Higher Transition Pressure Research Articles

High‐Pressure Molecular Sieving of High‐Humidity C2H4/C2H6 Mixture by a Hydrophobic Flexible Metal–Organic Framework

AbstractMolecular sieving is an ideal separation mechanism, but controlling pore size, restricting framework flexibility, and avoiding strong adsorption are all very challenging. Here, we report a flexible adsorbent showing molecular sieving at ambient temperature and high pressure, even under high humidity. While typical guest‐induced transformations are observed, a high transition pressure of 16.6 atm is observed for C2H4 at 298 K because of very weak C2H4 adsorption (~16 kJ mol−1). Also, C2H6 is completely excluded below the pore‐opening pressure of 7.7 atm, giving single‐component selectivity of ca. 300. Quantitative high‐pressure column breakthrough experiments using 1 : 1 C2H4/C2H6 mixtures at 10 atm as input confirm molecular sieving with C2H4 adsorption of 0.73 mmol g−1 or 32 cm3(STP) cm−3 and negligible C2H6 adsorption of 0.001(2) mmol g−1, and the adsorbent can be completely regenerated by inert gas purging. Furthermore, it is highly hydrophobic with negligible water adsorption, and the C2H4/C2H6 separation performance is unaffected at high humidity.

High-Pressure Molecular Sieving of High-Humidity C2H4/C2H6 Mixture by a Hydrophobic Flexible Metal-Organic Framework.

Molecular sieving is an ideal separation mechanism, but controlling pore size, restricting framework flexibility, and avoiding strong adsorption are all very challenging. Here, we report a flexible adsorbent showing molecular sieving at ambient temperature and high pressure, even under high humidity. While typical guest-induced transformations are observed, a high transition pressure of 16.6 atm is observed for C2H4 at 298 K because of very weak C2H4 adsorption (~16 kJ mol-1). Also, C2H6 is completely excluded below the pore-opening pressure of 7.7 atm, giving single-component selectivity of ca. 300. Quantitative high-pressure column breakthrough experiments using 1 : 1 C2H4/C2H6 mixtures at 10 atm as input confirm molecular sieving with C2H4 adsorption of 0.73 mmol g-1 or 32 cm3(STP) cm-3 and negligible C2H6 adsorption of 0.001(2) mmol g-1, and the adsorbent can be completely regenerated by inert gas purging. Furthermore, it is highly hydrophobic with negligible water adsorption, and the C2H4/C2H6 separation performance is unaffected at high humidity.

Effects of the intramolecular structural change on the superconducting phase of the TMTTF systems

The tetramethyltetrathiafulvalene (TMTTF) system is one of the key materials to reveal mechanisms for unconventional superconductivity which is driven by electronic correlation. Recent experimental studies show that only the SbF6 salt, which exhibit the strongest charge order among the PF6 and AsF6 and SbF6 salts have anomalously high superconducting phase transition pressure for the insulating-superconducting phase and superconducting-metallic phase transitions. To investigate the relationship between the superconductivity and the strength of the charge order of these salts, we constructed an extended Hubbard model with the periodic boundary condition that takes into account the site-deformation potential driven by the intramolecular structural change which causes the nonuniform charge order. The superconductivity was examined by the Drude weight and the charge gap obtained by exact diagonalization. It was found that site-deformation potentials driven by the intramolecular conformational change, even small changes, move the insulator-superconductor transition point to higher pressure, i.e. too large transfer integrals. Without the site-deformation potentials, the transfer integral at the transition point is too small as compared with those values deduced from the first-principles calculations at the structures under atmospheric pressure and it is difficult to reproduce the transition.

