Pressure-temperature Phase Diagram Research Articles

Pressure-induced metal–insulator transitions in chalcogenide NiS2-xSex

We report the temperature-dependent resistivity ρ(T) of chalcogenide NiS2-xSex (x = 0.1) using hydrostatic pressure as a control parameter in the temperature range of 4–300K. The insulating behavior of ρ(T) survives at low temperatures in the pressure regime below 7.5kbar, whereas a clear insulator-to-metallic transition is observed above 7.5kbar. Two types of magnetic transitions, from the paramagnetic (PM) to the antiferromagnetic (AFM) state and from the AFM state to the weak ferromagnetic (WF) state, were evaluated and confirmed by magnetization measurement. According to the temperature–pressure phase diagram, the WF phase survives up to 7.5kbar, and the transition temperature of the WF transition decreases as the pressure increases, whereas the metal–insulator transition temperature increases up to 9.4kbar. We analyzed the metallic behavior and proposed Fermi-liquid behavior of NiS1.9Se0.1.

Unexpected ground-state structures and properties of carbon nitride C3N at ambient and high pressures

Thermodynamically stable C3N phases in the pressure range of 0–100GPa are searched and calculated by ab initio evolutionary algorithm and density functional theory. Ground-state C3N phase at ambient pressure is predicted to be a layered structure with Cmmm space group. The previously reported Fd3¯m phase exists only in the narrow pressure range of 6.1–7.4GPa, which transforms into a P3¯m1 phase at 7.4GPa and then into a R3¯m phase at 40.7GPa. The increasing pressure drives the R3¯m phase to a C2/c phase at 76.9GPa. Dynamical and mechanical stabilities of C3N phases are checked and confirmed by phonon dispersion and elastic constant calculations. It is diverse of the electronic properties of C3N with Cmmm (zero band gap), P3¯m1 (wide gap of 4.55eV), R3¯m (wide gap of 4.54eV) and C2/c (narrow gap of 1.94eV). The three-dimensional strong covalent C2/c C3N is superhard with high hardness of 79.5 and 86.9GPa calculated by Chen and Gao models. Pressure-temperature phase diagram of C3N is firstly established and shows high temperature can significantly decrease synthesis pressures of C2/c phase, which gives a theoretical guidance for further experimental synthesis.

Oxygen adsorption on Pt-Pd nanoclusters by DFT and ab initio atomistic thermodynamics

Monometallic Pt and Pd, and bimetallic Pt-core/Pd12-shell and Pd-core/Pt12-shell were built and geometrically optimized by density functional theory calculations in consideration of oxygen adsorption. The different oxygen adsorption sites on the cluster were considered: top site on one shell atom, bridge site between two shell atoms and hollow site among three shell atoms. For each adsorption site, a few coverages were considered, both oxygen-rich condition and oxygen-poor condition. The pressure-temperature phase diagrams were determined for 13-atom Pt-Pd clusters with adsorbed oxygens in terms of ab initio atomistic thermodynamics method. The most stable phase structure and adsorption of oxygen at certain temperature and pressure were illustrated. Additionally the results showed that the cluster structures were most likely to be changed under oxygen-poor condition. This study is helpful to understand their catalyst processes in a few oxygen-involved reactions such as formic acid electro-oxidation, direct methanol fuel cell, oxygen reduction reaction in fuel cells, oxidation of sulfur dioxide and diesel oxidation and so on.

High-pressure and high-temperature synthesis of heavy lanthanide sesquisulfides Ln2S3 (Ln=Yb and Lu)

Detailed pressure-temperature phase diagrams of heavy lanthanide sesquisulfides Ln2S3 (Ln = Yb and Lu) have been investigated by in-situ x-ray diffraction experiments under high pressure and high temperature using synchrotron radiation and multi-anvil press. Based on the results of the in-situ observation, the single γ-phase (Th3P4-type structure, I4¯3d) samples of Ln2S3 (Ln = Yb and Lu) have been synthesized under high pressure. The physical properties of the compounds were studied by electrical resistivity, specific heat, and magnetic susceptibility measurements between 2 K and 300 K.

