Sequence Of Phase Transitions Research Articles

Modeling ferroelectric phase transitions with graph convolutional neural networks

Ferroelectric materials are widely used in functional devices, however, it has been a long-standing issue to achieve convenient and accurate theoretical modeling of them. Herein, a noval approach to modeling ferroelectric materials is proposed by using graph convolutional neural networks (GCNs). In this approach, the potential energy surface of ferroelectric materials is described by GCNs, which then serves as a calculator to conduct large-scale molecular dynamics simulations. Given atomic positions, the well-trained GCN model can provide accurate predictions of the potential energy and atomic forces, with an accuracy reaching up to 1 meV per atom. The accuracy of GCNs is comparable to that of <i>ab inito</i> calculations, while the computing speed is faster than that of <i>ab inito</i> calculations by a few orders. Benefiting from the high accuracy and fast prediction of the GCN model, we further combine it with molecular dynamics simulations to investigate two representative ferroelectric materials—bulk GeTe and CsSnI<sub>3</sub>, and successfully produce their temperature-dependent structural phase transitions, which are in good agreement with the experimental observations. For GeTe, we observe an unusual negative thermal expansion around the region of its ferroelectric phase transition, which has been reported in previous experiments. For CsSnI<sub>3</sub>, we correctly obtain the octahedron tilting patterns associated with its phase transition sequence. These results demonstrate the accuracy and reliability of GCNs in the modeling of potential energy surfaces for ferroelectric materials, thus providing a universal approach for investigating them theoretically.

Kinetics of hydrogen absorption/desorption in the Zr-2.5Nb alloy.

The Zr-2.5Nb alloy is a typical pressure tube material in heavy water nuclear reactors, and an increase of hydrogen isotope content in the alloy during service can pose major safety risks; hot vacuum extraction-mass spectrometry is an efficient method for evaluating hydrogen isotope concentrations in the Zr-2.5Nb alloy. This work investigates the kinetics and thermodynamic properties of deuterium (D) absorption and desorption of the Zr-2.5Nb alloy using the constant volume adsorption method and the hot vacuum extraction method. In addition to the previously reported volume contraction model, it was observed that at 600 °C and above, the reaction between D2 and Zr-2.5Nb is dominated by diffusion, while the reaction is predominantly influenced by surface adsorption and dissociation below 600 °C. Phase transition sequence of Zr-2.5Nb deuterides during non-isothermal desorption was established using quantitatively calibrated thermal desorption spectra combined with the phase transition process of deuteride decomposition. These results can provide important references for optimizing the process parameters of the hot vacuum extraction-mass spectrometry method.

New metallic ice phase under high pressure.

Crystal materials can exhibit novel properties under high pressure, which are completely different from properties under ambient conditions. Water ice has an exceptionally rich phase diagram with at least 20 known crystalline ice phases from experiments, where the high-pressure ice X and ice XVIII behave as an ionic state and a superionic state, respectively. Thus, the ice structures stabilized under high pressure are very likely to possess other novel properties. Herein, we constructed a sequence of hypothetical high-pressure ices whose structures were generated according to the topological frameworks of common metal oxides. Based on density functional theory calculations, the pressure-induced phase transition sequence is in order that the known Ag2O-Pn3̄m structure (ice X) transformed into a previously undiscovered TiO2_brookite-Pbca structure at a pressure of 300 GPa, followed by a transition to a new NaO2-Pa3 structure at a pressure of 2120 GPa. Hitherto unreported NaO2-Pa3 ice with a cubic structure is in the ionic state, where the oxygen atoms in NaO2-Pa3 have a face-centered cubic (fcc) sublattice, and the coordination number of H atoms increases to 3. These two structures are dynamically stable according to phonon spectrum analysis and remain stable at temperature of 100 K based on ab initio molecular dynamics (AIMD) simulations. More importantly, the NaO2-Pa3 ice exhibits novel metallic properties with a closing band gap above a pressure of 2600 GPa, owing to the electron orbital coupling of oxygen atoms in close proximity induced by pressure.

The identification of high-pressure phase transition sequence in selected tungstates and molybdates.

