Phase Transition Pressure Research Articles

Adsorption of Methane–Nitrogen mixtures with flexible Zeolitic Imidazolate Framework-7 (ZIF-7)

ZIF–7 is a pressure–regulated flexible metal–organic framework with a notably lower admission pressure for CH4 relative to N2, making it highly-prospective for curbing fugitive emissions of methane. Here, the separation of CH4 from N2 by an improved variant of ZIF–7 was studied over a range of pressures, temperatures, and compositions using multiple complementary techniques. Pure gas adsorption isotherms measured from (283 to 323) K at up to 16 MPa reveal this variant of ZIF-7 exhibits 50–60 % higher adsorption capacities for both CH4 and N2 in the large–pore phase than previous ZIF–7 samples. Binary mixture adsorption isotherms from dynamic column breakthrough experiments were consistent with those predicted from pure–gas isotherms using Ideal Adsorbed Solution Theory. Methane uptake increased significantly when the pore phase transition pressure for the mixture was reached, while the uptake of nitrogen remained small. Consequently, the equilibrium CH4-over–N2 selectivity of ZIF-7 is always larger than 5.5 and can increase to around 9, significantly above that of conventional adsorbents. Further characterisation showed the rate of the adsorption on ZIF–7 initially decreases as the pore phase transition begins before it increases significantly. Raman spectra acquired at various pressures along the adsorption isotherms revealed changes in certain vibrational modes associated with the structural change. Collectively, these observations indicate that the conceptually simple gating mechanism often used to describe the pore phase transitions in ZIF-7 in fact depends on interactions between the guest molecules and the adsorbent framework that continue to operate once the material is in the large–pore state.

Structural Phase Transitions of Mg2NiH4 Induced by NiH4 Molecular Ion Distortion and Metal Lattice Deformation under High Pressures.

Experimental observations indicate that pressure treatments transform the monoclinic and orthorhombic low-temperature phases of Mg2NiH4 into an unresolved pseudocubic phase even at room temperatures, resembling the temperature-induced phase transformation of the material. This pressure-induced phase transformation is anticipated to involve deformations of the tetrahedrally bonded NiH4 molecular ion and the metal lattice. However, the precise mechanism underlying this transformation remains unclear. To elucidate this behavior, this study adopted first-principles density functional theory calculations to investigate the effect of applied pressure on the observed phase transition. The results revealed that the phase transition from the low-temperature phases to the pseudocubic phase occurs at approximately 8-9 GPa. Furthermore, at this pressure, the metal lattice deforms into an antifluorite-like structure, and the NiH4 molecular ion undergoes substantial distortion, resulting in the alignment of hydrogen atoms along the crystal axes of the metal lattice. Furthermore, through a comprehensive structural exploration, we successfully identified several tetragonal and orthorhombic models that are energetically more stable than the low-temperature phases at low pressures. These models are potential candidates for elucidating the pseudocubic phase observed in experiments. Overall, our findings are anticipated to enhance the current understanding of the structural changes occurring during pressure and temperature-induced phase transitions.

Investigation on Melting Curves and Phase Diagrams for CaO3 Using Deep Learning Potentials.

Melting in the deep rocky parts of planets plays an important role in geological processes such as planet formation, seismicity, magnetic field generation, thermal convection, and crustal evolution. Such processes are the key way to understanding the dynamics of planetary interiors and the history as well as mechanisms of planetary evolution. We herein investigate the melting curves and pressure-temperature (P-T)-phase diagrams for CaO3, a candidate mineral for the lower mantle, by means of the deep learning potential model. Using first-principles, molecular dynamics, and quasi-harmonic approximation, the reliability of the deep learning potential model is verified by calculating the high-temperature and high-pressure equations of state and phase transition pressures for the orthorhombic and tetragonal structures of CaO3 described by space groups Aea2 and P4̅21m, respectively. The melting temperatures of 975, 850, and 755 K at zero pressure are obtained by the single-phase, void, and two-phase methods, respectively, and their melting temperatures are analyzed by the radial distribution function and mean-square displacement to analyze the melting process. Finally, the melting phase diagrams of CaO3 at 0-135 GPa were obtained by the two-phase method.

