Low-pressure Phase Research Articles

Structural phase transition in NH₄F under extreme pressure conditions.

Ammonium fluoride (NH₄F) exhibits a variety of crystalline phases depending on temperature and pressure. By employing Raman spectroscopy and synchrotron X-ray diffraction beyond megabar pressures (up to 140 GPa), we have here observed a novel dense solid phase of NH₄F, characterised by the tetragonal P4/nmm structure also observed in other ammonium halides under less extreme pressure conditions, typically a few GPa. Using detailed ab-initio calculations and reevaluating earlier theoretical models pertaining to other ammonium halides, we examine the microscopic mechanisms underlying the transition from the low-pressure cubic phase (P-43m) to the newly identified high-pressure tetragonal phase (P4/nmm). Notably, NH₄F exhibits distinctive properties compared to its counterparts, resulting in a significantly broader pressure range over which this transition unfolds, facilitating the identification of its various stages. Our analysis points to a synergistic interplay driving the transition to the P4/nmm phase, which we name phase VIII. At intermediate pressures (around 40 GPa), a displacive transition of fluorine ions initiates a tetragonal distortion of the cubic phase. Subsequently, at higher pressures (around 115 GPa), every second ammonium ion undergoes a rotational shift, adopting an anti-tetrahedral arrangement. This coupled effect orchestrates the transition process, leading to the formation of the tetragonal phase.

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The effect of continuous geomagnetic storms on enhancements of ultrarelativistic electrons in the Earth’s outer radiation belt

The effect of continuous geomagnetic storms on enhancements of ultrarelativistic electrons in the Earth’s outer radiation belt

Maximizing Efficiency in Compressed Air Energy Storage: Insights from Thermal Energy Integration and Optimization

Motivated by the suboptimal performances observed in existing compressed air energy storage (CAES) systems, this work focuses on the efficiency optimization of CAES through thermal energy storage (TES) integration. The research explores the dependence of CAES performance on power plant layout, charging time, discharging time, available power, and cavern volume. Hence, a range of solutions are examined, encompassing both solid and liquid TES options, alongside the potential utilization of external air heaters. Inefficiencies in solid TES due to significant retention of thermal power within the medium after complete discharge are identified and mitigated through optimization strategies. In addition, solutions to prevent ice formation at the low-pressure expander phase are suggested to avoid icing issues in CAES layouts with liquid TES. Through this comprehensive investigation, the study provides valuable insights into enhancing the efficiency and sustainability of CAES systems. By constructing a volume–power–time conversion table, the research contributes to the advancement of CAES technology, facilitating more efficient energy storage and utilization, thereby addressing critical challenges in the field of energy storage.

Role of GdO addition in the structural stability of cubic Gd2O3 at high pressures: Determination of the equation of states of GdO and Gd2O3

We report a high-pressure X-ray diffraction study on bixbyite-type Gd2O3 showing that the addition of 4.3 mol% of GdO increases the pressure stability of Gd2O3. In our experiments the onset of the phase transition to the previously reported high-pressure trigonal polymorph was detected upon compression at 15.1(1) GPa, with the low- and high-pressure phases coexisting up to 20 GPa. The observed transition is reversible upon decompression. Our finding extends the high-pressure stability of the low-pressure phase, which in previous studies was reported to undergo phase transitions from 2.6 to 13.4 GPa. This is important for many of the technological applications of Gd2O3 including gas turbine engines, light water reactors, and nuclear waste immobilization. Additionally, we find that GdO undergoes an irreversible amorphization beyond 15.1(1) GPa. Finally, accurate equations of state were determined from experiments and density-functional theory calculations. These calculations also provided information on elastic constants and moduli. A systematic discussion of the bulk modulus and its pressure derivative is also presented.

Irreversible pressure effect on phase transitions and bandgap narrowing of layered MoO3

Two-dimensional layered molybdenum trioxide (MoO3) holds significant importance in optoelectronics owing to its unique anisotropic structure and optical properties. Given that changes in structure can profoundly impact physical characteristics, it becomes imperative to explore new properties by means of acquiring different structures. Here, we report the irreversibility of structural evolution and optical properties of molybdenum trioxide under high pressure. The quenched MoO3 possesses a stable high-pressure phase MoO3-II, a narrowed bandgap (reduced by ∼25%) and a strong optical absorption at visible to infrared region compared to the initial low-pressure phase α-MoO3. These findings will facilitate the exploring of high-pressure phase materials under ambient conditions.

