Temperature-driven Phase Transition Research Articles

Unusual temperature-induced swelling of ionizable poly(N-isopropylacrylamide)-based microgels: experimental and theoretical insights into its molecular origin.

In the traditional view of temperature-driven volume phase transitions in PNIPAM-based microgel solutions, a monotonic and sharp decrease in the particle size occurs upon heating the solution to above the volume phase transition temperature (VPTT). However, at sufficiently high microgel concentrations and under low salt conditions, our dynamic light scattering experiments reveal an unexpected non-monotonic evolution of the particle size when increasing the solution temperature. These findings show that poly(N-isopropylacrylamide-co-methacrylic acid) (P(NIPAM-co-MAA)) microgels swell upon heating the solution in the temperature range where NIPAM is water-soluble (i.e., below the VPPT). Further heating the microgel solution leads to microgel collapse as typically observed at temperatures above the VPTT. This novel behavior depends on the particle and salt concentration. We have observed the expected monotonic temperature-response of P(NIPAm-co-MAA) microgel solutions at low particle density and high salt concentration. To gain insights into the molecular origin of the unusual behavior of these microgel solutions, we have combined nuclear magnetic resonance studies and molecular-level theoretical calculations of the system. A delicate balance between inter-particle steric compressions and intra-microgel physical interactions and chemical equilibria determines the size of these microgels. Both steric compression, due to finite density, and hydrogen bond formation in the interior of the microgels favors a more compact particle. On the contrary, at the pH of the experiments the acid-base equilibrium constrains the polymer charge to increase, which favors particle swelling due to intra-microgel electrostatic repulsions. This interplay between physical interactions and chemical equilibria occurring at the nanometer length-scale determines the unusual thermal-induced swelling of P(NIPAM-co-MAA) microgels.

Acoustic phonon behavior of PbWO4 and BaWO4 probed by low temperature Brillouin spectroscopy

Temperature dependent acoustic phonon behavior of PbWO4 and BaWO4 using Brillouin spectroscopy has been explained for the first time. Low temperature Brillouin studies on PbWO4 and BaWO4 have been carried out from 320–20K. In PbWO4, we observe a change in acoustic phonon mode behavior around 180K. But in the case of BaWO4, we have observed two types of change in acoustic phonon mode behavior at 240K and 130K. The change in Brillouin shift ω and the slope dω/dT are the order parameter for all kinds of phase transitions. Since we do not see hysteresis on acoustic phonon mode behavior in the reverse temperature experiments, these second order phase transitions are not related to structural phase change and could be related to acoustic phonon coupled electronic transitions. In PbWO4, the temperature driven phase transition at 180K could be due to changes in the environment around the lead vacancy (VPb2−) changes the electronic states. In the case of BaWO4, the phase transition at 240K shows the decrease in penetration depth of WO3 impurity. So it becomes more metallic. The transition at 130K could be the same electronic transitions as that of PbWO4 as function of temperature. The sound velocity and elastic moduli of BaWO4 shows that it could be the prominent material for acousto-optic device applications.

Multicanonical analysis of the plaquette-only gonihedric Ising model and its dual

The three-dimensional purely plaquette gonihedric Ising model and its dual are investigated to resolve inconsistencies in the literature for the values of the inverse transition temperature of the very strong temperature-driven first-order phase transition that is apparent in the system. Multicanonical simulations of this model allow us to measure system configurations that are suppressed by more than 60 orders of magnitude compared to probable states. With the resulting high-precision data, we find excellent agreement with our recently proposed nonstandard finite-size scaling laws for models with a macroscopic degeneracy of the low-temperature phase by challenging the prefactors numerically. We find an overall consistent inverse transition temperature of β∞=0.551334(8) from the simulations of the original model both with periodic and fixed boundary conditions, and the dual model with periodic boundary conditions. For the original model with periodic boundary conditions, we obtain the first reliable estimate of the interface tension σ=0.12037(18), using the statistics of suppressed configurations.

