Pressure-temperature Phase Diagram Research Articles

Pressure-temperature phase diagram of the EuRbFe4As4 superconductor

The pressure dependencies of the magnetic and superconducting transitions, as well as that of the superconducting upper critical field are reported for single crystalline EuRbFe$_4$As$_4$. Resistance measurements were performed under hydrostatic pressures up to 6.21 GPa and in magnetic fields up to 9 T. Zero-field-cool magnetization measurements were performed under hydrostatic pressures up to 1.24 GPa under 20 mT applied field. Superconducting transition temperature, $T_\text c$, up to 6.21 GPa and magnetic transition temperature, $T_\text M$, up to 1.24 GPa were obtained and a pressure-temperature phase diagram was constructed. Our results show that $T_\text c$ is monotonically suppressed upon increasing pressure. $T_\text M$ is linearly increased up to 1.24 GPa. For the studied pressure range, no signs of the crossing of $T_\text M$ and $T_\text c$ lines are observed. The normalized slope of the superconducting upper critical field is gradually suppressed with increasing pressure, which may be due to the continuous change of Fermi-velocity $v_F$ with pressure.

First-principles lattice dynamics and thermodynamic properties of pre-perovskite PbTiO3

It was recently found that nanowires of PbTiO3 synthesized through an intermediate pre-perovskite phase exhibit enhanced spontaneous polarization. Here we investigated the pre-perovskite PbTiO3 (PP-PTO) nanowire phase at finite temperatures employing first-principles quasiharmonic calculations. We calculated its band gap, phonon dispersions, phonon density of states, Debye temperature, and thermodynamic properties. The corresponding calculations for cubic and tetragonal PbTiO3 were also carried out for comparison. In the current calculations, the amount of imaginary frequencies associated with the ideal cubic PTO structure, i.e., a cubic cell shape with ion positions at the ideal cubic perovskite lattice sites, was decreased to a negligible level by employing a constrained cubic structure, a structure with the same cubic cell shape as the ideal cubic PTO structure but allowing the ion positions to relax to thermodynamically more stable tetragonal positions at 0 K. In contrast to the general observation that a higher volume phase would have relatively higher entropy, it is found that the PP-PTO phase possesses the lowest entropy while having the largest volume compared to cubic and tetragonal PbTiO3 phases. Furthermore, the temperature-pressure phase diagram for the three PbTiO3 phases was obtained, which demonstrates that PP-PTO could be stabilized under a large volume or a negative pressure. This study provides insights to experimentally synthesizing the PP-PTO phase and to better understanding its phase transition into the converted tetragonal PbTiO3 nanowires with enhanced piezoelectric and ferroelectric properties.

Salt effect on bilayer phase transitions of dipalmitoylphosphatidylglycerol in saline water under high pressure

ABSTRACTThe phase transitions of dipalmitoylphosphatidylglycerol (DPPG) bilayer membrane at high NaCl concentrations under high pressure were investigated to construct the temperature–pressure phase diagram and to determine phase-transition properties. The constructed phase diagrams exhibited qualitative resemblance to that of the dipalmitoylphosphatidylcholine (DPPC) bilayer membrane: they showed the gel-phase polymorphism including the pressure-induced bilayer interdigitation. The phase-transition properties of the DPPG bilayer membrane changed in a salt concentration-dependent manner. We discussed the salt effect on the DPPG bilayer membrane from the variation in interactions between the polar head groups of the PG molecules.

Variation in the nature of the neutral-ionic transition in DMTTF−QCl4 under pressure probed by NQR and NMR

We have investigated the microscopic charge/lattice/spin states in the neutral-ionic transition material $\mathrm{DMTTF}\ensuremath{-}{\mathrm{QCl}}_{4}$ under pressure by $^{35}\mathrm{Cl}\ensuremath{-}\mathrm{NQR}$ and $^{1}\mathrm{H}\ensuremath{-}\mathrm{NMR}$ measurements to reveal the mechanism of the phase transition and the nature of the pressure-temperature phase diagram. At ambient pressure, the $^{35}\mathrm{Cl}\ensuremath{-}\mathrm{NQR}$ frequency indicates low ionicity and shows neither discontinuous jump nor precursory decrease around the transition, suggesting that the spin degrees of freedom are not involved in a dimerization transition. These experimental features are consistent with the theoretically proposed picture that the electronic energy gain is acquired by modulations in transfer integrals (so-called Peierls mechanism) with the lattice dimerization. As pressure is applied, the ionicity is increased and the system is driven into an ionic state with the degree of charge transfer exceeding 0.5 above $\ensuremath{\sim}9\phantom{\rule{0.28em}{0ex}}\mathrm{kbar}$ at room temperature, where the $^{1}\mathrm{H}\ensuremath{-}\mathrm{NMR}$ captures paramagnetic spin fluctuations indicative of the emergence of a nondimeric ionic phase. Furthermore, $^{35}\mathrm{Cl}\ensuremath{-}\mathrm{NQR}$ and x-ray-diffraction measurements found a distinct antiferroelectric state at high pressures above 11 kbar. Based on these results, we propose a pressure-temperature phase diagram.