Non-volatile resistive switching in the Mott insulator (V1−xCrx)2O3

The discovery of non-volatile resistive switching in Mott insulators related to an electric-field-induced insulator to metal transition (IMT) has paved the way for their use in a new type of non-volatile memories, the Mott memories. While most of the previous studies were dedicated to uncover the resistive switching mechanism and explore the memory potential of chalcogenide Mott insulators, we present here a comprehensive study of resistive switching in the canonical oxide Mott insulator (V1−xCrx)2O3. Our work demonstrates that this compound undergoes a non-volatile resistive switching under electric field. This resistive switching is induced by a Mott transition at the local scale which creates metallic domains closely related to existing phases of the temperature-pressure phase diagram of (V1−xCrx)2O3. Our work demonstrates also reversible resistive switching in (V1−xCrx)2O3 crystals and thin film devices. Preliminary performances obtained on 880nm thick layers with 500nm electrodes show the strong potential of Mott memories based on the Mott insulator (V1−xCrx)2O3.

Recent progress on the high-pressure studies of FeSe single crystal

Among the iron-based superconductors, the structurally simplest FeSe and FeSe-derived materials have attracted enormous research interest recently due to its unusual electronic properties and the great tunability of the superconducting transition temperature <italic>T</italic>_c. For the bulk FeSe single crystal, it shows a relatively low <italic>T</italic>_c<x content-type=symbol>»</x>8 K within the unusual nonmagnetic nematic phase below <italic>T</italic>_s<x content-type=symbol>»</x>90 K. It has been reported that the application of high pressure can induce a long-range antiferromagnetic order below <italic>T</italic>_m<x content-type=symbol>»</x>20 K at <italic>P</italic><x content-type=symbol>»</x>1 GPa, and then achieve a high <italic>T</italic>_c<x content-type=symbol>»</x>37 K at <italic>P</italic><x content-type=symbol>»</x>8 GPa. It remains unclear how the high-<italic>T</italic>_c superconductivity is achieved in FeSe under pressure. In this regard, a detailed temperature-pressure phase diagram mapping out the exact relationship between the high-<italic>T</italic>_c superconductivity and the normal-state nematic order and the pressure-induced antiferromagnetic order is indispensable for achieving a better understanding. By taking advantage of the cubic anvil cell apparatus that can maintain an excellent hydrostatic pressure condition up to 15 GPa, we recently undertook a comprehensive high-pressure magnetotransport study on high-quality FeSe single crystals and obtained important results. In this article, we give a brief review on the recent progresses about the high-pressure studies of FeSe. By tracking the characteristic resistivity anomalies at the nematic order, the pressure-induced antiferromagnetic order, and the superconducting transition, we constructed for FeSe a comprehensive temperature-pressure phase diagram, which depicts explicitly the detailed evolution of these competing electronic orders. We observed a dome-shaped antiferromagnetic phase boundary <italic>T</italic>_m(<italic>P</italic>), and revealed the competitive relationship between these electronic orders. The superconducting <italic>T</italic>_c experiences two successive step increases at the critical pressures when the nematic order and the antiferromagnetic order are respectively suppressed, and finally the highest <italic>T</italic>_c^max=38.5 K is realized near <italic>P</italic>_c≈6 GPa when the antiferromagnetic order just collapses. We further measured the Hall effect of FeSe to extract the electronic structure information under high pressure. Our Hall data show that the Fermi surface of FeSe in the normal state undergoes a reconstruction at <italic>P</italic><x content-type=symbol>»</x>2 GPa, when the nematic order is suppressed and the antiferromagnetic order just emerges. The dominant charge carriers for the transport properties in the normal state change from the electron to the hole type. In addition, in the high-<italic>T</italic>_c phase near the optimal<italic> </italic><italic>T</italic>_c^max at <italic>P</italic>_c<x content-type=symbol>»</x>6 GPa, the Hall coefficient experiences a dramatic enhancement and the magnetoresistance exhibits anomalous scaling behaviors, signaling the presence of strongly enhanced inter-band spin fluctuations. These results suggest that the critical antiferromagnetic fluctuations play an important role for achieving the high-<italic>T</italic>_c superconductivity in FeSe. Aided by the first-principles calculations, we further confirmed that both electron-like and hole-like Fermi surfaces are present in FeSe under high pressures, in favor of the Fermi surface nesting scenario as in the FeAs-based high-<italic>T</italic>_c<italic> </italic>superconductors. These above results on FeSe suggest great similarities with other FeAs-based superconductors, and thus constitute an important step toward a unified understanding of iron-based superconductivity.