Unraveling the role of vaporization momentum in self-jumping dynamics of freezing supercooled droplets at reduced pressures

Supercooling of water complicates phase change dynamics, the understanding of which remains limited yet vital to energy-related and aerospace processes. Here, we investigate the freezing and jumping dynamics of supercooled water droplets on superhydrophobic surfaces, induced by a remarkable vaporization momentum, in a low-pressure environment. The vaporization momentum arises from the vaporization at droplet’s free surface, progressed and intensified by recalescence, subsequently inducing droplet compression and finally self-jumping. By incorporating liquid-gas-solid phase changes involving vaporization, freezing recalescence, and liquid-solid interactions, we resolve the vaporization momentum and droplet dynamics, revealing a size-scaled jumping velocity and a nucleation-governed jumping direction. A droplet-size-defined regime map is established, distinguishing the vaporization-momentum-dominated self-jumping from evaporative drying and overpressure-initiated levitation, all induced by depressurization and vaporization. Our findings illuminate the role of supercooling and low-pressure mediated phase change in shaping fluid transport dynamics, with implications for passive anti-icing, advanced cooling, and climate physics.

Effects of alternating pressure patterns on sacral skin blood flow responses in people with spinal cord injury.

Alternating pressure support surface (APSS) is a common support surface for treating pressure injury in individuals with spinal cord injury (SCI). However, conflicting results on the effectiveness of APSS have been reported and may be associated with inappropriate configurations of APSS. The objectives of this study were to compare the different pressure amplitudes (75/5 mmHg [alternating between 75 and 5 mmHg] vs. 65/15 mmHg) and cycle periods (5 min [4 cycles] vs. 2.5 min [8 cycles]) of alternating pressure on sacral skin blood flow responses in 10 individuals with SCI. Sacral skin blood flow during and after loading of four alternating pressure protocols was assessed using laser Doppler flowmetry and was normalised to the value before loading (10-min baseline, 20-min loading and 10-min recovery). The results demonstrated that during the high-pressure phase, there was a significant difference between the 75/5 and 65/15 mmHg protocols (0.3658 ± 0.0688 for 75/5 mmHg and 0.1702 ± 0.0389 for 65/15 mmHg, p < 0.05); and during the low-pressure phase, there was a significant difference between the 75/5 and 65/15 mmHg protocols (1.7184 ± 0.262 for 75/5 mmHg and 0.5916 ± 0.1378 for 65/15 mmHg, p < 0.05). There were no differences between cycle periods in skin blood flow responses. No adverse events were reported. Our finding indicates that the pressure amplitude of alternating pressure is a significant factor affecting sacral skin blood flow responses. An appropriate configuration of alternating pressure is needed to effectively increase skin blood flow and tissue viability in individuals with SCI.

Structural instability in LaNbO4 under compression

In this work, we report a high-pressure study on fergusonite-type LaNbO4. Powder x-ray diffraction and Raman spectroscopic experiments support the occurrence of a phase transition between 11 and 14 GPa. The transition takes place from a monoclinic fergusonite-type structure (space group I2/a) to another monoclinic structure (space group P21/c). The phase transition is reversible, and the high-pressure phase is isomorphic to the high-pressure phase of HoNbO4. The high-pressure phase remains stable up to 33.3 GPa, the highest pressure reached in the present measurements. Density-functional theory calculations found that in the pressure range of the studies; the high-pressure phase has a higher enthalpy than the low-pressure fergusonite phase. We propose that the high-pressure phase is metastable and it is observed because of non-hydrostatic conditions in the experiments. The pressure dependence of unit-cell parameters of the low-pressure phase and the room-temperature equation of state are reported. The pressure dependence of various Raman and IR frequencies as obtained from experiment and theory is also reported. For the fergusonite phase, we have also obtained the isothermal compressibility tensor, elastics constants, and elastic moduli.