First-principles calculations of phase transition, phonon spectra, thermodynamic properties and elasticity for PuO2 polymorphs

The first principles are utilized to calculate the phase transition, phonon spectra, thermodynamic properties, and elasticity of PuO2 under varying pressures. Under the compression condition, the anticipated pressure of phase transition from Fmm to Pnma is 26.7 GPa, from Pnma to Cmcm is 112.9 GPa, and from Cmcm to I4/mmm phase is 588.0 GPa. As the pressure decreases, the phase transition pressure of anticipating from Fmm phase to P42/mnm phase occur at -8.6 GPa. The phonon spectra of these six structures show that all phases are dynamically stable under the selected pressure. Meanwhile, the verification results of the mechanical stability criterion show that Fmm, Pnma, Cmcm and I4/mmm of PuO2 are mechanically stable, nevertheless P42/mnm is unstable in high pressure. The thermodynamic properties were then calculated to further research the patterns of variation in the coefficient of thermal expansion, bulk modulus, Gibbs free energy, and heat capacity with temperature, and these were compared with experimental data.

Effect of temperature on the symmetrization of AlOOH hydrogen bonds

Hydrogen bonding symmetrization of water-bearing minerals plays a key role in the water cycle of the deep Earth. The lower mantle is a high-temperature and high-pressure environment, and in this study a classical molecular dynamics simulation of the δ-AlOOH hydrogen bond symmetrization process is carried out using a deep learning potential model. Calculation the length of the HO bond reveals that AlOOH undergoes hydrogen bond symmetrization when the pressure reaches 9 GPa. The volume thermal expansion profile of AlOOH shows an anomalous phenomenon of negative correlation between phase transition pressure and temperature. The radial distribution functions gOH(r) and gHH(r) show the asymmetry of hydrogen bonding at 8 GPa and indicate the critical conditions for hydrogen bonding symmetry at 10 GPa. Calculations of the O1O2 bond lengths show that the critical pressure required for hydrogen bond symmetry increases with temperature. Our simulation results provide some insight into the hydrogen bonding symmetry of water-bearing minerals in the deep Earth’s.

Diverse kinetic pathways in shock-compressed phase transitions of a metallic single crystal

Substantial gaps in solid-solid phase boundaries under hydrostatic and uniaxial compression have recently garnered great attention, though the underlying physics remains unclear. In this study, through molecular dynamics simulations of shock-compressed fcc Cu single crystals, we report pronounced orientation-dependent fcc-to-bcc phase transition pressures following the trend [100] < [110] < [111] ≈ thermodynamic phase boundary. We uncover a fundamental crystallographic law that explains these phase boundary gaps, rooted in the classical orientational relationship of martensitic transformations: the degree of alignment between loading directions and the easiest atomic moving path plays a critical role in determining phase transition pathways. The complex, orientation-dependent phase transition pathways and the observed temperature equilibrium efficiency ranking [100] > [110] > [111] further support the validity of this crystallographic law. This law is broadly applicable to fcc crystals, indicating that phase composition can be controlled by the method of compression, providing a new framework for selective polymorph formation.

Appearance of topological phase in YAs semimetal under hydrostatic pressure and epitaxial strain

By means of hybrid density functional theory, we present the evolution of the topological phase in rare earth monopnictide YAs with hydrostatic pressure and epitaxial strain. This material exists in NaCl-type structure at ambient conditions and shows structural phase transition into CsCl-type structure at 56.54 GPa hydrostatic pressure. The epitaxial strain reduces the structure into a compressed tetragonal-type. The thermodynamical and dynamical stability of the material is established with the calculation of enthalpy and phonon band structure within structural phase transition, respectively. The topologically trivial phase of the material is observed at ambient pressure in agreement with previous reports. This material shows topological phase transition at 24.8 GPa applied hydrostatic pressure and 10% epitaxial strain. The band inversion at the X-point between d-Y and p-As orbitals is verified with the help of the product of parity analysis of all the filled bands. The presence of the Dirac cone in the (001) plane and the existence of topologically non-trivial states at M‾-point in the Fermi arc contour also established our claim. The Z2 indices are calculated with the help of the product of parities and a change in Z2 indices from (0; 000) to (1; 000) is also verified with the evolution of Wannier change centers under the conditions of applied hydrostatic pressure and epitaxial strain. The time reversal and inversion symmetries are preserved throughout the study and the topological phase transition at 24.8 GPa is much lower than the structural phase transition pressure i.e., 56.54 GPa.

