Coexistence Of Multiple Phases Research Articles

Nano-Ferroelectric for High Efficiency Overall Water Splitting under Ultrasonic Vibration.

Piezocatalysis, converting mechanical vibration into chemical energy, has emerged as a promising candidate for water-splitting technology. However, the efficiency of the hydrogen production is quite limited. We herein report well-defined 10 nm BaTiO3 nanoparticles (NPs) characterized by a large electro-mechanical coefficient which induces a high piezoelectric effect. Atomic-resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and scanning probe microscopy (SPM) suggests that piezoelectric BaTiO3 NPs display a coexistence of multiple phases with low energy barriers and polarization anisotropy which results in a high electro-mechanical coefficient. Landau free energy modeling also confirms that the greatly reduced polarization anisotropy facilitates polarization rotation. Employing the high piezoelectric properties of BaTiO3 NPs, we demonstrate an overall water-splitting process with the highest hydrogen production efficiency hitherto reported, with a H2 production rate of 655 μmol g-1 h-1 , which could rival excellent photocatalysis system. This study highlights the potential of piezoelectric catalysis for overall water splitting.

Autothermal dry reforming of methane with a nickel spinellized catalyst prepared from a negative value metallurgical residue

In this study, the performances of the nickel upgraded slag oxides (Ni-UGSO) catalyst on autothermal dry reforming (ATDR) of methane have been assessed. This catalyst, formulated from a negative value mining residue had been reported in recent studies and had shown good performances during methane steam reforming. At the experimental conditions range: T = 850 °C, molar ratios of CH4/O2 = 2 and CH4/CO2 = 3 and space velocity (GHSV) = 4500+/-100 ml/(h.gcat)STP, the catalyst displayed the best performances: 2 days stability without any deactivation, undetectable carbon formation, CH4 conversion of 98% and 98.8% (H2) and 95.5% (CO) yields. The apparent steady state operation is characterized by the coexistence of multiple phases in the catalyst structure such as iron, nickel, nickel oxide (NiO), nickel magnesium oxide (Ni,Mg)O, iron nickel NiFe and traces of spinel elements. At the tested temperatures and GHSV, the studied catalyst showed high activity (reaching near-chemical equilibrium state) with no detectable coke deposition. Moreover, the catalyst’s activity remained constant over time-on-stream. Ni-UGSO is derived from a Ni-decorated negative value metallurgical residue and its cost is well below all market-available reforming catalysts.This new low cost metallurgical waste-derived catalyst proves a potential good candidate for successful industrial dry reforming processes aimed at sustainable production of fuels from renewable sources i.e. biogas to liquid (BTL) fuels synthesis.

Probing lattice dynamics and electron-phonon coupling in the topological nodal-line semimetal ZrSiS

Topological materials provide an exclusive platform to study the dynamics of relativistic particles in table-top experiments and offer the possibility of wide-scale technological applications. ZrSiS is a newly discovered topological nodal-line semimetal and has drawn enormous interests. In this report, we have investigated the lattice dynamics and electron-phonon interaction in single crystalline ZrSiS using Raman spectroscopy. Polarization and angle resolved measurements have been performed and the results have been analyzed using crystal symmetries and theoretically calculated atomic vibrational patterns along with phonon dispersion spectra. Wavelength and temperature dependent measurements show the complex interplay of electron and phonon degrees of freedom, resulting in resonant phonon and quasielastic electron scatterings through inter-band transitions. Our high-pressure Raman studies reveal vibrational anomalies, which were further investigated from the high-pressure synchrotron x-ray diffraction (HPXRD) spectra. From HPXRD, we have clearly identified pressure-induced structural transitions and coexistence of multiple phases, which also indicate possible electronic topological transitions in ZrSiS. The present study not only provides the fundamental information on the phonon subsystem, but also sheds some light in understanding the topological nodal-line phase in ZrSiS and other iso-structural systems.

