Configurational Phase Transition Research Articles

First-principles prediction of configurational order-disorder phase transition in Mo2(Ti1−x V x )C2 MXene alloys

We investigated the thermodynamic stability of Mo2(Ti1−x V x )C2 MXenes (MTVCs), using the first-principles calculation based on density functional theory and cluster-expansion formalism. Our results suggest that Mo atoms prefer to substitute in the outer metal layers of MTVCs, and such occupying behavior is a crucial factor that determines the thermodynamic stability of MTVCs. On the other hand, Ti and V atoms residing in the inner metal layer of them are predicted to weakly interact with each other and their atomic configuration has a minimal impact on the stability of MTVCs. The transition temperatures between configurationally ordered and disordered phases of MTVCs have been predicted, and the possible existence of configurationally disordered MTVCs has also been discussed.

Differential configurational complexity and phase transitions of the BPS solutions in the O(3)-sigma model

Using a spherically symmetric ansatz, we show that the Chern–Simons O(3)-sigma model with a logarithmic potential admits topological solutions. This result is quite interesting since the Gausson-type logarithmic potential only predicted topological solutions in (1+1)D models. To accomplish our goal, the Bogomol’nyi–Prasad–Sommerfield (BPS) method is used, to saturate the energy and obtain the BPS equations. Next, we show by the numerical method is the graphical results of the topological fields, as well as, the magnetic field behavior that generates a flux given by Φflux=−Q/κ and the energy density of the structures of vortices. On the other hand, we evaluate the measure of the differential configurational complexity (DCC) of the topological structures, by considering the energy density of the vortex. This analysis is important because it will provide us with information about the possible phase transitions associated with the localized structures and it shows that our model only supports one phase transition.

Configurational entropy-induced phase transition in spinel LiMn2O4

The spinel-type LiMn2O4 is a promising candidate as cathode material for rechargeable Li-ion batteries due to its good thermal stability and safety. Experimentally, it is observed that in this compound there occur the structural phase transitions from cubic () to tetragonal (I41/amd) phase at slightly below room temperature. To understand the phase transition mechanism, we compare the Gibbs free energy between cubic phase and tetragonal phase by including the configurational entropy. Our results show that the configurational entropy contributes substantially to the stability of the cubic phase at room temperature due to the disordered Mn3+/Mn4+ distribution as well as the orientation of the Jahn–Teller elongation of the Mn3+O6 octahedron in the the spinel phase. Meanwhile, the phase transition temperature is predicted to be 267.8 K, which is comparable to the experimentally observed temperature. These results serve as a good complement to the experimental study, and are beneficial to the improving of the electrochemical performance of LiMn2O4 cathode.

Packing of stiff rods on ellipsoids: Geometry.

We suggest a geometrical mechanism for the ordering of slender filaments inside nonisotropic containers, using cortical microtubules in plant cells and the packing of viral genetic material inside capsids as concrete examples. We show analytically how the shape of the cell affects the ordering of phantom elastic rods that are not self-avoiding (i.e., self-crossing is allowed). We find that for oblate cells, the preferred orientation is along the equator, while for prolate spheroids with an aspect ratio close to 1, the orientation is along the principal (long axis). Surprisingly, at a high enough aspect ratio, a configurational phase transition occurs and the rods no longer point along the principal axis, but at an angle to it, due to high curvature at the poles. We discuss some of the possible effects of self-avoidance using energy considerations. These results are relevant to other packing problems as well, such as the spooling of filament in the industry or spider silk inside water droplets.

The first order L-G phase transition in liquid Ag and Ag-Cu alloys is driven by deviatoric strain

An undercooled liquid-phase (L-phase) can undergo a first order configurational phase transition to either a crystal phase (X-phase) or a metastable, configurationally heterogeneous, rigid glassy phase (G-phase). To investigate the underlying mechanism of the L-G transition, we employ molecular dynamics simulations to study G-phase formation in a binary Cu-Ag system. We find that G-phase formation is driven by the reduction of local distortion energy arising from deviatoric strains in the liquid phase and demonstrate its local distribution. Reduction of distortion energy contributes over 80% of the latent heat of the L-G transition, suggesting that condensation of spatially varying random elastic fields in the liquid is primarily responsible for the first order L-G transition. By applying this analysis to crystallization and G-phase formation in elementary Ag, we show that deviatoric strain energy is the dominant driving force for the L-G and L-X transition also in the case of the pure metal.