Superconductivity induced by external pressure in Eu3−xSrxBi2S4F4 (x = 1, 2) compounds

We have studied the temperature-pressure phase diagram of two materials Eu3−xSrx Bi2S4F4 (x = 1 and x = 2) by electrical resistivity and magnetic measurements down to 2 K. Semiconducting resistive behavior observed in both the materials under ambient conditions transforms into metallic behavior as externally applied pressure gradually increases. Superconductivity is observed in both the materials at and above applied pressure P = 2.37 GPa. Under the highest pressure P ∼ 2.9 GPa applied in our measurements, Tc is ∼9.8 K in Eu2SrBi2S4F4 (x = 1) and 8.2 K in EuSr2Bi2S4F4 (x = 2). Upper critical field Hc2(0) ∼ 3.04 T (x = 1) and 1.17 T (x = 2) is estimated from magnetic field dependent resistivity measurements at 2.9 GPa. Using the Arrhenius equation, we estimate the thermally activated flux flow activation energy U0 as 116 K in Eu2SrBi2S4F4 and 39 K in EuSr2Bi2S4F4. At 2 K, DC magnetic susceptibility measurements indicate S-type paramagnetic behavior.

The Full Pressure–Temperature Phase Envelope of a Mixture in 1000 Microfluidic Chambers

AbstractKnowing the thermodynamic state of complex mixtures—liquid, gas, supercritical or two‐phase—is essential to industrial chemical processes. Traditionally, phase diagrams are compiled piecemeal from individual measurements in a pressure–volume–temperature cell performed in series, where each point is subject to a long fluid equilibrium time. Herein, 1000 microfluidic chambers, each isolated by a liquid piston and set to a different pressure and temperature combination, provide the complete pressure–temperature phase diagram of a hydrocarbon mixture at once, including the thermodynamic phase envelope. Measurements closely match modeled values, with a standard deviation of 0.13 MPa between measurement and model for the dew and bubble point lines, and a difference of 0.04 MPa and 0.25 °C between measurement and model for the critical point.

The Full Pressure-Temperature Phase Envelope of a Mixture in 1000 Microfluidic Chambers.

Knowing the thermodynamic state of complex mixtures-liquid, gas, supercritical or two-phase-is essential to industrial chemical processes. Traditionally, phase diagrams are compiled piecemeal from individual measurements in a pressure-volume-temperature cell performed in series, where each point is subject to a long fluid equilibrium time. Herein, 1000 microfluidic chambers, each isolated by a liquid piston and set to a different pressure and temperature combination, provide the complete pressure-temperature phase diagram of a hydrocarbon mixture at once, including the thermodynamic phase envelope. Measurements closely match modeled values, with a standard deviation of 0.13 MPa between measurement and model for the dew and bubble point lines, and a difference of 0.04 MPa and 0.25 °C between measurement and model for the critical point.

Pressure-induced half-collapsed-tetragonal phase in CaKFe4As4

We report the temperature-pressure phase diagram of CaKFe$_4$As$_4$ established using high pressure electrical resistivity, magnetization and high energy x-ray diffraction measurements up to 6 GPa. With increasing pressure, both resistivity and magnetization data show that the bulk superconducting transition of CaKFe$_4$As$_4$ is suppressed and then disappears at $p$ $\gtrsim$ 4 GPa. High pressure x-ray data clearly indicate a phase transition to a collapsed tetragonal phase in CaKFe$_4$As$_4$ under pressure that coincides with the abrupt loss of bulk superconductivity near 4 GPa. The x-ray data, combined with resistivity data, indicate that the collapsed tetragonal transition line is essentially vertical, occuring at 4.0(5) GPa for temperatures below 150 K. Band structure calculations also find a sudden transition to a collapsed tetragonal state near 4 GPa, as As-As bonding takes place across the Ca-layer. Bonding across the K-layer only occurs for $p$ $\geq$ 12 GPa. These findings demonstrate a new type of collapsed tetragonal phase in CaKFe$_4$As$_4$: a half-collapsed-tetragonal phase.