Effects of residual strain on magnetoresistance in heterostructures of La0.9Ca0.1MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3

Epitaxial thin films of La0.9Ca0.1MnO3 were grown on (100) oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) single-crystal substrates by pulsed laser deposition. The heterostructure composed of manganite and ferroelectric would enable us to control and study the electric/magnetic properties of manganite by means of tunable strain induced via the piezoelectric effect of PMN-PT. X-ray diffraction revealed that the ferroelectric poling remarkably modify the in-plane tensile strain introduced in the La0.9Ca0.1MnO3 film, which gives rise to tune resistance and Curie temperature TC. It is also found that due to poling of PMN-PT the magnetoresistance of La0.9Ca0.1MnO3 is reduced in the paramagnetic state, but increased in ferromagnetic state. The observed effects of residual strain on magnetoresistance could be explained by the Jahn-Teller distortion and polarization effect on the coexistence of multiple phases.

Electrocaloric response near room temperature in Zr- and Sn-doped BaTiO 3 systems

The electrocaloric effect (ECE) in (1-x)BaZr0.18 Ti0.82O3-(x)BaSn0.11Ti0.89O3 (BZT18-BST11, 0.1≤x≤0.5) ceramics is investigated near room temperature using a calorimetry method. The ceramics exhibit relaxor-like ferroelectric characteristics and by merging phases, a large electrocaloric (EC) response is observed in the system. The largest entropy change is 4.8 Jkg(-1) K(-1) (along with a temperature change of 3.5 K), which is induced under an electric field of 10 MV m(-1) for the 0.8 BaZr0.18Ti0.82O3-0.2 BaSn0.11Ti0.89O3 ceramics. This result reveals that the coexistence of multiple phases improves the ECE of the ceramics, which provides an effective route to achieve a large EC response using a small electric field.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.

Low temperature preparation and electric properties of highly (100)-oriented (Na0.85K0.15)0.5Bi0.5TiO3 thin films prepared by a sol–gel route

Lead-free (Na0.85K0.15)0.5Bi0.5TiO3 thin films with different spin-coating layers were fabricated by a sol–gel process using a 15nm-thick Pb0.8La0.1Ca0.1Ti0.975O3 seed layer. The phase composition and electrical properties of the thin films were investigated in detail. X-ray diffraction analysis indicated that low-temperature growth of (Na0.85K0.15)0.5Bi0.5TiO3 thin films—as low as 500°C—was successfully achieved. The films, comprising six layers, featured both rhombohedra and tetragonal phases near the morphotropic phase boundary. In contrast, the films fabricated with 9 and 12 layers consisted of single phase only. Additionally, a slow interfacial diffusion was achieved, owing to the low crystallization temperature. The six-layer thin film that crystallized at a low temperature exhibited enhanced electrical properties when the external electric field was small, which was attributed to the co-existence of multiple phases and an unhindered interface.

Nanoscale phase mixture in uniaxial strained BiFeO3 (110) thin films

A strain-induced nanoscale phase mixture in epitaxial BiFeO3 (110) films is investigated. High resolution synchrotron x-ray diffraction shows that a monoclinic M2 phase (orthorhombic-like, with a c/a ∼ 1.01) coexists as the intermediate phase between monoclinic M1 phase (tetragonal-like, with a c/a ∼ 1.26) and monoclinic M3 phase (rhombohedral-like, with a c/a ∼ 1.00), as the film thickness increases from 10 to 190 nm. Cross-sectional transmission electron microscopy images reveal the evolution of domain patterns with coexistence of multiple phases. The different ferroelectric polarization directions of these phases, as shown by piezoelectric force microscopy, indicate a strong potential for high electromechanical response. The shear strain ϵ13 is found to be a significant driving factor to reduce strain energy as film thickness increases, according to our theoretical calculations based on the measured lattice parameters. The nanoscale mixed phases, large structure distortions, and polarization rotations among the multiple phases indicate that (110)-oriented epitaxial films provide a promising way to control multifunctionalities of BiFeO3 and an alternative direction to explore the rich physics of perovskite system.

Theoretical Analysis of Multiple Phase Coexistence in Polyelectrolyte Blends

Advanced materials for energy storage and process technology are globally in high demand. Promising components for such materials are polymer blends and copolymers, because of their ability to self-assemble into nanostructured materials that combine desirable material properties, such as high mechanical stability with ion-selective conductivity. It remains a challenge to predict the nanoscale structure in charged polymers, because of intricate ionic correlations that can influence the structure at many length scales. Here we present a free energy analysis of charged polymer blends with electrolytes of the primitive model, by combining a Flory–Huggins-type theory with liquid-state methods, relying on the Ornstein–Zernike equation. We find that different mechanisms, driven by entropy and electrostatics, can stimulate or repress phase separation in blends. These mechanisms can enable the existence of a triple point, where three phases can coexist in an effective two-component blend. We analyze the influence ...