Tungstates and molybdates possessing the scheelite- and wolframite-type (if present) structures hold a significant functional value. Their high-pressure phase diagrams are very complicated and controversial, and even some parts have not been characterized yet. In this study, we investigate the sequence of pressure driven structural phase transitions up to 100 GPa in these tungstate and molybdate families via first-principles structure predictions. Based on our structural predictions, it is possible for isostructural tungstates and molybdates to exhibit a phase transition sequence that is either similar or identical. Examples of these compounds are CaWO4, CaMoO4, and CdMoO4, in addition to EuWO4 and EuMoO4. However, the phase transition sequences of some tungstates and molybdates, especially those with different divalent cations, display noteworthy variations, revealing the intricate influence of ionic radii and electronic properties on crystal configurations. To obtain a deeper understanding of the high-pressure phase transition behavior of tungstates and molybdates, we analyze the high-pressure phase diagrams of MgWO4, SrWO4, and CaMoO4, representative examples of wolframite-type tungstate, scheelite-type tungstate, and scheelite-type molybdate, respectively, using x-ray powder diffraction. Our x-ray diffraction experiments and structure predictions consistently verify that the orthorhombic Cmca phase is a high-pressure phase of SrWO4. Structural configurations and mechanical properties of these predicted structures are discussed, and electronic properties are given. This study could have important implications for the fields of seismology and geophysics, as well as the utilization of these materials in various capacities, such as photocatalysts, photoanodes, and phosphors.

Thickness dependence of optical and electronic properties of FeCl2 films under high pressure

In this study, we propose an unconventional approach to investigate the pressure-induced changes in crystal structure, electrical transport, and optical bandgap by examining bulk FeCl2 and FeCl2 films with a thickness of 600 nm. Our analysis involved Raman spectroscopy, electrical transport measurements, and Ultraviolet–visible (UV–vis) absorption spectroscopy. Analyzing the Raman spectroscopy data, we discovered subtle distinctions in the sequence of structural phase transitions between bulk FeCl2 and the 600 nm thick film of FeCl2 under varying pressures. Specifically, while bulk FeCl2 underwent an insulating-to-metal transition at 52 GPa, the film of FeCl2 maintained its semiconductor behavior even at pressures as high as 60 GPa. Furthermore, by studying the optical absorption spectra, we observed a consistent decrease in the direct band gap of bulk FeCl2 with increasing pressure. In contrast, the film of FeCl2 exhibited an increase in the optical band gap beyond 34.1 GPa. These remarkable findings offer valuable insights into the manipulation of the mechanical, electrical, and optical properties of layered nanomaterials.

Ultrahigh pressure phase stability of AlB2-type and CaC2-type structures with respect to Fe2P-type and Ni2In-type structures of zirconia

Using density-functional theory, we have performed first-principles calculations to test the phase stability of the hexagonal AlB2-type and tetragonal CaC2-type phases at ultrahigh pressures with respect to the experimentally observed hexagonal Fe2P-type phase and the recently predicted (as post-Fe2P) hexagonal Ni2In-type phase of ZrO2. The phase relations among the four phases have been thoroughly investigated to better understand the high-pressure behavior of ZrO2, especially the upper part of the pressure phase transition sequence. Our enthalpy calculations revealed that the transformation from Ni2In phase to either AlB2 phase or CaC2 phase is unlikely to happen. On the other hand, a direct phase transition from Fe2P phase to Ni2In, CaC2 and AlB2 phases is predicted to occur at 325 GPa, 505 GPa and 1093 GPa, respectively. A deep discussion has been made on the Fe2P → Ni2In and Fe2P → CaC2 transitions in terms of the volume change, the coordination number (CN) change, and the band gap change to obtain a better prediction of the favored post-Fe2P phase of ZrO2. Additionally, the equation of state (EOS) parameters for each phase have been computed using Birch-Murnaghan EOS. To further investigate the phase stability testing, we have studied the components of the enthalpy difference to explore their effect on our findings, and found that all predicted transitions from Fe2P phase are driven by the volume reduction effect when compared to the slight effect of the electronic energy gain.

Pressure-Induced Phase Diagram and Electronic Structure Evolves during the Insulator-Metal Transition of Bulk BiFeO3.

BiFeO3 is the most widely known multiferroic at room temperature, possessing both ferroelectricity and antiferromagnetism. It has high Curie temperature and Néel temperature, i.e., 1103 and 643 K, respectively. Despite these unique properties, the pressure-induced phase diagram of bulk BiFeO3 has remained controversial. Based on the ab initio evolutionary algorithm, we systematically searched for the potential stable structures of bulk BiFeO3 at 0-50 GPa. It is identified that there are five pressure-induced phase transition sequences R3c-G-AFM →(5GPa) C2/m-G-AFM →(15GPa) Pnma-G-AFM →(24GPa) Pnma-FM →(35GPa) Imma-FM →(45GPa) Cmcm-FM, which provided a comprehensive pressure-induced phase diagram. As the pressure increases, we discovered an interesting phenomenon: a pressure-induced magnetic sequence transition, i.e., BiFeO3 transitions from an antiferromagnetic to a ferromagnetic sequence. Concurrently, the electronic structure evolves during the insulator-metal transition, influenced not only by the pressure but also by the phase transition. Our research has elucidated the long-standing question of the phase transition sequence of the BiFeO3 system under pressure and provided theoretical support for the insulator-metal transition.