Superconductivity in CeBeH8 and CeBH8 at moderate pressures

High-pressure structural searches of superhydrides CeBeH8 and CeBH8 were performed under ambient pressure up to 300 GPa. We identify -CeBeH8 with a superconducting transition temperature T c of 56 K at 10 GPa. Two more phases with spacegroup and , were investigated within the increasing pressures. CeBH8 shows a similar phase transition process as CeBeH8 but with higher transition pressures and higher T c . -CeBH8 is predicted to be superconducting above 120 GPa with a maximum T c of 118 K at 150 GPa. -CeBH8 and -CeBH8 are dynamically stable above 120 GPa and 100 GPa, respectively. The maximum T c is 123 K at 195 GPa for -CeBH8, and 115 K at 350 GPa for -CeBH8. Our work enriches the family of ternary hydrides and may provide a useful guideline for further search for superconducting hydrides at low and moderate pressures.

Unified and ultimate high-pressure phase of several nanolaminate Mn+1AXn (n = 1, 2, 3, etc.) ceramics from first principles

We did extensive research for the nanolaminate Mn+1AXn (n = 1, 2, 3, etc.) ceramics on their structural stability, in which the “M”, “A” and “X” dependence of the phase transition pressure is concluded. Mo2GaC presents multi-phase co-existence status at high pressure. The hexagonal P63/mmc to tetragonal P4/mmm transition is the common direct route for the N-containing MAX due probably to the high transition pressure. P63/mmc (No.194, α, β) α → β → P4/mmm transition might be the common route for the C-containing MAX due probably to the low α → β transition pressure. Nb2InN/Nb2GaN and Mo2InN present c-axis abnormal elongation at low pressures.

Unraveling the Atomic Structure of Bulk Binary Ga-Te Glasses with Surprising Nanotectonic Features for Phase-Change Memory Applications.

Binary Ge-Te and ternary Ge-Sb-Te systems belong to flagship phase-change materials (PCMs) and are used in nonvolatile memory applications and neuromorphic computing. The working temperatures of these PCMs are limited by low-T glass transition and crystallization phenomena. Promising high-T PCMs may include gallium tellurides; however, the atomic structure and transformation processes for amorphous Ga-Te binaries are simply missing. Using high-energy X-ray diffraction and Raman spectroscopy supported by first-principles simulations, we elucidate the short- and intermediate-range order in bulk glassy GaxTe1-x, 0.17 ≤ x ≤ 0.25, following their thermal, electric, and optical properties, revealing a semiconductor-metal transition above melting. We also show that a phase change in binary Ga-Te is characterized by a very unusual nanotectonic compression with the high internal transition pressure reaching 4-8 GPa, which appears to be beneficial for PCM applications increasing optical and electrical contrast between the SET and RESET states and decreasing power consumption.

Effects of Back Pressure on Flow Regime and Suction Performance of Gas–Liquid Swirl Ejector

Abstract The back pressure of a gas–liquid swirl ejector is a critical parameter that affects the flow regime and entrainment performance of the system. In the case of a weak swirl injection, the motive jet carries relatively higher axial momentum thus remains in a single phase and the liquids appear glassy. With increasing in the back pressure of the bubble flow, the suction rate drops rapidly. For a strong swirl injection, the liquid jet is disintegrated due to higher angular momentum and the spray is atomized. Augmentation in the back pressure also causes the reduction in the suction rate but it tends to grow more gradual than the weak swirl injection. As a result, the suction rate of the strong swirl is greater than that of the weak swirl in the majority of the back-pressure range. However, owing to the high transition pressure, only the weak swirl entrains the air in the bubble flow regime at low back pressure. The relationship between the suction and the swirl intensity is not fixed and is influenced by the back pressure.