High-pressure polymorph of Co3P2O8: phase transition to an olivine-related structure.

The monoclinic polymorph of Co3P2O8 (space group P21/c), isomorphic to farringtonite (Mg3P2O8) type orthophosphates, was studied up to 21 GPa using synchrotron powder X-ray diffraction and density-functional theory simulations to investigate the influence of pressure in the crystal structure. This study revealed a pressure induced structural phase transition for monoclinic cobalt phosphate, Co3P2O8, and the details of crystal structure of the new high-pressure polymorph were delineated. The evolution of XRD pattern with pressure indicate that the onset of a phase transition occurs around 2.90(5) GPa, and the low- and high- pressure phases coexist up to 10.3(1) GPa. The high-pressure phase also has a monoclinic lattice (space group P21/c), and a discontinuous change of unit-cell volume of about 6.5% occurs at the transition. A reorganization of atomic positions with a change in the cobalt coordination sphere occurs in the phase transition. Notably, the high-pressure polymorph has a defect-olivine-type structure like chopinite-type orthophosphates. Using a third-order Birch-Murnaghan equation of state, the bulk moduli of the low pressure (LP) phase (75(2) GPa) and high-pressure (HP) phase (92(2) GPa) were determined. For the low-pressure polymorph, the principal axes of compression and their compressibility were also determined. Density-functional theory calculations also provided the frequencies of Raman- and infrared-active modes which can be used for mode assignment in future experiments.

Assessing structure of Mg3Bi2−xSbx (0 ≤ x ≤ 2) at pressures below 40 GPa

Mg3Bi2–xSbx (0 ≤ x ≤ 2) have gained significant attention due to their potential in thermoelectric (TE) applications. However, there has been much debating regarding their structural properties and phase diagram as a function of pressure, which is crucial for understanding of their TE properties. Here, we investigate a unified phase diagram of Mg3(Bi,Sb)2 materials up to 40 GPa at room temperature using high-pressure X-ray diffraction. Two high-pressure phases with the structural transition succession of P3¯m1 → C2/m → P21/n are observed, which is valid for all Mg3Bi2–xSbx (0 ≤ x ≤ 2) compounds. We further explore the low-pressure phase P3¯m1 and report that alloying does not change the quasi-isotropic compression of the unit lattice parameters nor has effect on the anisotropic bond compressibility, as recently reported for the end-members. Our study presents a comprehensive picture of Mg3Bi2–xSbx as a function of pressure and chemical composition providing a solid foundation for the future experimental and theoretical studies searching for the most efficient TE compound in Mg3(Bi,Sb)2.

Investigation of the Low-Pressure Phase Behavior and SAFT Modeling of 1-Alcohol and n-Alkane Binary Systems

Investigation of the Low-Pressure Phase Behavior and SAFT Modeling of 1-Alcohol and <i>n</i>-Alkane Binary Systems

High-pressure effect on the superconductivity of NaAlGe and pressure-induced structural phase transition

The structural, electronic, phonon, and electron–phonon properties of NaAlGe have been investigated under high pressure by means of density-functional theory simulations. The calculated electronic structure and density of states reveal that the electronic states near the Fermi level, which are responsible for electrical conductivity, are mostly composed of Ge 4p states. The largest contribution to the average electron–phonon coupling parameter is from Ge-related vibrations, and this parameter is calculated to be λ∼ 0.609. The superconducting critical temperature at ambient pressure is calculated to be Tc = 2.31 K. This temperature agrees with the previously experimentally obtained value of 1.80 K. The electronic density of states at the Fermi energy, λ, and Tc decrease as pressure is increased, and superconductivity is suppressed at 12 GPa. We also report the phonon dispersion at different pressures showing that NaAlGe develops a dynamical instability at 13 GPa favoring a structural phase transition Based on enthalpy calculations the crystal structure of the high-pressure phase has been proposed to be orthorhombic and described by space group Pnma. This phase does not exhibit superconductivity. The pressure dependence of unit-cell parameters and Raman- and infrared-active phonons of the low-pressure and high-pressure phase is also reported.