Weyl Fermions and broken symmetry phases of laterally confined 3He films

Broken symmetries in topological condensed matter systems have implications for the spectrum of Fermionic excitations confined on surfaces or topological defects. The Fermionic spectrum of confined (quasi-2D) 3He-A consists of branches of chiral edge states. The negative energy states are related to the ground-state angular momentum, , for Cooper pairs. The power law suppression of the angular momentum, for , in the fully gapped 2D chiral A-phase reflects the thermal excitation of the chiral edge Fermions. We discuss the effects of wave function overlap, and hybridization between edge states confined near opposing edge boundaries on the edge currents, ground-state angular momentum and ground-state order parameter of superfluid 3He thin films. Under strong lateral confinement, the chiral A phase undergoes a sequence of phase transitions, first to a pair density wave (PDW) phase with broken translational symmetry at . The PDW phase is described by a periodic array of chiral domains with alternating chirality, separated by domain walls. The period of PDW phase diverges as the confinement length . The PDW phase breaks time-reversal symmetry, translation invariance, but is invariant under the combination of time-reversal and translation by a one-half period of the PDW. The mass current distribution of the PDW phase reflects this combined symmetry, and originates from the spectra of edge Fermions and the chiral branches bound to the domain walls. Under sufficiently strong confinement a second-order transition occurs to the non-chiral ‘polar phase’ at , in which a single p-wave orbital state of Cooper pairs is aligned along the channel.

Deciphering the sequence of phase transitions in KNiCl3

Deciphering the sequence of phase transitions in KNiCl₃

Negative linear compressibility and tuneable phase transition sequence in selenium and selected high-entropy oxides under high-pressure conditions via compressional rate control

Negative linear compressibility and tuneable phase transition sequence in selenium and selected high-entropy oxides under high-pressure conditions <i>via</i> compressional rate control

Peculiar dynamics of polar states at the morphotropic phase boundary of antiferroelectric Pb(Zr1-xTix)O3

The coexistence of several structural instabilities in Zr-rich Pb(Zr,Ti)O3 leads to the possibility of condensing them successively to trigger a sequence of phase transitions, instead of reaching directly the AFE phase, as in pure PbZrO3. This is especially noticeable in compositions near the antiferroelectric morphotropic phase boundary and the tricritical point around room temperature, as PZT 95/5, where three phases are energetically available. To check the thermal development of these different phases, differential scanning calorimetry, dielectric spectroscopy and Raman scattering experiments were carried on antiferroelectric morphotropic-adjacent ceramics PZT 96/4 and 95/5. They revealed a complicated and hysteretic thermal behaviour due to the complex microstructure and the coexistence of regions with different phases within the samples. On heating and cooling the ceramics show two intermediate polar phases between the cubic paraelectric and the low temperature antiferroelectric phase, which are attained under different experimental conditions and specific pre-history. Our results suggest that ceramics with composition near PZT 95/5 are potential materials for novel room temperature device applications. An innovative approach of phase-control is proposed, based on the thermal behaviour of the intermediate polar states observed and using small temperature gradients by appropriate heating-cooling cycles.

Densest-known packings and phase behavior of hard spherical capsids.

By mostly using Monte Carlo numerical simulation, this work investigates the densest-known packings and phase behavior of hard spherical capsids, i.e., hard infinitesimally thin spherical caps with a subtended angle larger than the straight angle. The infinitely degenerate densest-known packings are all characterized by hard spherical capsids that interlock and can be subdivided into three families. The first family includes crystalline packings that are constructed by suitably rotating and stacking layers of hexagonally arranged and suitably tilted hard spherical capsids; depending on the successive rotations, the crystalline packings of this family can become the face-centered cubic crystal, the hexagonal close-packed crystal, and their infinitely degenerate variants in the hard-sphere limit. The second family includes crystalline packings that are characterized by rhombic motifs; they all become the face-centered cubic crystal in the hard-sphere limit. The third family includes crystalline packings that are constructed by suitably shifting and stacking layers in which hard spherical capsids are arranged in tightly packed, straight or zigzag, columns; depending on the successive shifts, the crystalline packings of this family can become the face-centered cubic crystal, the hexagonal close-packed crystal, and their infinitely degenerate variants in the hard-sphere limit. In the plane number density vs subtended angle, the phase diagram of hard spherical capsids features a hexagonal columnar liquid-crystalline phase, toward the hard-hemispherical-cap limit, and a plastic-crystalline phase, toward the hard-sphere limit, in addition to the isotropic fluid phase and crystalline phases. On departing from the hard-sphere limit, the increasing propensity of hard spherical capsids to interlock progressively disfavors the plastic-crystalline phase while favoring auto-assemblage into mostly dimeric interlocks in the denser isotropic fluid phase so that a purely entropic isotropic-fluid-plastic-crystal-isotropic-fluid re-entrant sequence of phase transitions is observed in systems of hard spherical capsids with a subtended angle intermediate between the straight angle and the complete angle.