The effects of conductive nano fillers alignment on the dielectric properties of copolymer matrix

This research focuses on the improvement of the dielectric and energy harvesting properties of piezoelectric P(VDF-TrFE) copolymer matrix by the alignment of conductive reduced graphene oxide nano fillers. The dispersion and the morphology of the conductive nano fillers on the co-polymer matrix were characterized by scanning electron microscopy which showed a configurational phase transition due to highly conductive nano channel formation, steric hindrance, excluded volume interaction van-der-walls forces between adjacent reduced graphene oxide sheets. Five different piezoelectric nanocomposites were prepared by varying the reduced graphene oxide contents in P(VDF-TrFE) matrix to realize its optimum concentration in the matrix. From our analysis, we observed that, an optimized morphological structure plays a vital role in the formation of polar electroactive β phase on the co-polymer matrix through the good dispersion, filler alignment and interfacial interaction of reduced graphene oxide nano fillers. The as prepared nanocomposite film showed an enhanced crystallinity (50 ∼ 52%), dielectric constant (72 at 1 kHz), piezoelectric charge constant (−23 pC/N) with an output power of 3.2 µW at 1.8 MΩ load for 2 N mechanical force. All the outputs were observed without applying poling process. We expect that our synthesized self-poled nanocomposite can be a useful candidate for energy harvesting applications.

Configurational phase transition in AuxCu1−x nano-alloy: first principle and Monte-Carlo calculations

ABSTRACTWe report on the calculation of phase transition in the configurational state (i.e. atomic arrangement) of AuxCu1−x nanoparticle (or nano-alloy). The cluster expansion, represented by the formation energies of several atomic configurations, has been determined from full potential, linearized augmented-plane-wave first-principle calculation. The cluster expansion was then used as a Hamiltonian for the Monte-Carlo calculation of the phase transition between the configurational ground states (gs). Clear observation of plateau structures, associated with the calculated ‘gs’, in the plane of the composition (x) versus the chemical potential, have been observed at low temperatures <100 K. The plateaus smeared out at higher temperatures, giving rise to the order–disorder transition as the temperature rises. The critical temperature for this transition was estimated at 100 K, agreeing with the reported trend of the transition temperature with the nano-alloy size. Our results are thus relevant to applications concerning deposition of dispersed (i.e. well separated) nano-alloys.

Transverse single-file diffusion and enhanced longitudinal diffusion near a subcritical bifurcation.

A quasi-one-dimensional system of repelling particles undergoes a configurational phase transition when the transverse confining potential decreases. Below a threshold, it becomes energetically favorable for the system to adopt one of two staggered raw patterns, symmetric with respect to the system axis. This transition is a subcritical pitchfork bifurcation for short range interactions. As a consequence, the homogeneous zigzag pattern is unstable in a finite zigzag amplitude range [h_{C1},h_{C2}]. We exhibit strong qualitative effects of the subcriticality on the thermal motions of the particles. When the zigzag amplitude is close enough to the limits h_{C1} and h_{C2}, a transverse vibrational soft mode occurs which induces a strongly subdiffusive behavior of the transverse fluctuations, similar to single-file diffusion. On the contrary, the longitudinal fluctuations are enhanced, with a diffusion coefficient which is more than doubled. Conversely, a simple measurement of the thermal fluctuations allows a precise determination of the bifurcation thresholds.

Schottky-like anomalies and phase transitions in ideal heteropolymer proteinogenic chains in terms of size and single-residue helical propensity

Proteins belong to a class of biological macromolecules capable to undergo thermodynamic phase transitions from random one-dimensional to three-dimensional non-periodic structures. For a large class of proteins, such configurational phase transitions can be directly associated with a passage from poor or non-biologically active states to highly biologically-active conformations. In this work we present a simple model suitable to study analytical and numerically thermodynamic properties for an ideal heteropolymer chain X1−X2⋯−XN composed by N aminoacid residues. This model includes as relevant parameters the single-residue helical propensity ωj associated to residue Xj and characteristic energies, ϵj and Uj corresponding to native and non-native contact energies, respectively. We derive analytical expressions for the partition function Z when two types of energy spectra E are considered. One corresponding to all-or-none transitions typically observed in protein folding and a second one which considers partial contributions to the folded state. Specific heat Cv, configurational entropy S, average energy 〈E〉 and average occupancy number N are discussed in order to analyze the onset of thermodynamic phase transitions and Schottky-like anomalies in polypeptide chains of different size.