Hydrogen-Doped Polymeric Carbon Monoxide at High Pressure

The ability to control materials stability, bonding, and transformation by thermo-mechanical and chemical means is significant for development of high-energy-density extended solids. We report that doping hydrogen (∼10%) in carbon monoxide (CO) can greatly lower the polymerization pressure of CO and enhance the stability of recovered polymeric CO products at ambient conditions. Hydrogen-doped CO crystallizes into well-grown dendrites of β-CO-like phase at 3.2 GPa, which polymerizes to highly unsaturated black polymer (phase I) at ∼4.7 (5.8) GPa. Upon further compression, this highly colored polymer transforms into a translucent 3D network structure (phase II) at 6–7 (10–17) GPa and then a transparent 2D layer structure (phase III) at 20–30 (30–60) GPa. A similar series of transformations are also found in pure CO but at considerably higher transition pressures, as noted in parentheses. All polymeric phases are recoverable at ambient conditions, exhibiting an array of phase stability and novel properties such as chemically unstable phase I, highly luminescent phase II, and highly transparent layered phase III. The density of recovered products ranges from ∼2.3 g/cm3 to 3.6 g/cm3, depending on the pressure recovered. The recovered products are highly disordered but slowly decompose to crystalline solids of anhydrous polymeric oxalic acid while exhibiting interesting crystal morphologies such as nm-cobs, nm-lamellar layers, and μm-bales. The present first-principles MD simulations suggest that the polymerization occurs at 6 (or 10) GPa in H2-doped (or pure) CO. While not directly participating in the reaction, the role of H2 molecules is to enhance the mobility of CO molecules leading to the polymerization.

Conventional empirical law reverses in the phase transitions of 122-type iron-based superconductors.

Phase transition of solid-state materials is a fundamental research topic in condensed matter physics, materials science and geophysics. It has been well accepted and widely proven that isostructural compounds containing different cations undergo same pressure-induced phase transitions but at progressively lower pressures as the cation radii increases. However, we discovered that this conventional law reverses in the structural transitions in 122-type iron-based superconductors. In this report, a combined low temperature and high pressure X-ray diffraction (XRD) measurement has identified the phase transition curves among the tetragonal (T), orthorhombic (O) and the collapsed-tetragonal (cT) phases in the structural phase diagram of the iron-based superconductor AFe2As2 (A = Ca, Sr, Eu, and Ba). The cation radii dependence of the phase transition pressure (T → cT) shows an opposite trend in which the compounds with larger ambient radii cations have a higher transition pressure.

Pressure-induced transitions in solid nitrogen: Role of dispersive interactions

We have investigated the transition from molecular to fully covalent phases of solid nitrogen with an advanced ab initio quantum-mechanical method for crystals that rigorously describes weak dispersive interactions. It is demonstrated that the thermodynamic domain of stability of the cubic gauche phase (a promising high-energy- density material) extends down to pressures of about 60 GPa and that the high experimental transition pressure of 110 GPa is largely due to kinetic barriers which prevent the dissociation of triply bonded ${\mathrm{N}}_{2}$ molecules. The role of temperature is complex: it helps the kinetics of the transition to fully covalent phases while it stabilizes the molecular phases through the entropic term.

Ambient- and low-temperature synchrotron x-ray diffraction study of BaFe2As2and CaFe2As2at high pressures up to 56 GPa

We report on high-pressure powder synchrotron x-ray diffraction studies on $M$Fe${}_{2}$As${}_{2}$ ($M$$=$Ba, Ca) over a range of temperatures and pressures up to about 56 GPa using a membrane diamond-anvil cell. Our data indicate a phase transition to a collapsed tetragonal phase in both compounds upon compression. The data at 300 K are measured in both pressure-increasing and -decreasing cycles. Our measurements show that at 300 K in the Ba compound, the transition occurs at 27 GPa, which is much higher than the transition pressure of 1.7 GPa in the Ca compound. At low temperature, we could obtain data only in the pressure-increasing cycle, therefore a precise transition pressure is not identified. At a low temperature of 33 K, the transition to the tetragonal phase in the Ba compound starts, upon compression, at about 29 GPa, which is much higher than the transition pressure of 0.3 GPa at 40 K as known in the case of the Ca compound. The much higher transition pressure in the Ba compound may be due to its larger unit-cell volume at ambient pressure. It is important to note that the transition in both compounds occurs when they are compressed to almost the same value of the unit-cell volume and attain similar ${c}_{t}/{a}_{t}$ ratios. We also show that the FeAs${}_{4}$ tetrahedra are much less compressible and more distorted in the collapsed tetragonal phase than their nearly regular shape in the ambient pressure phase. We present a detailed analysis of the pressure dependence of the structures as well as equations of state in these important BaFe${}_{2}$As${}_{2}$ and CaFe${}_{2}$As${}_{2}$ compounds.