Heavy-fermion superconductivity in CeAg2Si2 – Interplay of spin and valence fluctuations

We present the pressure-temperature phase diagram of the antiferromagnet CeAg2Si2 established via resistivity and calorimetry measurements under quasi-hydrostatic conditions up to 22.5GPa. With increasing pressure, the Néel temperature [TN(p=0)=8.6K] slowly increases up to TN=13.4K at 9.4GPa and then vanishes abruptly at the magnetic critical pressure pc∼13GPa. For the first time, heavy fermion superconductivity is observed in CeAg2Si2. Superconductivity emerges at ∼11 GPa and persists over roughly 10GPa. Partial- and bulk-transition temperatures are highest at p=16GPa, with a maximal Tcbulk=1.25K. In the pressure region of superconductivity, Kondo and crystal-field splitting energies become comparable and resistivity exhibits clear signatures of a Ce-ion valence crossover. The crossover line is located at a rapid collapse in resistivity as function of pressure and extrapolates to a valence transition critical endpoint at critical pressure and temperature of pcr∼17GPa and Tcr∼−13K, respectively. Both critical spin and valence fluctuations may build up superconductivity in CeAg2Si2.

Back to the Structural and Dynamical Properties of Neutral-Ionic Phase Transitions

Although the Neutral-Ionic transition in mixed stack charge-transfer crystals was discovered almost forty years ago, many features of this intriguing phase transition, as well as open questions, remain at the heart of today’s science. First of all, there is the most spectacular manifestation of electronic ferroelectricity, in connection with a high degree of covalency between alternating donor and acceptor molecules along stacks. In addition, a charge-transfer instability from a quasi-neutral to a quasi-ionic state takes place concomitantly with the stack dimerization, which breaks the inversion symmetry. Moreover, these systems exhibit exceptional one-dimensional fluctuations, with an enhancement of the effects of electron-lattice interaction. This may lead to original physical pictures for the dynamics of pre-transitional phenomena, as the possibility of a pronounced Peierls-type instability and/or the generation of unconventional non-linear excitations along stacks. Last but not least, these mixed stack charge-transfer systems constitute a valuable test bed to explore some of the key questions of ultrafast photo-induced phenomena, such as multiscale dynamics, selective coherent excitations and non-linear responsiveness. These different aspects will be discussed through the structural and dynamical features of the neutral-ionic transition, considering old and recent results, open questions and future opportunities. In particular, we revisit the structural changes and symmetry considerations, the pressure-temperature phase diagrams and conclude by their interplay with the photo-induced dynamics.

Pressure effects on the electronic properties in CeCoIn5: A first-principle study

Superconducting heavy fermions are exotic materials with strong electronic correlations. The temperature-pressure phase diagrams of some of these materials show a complex interplay between superconductivity and magnetism that is essential to understand the physical properties of these systems. In this work, first principle calculations are performed in order to study the pressure effects on the electronic correlations in the CeCoIn5 system, which is superconducting at ambient pressure with Tc = 2.3 K. The density functional theory (DFT) method was used to include on-site coulomb repulsions (U) at the d (Co and In) and f (Ce) electrons of the CeCoIn5 compound. External applied pressures were simulated by correlating an applied pressure with a reduction of the volume of the unit cell, but keeping constant the c/a relation, as reported in experiments. Our findings reveal that the U parameters for all atomic species increase linearly with the pressure (P), being this effect higher for the f-electrons of the cerium ions, where dU/dP=1.2 eV/GPa. In summary, these results not only suggest that the pressure effect can be correlated with an increase in the electronic correlations in the CeCoIn5 compound, as also, the work allows quantify this effect.

Molecular Simulation of the Phase Diagram of Methane Hydrate: Free Energy Calculations, Direct Coexistence Method, and Hyperparallel Tempering.