X-ray Absorption Spectroscopic Characterization of Solid Acid-base Catalysts

Recent X-ray absorption spectroscopic studies of solid acid-base catalysts were reviewed, which have been carried out to investigate the effects of lanthanum-ion addition to alumina for improving the thermal stability, acidity generation mechanism of siliceous mesoporous silica, or state of tungsten oxide species in WOx–ZrO2 strong solid acid. Remarks on EXAFS analytical procedures were also discussed about possibility of coexistence of multiple phases, mismatch between radial distribution functions and Fourier transforms of EXAFS, and effects of multiple excitations in an X-ray absorption spectrum.

Computational Modeling and Prediction of the Complex Time-Dependent Phase Behavior of Lyotropic Liquid Crystals under in Meso Crystallization Conditions

Membrane-bound proteins comprise a very important class of drug targets. Solution of their structures by X-ray crystallography has been hampered by difficulties in crystallizing them in biologically relevant conformations. Novel amphiphilic materials that form bicontinuous cubic phases are being used to support the growth of crystals. However the cubic phase may transit to other lipidic mesophase structures under the influence of the different components within the crystallization screen. Furthermore the mesophases may evolve with time, a process that is poorly understood but potentially critical for controlled crystal growth. Recent advances in high-throughput screening of lipid systems have allowed us to generate a large body of data on the influence of screen components on the cubic phase. However it has been difficult to deconvolute individual effects in the multicomponent system present during a crystallization trial. We have therefore developed robust and predictive computational models that predict how the phase behavior of lyotropic liquid crystals changes over time and under the influence of crystallization additives. Our work demonstrates that the complex phase behavior of amphiphilic nanostructured nanoparticles can be captured with high accuracy using modern, robust machine learning methods. We predicted the existence of individual nanophases with accuracies of 98–99% and the complex coexistence of multiple phases to a similar accuracy using nonlinear models: linear models were not as effective and robust. This approach also allowed us to determine which crystallization screen components were most relevant to the temporal evolution of individual mesophases.

Modulated superconductivity due to vacancy and magnetic order inAyFe2−x/2Se2[A=Cs, K, (Tl,Rb), (Tl,K)] iron-selenide superconductors

We present a calculation of a 'modulated' superconducting state in iron-selenide superconductors. The zero-momentum $d-$wave pairing breaks the translational symmetry of the conventional BaFe$_2$Se$_2$-like crystal of $I4/mmm$ space group. This pairing state becomes compatible when the Fe vacancies form an ordered state and the crystal symmetry changes to a low-temperature $I4/m$ one. For the specific case of an incommensurate vacancy order at $Q_v=(1/5, 3/5)$ in K$_{0.82(2)}$Fe$_{1.626(3)}$Se$_2$, we find that it induces a block checkerboard antiferromagnetic phase at wavevector ${\bf Q}_m=4{\bf Q}_v$. The coexistence of vacancy order and magnetic order leads to a reconstructed ground state which naturally couples to the $d-$wave superconductivity in a uniform phase in what we propose will be a general coupling for all iron-selenide superconductors. Our results agree with numerous experimental data available to date. We thus suggest that the incommensurability leads to uniform coexistence of multiple phases as a viable alternative to a nanoscale phase separation in high-$T_c$ superconductors and play an important role in the enhancement of superconductivity.