Pressure promoted low-temperature melting of metal-organic frameworks.

Metal-organic frameworks (MOFs) are microporous materials with huge potential for chemical processes. Structural collapse at high pressure, and transitions to liquid states at high temperature, have recently been observed in the zeolitic imidazolate framework (ZIF) family of MOFs. Here, we show that simultaneous high-pressure and high-temperature conditions result in complex behaviour in ZIF-62 and ZIF-4, with distinct high- and low-density amorphous phases occurring over different regions of the pressure-temperature phase diagram. In situ powder X-ray diffraction, Raman spectroscopy and optical microscopy reveal that the stability of the liquid MOF state expands substantially towards lower temperatures at intermediate, industrially achievable pressures and first-principles molecular dynamics show that softening of the framework coordination with pressure makes melting thermodynamically easier. Furthermore, the MOF glass formed by melt quenching the high-temperature liquid possesses permanent, accessible porosity. Our results thus imply a route to the synthesis of functional MOF glasses at low temperatures, avoiding decomposition on heating at ambient pressure.

Superconductivity, electronic phase diagram, and pressure effect in Sr1−xPrxFBiS2

Based on a combination of X-ray diffraction, electrical transports, magnetic susceptibility, specific heat, and pressure-effect measurements, we report the results of experiments on a series of BiS2-based Sr1− x Pr x FBiS2 superconductors with the maximum T c of 2.7 K for x =0.5 and at ambient pressure. Superconductivity appears only for 0.4≤ x ≤0.7 whereas the normal-state resistivity shows the semiconducting-like behaviors. The magnetic susceptibility χ( T ) displays the low superconducting shielding volume fractions and C ( T ) shows no distinguishable anomaly near T c, which suggests a filamentary superconductivity in the Pr-doped polycrystalline samples. By varying doping concentrations, an electronic phase diagram is established. Upon applying pressure on the optimally doped Sr0.5Pr0.5FBiS2 system, T c is abruptly enhanced, reaches 8.5 K at the critical pressure of P c= 1.5 GPa, and increases slightly to 9.7 K at 2.5 GPa. Accompanied by the enhancement of superconductivity from the low- to the high- T c phases, the normal state undergoes a semiconductor-to-metal transition when under pressure. This scenario may be linked to enhanced overlap of the Bi-6 p and S- p orbitals, which contributes to the enhanced superconductivity above P c. The pressure-temperature phase diagram for Sr0.5Pr0.5FBiS2 is also presented.

77Se-NMR Study under Pressure on 12%-S Doped FeSe

The 12%-S doped FeSe system has a high Tc of 30 K at a pressure of 3.0 GPa. We have successfully investigated its microscopic properties for the first time via $^{77}$Se-NMR measurements under pressure. The antiferromagnetic (AFM) fluctuations at the optimal pressure (~3 GPa) exhibited unexpected suppression compared with the AFM fluctuations at ambient pressure, even though the optimal pressure is close to the phase boundary of the AFM phase induced at the high-pressure region. In addition, we revealed that the SC phase at an applied field of 6.02 T exhibited a remarkable double-dome structure in the pressure-temperature phase diagram, unlike the SC phase at zero field.

Characteristics and Structure of Hopeite-Mineral (Type A3(PO4)2.4H2O)

The aim of the study was to gain more information about the structural changes during the dewatering reactions of the above compounds. The paper is focus on the low-temperature absorption of hopeite clay mineral, afterward the phases are examined for the dependence of temperature at different pressures. Hopeite shows signs of a phase transformation. Their use for the dental cement industry as a lubricant for cold-rolling mills is to be emphasized, such as coating with phosphate mineral or carbonaceous crystal. This is shown by the appearance of the new reflexes. The observed changes take place below the dewatering known from the literature. The phase change was investigated using a neutron scattering experiment. A temperature-pressure phase diagram of the dewatering could be set up in the range of 275 K to 380 K and 10-3 mBar to 103 mBar with the temperature-dependent powder diffractometer at different pressures and the temperature-dependent neutron scattering experiment. In this work, it has been proved that the dewatering of the hopeite is dependent on pressure and that the dewatering process of the hopeite is a reversible process.