Analysis of multi-step transitions in spin crossover nanochains

The temperature driven phase transition occurring in spin crossover nanochains has been studied by an Ising-like model considering both short-range and long-range interactions. Various types of spin crossover profiles have been described in this framework, including a novel three-step transition identified in a nanosystem with eight molecules, which is modeled for the first time. A special interest has been also given to stepwise transitions accompanied by two hysteresis loops. The edge and size effects on spin crossover behavior have been investigated in order to get a deeper insight of the underlying mechanisms involved in these unusual spin transitions.

New Phase Transitions of the Ising Model on Cayley Trees

We show that the nearest neighbors Ising model on the Cayley tree exhibits new temperature driven phase transitions. These transitions holds at various inverse temperatures different from the critical one. They are depicted by a change in the number of Gibbs states as well as by a drastic change of the behavior of free energies at these new transition points.

Reinterpretation of the bond-valence model with bond-order formalism: An improved bond-valence-based interatomic potential for PbTiO3

We present a modified bond-valence model of PbTiO${}_{3}$ based on the principles of bond-valence and bond-valence vector conservation. The relationship between the bond-valence model and the bond-order potential is derived analytically in the framework of a tight-binding model. An energy term, bond-valence vector energy, is introduced into the atomistic model and the potential parameters are reoptimized. This model potential can be applied both to canonical-ensemble (NVT) and isobaric-isothermal ensemble (NPT) molecular dynamics (MD) simulations. This model reproduces the experimental phase transition in NVT MD simulations and also exhibits the experimental sequence of temperature-driven and pressure-driven phase transitions in NPT simulations. We expect that this improved bond-valence model can be applied to a broad range of inorganic materials.

Investigation of Temperature-Driven Ferroelectric Phase Transition via Modified Heisenberg Model: The Monte Carlo Simulation

This work studies the temperature-driven ferroelectric phase transition of ferroelectric polarization under the absence of electric field. The modified Heisenberg model in three dimensions was considered and simulated via Monte Carlo simulation. The Metropolis algorithm and the periodic boundary condition were employed. The dependence of electric polarization on temperature was investigated to define the ferroelectric phases and their structural phase transitions. From the results, with well-defined set of relevant temperature parameters, the phase dependent polarization-behavior was found with a sudden change in its behaviors at the transition points. The structure factors were also considered and supported these phase changes. This conclusively pinpointed the important of temperature-dependent parameters in modeling ferroelectric materials.

Improving the efficiency of Monte Carlo simulations of systems that undergo temperature-driven phase transitions

Recently, Velazquez and Curilef proposed a methodology to extend Monte Carlo algorithms based on a canonical ensemble which aims to overcome slow sampling problems associated with temperature-driven discontinuous phase transitions. We show in this work that Monte Carlo algorithms extended with this methodology also exhibit a remarkable efficiency near a critical point. Our study is performed for the particular case of a two-dimensional four-state Potts model on a square lattice with periodic boundary conditions. This analysis reveals that the extended version of Metropolis importance sampling is more efficient than the usual Swendsen-Wang and Wolff cluster algorithms. These results demonstrate the effectiveness of this methodology to improve the efficiency of MC simulations of systems that undergo any type of temperature-driven phase transition.

Role of Dynamical Instability in theAb InitioPhase Diagram of Calcium

In the 32-119 GPa pressure range and at room temperature, a simple cubic phase was reported for calcium in many different experiments. Standard linear response theory, both within density functional perturbation theory and frozen phonon calculations, presents dynamical instabilities for the simple cubic structure in the whole pressure range. Many other possible candidate phases, as well as several possible stabilization mechanisms for the simple cubic phase, have been proposed as the result of ab initio predictions but the role of temperature on the relative stability of the different phases has not been systematically investigated. We revisit the stability of the three most important candidate phases of calcium for the intermediate pressure range and for various temperatures, taking explicitly into account thermal corrections relative to electronic as well as phononic entropy and anharmonic contributions. This corrects the discrepancies among previous theoretical results and experiments and presents a different picture of the temperature driven phase transition, which results from dynamical anharmonic stabilization of simple cubic and destabilization of the tetragonal phase.