Thermal conductivity of hydrate‐bearing sediments

A thorough understanding of the thermal conductivity of hydrate‐bearing sediments is necessary for evaluating phase transformation processes that would accompany energy production from gas hydrate deposits and for estimating regional heat flow based on the observed depth to the base of the gas hydrate stability zone. The coexistence of multiple phases (gas hydrate, liquid and gas pore fill, and solid sediment grains) and their complex spatial arrangement hinder the a priori prediction of the thermal conductivity of hydrate‐bearing sediments. Previous studies have been unable to capture the full parameter space covered by variations in grain size, specific surface, degree of saturation, nature of pore filling material, and effective stress for hydrate‐bearing samples. Here we report on systematic measurements of the thermal conductivity of air dry, water‐ and tetrohydrofuran (THF)‐saturated, and THF hydrate–saturated sand and clay samples at vertical effective stress of 0.05 to 1 MPa (corresponding to depths as great as 100 m below seafloor). Results reveal that the bulk thermal conductivity of the samples in every case reflects a complex interplay among particle size, effective stress, porosity, and fluid‐versus‐hydrate filled pore spaces. The thermal conductivity of THF hydrate–bearing soils increases upon hydrate formation although the thermal conductivities of THF solution and THF hydrate are almost the same. Several mechanisms can contribute to this effect including cryogenic suction during hydrate crystal growth and the ensuing porosity reduction in the surrounding sediment, increased mean effective stress due to hydrate formation under zero lateral strain conditions, and decreased interface thermal impedance as grain‐liquid interfaces are transformed into grain‐hydrate interfaces.

Phase coexistence and line tension in ternary lipid systems

The ternary system consisting of cholesterol, a saturated lipid, and an unsaturated one exhibits a rich phase behavior with multiple phase coexistence regimes. Remarkably, phase separation even occurs when each of the three binary systems consisting of two of these components is a uniform mixture. We use a Flory-Huggins like model in which the phase separation of the ternary system is a consequence of an interaction between all three components to describe the system. From the associated Gibbs free energy we calculate phase diagrams, spinodals, and critical points. Moreover, we use a Van der Waals/Cahn-Hilliard like construction to derive an expression for the line tension between coexisting phases. We show how the line tension depends on the position in the phase diagram, and give an explicit expression for the concentration profile at the phase boundary.

Microcanonical analysis of association of hydrophobic segments in a heteropolymer

Using the replica-exchange multicanonical Monte Carlo simulation, the intra-association of hydrophobic segments in a heteropolymer was numerically investigated by the microcanonical analysis method. We demonstrated that the microcanonical entropy shows the features of one or multiple convexes in the association transition region depending on the number and distribution of hydrophobic segments in the chain. We found that one or multiple negative specific heats imply a first-order-like transition with the coexistence of multiple phases.

Strain effects in perovskite manganites

Colossal magnetoresistance (CMR) - the dramatic reduction of electrical resistivity in a magnetic field and charge ordering (CO) in rare earth manganites of the type Ln(1-x)A(x)MnO(3) [Ln: rare earth, A: divalent cation] are manifestations of the intricate relation between orbital, spin, charge and lattice degrees of freedom. Recent studies indicate that the inhomogeneous state of mixed-valence manganites - evidenced by the presence of texture and multiple phase coexistence - is important for the CMR property of manganites. Theoretical models that explain the multiphase coexistence are based on quenched disorder or strain. Here we show that lattice strain due to the Jahn-Teller (IT) distortions of MnO6 octahedra and their tilt rotations are not sufficient to provide a unique structure-property relation. We present evidence that the science of manganites should take into account shear distortions of the MnO6 octahedra as well. Pressure evolution of the lattice strain of Nd0.5Ca0.5MnO3 shows a minimum around 7 GPa, with the same lattice strain above and below this pressure achieved by shear- and JT-type distortions, respectively. In general, a particular lattice strain of manganites can be achieved by different combinations of JT-type and shear-type distortions of the MnO6 octahedra, together with their tilts, which suggest a plausible description of the inhomogeneous state in manganites as one where phases with differently distorted states having the same lattice strain are preserved. (C) 2007 Published by Elsevier Ltd.