Unexpected Observation of Disorder and Multiple Phase-Transition Pathways in Shock-Compressed Zr.

The response of materials under dynamic compression involves a complex interplay of various deformation mechanisms aimed at relieving shear stresses, yielding a remarkable diversity in material behavior. In this Letter, we utilize femtosecond x-ray diffraction coupled with nanosecond laser compression to reveal an intricate competition between multiple shear-relieving mechanisms within an elemental metal. Our observations in shocked-compressed single-crystal Zr indicate a disorder-mediated shear relaxation at lower pressures. Above the phase-transition pressure, we observe the increasing contribution of structural phase transition in relieving shear stress. We detect not one but three concurrent pathways during the transition from the hcp to a hex-3 structure. These complex dynamics are partially corroborated through multimillion-atom molecular dynamics simulations employing a machine-learned interatomic potential. Our observation of multiple concurrent pathways and disorder during shock compression underscore the far greater intricacies in the dynamic response of metals than previously assumed.

Influence of gold-selenium precursor ratio on synthesis and structural stability of α- and β- AuSe

Gold selenide (AuSe) is a multilayer compound that has yet to be thoroughly studied. The colloidal synthesis and characterization of gold selenide nanoparticles are described, emphasizing the effect of different gold-to-selenium precursor ratios and temperatures on the crystal structure and form. The structural characterization is done using an X-ray diffraction pattern. The coexistence of the α- and β-AuSe phases is observed in all synthesized samples. Our research indicates that the β-AuSe phase was predominantly found in the AuSe preparation process, which involved the reaction of metallic gold with elemental Se in oleylamine at a 1:1 precursor ratio of Au/Se. On the other hand, the mixed phase, which was dominated by the α-phase, was found in the selenium-rich synthesis, which had a 1:4 precursor ratio of Au/Se. The morphologies of the mainly α-AuSe sample are nanobelts, whereas the primarily β-AuSe phase sample has a nanoplate-like structure, according to the TEM and SEM data. All of the samples had Raman vibrational modes with mixed phases. The effect of high pressure on as-prepared AuSe samples has been studied in this work. The introduction of external pressure and temperature allows both phases to transition. The rising pressure reduces the presence of β-phase in the mixed phase sample, which has a pre-dominating α-phase, leaving only a substantial α-phase in the AuSe sample. The phase transition pressure was observed using Raman scattering. Our findings show that 2D AuSe has a lot of promise for multifunctional applications, encouraging more research on these systems.

High pressure phase transition and its influence on structural, mechanical properties and elastic anisotropy of [formula omitted]: An ab-initio study

Lithium carbide (Li2C2), a promising tritium breeding material with potential applications in magnetic confinement-based fusion reactor blankets, has drawn significant attention due to its recent utilization in lithium-ion batteries. In the current study, we employed first-principles calculations with van der Waals (vdW) force correction to explore the structural and mechanical properties of Immm and Pnma phases of Li2C2 subjected to external hydrostatic pressure. It is demonstrated that inclusion of vdW effects accurately predicts Immm→Pnma phase transition at 16 GPa, aligning closely with experimental transition pressure. In contrast, PBE-GGA calculations underestimate the same by about 21%. Current paper is the first comprehensive study of pressure evolution of mechanical properties and strength determining parameters of Li2C2. Interestingly, this inherently brittle material exhibits ductile characteristics beyond 7.5 GPa. The paper also offers a quantitative comparison of its strength parameters with other prominent tritium breeding materials. Finally, the strong directional dependence due to the presence of C2 dimer is investigated by analyzing the pressure variation of elastic anisotropy. The study has its relevance in assessing structural integrity of Li2C2 during inadvertent buildup of tritium and helium that would generate stress in the solid breeder.