Size-driven ferroelectric–paraelectric phase transition in TGS nanocomposites

Dielectric properties of porous glass nanocomposites with TGS crystals embedded into six porous matrices with average pore size from 5 to 312 nm were investigated in the temperature range from 280 to 380 K at selected frequencies. The results are discussed based on the effect of the particle size on the phase transition temperature of TGS nanocomposites. Temperature–size phase diagram of TGS composites was derived. Non-monotonic character of the temperature-driven phase transition (T p) with the decreasing particle size was determined. The nature of the T p variation can be ascribed to the size-effect theoretically predicted by Zhong et al. (Phys Rev B 50:698–703, 1994).

Flexible Thermochromic Window Based on Hybridized VO2/Graphene

Large-scale integration of vanadium dioxide (VO2) on mechanically flexible substrates is critical to the realization of flexible smart window films that can respond to environmental temperatures to modulate light transmittance. Until now, the formation of highly crystalline and stoichiometric VO2 on flexible substrate has not been demonstrated due to the high-temperature condition for VO2 growth. Here, we demonstrate a VO2-based thermochromic film with unprecedented mechanical flexibility by employing graphene as a versatile platform for VO2. The graphene effectively functions as an atomically thin, flexible, yet robust support which enables the formation of stoichiometric VO2 crystals with temperature-driven phase transition characteristics. The graphene-supported VO2 was capable of being transferred to a plastic substrate, forming a new type of flexible thermochromic film. The flexible VO2 films were then integrated into the mock-up house, exhibiting its efficient operation to reduce the in-house temperature under infrared irradiation. These results provide important progress for the fabrication of flexible thermochromic films for energy-saving windows.

Tracking competitive lattice distortions in strongly correlated VO2-based systems: A temperature-dependent EXAFS study

Temperature-driven phase transitions of V1−xCrxO2 (with x = 0.00 and 0.007) have been studied by means of vanadium K-edge X-ray absorption spectroscopy between 50 and 350 K. X-ray absorption near-edge structure (XANES) reveals the electronic changes occurring across the metal-insulator transition at ∼350 K. Extended X-ray absorption fine structure (EXAFS) spectroscopy enlightens the modification occuring to the VO6 octahedra across the different monoclinic (M1, M2, M3) and rutile (R) structures. The EXAFS results provide a complete local structural characterization of the competing lower-symmetry phases of the V1−xCrxO2 (with x = 0.00 and 0.007) and shed light on the major role of lattice relaxation energy in the degradation properties of several devices based on the metal-insulator transition.

Structural study of SrPrZnRuO6, SrPrCoRuO6, SrPrMgRuO6 and SrPrNiRuO6 double perovskite oxides by symmetry-adapted mode analysis

Powder SrPrZnRuO6, SrPrCoRuO6, SrPrMgRuO6 and SrPrNiRuO6 double perovskites were synthesized by the solid-state reaction method, and their crystal structure was investigated by Rietveld analysis (using the symmetry-mode procedure) by x-ray, synchrotron and neutron powder diffraction data. SrPrMRuO6 materials are monoclinic perovskites at room temperature, adopting the space group P21/n, , c ≈ 2ap, Z = 2. The unit cell parameters increase through the series as the B-cation size increases. The tilting of the octahedra is associated with irreducible representations GM+4 and X+3. In the studied series, Sr2+/Pr3+ are in eight-fold coordination and are displaced from the center of the Sr/PrO8 polyhedron along [010] by the X+5(A10) mode. The size of the first coordination sphere of Sr/Pr increases and the second coordination sphere reduces with temperature, suggesting an actual change in coordination. While changing the interatomic distances, the distortion of the structure diminishes, as observed in distortion mode amplitudes and octahedral tilt angles. The temperature driven phase transition was observed for Ni and Mg materials at ≈1025 K and 950 K, respectively. Tilting of the octahedra in the trigonal phase is associated with GM+4, which is the unique active mode associated with the not experimentally observed phase transition.