Effect of Al content on the phase structure and the hydrogenation characteristic of La(Mg 1- xAl x) alloys

La(Mg 1- x Al x ) ( x = 0.2, 0.4, 0.6, 0.8) alloys have been prepared using induction melting followed by annealing. It is found that partial substitution of Mg by Al does not lead to a change in crystal structure, and the alloys have a single LaMg phase when x ≤ 0.4. The lattice parameter of the LaMg phase decreases obviously after the partial substitution of Mg by Al. However, further substitution of Mg by Al leads to the coexistence of multiple phases when x ≥ 0.6. The alloys consist of the LaMg, LaAl, LaAl 2, and La 5Al 4 phases. The LaMg phase decreases, whereas the La 5Al 4 phase increases with the increase in x. The Al-substituted La(Mg 0.6Al 0.4) alloy can be hydrogenated into the tetragonal LaH 3, cubic LaH 3, MgH 2, and LaAl under 5 MPa at 473 K for 5 d.

Structural and hydriding studies of Ca(Al 1− xNi x) 2 alloys

The phase relations and hydriding characteristics of the Ca(Al 1− x Ni x ) 2 ( x = 0, 0.1, 0.2, 0.3 and 0.4) alloys were studied. It was found that partially substituting Al by Ni in CaAl 2 alloy leads to the co-existence of multiple phases. When x = 0.1, 0.2 and 0.3, the alloys consist of C15, Ca 13Al 14 and NiAl phases. The amount of the C15 phase decreases with the increase of x. Further increasing x to 0.4, the C15 phase disappears entirely and the alloy contains Ca 8Al 3, NiAl and Ca. The partial substitution of Al by Ni cannot effectively improve the hydrogen storage properties of CaAl 2 because the lattice parameter of the C15 phase decreases slightly after the substitution.

Voltammogram spikes interpreted as envelopes of spikes resulting from electrode crystals of various sizes: Application to the UPD of Cu on Au(111)

We study and explain shapes of voltammogram spikes, observed during underpotential deposition (UPD) on electrode surfaces, as averaged envelopes of mutually shifted spikes associated with first-order phase transitions that occur in crystalline domains of various sizes that are formed on the electrode surface. This concept, already used in our previous work for two-phase systems and symmetric voltammogram spike shapes, is here substantially generalized to systems with multiple-phase coexistence and asymmetric spike shapes, using the rigorous statistical mechanical techniques of Borgs and Kotecký. Rather than mere numerical plots, we extract explicit functions that accurately describe the spike shapes. For the sake of clarity, we present our analysis and apply our results to fit the voltammogram of the UPD of Cu on Au(111) in sulfuric acid medium. This voltammogram shows two distinct spikes with a broad foot region near the spike at higher potentials. As was done in earlier treatments, we explain each of the two spikes as a result of a first-order transition. Here, though, the spikes are obtained as envelopes of closely spaced spikes resulting from crystals of various sizes. In contrast to earlier studies, however, we also explain the foot region in the same way. The foot’s shape, despite its large width and small height, can be equally well obtained as an envelope of shifted crystal spikes that are broader and smaller than those giving rise to the two distinct spikes. We achieve very good agreement with experiment.

Phase transitions and the weak relaxor ferroelectric phase in Ba7/8(La0.5Na0.5)1/8TiO3

In order to elucidate the correlation between the phase transitions and presence ofa relaxor phase in the composition Ba1−x(La0.5Na0.5)xTiO3, intermediate betweenBaTiO3 and (La0.5Na0.5)TiO3, measurements of the dielectric permittivity inthe temperature range 10–500 K in a heating–cooling cycle and powderx-ray diffraction measurements down to 150 K have been carried out onBa7/8(La0.5Na0.5)1/8TiO3, which has three phase transitions and weak relaxorferroelectric character below 500 K. The hysteresis observed in the dielectricpermittivity in the heating–cooling cycle and a weak tetragonality deduced from thec/aratio being very close to the cubic ratio for the tetragonal phase leadto the prediction of coexistence of multiple phases. The shrinkages ofthe temperature regions of the low-symmetry phases with increasingx inBa1−x(La0.5Na0.5)xTiO3 confirm this prediction. The deformation mode changes gradually fromΓ15 toR25with increase of x.The relation between the deformation mode and the phase transitions is alsodiscussed.

Hofstadter butterfly as quantum phase diagram

The Hofstadter butterfly is viewed as a quantum phase diagram with infinitely many phases, labeled by their (integer) Hall conductance, and a fractal structure. We describe various properties of this phase diagram: We establish Gibbs phase rules; count the number of components of each phase, and characterize the set of multiple phase coexistence.