A comparative study for structural and electronic properties of single-crystal ScN

A comparative study by FP-LAPW calculations based on DFT within LDA, PBE-GGA, EV$_{ex}$-PW$_{co}$-GGA, and EV$_{ex}$-GGA-LDA$_{co}$ schemes is introduced for the structural and electronic properties of ScN in RS, ZB, WZ, and CsCl phases. According to all approximations used in this work, the RS phase is the stable ground state structure and makes a transition to CsCl phase at high transition pressure. While PBE-GGA and EV$_{ex}$-PW$_{co}$-GGA's have provided better structural features such as equilibrium lattice constant and bulk modulus, only EV$_{ex}$-PW$_{co}$-GGA and EV$_{ex}$-GGA-LDA$_{co}$'s have given the non zero, positive indirect energy gap for RS-ScN, comparable with the experimental ones. The indirect band gap of ScN in RS phase is enlarged to the corresponding measured value by EV$_{ex}$-PW$_{co}$-GGA+U$^{SIC}$ calculations in which the Coulomb self and exchange-correlation interactions of the localized d-orbitals of Sc have been corrected by the potential parameter of U. The EV$_{ex}$-PW$_{co}$-GGA calculations have also provided good results for the structural and electronic features of ScN in ZB, WZ, and CsCl phases comparable with the theoretical data available in the literature. EV$_{ex}$-PW$_{co}$-GGA and EV$_{ex}$-PW$_{co}$-GGA+U$^{SIC}$ schemes are considered to be the best ones among the others when the structural and electronic features of ScN are aimed to be calculated by the same exchange-correlation energy approximations.

High-pressure resistance study of NpSb up to 23 GPa.

The resistance versus pressure of NpSb single crystals has been measured up to 23 GPa and as a function of temperature from 1.3 to 300 K. Up to 2.3 GPa the N\'eel temperature, ${\mathit{T}}_{\mathit{N}}$, is found to decrease at a rate of about -11.5 K/GPa. We suggest that the direct effect of pressure on the spin may play a significant part in this variation. Between 2.3 and 2.7 GPa a dramatic change in the resistance versus temperature behavior below ${\mathit{T}}_{\mathit{N}}$ indicates some electronic or magnetic transformation in NpSb. At higher pressure the NaCl to tetragonal structural phase transition is observed by a decrease in the room temperature resistance and a maximum in the low-temperature resistance. The transition pressure is determined to be between 6 and 8 GPa and the transition shows considerable hysteresis on releasing the pressure. This is compared to previous data from high-pressure x-ray diffraction experiments which show a higher transition pressure. The high-pressure phase seems likely to be nonmagnetic and we show indirect evidence for delocalization of the 5f electrons at the phase transition.

3-sample, 6-lead Bridgman anvil assembly for resistivity measurements to 100 Kbar

An analysis of the mechanical behavior of gaskets in a Bridgman anvil under high pressure clearly reveals an inherent source of nonreproducibility in the stress system from run to run. This problem has been shown to be overcome by designing a 3-sample, 6-lead cell in which a calibrant is also mounted along with the sample. The cell is described, and results on pure antimony clearly establish that the structural transition in antimony (at room temperature) occurs, at exactly the high bismuth transition pressure.

Polymorphism of solids under pressure

The fundamental thermodynamic concepts involved in high pressure polymorphism of solids with reference to both the macroscopic and micro scopic aspects have been reviewed. A thermodynamic analysis involving a pressure invariable characteristic compressibility, has led to an expression for the high pressure transition pressure of solids entirely in terms of well known thermodynamic quantities. A further study of the concept of entropy, interpreted as the ‘degree of randomness’ of a system, indicates that a specific entropy change has to occur before transition from one stable phase to another can take place. Different structure calls for different values of entropy change. Structures of CN=6 involve a higher critical entropy change than those of CN=4. By correlating the radius ratio with the critical entropy change, the transition pressures of inorganic compounds which have the coordination number of four and six are predictable.