Thermal conductivity of the lowermost mantle governs the heat flow out of the core energizing planetary-scale geological processes. Yet, there are no direct experimental measurements of thermal conductivity at relevant pressure–temperature conditions of Earth's core–mantle boundary. Here we determine the radiative conductivity of post-perovskite at near core–mantle boundary conditions by optical absorption measurements in a laser-heated diamond anvil cell. Our results show that the radiative conductivity of Mg0.9Fe0.1SiO3 post-perovskite (~1.1 W/m/K) is almost two times smaller than that of bridgmanite (~2.0 W/m/K) at the base of the mantle. By combining this result with the present-day core–mantle heat flow and availablemore » estimations on the lattice thermal conductivity we conclude that post-perovskite is at least as abundant as bridgmanite in the lowermost mantle which has profound implications for the dynamics of the deep Earth.« less

Used nuclear fuel (UNF) reprocessing represents a unique challenge when dealing with radionuclides such as isotopes of ⁸⁵Kr and ¹²⁹I₂, due to their volatility and long half-life. However, efficient capture of ¹²⁹I₂ (t_1/2 = 15.7 x 10⁶ years) from the nuclear waste stream can help to reduce the risk of releasing I₂ radionuclide into the environment and mitigate concerns about human health problems. Metal organic frameworks (MOFs) have been reported to be potential I₂ adsorbents: but the effect of water vapor, generally present in the reprocessing off-gas stream is rarely taken into account. Moisture stable porous MOFs, which can selectivelymore » adsorb I₂ in presence of water vapor is thus of great interest. Herein, the I₂ adsorption performance of two microporous MOFs is reported in presence of different humidity. I…π phenyl ring interactions are mainly responsible for the adsorption as revealed by single crystal XRD« less

Hydrogen-rich hydrides attract great attention due to recent theoretical (1) and then experimental discovery of record high-temperature superconductivity in H₃S [T_c = 203 K at 155 GPa (2)]. Here we search for stable uranium hydrides at pressures up to 500 GPa using ab initio evolutionary crystal structure prediction. Chemistry of the U-H system turned out to be extremely rich, with 14 new compounds, including hydrogen-rich UH₅, UH₆, U₂H₁₃, UH₇, UH₈, U₂H₁₇, and UH₉. Their crystal structures are based on either common face-centered cubic or hexagonal close-packed uranium sublattice and unusual H₈ cubic clusters. Our high-pressure experiments at 1 to 103more » GPa confirm the predicted UH₇, UH₈, and three different phases of UH₅, raising confidence about predictions of the other phases. Many of the newly predicted phases are expected to be high-temperature superconductors. The highest-T_c superconductor is UH₇, predicted to be thermodynamically stable at pressures above 22 GPa (with T_c = 44 to 54 K), and this phase remains dynamically stable upon decompression to zero pressure (where it has T_c = 57 to 66 K).« less

Capture of highly volatile radioactive iodine is a promising application of metal–organic frameworks (MOFs), thanks to their high porosity with flexible chemical architecture. Specifically, strong charge-transfer binding of iodine to the framework enables efficient and selective iodine uptake as well as its long-term storage. As such, precise knowledge of the electronic structure of iodine is essential for a detailed modeling of the iodine sorption process, which will allow for rational design of iodophilic MOFs in the future. Here we probe the electronic structure of iodine in MOFs at variable iodine···framework interaction by Raman and optical absorption spectroscopy at high pressuremore » (P). The electronic structure of iodine in the straight channels of SBMOF-1 (Ca-sdb, sdb = 4,4'-sulfonyldibenzoate) is modified irreversibly at P > 3.4 GPa by charge transfer, marking a polymerization of iodine molecules into a 1D polyiodide chain. In contrast, iodine in the sinusoidal channels of SBMOF-3 (Cd-sdb) retains its molecular (I₂) character up to at least 8.4 GPa. Such divergent high-pressure behavior of iodine in the MOFs with similar port size and chemistry illustrates adaptations of the electronic structure of iodine to channel topology and strength of the iodine···framework interaction, which can be used to tailor iodine-immobilizing MOFs.« less

The iron spin transition directly affects properties of lower mantle minerals and can thus alter geophysical and geochemical characteristics of the deep Earth. While the spin transition in ferropericlase has been documented at P ~60 GPa and 300 K, experimental evidence for spin transitions in other rock–forming minerals, such as bridgmanite and post–perovskite, remains controversial. Multiple valence, spin, and coordination states of iron in bridgmanite and post–perovskite are difficult to resolve with conventional spin probing techniques. Optical spectroscopy, on the other hand, can discriminate between high and low spin and between ferrous and ferric iron at different sites. Here wemore » establish the optical signature of low spin Fe³⁺O₆, a plausible low spin unit in bridgmanite and post–perovskite, by optical absorption experiments in diamond anvil cells. We show that the optical absorption of Fe³⁺O₆ in new aluminous phase (NAL) is very sensitive to the iron spin state and may represent a model behavior of bridgmanite and post–perovskite across the spin transition. Specifically, an absorption band centered at ~19,000 cm^–1 is characteristic of the ²T_2g → ²T_1g (²A_2g) transition in low spin Fe³⁺ in NAL at 40 GPa, constraining the crystal field splitting energy of low spin Fe³⁺ to ~22,200 cm^–1, which we independently confirm by first–principles calculations. Together with available information on the electronic structure of Fe³⁺O₆ compounds, we show that the spin–pairing energy of Fe³⁺ in an octahedral field is ~20,000–23,000 cm^–1. Furthermore, this implies that octahedrally coordinated Fe³⁺ in bridgmanite is low spin at P > ~40 GPa.« less