Different molecular simulation strategies are used to assess the stability of methane hydrate under various temperature and pressure conditions. First, using two water molecular models, free energy calculations consisting of the Einstein molecule approach in combination with semigrand Monte Carlo simulations are used to determine the pressure-temperature phase diagram of methane hydrate. With these calculations, we also estimate the chemical potentials of water and methane and methane occupancy at coexistence. Second, we also consider two other advanced molecular simulation techniques that allow probing the phase diagram of methane hydrate: the direct coexistence method in the Grand Canonical ensemble and the hyperparallel tempering Monte Carlo method. These two direct techniques are found to provide stability conditions that are consistent with the pressure-temperature phase diagram obtained using rigorous free energy calculations. The phase diagram obtained in this work, which is found to be consistent with previous simulation studies, is close to its experimental counterpart provided the TIP4P/Ice model is used to describe the water molecule.

Correlated electron state in CeCu2Si2 controlled through Si to P substitution

CeCu$_2$Si$_2$ is an exemplary correlated electron metal that features two domes of unconventional superconductivity in its temperature-pressure phase diagram. The first dome surrounds an antiferromagnetic quantum critical point, whereas the more exotic second dome may span the termination point of a line of $f$-electron valence transitions. This behavior has received intense interest, but what has been missing are ways to access the high pressure behavior under milder conditions. Here we study Si $\rightarrow$ P chemical substitution, which compresses the unit cell volume but simultaneously weakens the hybridization between the $f$- and conduction electron states and encourages complex magnetism. At concentrations that show magnetism, applied pressure suppresses the magnetic ordering temperature and superconductivity is recovered for samples with low disorder. These results reveal that the electronic behavior in this system is controlled by a nontrivial combination of effects from unit cell volume and electronic shell filling. Guided by this topography we discuss prospects for uncovering a valence fluctuation quantum phase transition in the broader family of Ce-based ThCr$_2$Si$_2$-type materials through chemical substitution.

Phase behavior of C3H8–CO2–heavy oil systems in the presence of aqueous phase under reservoir conditions

Phase behaviors including phase boundaries, volumes, and compositions of reservoir fluids during solvent(s)-assisted heavy oil recovery processes have been experimentally and theoretically determined in the presence of an aqueous phase. More specifically, a water-rich aqueous phase (A), an oil-rich liquid phase (L), and a solvents-rich vapor phase (V) can coexist under certain reservoir conditions. The phase boundary between AL and ALV on a pressure–temperature phase diagram, i.e., three-phase bubblepoint pressure, has been measured for C3H8–CO2–water–heavy oil systems at temperature ranging from 298.15K to 383.15K. The phase volumes of A+L and V at thermodynamic equilibrium state have been determined at temperatures of 321.55K and 344.95K, respectively. Moreover, the fluids in both L and V phases are sampled in an isobaric manner to perform the compositional analyses by using gas chromatography (GC) method. Meanwhile, two heavy oil samples are theoretically characterized as the multiple pseudocomponents. The Peng–Robinson equation of state (PR EOS) together with two recently developed alpha functions for water and non-water component(s) is applied as the primary thermodynamic model. A previously developed binary interaction parameter (BIP) correlation for CO2–water pair is combined with the van der Waals’ mixing rule to improve the phase behavior prediction for water-contained system. A volume translation method proposed by Peneloux et al. (1982) is then incorporated to correct the calculated phase volume. Without tuning any parameters, the developed water-associated and water-free mathematical models are found to be able to accurately reproduce the measured phase boundaries in the presence and absence of the aqueous phase, respectively. The three-phase bubblepoint pressure is found to be reduced in the presence of water. More accurate prediction can be achieved by considering the effect of aqueous phase. Finally, the GC analyses and flash calculations demonstrate that CO2 is more easily to be vaporized than alkane solvent (i.e., C3H8) when phase splitting occurs for a C3H8–CO2–water–heavy oil system.

Topotactic Metal-Insulator Transition in Epitaxial SrFeOx Thin Films.