High-pressure phase transitions of Fe-bearing orthopyroxene revealed by Raman spectroscopy

Abstract Orthopyroxene is one of the dominant minerals in the Earth’s upper mantle. In this study, we used Raman spectroscopy to investigate the lattice vibration and phase transition of orthopyroxene with four different compositions using diamond-anvil cells up to 34 GPa at 300 K. Our orthopyroxene samples contain 0 (En100), 9% (En91Fs9), 11% (En86Fs11), and 21% (En74Fs21) Fe. At ambient conditions, the Raman modes exhibit a negative dependence on the Fe content, with the exception of the modes at ~850 and 930 cm–1. In contrast, these two Raman modes increase with increasing the Fe content. The phase transition from metastable α- to β-phase was observed at 12.9–15 GPa for samples with &amp;lt;21 mol% Fe and varying Fe content has a minor effect on the phase transition pressure. Besides Fe, incorporation of 2–24 mol% Al can cause an increase in the phase transition pressure from 10–13 to 14–16 GPa. At 29–30.1 GPa, we observed the second apparent change in the Raman spectra for all four investigated samples. For Fe-bearing orthopyroxene, this change in the Raman spectra and frequency shift is associated with the phase transition from β- to γ-phase, whereas for En100, it should be caused by the change of coordination number of Si from 4 to 6 or the presence of α-popx phase. Using the obtained Raman frequency shifts, we calculated the Grüneisen parameters at high pressures. These parameters are useful for understanding the thermoelastic properties of orthopyroxene at high pressures.

Ab initio investigation of electronic structure and magnetic transformation during the bcc to hcp transition in Fe induced by pressure

The pressure-induced structural transition from a body-centered cubic (bcc) to a hexagonal close-packed (hcp) crystal in Fe is studied through ab initio electronic structure calculations, combining with the thermodynamic and kinetic phase-transition methods. According to the energy-volume relationship and thermodynamic criterion, the reasonable phase transition pressure is calculated to be about 13.88 GPa. The SSNEB method is employed to examine kinetic structural and barrier changes along bcc-hcp transition under various pressures. The transformation path involves the deformation of (110)bcc into the tightly packed (0001)hcp, accompanied by the relative slip of (110)bcc along the [1̅10]bcc direction, leading to the formation of the hcp structure. Our results indicate a reduction in the transition barrier of bcc-hcp with increasing pressure. Under the thermodynamic phase transition pressure, the bcc-hcp phase transition barrier is still as high as 140 meV, implying that pure hydrostatic pressure is not a sufficient condition to drive the phase transition. In addition, different from the continuous change of volume, the magnetism of the structure undergoes a magnetic mutation at the transition state, and the phase transition process involves complex magnetic transitions.

Mechanical and Thermal Behavior of Semiconducting Cadmium Oxide at High-Pressure

Abstract The effect of high pressures on the mechanical and thermal properties up to phase transition pressure for cubic rock-salt cadmium oxide (CdO) semiconducting compound is presented in this research. The calculations are based on the equation of state (EOS) parameters measured at ambient conditions reported in the literature. The approach used here was previously applied successfully for other materials. Our study shows that the parameters reflect the rigidity of the semiconductor and the melting temperature increases with increasing pressure, while some other parameters decrease gradually for our material of interest. A comparative analysis of some quantities for CdO compound under high pressure was done. Similar behaviors were observed in the literature for particular materials with particular structures. Furthermore, we calculated both the fracture toughness and the fracture surface energy, which are at around 1.23 MPa.m1/2 and 5.14 J.m−2, respectively.

Characterization and modeling of supercritical CO2 pulse pressures: Effects of activator mass and discharge plate thickness