Dynamical Quantum Phase Transitions in the Transverse-Field Ising Model

A phase transition indicates a sudden change in the properties of a large system. For temperature-driven phase transitions this is related to nonanalytic behavior of the free energy density at the critical temperature: The knowledge of the free energy density in one phase is insufficient to predict the properties of the other phase. In this Letter we show that a close analogue of this behavior can occur in the real time evolution of quantum systems, namely nonanalytic behavior at a critical time. We denote such behavior a dynamical phase transition and explore its properties in the transverse-field Ising model. Specifically, we show that the equilibrium quantum phase transition and the dynamical phase transition in this model are intimately related.

Template synthesis, X-ray structure, spectral and redox properties of the paramagnetic alkylboron-capped cobalt(II) clathrochelates and their diamagnetic iron(II)-containing analogs

The n-butyl- and n-hexadecylboron-capped macrobicyclic iron and cobalt(II) tris-dioximates were synthesized by the direct template condensation of the aliphatic and aromatic α-dioximes with alkylboronic acids on either a Fe2+ or a Со2+ cation as a matrix. The clathrochelates obtained were characterized using elemental analysis, MALDI-TOF mass spectrometry, IR, UV–Vis, 1H and 13C{1H} NMR, 57Fe Mössbauer and EPR spectroscopies, magnetochemistry, and by X-ray crystallography; their redox properties were studied by cyclic voltammetry (CV). These low-spin complexes undergo no spin transitions in the temperature range from 5 to 300 K. Their trigonal prismatic (TP)–trigonal antiprismatic (TAP) molecular geometry is mainly governed by the nature of an encapsulated metal ion and that of their ribbed substituents. The encapsulated cobalt(II) ions are shifted from the centers of their CoN6 coordination polyhedra due to Jahn–Teller distortion, and their anisotropic displacement parameters proved the energetical preference for only one distorted structure. The hexadecylboron-capped clathrochelates undergo the temperature-driven phase transitions. The EPR and NMR data suggest that the length of the apical alkyl groups has no impact on the magnetic properties of the cobalt complexes, whereas that of the ribbed substituents affects a distribution of the spin density. The CV data showed that these electron-donor groups stabilize the oxidized and destabilize the reduced clathrochelate species.

Thermodynamics of spin-orbit-coupled Bose-Einstein condensates

In this paper we develop a quantum field approach to reveal the thermodynamic properties of the trapped BEC with the equal Rashba and Dresselhaus spin-orbit couplings. In the experimentally-feasible regime, the phase transition from the separate phase to the single minimum phase can be well driven by the tunable temperature. Moreover, the critical temperature, which is independent of the trapped potential, can be derived exactly. At the critical point, the specific heat has a large jump and can be thus regarded as a promising candidate to detect this temperature-driven phase transition. In addition, we obtain the analytical expressions for the specific heat and the entropy in the different phases. In the single minimum phase, the specific heat as well as the entropy are governed only by the Rabi frequency. However, in the separate phase with lower temperature, we find that they are determined only by the strength of spin-orbit coupling. Finally, the effect of the effective atom interaction is also addressed. In the separate phase, this effective atom interaction affects dramatically on the critical temperature and the corresponding thermodynamic properties.