Stability, diffusion and interactions of nonlinear excitations in a many body system

When repelling particles are confined in a quasi-one-dimensional trap by a transverse potential, a configurational phase transition happens. All particles are aligned along the trap axis at large confinement, but below a critical transverse confinement they adopt a staggered row configuration (zigzag phase). This zigzag transition is a subcritical pitchfork bifurcation in extended systems and in systems with cyclic boundary conditions in the longitudinal direction. Among many evidences, phase coexistence is exhibited by localized nonlinear patterns made of a zigzag phase embedded in otherwise aligned particles. We give the normal form at the bifurcation and we show that these patterns can be described as solitary wave envelopes that we call bubbles. They are stable in a large temperature range and can diffuse as quasi-particles, with a diffusion coefficient that may be deduced from the normal form. The potential energy of a bubble is found to be lower than that of the homogeneous bifurcated phase, which explains their stability. We observe also metastable states, that are pairs of solitary wave envelopes which spontaneously evolve toward a stable single bubble. We evidence a strong effect of the discreteness of the underlying particles system and introduce the concept of topological frustration of a bubble pair. A configuration is frustrated when the particles between the two bubbles are not organized in a modulated staggered row. For a nonfrustrated (NF) bubble pair configuration, the bubbles interaction is attractive so that the bubbles come closer and eventually merge as a single bubble. In contrast, the bubbles interaction is found to be repulsive for a frustrated (F) configuration. We describe a model of interacting solitary wave that provides all qualitative characteristics of the interaction force: it is attractive for NF-systems, repulsive for F-systems, and decreases exponentially with the bubbles distance.

First-principles study of configurational disorder inB4Cusing a superatom-special quasirandom structure method

Configurationally disordered crystalline boron carbide, B$_{4}$C, is studied using first-principles calculations. We investigate both dilute and high concentrations of carbon-boron substitutional defects. For the latter purpose, we suggest a superatom's picture of the complex structure and combine it with a special quasi-random structure approach for disorder. In this way, we model a random distribution of high concentrations of the identified low-energy defects: 1) Bipolar defects and 2) Rotation of icosahedral carbon among the three polar-up sites. Additionally, the substitutional disorder of the icosahedral carbon at all six polar sites, as previously discussed in the literature, is also considered. Two configurational phase transitions from the ordered to the disordered configurations are predicted to take place upon increasing temperature using a mean-field approximation for the entropy. The first transition, at 870 K, induces substitutional disorder of the icosahedral carbon atoms among the three polar-up sites, meanwhile the second transition, at 2325 K, reveals the random substitution of the icosahedral carbon atoms at all six polar sites coexisting with bipolar defects. Already the first transition removes the monoclinic distortion existing in the ordered ground state configuration and restore the rhombohedral system (R3m). The restoration of inversion symmetry yielding the full rhombohedral symmetry (R-3m) on average, corresponding to what is reported in the literature, is achieved after the second transition. The electronic density of states, obtained from the disordered phases indicates a sensitivity of band gap to the degree of configurational disorder in B$_{4}$C.

Noisy zigzag transition, fluctuations, and thermal bifurcation threshold

We study the zigzag transition in a system of particles with screened electrostatic interaction, submitted to a thermal noise. At finite temperature, this configurational phase transition is an example of noisy supercritical pitchfork bifurcation. The measurements of transverse fluctuations allow a complete description of the bifurcation region, which takes place between the deterministic threshold and a thermal threshold beyond which thermal fluctuations do not allow the system to flip between the symmetric zigzag configurations. We show that a divergence of the saturation time for the transverse fluctuations allows a precise and unambiguous definition of this thermal threshold. Its evolution with the temperature is shown to be in good agreement with theoretical predictions from noisy bifurcation theory.

