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 H-8 cubic clusters. Our high-pressure experiments at 1 to 103 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-Tc 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).

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 pressure (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 (I-2 ) 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.

Half-Heusler compounds have recently been identified as promising thermoelectric materials, but their relatively high thermal conductivities impede the further improvement of thermoelectric performance. The knowledge of phonon vibrational properties provides a fundamental understanding of the thermal transport behavior of solids and thus could serve as a guidance on further suppressing the thermal conductivity. Herein, a highly efficient p-type half-Heusler thermoelectric alloy FeNb0.8Ti0.2Sb is taken as an example to explore its phonon vibrational properties. Phonon spectrum with the frequencies down to 10 cm(-1) and its evolution with pressure for the studied material are provided by Raman scattering. It is found that two vibrational modes with the frequency > 200 cm(-1) display a common mode frequency increase with increasing pressure. Based on the bulk modulus from synchrotron X-ray powder diffraction and phonon frequency shifts, the mode Gruneisen parameters are obtained. Our results establish characteristic phonon vibrational properties of this high-performance half-Heusler thermoelectric alloy. Published by AIP Publishing.

Layered molybdenum dichalchogenides differ from the classic example of bilayer graphene with their unique electronic properties: the application of pressure can continuously tune electronic structure since the band gap is controlled by delicate interlayer interaction. Here, we have performed measurements of Raman scattering, synchrotron x-ray diffraction, electrical conductivity, and Hall coefficient combined with density functional theory calculations to synthetically study the pressure effect on 2H-MoTe2. Both the experiments and calculations consistently demonstrate that MoTe2 undergoes a semiconductor-to-metallic (S-M) transition above 10 GPa. Unlike MoS2, the S-M transition is driven by the gradual tunability of electric structure and band gap without structural transition. The applied pressure also effectively enhances conductivity and carrier concentration while reducing the mobility, which makes MoTe2 more suitable for applications than most other transition-metal dichalchogenides and allows it to be applied in strain-modulated optoelectronic devices.

Diatomic nitrogen is an archetypal molecular system known for its exceptional stability and complex behavior at high pressures and temperatures, including rich solid polymorphism, formation of energetic states, and an insulator-to-metal transformation coupled to a change in chemical bonding. However, the thermobaric conditions of the fluid molecular-polymer phase boundary and associated metallization have not been experimentally established. Here, by applying dynamic laser heating of compressed nitrogen and using fast optical spectroscopy to study electronic properties, we observe a transformation from insulating (molecular) to conducting dense fluid nitrogen at temperatures that decrease with pressure and establish that metallization, and presumably fluid polymerization, occurs above 125 GPa at 2500 K. Our observations create a better understanding of the interplay between molecular dissociation, melting, and metallization revealing features that are common in simple molecular systems.

In their comment, Desjarlais et al. claim that a small temperature drop occurs after isentropic compression of fluid deuterium through the first-order insulator-metal transition. We show that their calculations do not correspond to the experimental thermodynamic path, and that thermodynamic integrations with parameters from first-principles calculations produce results in agreement with our original estimate of the temperature drop.

We describe a new integrated optical spectroscopy facility for high-pressure research in materials research and mineral science located at the beamline BL01B of the Shanghai Synchrotron Radiation Facility. The system combines infrared synchrotron Fourier-Transform spectroscopy with broadband laser visible/near infrared and conventional laser Raman spectroscopy in one instrument. The system utilizes a custom-built microscope optics designed for a variety of diamond anvil cell experiments, which include low-temperature and ultrahigh pressure studies. We demonstrate the capabilities of the facility for studies of a variety of high-pressure phenomena such as phase and electronic transitions and chemical transformations.

The phase diagrams of Na2CO3 and K2CO3 have been determined with multianvil (MA) and diamond anvil cell (DAC) techniques. In MA experiments with heating, gamma-Na2CO3 is stable up to 12 GPa and above this pressure transforms to P6(3)/mcm-phase. At 26 GPa, Na2CO3-P6(3)/mcm transforms to the new phase with a diffraction pattern similar to that of the theoretically predicted Na2CO3-P21/m. On cold compression in DAC experiments, gamma-Na2CO3 is stable up to the maximum pressure reached of 25 GPa. K2CO3 shows a more complex sequence of phase transitions. Unlike gamma-Na2CO3, gamma-K2CO3 has a narrow stability field. At 3 GPa, K2CO3 presents in the form of the new phase, called K2CO3-III, which transforms into another new phase, K2CO3-IV, above 9 GPa. In the pressure range of 9-15 GPa, another new phase or the mixture of phases III and IV is observed. The diffraction pattern of K2CO3-IV has similarities with that of the theoretically predicted K2CO3-P2(1)/m and most of the diffraction peaks can be indexed with this structure. Water has a dramatic effect on the phase transitions of K2CO3. Reconstruction of the diffraction pattern of gamma-K2CO3 is observed at pressures of 0.5-3.1 GPa if the DAC is loaded on the air.

Alternative technologies are required in order to meet a worldwide demand for clean non-polluting energy sources. Thermoelectric generators, which generate electricity from heat in a compact and reliable manner, are potential devices for waste heat recovery. However, thermoelectric performance, as encapsulated by the figure of merit ZT, has remained at around 1.0 at room temperature, which has limited practical applications. Here, we study the effects of pressure on ZT in Cr-doped PbSe, which has a maximum ZT of less than 1.0 at a temperature of about 700 K. By applying external pressure using a diamond anvil cell, we obtained a room-temperature ZT value of about 1.7. From thermoelectric, magnetoresistance and Raman measurements, as well as density functional theory calculations, a pressure-driven topological phase transition is found to enable this enhancement. Experiments also support the appearance of a topological crystalline insulator after the transition. These findings point to the possibility of using compression to increase not just ZT in existing thermoelectric materials, but also the possibility of realizing topological crystalline insulators.

We performed Raman and infrared (IR) spectroscopy measurements of hydrogen at 295 K up to 280 GPa at an IR synchrotron facility of the Shanghai Synchrotron Radiation Facility (SSRF). To reach the highest pressure, hydrogen was loaded into toroidal diamond anvils with 30-mu m central culet. The intermolecular coupling has been determined by concomitant measurements of the IR and Raman vibron modes. In phase IV, we find that the intermolecular coupling is much stronger in the graphenelike layer (G layer) of elongated molecules compared to the Br2-like layer (B layer) of shortened molecules and it increases with pressure much faster in the G layer compared to the B layer. These heterogeneous lattice dynamical properties are unique features of highly fluxional hydrogen phase IV.