Utilizing a bespoke CO2 phase transition pulse pressure experimental system, we conducted pulse pressure characterization tests across various activator masses, CO2 filling pressures, and energy discharge plate thicknesses. This approach enabled us to ascertain the pulse pressure's response characteristics and variation patterns under diverse conditions. The formula for calculating the peak supercritical CO2 pulse pressure was deduced by modeling the ultimate load calculation of the clamped circular plate, and then the time-course expression of the supercritical CO2 phase transition pulse pressure and energy was carried out by introducing the time factor and taking into account the parameters of the activator mass and the thickness of the energy discharging plate. Our findings reveal a four-stage pressure evolution in the cracking tube during initiation: a gradual increase, a rapid spike, swift attenuation, and eventual negative pressure formation. The activator mass and discharge plate thickness critically influence the peak pressure's timing and magnitude. Specifically, increased activator mass hastens peak pressure onset, while a thicker discharge plate amplifies it. The errors between calculated and experimental values for peak supercritical CO2 phase transition pressure fall within −5%–5%. Furthermore, the pressure peak and arrival time model demonstrates less than 10% error compared to experimental data, affirming its strong applicability. These insights offer theoretical guidance for controlling phase transition pressure and optimizing energy in supercritical CO2 systems.

Hydrostatic pressure response of semiconducting GaSb applying a semi-empirical approach

In addition to experimental investigations, semi-empirical approaches prove valuable for replicating experimental results and predicting material behavior under high-pressure conditions. This study investigates the variation of unit cell volume in cubic zinc blende Gallium antimonide (GaSb) up to the phase transition pressure, relying solely on structural parameters from literature measurements at ambient conditions. The research extends to inspect the impact of high pressure on various properties, including mass density, bulk modulus, bulk sound wave velocity, microhardness, first-order pressure derivative of the isothermal bulk modulus, Grüneisen parameter, and Debye temperature for the GaSb compound. The bulk modulus and microhardness show nearly linear increases with pressure, while sound wave velocity and Debye temperature exhibit nonlinear variations. The first-order pressure derivative of the bulk modulus and Grüneisen parameter gradually decreased in the material under study. Similar trends under pressure are found in literature for materials with different crystallographic structures. Additionally, the study predicts and analyzes extra physical parameters of GaSb, comparing them with existing literature data.

The effect of f-electrons on the structural phase transition, mechanical and electronic properties in light rare-earth bismuthides

ABSTRACT The electronic structure, phase transition pressure and mechanical properties of light rare-earth bismuthides (PrBi, NdBi, PmBi and SmBi) have been investigated by full geometry optimization using Vienna Ab-Initio Simulation Package (VASP) which has been used in combination with the projector augmented wave (PAW) method and the generalized gradient approximation (GGA) over the local density approximation (LDA) for exchange-correlation functional. We focus our attention on the lattice constants and elastic constants of these compounds. These compounds undergo a phase transition from the NaCl (B1) structure to the body-centered tetragonal (BCT) structure. We also report the density of states (DOS) of these rare earth bismuthides. In the case of PrBi and NdBi, the 4f states are treated as valence states. The effects of on-site Coulomb repulsion and the spin-orbit coupling (SOC) on the electronic structure are also studied.

First principles study of the electronic structure, mechanical and optical properties of cubic perovskite NaCaCl3 under pressure

Based on the first-principles methods, the electronic, mechanical and optical properties of NaCaCl3 crystal were investigated under various pressures for the first time. At 0 GPa, the lattice constant and bulk modulus were calculated to be 5.278 Å and 27.011 GPa, respectively. Analysis of Poisson's ratio and Pugh's ratio indicated that NaCaCl3 exhibits ductile characteristics, which increases with the increasing pressure. Based on the crystal mechanical stability condition, the phase transition pressure of NaCaCl3 was calculated to be 5.12 GPa. The electronic property demonstrated that NaCaCl3 exhibits an indirect and wide band gap insulator (3.84 eV) at 0 GPa, and the band gap increases with the increasing pressure. The results of optical properties show that all spectrums of NaCaCl3 exhibit a blue shift when the pressure increases. The excellent ultraviolet absorption suggests that NaCaCl3 has a potential application in vacuum ultraviolet devices. And it is also suitable as transparent coating and window materials in the infrared and visible regions due to small reflectivity and absorption.

Revealing the Substrate Constraint Effect on the Thermodynamic Behaviour of the Pd-H through Capacitive-Based Hydrogen-Sorption Measurement.