Understanding the Hydration Process of Salts: The Impact of a Nucleation Barrier

The solid-state hydration of salts has gained particular interest within the frame of thermochemical energy storage. In this work, the water vapor pressure-temperature (p-T) phase diagram of the following thermochemical salts was constructed by combining equilibrium and nonequilibrium hydration experiments: CuCl 2 , K 2 CO 3 , MgCl 2 ·4H 2 O, and LiCl. The hydration of CuCl 2 and K 2 CO 3 involves a metastable zone of ca. 10 K, and the induction times preceding hydration are well-described by classical homogeneous nucleation theory. It is further shown for K 2 CO 3 (metastable) and MgCl 2 ·4H 2 O (not metastable) through solubility calculations that the phase transition is not mediated by bulk dissolution. We conclude that the hydration proceeds as a solid-solid phase transition, mobilized by a wetting layer, where the mobility of the wetting layer increases with increasing vapor pressure. In view of heat storage application, the finding of metastability in thermochemical salts reveals the impact of nucleation and growth processes on the thermochemical performance and demonstrates that practical aspects like the output temperature of a thermochemical salt are defined by its metastable zone width (MZW) rather than its equilibrium phase diagram. Manipulation of the MZW by e.g. prenucleation or heterogeneous nucleation is a potential way to raise the output temperature and power on material level in thermochemical applications. © Copyright 2019 American Chemical Society.

Multithermal-Multibaric Molecular Simulations from a Variational Principle

We present a method for performing multithermal-multibaric molecular dynamics simulations that sample entire regions of the temperature-pressure (TP) phase diagram. The method uses a variational principle [Valsson and Parrinello, Phys. Rev. Lett. 113, 090601 (2014)PRLTAO0031-900710.1103/PhysRevLett.113.090601] in order to construct a bias that leads to a uniform sampling in energy and volume. The intervals of temperature and pressure are taken as inputs and the relevant energy and volume regions are determined on the fly. In this way the method guarantees adequate statistics for the chosen TP region. We show that our multithermal-multibaric simulations can be used to calculate all static physical quantities for all temperatures and pressures in the targeted region of the TP plane. We illustrate our approach by studying the density anomaly of TIP4P/ice water.

Negative pressure effects on molecular dynamics and phase diagram of glass-forming nematic liquid crystal 4-cyano-3-fluorophenyl 4-butylbenzoate (4CFPB) confined in nanopores

Here, we show the method to determine the pressure conditions for liquid crystal restricted in nanochannels. For this purpose, broadband dielectric spectroscopy studies of 4CFPB glass-forming nematic liquid crystal at various pressure conditions and confined in non-intersecting nanopores of mean diameters 6 nm and 8 nm were combined. The pressure dependence of 4CFPB clearing point occurred to be linear and enabled to find pressure conditions in restricted spatial geometry. The pressure evolution of glass transition temperature was non-linear and was described well using the Andersson-Andersson law. Based on the presented results, the temperature-pressure phase diagram was constructed.

T–P–X Phase Diagram of the Water–Hydrogen System at Pressures up to 10 kbar

The stability field of the C0 hydrogen hydrate in the pressure–temperature phase diagram of the water–hydrogen system is studied by volumetry under hydrogen excess. This stability field is confined by the sII hydrate stability field at the low-pressure side; by the liquid stability field at the high-temperature side; and by the C1 hydrate stability field at the high-pressure side. The pressure of the C0 ↔ C1 phase equilibrium is temperature-independent in the studied temperature range from −20 to +18 °C. The corresponding equilibrium line in the phase diagram terminates at the nonvariant quadruple point “C0 hydrate + C1 hydrate + liquid H2O + H2 gas” (or Q3) at 7.7(3) kbar and +22(2) °C. The volume change accompanying the C0 → C1 phase transition, and the dTdPH2 slopes of the melting lines of the C0 and C1 hydrates are measured by volumetry. Thermodynamic considerations are used to estimate the hydrogen content H2/H2O ≈ 0.53(5) and 0.23(5) of the C0 and C1 hydrates, respectively, near the Q3 point.

Hyperbolic Pressure-Temperature Phase Diagram of the Zinc-Finger Protein apoKti11 Detected by NMR Spectroscopy.