Pressure-Induced Structural Phase Transition of Co-Doped SnO2 Nanocrystals

Co-doped SnO2 nanocrystals (with a particle size of 10 nm) with a tetragonal rutile-type (space group P42/mnm) structure have been investigated for their use in in situ high-pressure synchrotron angle dispersive powder X-ray diffraction up to 20.9 GPa and at an ambient temperature. An analysis of experimental results based on Rietveld refinements suggests that rutile-type Co-doped SnO2 undergoes a structural phase transition at 14.2 GPa to an orthorhombic CaCl2-type phase (space group Pnnm), with no phase coexistence during the phase transition. No further phase transition is observed until 20.9 GPa, which is the highest pressure covered by the experiments. The low-pressure and high-pressure phases are related via a group/subgroup relationship. However, a discontinuous change in the unit-cell volume is detected at the phase transition; thus, the phase transition can be classified as a first-order type. Upon decompression, the transition has been found to be reversible. The results are compared with previous high-pressure studies on doped and un-doped SnO2. The compressibility of different phases will be discussed.

Proof-of-Principle Experiment for Testing Strong-Field Quantum Electrodynamics with Exotic Atoms: High Precision X-Ray Spectroscopy of Muonic Neon.

To test bound-state quantum electrodynamics (BSQED) in the strong-field regime, we have performed high precision x-ray spectroscopy of the 5g-4f and 5f- 4d transitions (BSQED contribution of 2.4 and 5.2eV, respectively) of muonic neon atoms in the low-pressure gas phase without bound electrons. Muonic atoms have been recently proposed as an alternative to few-electron high-Z ions for BSQED tests by focusing on circular Rydberg states where nuclear contributions are negligibly small. We determined the 5g_{9/2}- 4f_{7/2} transition energy to be 6297.08±0.04(stat)±0.13(syst) eV using superconducting transition-edge sensor microcalorimeters (5.2-5.5eV FWHM resolution), which agrees well with the most advanced BSQED theoretical prediction of 6297.26eV.

Exchange field enhanced upper critical field of the superconductivity in compressed antiferromagnetic EuTe2

Understanding the interplay between superconductivity and magnetism has been a longstanding challenge in condensed matter physics. Here we report high pressure studies on the C-type antiferromagnetic semiconductor EuTe2 up to 36.0 GPa. A structural transition from the I4/mcm to the C2/m space group is identified at ~16 GPa. Superconductivity is observed above ~5 GPa in both structures. In the low-pressure phase, magnetoresistance measurements reveal strong couplings between the local moments of Eu2+ and the conduction electrons of Te 5p orbits. The upper critical field of superconductivity is well above the Pauli limit. While EuTe2 becomes nonmagnetic in the high-pressure phase and the upper critical field drops below the Pauli limit. Our results demonstrate that the high upper critical field of EuTe2 in the low-pressure phase is due to the exchange field compensation effect of Eu2+ and the superconductivity in both structures may arise in the framework of the Bardeen-Cooper-Schrieffer theory.

Equations of State and Crystal Structures of KCaPO4, KSrPO4, and K2Ce(PO4)2 under High Pressure: Discovery of a New Polymorph of KCaPO4.

We have studied by means of angle-dispersive powder synchrotron X-ray diffraction the structural behavior of KCaPO4, SrKPO4, and K2Ce(PO4)2 under high pressure up to 26, 25, and 22 GPa, respectively. For KCaPO4, we have also accurately determined the crystal structure under ambient conditions, which differs from the structure previously reported. Arguments supporting our structural determination will be discussed. We have found that KCaPO4 undergoes a reversible phase transition. The onset of the transition is at 5.6 GPa. It involves a symmetry decrease. The low-pressure phase is described by space group P3̅m1 and the high-pressure phase by space group Pnma. For KSrPO4 and K2Ce(PO4)2, no evidence of phase transitions has been found up to the highest pressure covered by the experiments. For the three compounds, the linear compressibility for the different crystallographic axes and the pressure-volume equation of states are reported and compared with those of other phosphates. The three studied compounds are among the most compressible phosphates. The results of the study improve the knowledge about the high-pressure behavior of complex phosphates.