Topotactic phase transformation enables structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for elucidating the redox reaction and oxygen diffusion within transition metal oxides. In addition, it enables tuning of the emergent physical properties of complex oxides, through strong interaction between the lattice and electronic degrees of freedom. In this communication, the electronic structure evolution of SrFeOx epitaxial thin films is identified in real-time, during the progress of reversible topotactic phase transformation. Using real-time optical spectroscopy, the phase transition between the two structurally distinct phases (i.e., brownmillerite and perovskite) is quantitatively monitored, and a pressure-temperature phase diagram of the topotactic transformation is constructed for the first time. The transformation at relatively low temperatures is attributed to a markedly small difference in Gibbs free energy compared to the known similar class of materials to date. This study highlights the phase stability and reversibility of SrFeOx thin films, which is highly relevant for energy and environmental applications exploiting the redox reactions.

Relationship between the two-component system 1-Br-adamantane + 1-Cl-adamantane and the high-pressure properties of the pure components

The temperature-composition phase diagram of the two-component system 1-Br-adamantane and 1-Cl-adamantane has been determined by means of thermal analysis techniques and X-ray powder diffraction from the low-temperature phase to the liquid state.The crossed isopolymorphism formalism has been applied to the two-component system to infer the normal pressure properties of the orthorhombic metastable phase of 1-Cl-adamantane at normal pressure. The experimental pressure-temperature phase diagrams for the involved compounds are related to the two-phase equilibria determined at normal pressure and inferences about the monotropic behavior of the aforementioned orthorhombic phase are discussed.

The α-γ-ε triple point and phase boundaries of iron under shock compression

The phase transition of iron under shock compression has attracted much attention in recent decades because of its importance in fields such as condensed matter physics, geophysics, and metallurgy. At room temperature, the transition of iron from the α-phase (bcc) to the ε-phase (hpc) occurs at a stress of 13 GPa. At high temperature, a triple point followed by transformation to the γ-phase (fcc) is expected. However, the details of the high-temperature phase transitions of iron are still under debate. Here, we investigate the phase-transition behavior of polycrystalline iron under compression from room temperature to 820 K. The results show that the shock-induced phase transition is determined unequivocally from the measured three-wave-structure profiles, which clearly consist of an elastic wave, a plastic wave, and a phase-transition wave. The phase transition is temperature-dependent, with an average rate Δσtr/ΔT of −6.91 MPa/K below 700 K and −34.7 MPa/K at higher temperatures. The shock α-ε and α-γ phase boundaries intersect at 10.6 ± 0.53 GPa and 763 K, which agrees with the α-ε-γ triple point from early shock wave experiments and recent laser-heated diamond-anvil cell resistivity and in situ X-ray diffraction data but disagrees with the shock pressure–temperature phase diagram reported in 2009 by Zaretsky [J. Appl. Phys. 106, 023510 (2009)].

Pressure-Induced Charge-Order Melting and Reentrant Charge Carrier Localization in the Mixed-Valent Pb3Rh7O15**Supported by the “Shi-Pei Ji Hua”, the National Science Foundation of China under Grant Nos 51402019 and 11574377, the Beijing Natural Science Foundation under Grant No 2152011, the National Basic Research Program of China under Grants No 2014CB921500, the Strategic Priority Research Program and Key Research Program of Frontier

The mixed-valent PbRhO undergoes a Verwey-type transition at K, below which the development of Rh/Rh charge order induces an abrupt conductor-to-insulator transition in resistivity. Here we investigate the effect of pressure on the Verwey-type transition of PbRhO by measuring its electrical resistivity under hydrostatic pressures up to 8 GPa with a cubic anvil cell apparatus. We find that the application of high pressure can suppress the Verwey-type transition around 3 GPa, above which a metallic state is realized at temperatures below ∼70 K, suggesting the melting of charge order by pressure. Interestingly, the low-temperature metallic region shrinks gradually upon further increasing pressure and disappears completely at P > 7 GPa, which indicates that the charge carriers in PbRhO undergo a reentrant localization under higher pressures. We have constructed a temperature-pressure phase diagram for PbRhO and compared to that of FeO, showing an archetype Verwey transition.