For a comprehensive understanding of the thermodynamic state functions describing the stability of a protein, the influence of the intensive properties of temperature and pressure has to be known. With the zinc-finger-containing Kti11, we found a suitable protein for this purpose because folding and unfolding transitions occur at an experimentally accessible temperature (280-330 °K) and pressure (0.1-240 MPa) range. We solved the crystal structure of the apo form of Kti11 to reveal two disulfide bonds at the metal-binding site, which seals off a cavity in the β-barrel part of the protein. From a generally applicable proton NMR approach, we could determine the populations of folded and unfolded chains under all conditions, leading to a hyperbolic pressure-temperature phase diagram rarely observed for proteins. A global fit of a two-state model to all derived populations disclosed reliable values for the change in Gibbs free energy, volume, entropy, heat capacity, compressibility, and thermal expansion upon unfolding. The unfolded state of apoKti11 has a lower compressibility compared to the native state and a smaller volume at ambient pressure. Therefore, a pressure increase up to 200 MPa reduces the population of the native state, and above this value, the native population increases again. Pressure-induced chemical-shift changes in two-dimensional 1H-15N NMR spectra could be employed for a molecular interpretation of the thermodynamic properties of apoKti11.

Grüneisen parameter and thermal expansion near magnetic quantum critical points in itinerant electron systems

Complete expressions of the thermal-expansion coefficient $\alpha$ and the Gr\"{u}neisen parameter $\Gamma$ are derived on the basis of the self-consistent renormalization (SCR) theory. By considering zero-point as well as thermal spin fluctuation under the stationary condition, the specific heat for each class of the magnetic quantum critical point (QCP) specified by the dynamical exponent $z=3$ (FM) and $z=2$ (AFM) and the spatial dimension ($d=3$ and $2$) is shown to be expressed as $C_{V}=C_a-C_b$, where $C_a$ is dominant at low temperatures, reproducing the past SCR criticality endorsed by the renormalization group theory. Starting from the explicit form of the entropy and using the Maxwell relation, $\alpha=\alpha_a+\alpha_b$ (with $\alpha_a$ and $\alpha_b$ being related to $C_a$ and $C_b$, respectively) is derived, which is proven to be equivalent to $\alpha$ derived from the free energy. The temperature-dependent coefficient found to exist in $\alpha_b$, which is dominant at low temperatures, contributes to the crossover from the quantum-critical regime to the Curie-Weiss regime and even affects the quantum criticality at 2d AFM QCP. Based on these correctly calculated $C_{V}$ and $\alpha$, Gr\"{u}neisen parameter $\Gamma=\Gamma_a+\Gamma_b$ is derived, where $\Gamma_a$ and $\Gamma_b$ contain $\alpha_a$ and $\alpha_b$, respectively. The inverse susceptibility coupled to the volume $V$ in $\Gamma_b$ gives rise to divergence of $\Gamma$ at the QCP for each class even though characteristic energy scale of spin fluctuation $T_0$ is finite at the QCP, which gives a finite contribution in $\Gamma_a=-\frac{V}{T_0}\left(\frac{\partial T_0}{\partial V}\right)_{T=0}$. General properties of $\alpha$ and $\Gamma$ including their signs as well as the relation to $T_0$ and the Kondo temperature in temperature-pressure phase diagrams of Ce- and Yb-based heavy electron systems are discussed.

Phase identification of natural gas system with high CO2 content through simulation approach using Peng-Robinson model

The phase behavior for natural gas mixtures with high CO2 content and heavier hydrocarbons up to pentane for cryogenic separation has been obtained through simulation using Peng-Robinson (PR) model. The bubble point and dew point curves which define the liquid-vapor and solid-vapor domains for separation are obtained by generating the pressure-temperature phase diagram. The simulations are performed using Aspen HYSYS. The validation for the use of PR model is done by comparing the results of CO2 freeze-out temperature predictions for PR and Soave-Redlich-Kwong (SRK) model with the experimental data available in the literature.

First-principles study of structural phase transformation and dynamical stability of cubic AlN semiconductors

Phase transformation and stability of cubic aluminium nitride (AlN) phases such as zinc-blende and rock-salt have been investigated using first-principles calculations based on density functional perturbation theory (DFPT) within quasi-harmonic approximation (QHA). The phonon dispersion relations of both the cubic phases have been calculated at various high-symmetry points of the Brillouin Zone. The pressure and volume dependence of phonon frequencies have been investigated. The application of pressure results in opposite trend of transverse acoustic (TA) phonon frequencies for rock-salt and zinc-blende AlN phases. The TA frequencies found to increase for the former one and decrease for the latter one with the increase in pressure. The dynamical instability results in a volume expansion of rock-salt AlN close to the equilibrium volume of zinc-blende AlN. Phase transformation of these cubic phases is further investigated by computing an equilibrium pressure-temperature phase diagram within QHA. The cubic rock-salt AlN is found to form at high pressures and temperatures than zinc-blende AlN. The temperature dependence of lattice constant and the corresponding volume thermal expansion coefficient of both the cubic phases have been investigated.