Bragg–Williams Theory for Particles with a Size-Modulating Internal Degree of Freedom

The field of soft matter teems with molecules and aggregates of molecules that have internal size-modulating degrees of freedom. Proteins, peptides, microgels, polymers, micelles, and even some colloids can exist in multiple—often just two dominating—states with different effective sizes, where size can refer to the volume or to the cross-sectional area for particles residing on surfaces. The size-dependence of their accessible states renders the behavior of these particles pressure-sensitive. The Bragg–Williams model is among the most simple mean-field methods to translate the presence of inter-particle interactions into an approximate phase diagram. Here, we extend the Bragg–Williams model to account for the presence of particles that are immersed in a solvent and exist in two distinct states, one occupying a smaller and the other one a larger size. The basis of the extension is a lattice–sublattice approximation that we use to host the two size-differing states. Our model includes particle–solvent interactions that act as an effective surface tension between particles and solvent and are ignorant of the state in which the particles reside. We analyze how the energetic preference of the particles for one or the other state affects the phase diagrams. The possibility of a single phase-two phases-single phase sequence of phase transitions as a function of increasing temperature is demonstrated.

Origin of reentrant relaxor formation in ferroelectric solid solutions

Nanoscale compositional heterogeneity created by doping in ferroelectric systems leads to the formation of conventional relaxors in most cases, but reentrant relaxors in some unusual cases. It has remained a long-standing puzzle why reentrant relaxors rather than conventional relaxors form in these unusual cases. In this study, we use a binary ferroelectric system having a solid solution of a ferroelectric with cubic (C) to tetragonal (T) transition at one end and a ferroelectric with C to rhombohedral (R) transition at the other end, with nanoscale compositional heterogeneities, to reveal the origin of the reentrant relaxor transition. Our phase field simulations based on Landau theory demonstrate that the reentrant relaxor transitions in such a system are manifested by the formation of R nanodomains in the T microdomains upon cooling at compositions near the T/R phase boundary, which is accompanied by frequency-dependent permittivity peaks below ${T}_{\mathrm{C}}$. We found that the difference in phase transition sequence at different local compositions near the T/R phase boundary created by point defect doping is essential for the formation of reentrant relaxors. This work unravels the general conditions for the formation of reentrant relaxors and may shed light on the origin of other reentrant ferroic glasses.

Low-Temperature Fabrication of Plate-like α-Al2O3 with Less NH4F Additive.

Fluorinated compounds are effective mineralization agents for the fabrication of plate-like α-Al2O3. However, in the preparation of plate-like α-Al2O3, it is still an extremely challenging task to reduce the content of fluoride while ensuring a low synthesis temperature. Herein, oxalic acid and NH4F are proposed for the first time as additives in the preparation of plate-like α-Al2O3. The results showed that plate-like α-Al2O3 can be synthesized at a low temperature of 850 °C with the synergistic effect of oxalic acid and 1 wt.% NH4F. Additionally, the synergistic effect of oxalic acid and NH4F not only can reduce the conversion temperature of α-Al2O3 but also can change the phase transition sequence.

Structural studies and magnetism of Dy6Ni2.43Sn0.5 stannide

Intermetallic compound Dy6Ni2.43Sn0.5 was prepared by arc melting and annealing at 873 K. It was characterized by X-ray powder diffraction, differential thermal analysis, and electron probe microanalysis. The crystal structure of low temperature Dy6Ni2.43Sn0.5 phase belongs to the orthorhombic Ho6Co2Ga structure type (space group Immm, a = 0.93116(1) nm, b = 0.94993(1) nm, c = 0.98947(1) nm). Crystal structure refinements showed the deviation from the ideal 6:2:1 stoichiometry corresponding to the formula Dy6Ni2.43Sn0.5. It exhibits a sequence of magnetic phase transitions; antiferromagnetic ordering sets in at 60 K, while further order-order magnetic phase transitions take place at lower temperatures.