Glassy atomic vibrations and blurry electronic structures created by local structural disorders in high-entropy metal telluride superconductors

Recently, robustness of superconductivity to external pressure has been observed in high-entropy (HE) metal telluride (MTe). Here, we investigated the atomic displacement parameters (Uiso), atomic vibration characteristics, and electronic structure of MTe with various configurational entropies of mixing (ΔSmix) at the M site to understand the origin of the robustness of the superconductivity. Uiso clearly increased by M-site alloying with ΔSmix ≥ 1.1R, which is evidence of the created local disorder in the NaCl-type (low-pressure) phases. The simulated vibrational density of states (DOS) shows remarkable broadening with ΔSmix ≥ 1.1R, which indicates the glassy characteristics of atomic vibrations in the NaCl-type phases. In the calculated electronic structure for the CsCl-type (high-pressure) phases, a blurry electronic band structure appears with increasing ΔSmix, which indicates the evolution of blurry electronic states in HE MTe with the CsCl-type structure. The estimated electronic DOS at the Fermi energy cannot explain the changes in Tc for HE MTe when assuming conventional electron-phonon superconductivity, but the conventional explanation seems to work for PbTe. Therefore, the pairing mechanisms in HE MTe are affected by the glassy phonon and/or blurry electronic states, and the robustness of superconductivity possibly originates from the unique electron-phonon coupling.

Pressure tuning of structure, magnetic frustration, and carrier conduction in the Kitaev spin liquid candidate Cu2IrO3

The layered honeycomb lattice iridate Cu$_2$IrO$_3$ is the closest realization of the Kitaev quantum spin liquid, primarily due to the enhanced interlayer separation and nearly ideal honeycomb lattice. We report pressure-induced structural evolution of Cu$_2$IrO$_3$ by powder x-ray diffraction (PXRD) up to $\sim$17 GPa and Raman scattering measurements up to $\sim$25 GPa. A structural phase transition (monoclinic $C2/c \: \rightarrow$ triclinic $P\bar{1}$) is observed with a broad mixed phase pressure range ($\sim$4 to 15 GPa). The triclinic phase consists of heavily distorted honeycomb lattice with Ir-Ir dimer formation and a collapsed interlayer separation. In the stability range of the low-pressure monoclinic phase, structural evolution maintains the Kitaev configuration up to 4 GPa. This is supported by the observed enhanced magnetic frustration in dc susceptibility without emergence of any magnetic ordering and an enhanced dynamic Raman susceptibility. High-pressure resistance measurements up to 25 GPa in the temperature range 1.4--300 K show resilient non-metallic $R$($T$) behaviour with significantly reduced resistivity in the high-pressure phase. The Mott 3D variable-range-hopping conduction with much reduced characteristic energy scale $T_0$ suggests that the high-pressure phase is at the boundary of localized-itinerant crossover. Using first-principles density functional theoretical (DFT) calculations, we find that at ambient pressure $\rm Cu_2IrO_3$ exists in monoclinic $P2_1/c$ phase which is energetically lower than $C2/c$ phase (both the structures are consistent with experimental XRD pattern). DFT reveals structural transition from $P2_1/c$ to $P\bar{1}$ structure at 7 GPa (involving dimerization of Ir-Ir bonds) in agreement with experimentally observed transition pressure.

High-Pressure X-ray Diffraction Study of Orthorhombic Ca2Zr5Ti2O16.

The orthorhombic polymorph of Ca2Zr5Ti2O16 (space group Pbca) has been studied by powder X-ray diffraction under high pressures up to 30 GPa using synchrotron radiation. We have found evidence of a structural phase transition at 12-13 GPa. The phase transition causes an enhancement of the crystal symmetry. The high-pressure phase is tetragonal, being described by space group I41/acd. The space groups of the high- and low-pressure phases have a group/subgroup relationship. However, the phase transition is accompanied by a discontinuous change in the unit-cell volume, indicating that the phase transition can be classified as first order. We have also performed density functional theory calculations. These simulations support the occurrence of the orthorhombic-to-tetragonal transition. The pressure-volume equation of state and axial compressibilities have been determined for both polymorphs. The results are compared with previous studies in related oxides.