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

In this work, structural transformation, elastic behaviors, phonon spectra, and thermodynamic properties upon hydrostatic compression of hafnium are investigated by using first-principle density functional theory. Our calculated equilibrium structural parameters and the transition pressures of α→ω→β are well consistent with experiments. At ambient pressure, the elastic constants indicate that the α and ω phases are mechanically stable while the β phase is unstable. The thermal properties including bulk modulus, entropy, and heat capacity in wide range of pressure and temperature for α-Hf are predicted. We present results of the phonon spectra and phonon density of states for the three experimentally observed phases and discuss the effects of pressure and temperature on those properties. The β-Hf is mechanically and dynamically stable at high pressure or at high temperature. Employing the quasiharmonic approximation as well as the Debye model, we calculate the Gibbs free energy and predict the pressure-temperature phase diagram.

A new higher-order Landau–Ginzburg–Devonshire theory for KNbO3 crystal

ABSTRACTA Landau-Ginzburg-Devonshire(LGD) theory based on a structurally stable 8th-order potential with three varying expansion coefficients (a1, a11 and a2) was proposed for phenomenological description for KNbO3. The phase diagrams in the space of phenomenological parameters of the LGD potential model with co-dimension (Codim) of 2 and 3, invariant under the cubic point group m3m, were constructed. The values of the phenomenological coefficients of the potential model were determined by fitting experimental data. The theoretical pressure-temperature phase diagram which can explain the experimentally observed cubic-tetragonal-orthorhombic triple point and cubic-orthorhombic phase transition was constructed. Temperature dependences of spontaneous strain, lattice constants, dielectric constants and spontaneous polarization calculated with this potential model were in good agreement with the experimental data. Furthermore, the influence of the hydrostatic pressure on the dielectric constant (ε33) in the vicinity of the triple point and the dependence of piezoelectric coefficients on the applied electric field were predicted.

Ab initiocalculations of carbon and boron nitride allotropes and their structural phase transitions using periodic coupled cluster theory

We present an ab-initio study of boron nitride as well as carbon allotropes. Their relative thermodynamic stabilities and structural phase transitions from low- to high-density phases are investigated. Pressure-temperature phase diagrams are calculated and compared to experimental findings. The calculations are performed using quantum chemical wavefunction based as well as density functional theories. Our findings reveal that predicted energy differences often depend significantly on the choice of the employed method. Comparison between calculated and experimental results allows for benchmarking the accuracy of various levels of theory. The produced results show that quantum chemical wavefunction based theories allow for achieving systematically improvable estimates. We find that on the level of coupled cluster theories the low- and high-density phases of boron nitride become thermodynamically degenerate at 0 K. This is in agreement with recent experimental findings, indicating that cubic boron nitride is not the thermodynamically stable allotrope at ambient conditions. Furthermore we employ the calculated results to assess transition probabilities from graphitic low-density to diamond-like high-density phases in an approximate manner. We conclude that the stacking order of the parent graphitic material is crucial for the possible formation of meta-stable wurtzite boron nitride and hexagonal carbon diamond also known as lonsdaleite.

Ab initio phase diagram of PbSe crystals calculated with the random phase approximation

Understanding the phase behavior of semiconductor materials is important for applications in solid state physics and nanoscience. Accurate experimental data is often difficult to obtain due to strong kinetic effects. In this work, we calculate the temperature-pressure phase diagram of lead selenide (PbSe) using the random phase approximation (RPA), an accurate wavefunction based many-body technique. We consider three crystalline phases, the low pressure B1 phase (NaCl-type), the intermediate B33 phase (CrB-type), and the high pressure B2 phase (CsCl-type). The electronic contributions to the free energy (at T=0K) are calculated in the Born-Oppenheimer approximation using the RPA, whereas phononic contributions are computed in the quasi-harmonic approximation using DFT and the PBEsol functional. At room temperature, we find transition pressures of 4.6 +/- 0.3 GPa for the B1-B33 transition and 18.7 +/- 0.3 GPa for the B33-B2 transition, in good agreement with experiments. In contrast to the interpretation of recent experiments, we observe a negative Clapeyron slope for both transitions. Gibbs free energy differences between competing structures have small gradients close to coexistence, consistent with pronounced hysteresis observed in experiments. The phase diagram presented in this work can serve as a reference for future studies of PbSe and should prove useful in the development of accurate and efficient force fields.