Pressure-induced successive phase transitions and Fano resonance engineering in lead-free piezoceramics KNbO3

We carried out detailed high-pressure Raman scattering studies on lead-free perovskite piezoceramics KNbO3 (KNO) with a smaller pressure interval so that the successive phase transition pressures are precisely determined (PO→T: 8.2 GPa; PT→C: 10.9 GPa). The pressure-quenched phase does not return to the ambient orthorhombic phase completely. In this work, the pressure-induced phase transition sequence is discussed in detail. Also, based on the pressure-dependent Raman spectra of KNO, we have discovered clear evidence of a pressure-induced Fano resonance. It reveals the role played by the Raman-active continuum and discrete energy states in the condensed matter system under high pressure and enables the high-pressure Fano resonances engineering in arbitrary geometries [q∈(−∞, 0)]. These findings highlight the opportunity for designing and tuning the wave transmission and optical switch properties of lead-free piezoceramics KNO via high pressure.

Effect of flexibility of arms on the nematic ordering of V-shaped molecules

The nematic ordering of a melt of V-shaped molecules composed of two semiflexible arms connected by one end at an external angle α is inspected within the Landau theory of phase transitions. The conformational statistics of arms is described by a discrete worm-like chain model. It is found that for a sufficiently high stiffness of the arms the phase diagram contains the regions of stability of isotropic (I), prolate (NU+) and oblate (NU−) uniaxial, and biaxial (NB) nematic phases. The isotropic–uniaxial nematic and uniaxial nematic – biaxial nematic phase transitions are of the first and second order, respectively. At lowering the stiffness of arms the stability area of NB phase decreases; the re-entrant NB phase becomes possible in the certain range of angles α and the sequence of phase transitions I−NU−−NB−NU+−NB occurs. At a sufficiently low stiffness, the stability region of the nematic biaxial phase disappears completely, and phase diagrams display only the first order transitions between I−NU+, I−NU−, and NU+−NU− phases. A further decrease in the stiffness of arms leaves only the first order I−NU+ transition in the phase diagrams. The tricritical points appear at the curves of NU−−NB transition in the phase diagrams in certain intervals of values of the stiffness parameter.

The Role of MX Carbonitrides for the Particle-Stimulated Nucleation of Ferrite in Microalloyed Steel

The role of titanium and vanadium carbonitrides (Ti,V)(C,N) of the MX series on the austenite-to-ferrite transformation in microalloyed steels and their potential of acting as pre-nuclei for intragranular ferrite nucleation is examined experimentally and by computational simulation. Thermal treatments and single-hit compression tests are performed on a dilatometer and a Gleeble® 3800 thermomechanical simulator to investigate the phase transition and precipitation sequences within microalloyed steel. The analysis of the microstructure and examination of formed precipitates is carried out by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In situ observations are realized via high-temperature laser scanning confocal microscopy (HT-LSCM). The experimental results are compared to kinetic precipitation simulations for MX precipitates’ particle-stimulated intragranular ferrite formation using an on-particle nucleation model for ferrite on the surface of the MX carbonitrides. A particular focus is the comparison of precipitation trends between non-deformed and deformed samples. The physically appropriate modeling of MX precipitation kinetics in combination with intragranular ferrite formation serves as a basis for future industrial process optimizations without extensive experimental work.

Joint experimental and theoretical study of bulk Y2O3 at high pressure

We report a joint experimental and theoretical study of the structural and vibrational properties of C-type bulk Y2O3 under hydrostatic compression. The combination of high-pressure X-ray diffraction and Raman scattering experimental measurements with ab initio theoretical calculations on bulk Y2O3 allows us to confirm the cubic (C-type) – monoclinic (B-type) – trigonal (A-type) phase transition sequence on the upstroke and the trigonal-monoclinic phase transition on the downstroke. This result reconciles with the results already found in related rare-earth sesquioxides of cations with similar ionic radii as Y, such as Ho2O3 and Dy2O3, and ends with the controversy regarding the existence of the intermediate monoclinic phase between the cubic and trigonal phases in pure bulk Y2O3 on the upstroke. As a byproduct, the good agreement between experimental and calculated results allows us to use extensive theoretical data to discuss the structural and vibrational behavior of the three phases of Y2O3 under compression, thus allowing a more detailed understanding of the effect of pressure on rare-earth sesquioxides than previous studies.