Phase diagrams of strained Ca1–xCexMnO3 films

The transport and magnetic properties were examined of thin films of the electron-doped manganite Ca1−xCexMnO3 (CCMO; 0 ≤ x ≤ 0.1) deposited on three different single-crystalline oxide substrates: YAlO3, NdAlO3, and LaSrAlO4, which subject the films to a compressive strain, a practical lattice matching, and a tensile strain, respectively. The temperature dependence of the activation energy extracted from the resistivity–temperature curves clearly indicated the phase transitions in the CCMO films. A comparison with magnetization measurements revealed that regardless of the substrates, the temperature-driven phase transition of the CCMO films changed from the G-type antiferromagnetic transition at x = 0 to the canted antiferromagnetic transition and to the charge- and/or orbital-ordered transition, as x was increased. In the canted antiferromagnetic phase, the epitaxial strains have significant impact not only on the transport properties but also on the magnetic properties such as the spontaneous magnetization and the magnetic anisotropy, confirming that the competition between antiferromagnetic super-exchange and ferromagnetic double-exchange interactions results in the canted antiferromagnetic state in the electron-doped CCMO films. Based on the results of the transport and magnetic measurements, the temperature versus band-filling phase diagrams of the strained CCMO films on three different substrates were established.

Topological crystalline insulator states in Pb1−xSnxSe

Topological insulators are a class of quantum materials in which time-reversal symmetry, relativistic effects and an inverted band structure result in the occurrence of electronic metallic states on the surfaces of insulating bulk crystals. These helical states exhibit a Dirac-like energy dispersion across the bulk bandgap, and they are topologically protected. Recent theoretical results have suggested the existence of topological crystalline insulators (TCIs), a class of topological insulators in which crystalline symmetry replaces the role of time-reversal symmetry in ensuring topological protection. In this study we show that the narrow-gap semiconductor Pb(1-x)Sn(x)Se is a TCI for x = 0.23. Temperature-dependent angle-resolved photoelectron spectroscopy demonstrates that the material undergoes a temperature-driven topological phase transition from a trivial insulator to a TCI. These experimental findings add a new class to the family of topological insulators, and we anticipate that they will lead to a considerable body of further research as well as detailed studies of topological phase transitions.

Fast Protein-Binding Nanoparticles

Chemistry One possible use of synthetic polymeric nanoparticles is to target and bind protein in vivo. As such, these nanoparticles have potential for medical applications, but often their performance is lower in vivo than in vitro because of slow binding kinetics and nonspecific protein binding. Hoshino et al. borrowed the concept of “induced fit” for enzymatic binding to substrates to improve the binding kinetics of a polymeric nanoparticle. They used a poly- N -isopropylacrylamide (PNIPam) backbone, which has a temperature-driven phase transition from a flexible random coil to a rigid conformation. The protein target, concanavalin A, binds the sugar mannose. To prepare synthetic polymeric nanoparticles that recognized the target protein through multipoint interactions, the authors synthesized PINPam nanoparticles copolymered with p -acrylamidophenyl-α-d-mannopyranoside. Nanoparticles in the flexible conformation have faster binding kinetics than those in the rigid conformation, but the fastest kinetics was observed at the transition temperature between the swollen and collapsed phases. J. Am. Chem. Soc. 10.1021/ja306053s (2012).

Polarization of poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) thin films revealed by emission spectroscopy with computational simulation during phase transition

The electronic structure and self-polarization of P(VDF-TrFE) Langmuir-Blodgett nanofilms were analyzed under temperature-driven phase transitions, according to their thickness, composition, and structural conformation. Both thermo-stimulated exoelectron emission (TSEE) spectroscopy and computational simulation, including quantum-chemical calculations from first principles, were carried out. PVDF and composite P(VDF-TrFE) (70:30) molecular chains as Trans and Gauche conformers, as well as crystal cells, were modeled for these TSEE analyses. The quantum-chemical calculations and the computational simulation were based on the density functional theory (DFT) as well as semi-empirical (PM3) methods. It was demonstrated that the energy of electron states, as well as the total energies of the studied P(VDF-TrFE) molecular clusters during phase transformation, is influenced by electron work function and electron affinity. Analysis was performed by combining TSEE experimental data with the computational data of the molecular models, demonstrating the effectiveness of this joint approach. For the first time, TSEE was used for contactless measurements of nanofilm polarization, and characterization of the phase transition. The proposed new method can be widely applied in nanobiomedicine, particularly in development of new bone bio-implants, including built-in sensors (new smart nanotechnology).