Mechanical constraints imposed on the Pd-H system can induce significant strain upon hydrogenation-induced expansion, potentially leading to changes in the thermodynamic behavior, such as the phase-transition pressure. However, the investigation of the constraint effect is often tricky due to the lack of simple experimental techniques for measuring hydrogenation-induced expansion. In this study, a capacitive-based measurement system is developed to monitor hydrogenation-induced areal expansion, which allows us to control and evaluate the magnitude of the substrate constraint. By using the measurement technique, the influence of substrate constraint intensity on the thermodynamic behavior of the Pd-H system is investigated. Through experiments with different constraint intensities, it is found that the diffefrence in the constraint intensity minimally affects the phase-transition pressure when the Pd-H system allows the release of constraint stress through plastic deformation. These experiments can improve the understanding of the substrate constraint behaviours of Pd-H systems allowing plastic deformation while demonstrating the potential of capacitive-based measurement systems to study the mechanical-thermodynamic coupling of M-H systems.

Regulation of the Ice Ⅰ to Ice III high pressure phase transition meta-stability in milk and its bactericidal effects

This study was focused on a novel approach of creating perturbations under high pressure (HP) meta-stable Ice Ⅰ to Ice Ⅲ phase transition and its bactericidal effects. Experiments were carried out under subzero high pressure processing conditions using Escherichia coli suspended in milk, and the microbial inactivation before and after the meta-stable state regulation was compared. The phase transition position of unperturbed milk was 302 MPa/–37.5 °C. The volume change resulting from the phase transition was employed as the perturbation mechanism. Glucose (5 %, 20 %) and sodium chloride solutions (5 %, 20 %) were used as regulatory sources. Glucose solutions accelerated the phase change of the milk better than the sodium chloride solution and resulted in an optimum phase transition position of milk at 243 MPa/–30.6 °C. The induced perturbations accelerated meta-stable transformation and enhanced the microbial destruction. At 330 MPa/3s, compared to the unfrozen samples, the lethality of E. coli in the frozen-regulated samples significantly increased by 1.79 log. The relationship between the E. coli inactivation within the phase change pressure range and the pressure was not continuous, but a segmented one, both before and after meta-stable state regulation. A higher level of E. coli destruction was accomplished by a 5 min pressure-holding of frozen samples at 220 MPa and 280 MPa as compared to the one-pulse and two-pulses treatments without holding time. The maximum lethality of 6.73 log was achieved at 280 MPa/5 min in the frozen-regulated application.

Phase Behavior of Biodegradable Poly(L-lactic acid) in Supercritical Solvents and Cosolvents

Poly(L-lactic acid) (PLLA), a biodegradable polymer from plant sources, is widely employed in biomedical applications for its biocompatibility and versatile mechanical properties. However, comprehending its phase behavior in various solvents poses a challenge for broader applications. In this study, the phase behavior of the polymer was investigated in various solvent and cosolvent media and combinations at a wide range of temperatures and pressures to gain insight into the polymer properties. PLLA of varying intrinsic viscosity (IV): [0.15dl/g, with an estimated weight average molecular weight (Mw) range of approximately: 1,600 to 2,400g/mol; 1.0dl/g (Mw): 100,000g/mol); and 1.8dl/g (Mw): 80,000 to 100,000g/mol] was employed for this study. The phase behavior of the polymer across different compositions of PLLA and solvent (CH2F2, CHF3, CO2, DME, F-22) in a wide range of temperatures from 333 to 425K and high pressure 1.55 to 172MPa was investigated. Miscibility of PLLA in DME and F-22 is predicted to be higher with increasing in concentration than the other cosolvents. Furthermore, in the case of the supercritical solvent CHF3 and DME the phase transition pressure curve of PLLA was observed to exhibit a lower critical solution temperature (LCST) type with a positive gradient with increasing temperature. However, lower phase transition pressure was noticed for lower molecular weight PLLA in supercritical solvent CHF3 and DME, i.e., of the order IV 0.15dl/g<IV 1.0dl/g<IV 1.8dl/g. The phase transition pressure of PLLA (IV 1.8dl/g) in solvent CO2 with increasing the concentration of co-solvent (F-22, DME) from 0 to ~97wt% shifted from UCST to LCST type. The LCST curve of PLLA (IV 0.15) in DME intersects with the vapor(V)-liquid(L) curve at T: 353K and P below 5MPa, indicating the coexistence of V+L phases.