Thermodynamics of ideal proteinogenic homopolymer chains as a function of the energy spectrum E, helical propensity ω and enthalpic energy barrier

A reformulation and generalization of the Zwanzig model (ZW model) for ideal homopolymer chains poly-X, where X represents any of the twenty naturally occurring proteinogenic amino acid residues is presented. This reformulation and generalization provides a direct connection between coarse-grained parameters originally proposed in the ZW model with variables from the Lifson–Roig (LR) theory, such as the helical propensity per residue ω, and new variables introduced here, such as the energy gap Δ between unfolded and folded structures, as well as the ratio f of the energy scales involved. This enables us to discover the relevance of the energy spectrum E to the onset of configurational phase transitions. From the configurational partition function \U0001d4ac, thermodynamic properties such as the configurational entropy S, specific heat Cv and average energy 〈E〉 are calculated in terms of the number of residues K, temperature T, helical propensity ω and energy barrier ΔH for different poly-X chains in vacuo. Results obtained here provide substantial evidence that configurational phase transitions for ideal poly-X chains correspond to first-order phase transitions. An anomalous behavior of the thermodynamic functions 〈E〉, Cv, S with respect to the number K of residues is also highlighted. On-going methods of solution are outlined.

Entropy driven phase transition in itinerant antiferromagnetic monolayers

Based on Monte Carlo calculations, the magnetic ordering in itinerant antiferromagnetic monolayers with geometric frustration has been analyzed. For description of the itinerant magnetism exchange interactions beyond the Heisenberg model such as the biquadratic exchange and the four-spin exchange interaction have been taken into account. We demonstrate that the higher-order exchange interactions remove the structural degeneracy and lead to configurational phase transitions at finite temperatures. A new, entropy driven structural phase transition in $\mathrm{V}∕\mathrm{Ag}(111)$ is predicted theoretically.

A statistical approach to close packing of elastic rods and to DNA packaging in viral capsids

We propose a statistical approach for studying the close packing of elastic rods. This phenomenon belongs to the class of problems of confinement of low dimensional objects, such as DNA packaging in viral capsids. The method developed is based on Edwards' approach, which was successfully applied to polymer physics and to granular matter. We show that the confinement induces a configurational phase transition from a disordered (isotropic) phase to an ordered (nematic) phase. In each phase, we derive the pressure exerted by the rod (DNA) on the container (capsid) and the force necessary to inject (eject) the rod into (out of) the container. Finally, we discuss the relevance of the present results with respect to physical and biological problems. Regarding DNA packaging in viral capsids, these results establish the existence of ordered configurations, a hypothesis upon which previous calculations were built. They also show that such ordering can result from simple mechanical constraints.

The configurational phase transitions of flexible polymers in planar mixed flows near simple shear

We examine the configurational phase transitions of flexible polymer chains in planar mixed flows. We distinguish three distinct phases: the coiled state (C) in which the flow is unable to deform the chain significantly from its equilibrium configuration, the tumbling state (T) in which the chain undergoes a tumbling motion and the chain fluctuations are comparable to the full chain contour length as in shear flow, and the fully stretched state (S) in which the chain is fully stretched in its steady configuration and the chain fluctuations are small compared to the full chain contour length as in extensional flow. As the mixed flow parameter α changes, the chain undergoes changes in configuration from C to T to S. We consider separately the phase transitions from C to T (C-T) and from T to S (T-S). We use physical arguments and supporting Brownian dynamics simulations to derive the range in the parameter α (as a function of the flow strength Wi and chain molecular weight N) over which such phase transitions occur in the high flow strength limit.

Critical chain dynamics near the interdigitated–noninterdigitated chain configurational phase transition in decylammonium chloride

We have carried out a H1 nuclear magnetic resonance line shape analysis near the irreversible interdigitated-to-noninterdigitated chain configurational phase transition in decylammonium chloride (C10H21NH3Cl), a model biomembrane. The splitting parameter corresponding to the nematic order of the hydrocarbon chain shows a two-dimensional Ising behavior and indicates a chain defect motion in addition to the chain reorientational motion. The origin of the low-frequency critical fluctuation is attributed to a critical slowing down of the collective gear-like reorientation of the rigid hydrocarbon chain.

Low-Frequency Chain Dynamics in Decylammonium Chloride

Proton nuclear magnetic resonance (NMR) relaxation measurements were carried out to investigate the low-frequency hydrocarbon chain dynamics in decylammonium chloride (C 10 H 21 NH 3 Cl), a model biomembrane undergoing an irreversible structural phase transition. It is demonstrated by systematic proton NMR study of spin-spin relaxation and rotating frame spin-lattice relaxation that a low-frequency chain dynamics is responsible for the critical dynamics associated only with the interdigitated-to-noninterdigitated chain configurational phase transition.

Low-frequency collective chain dynamics in a model biomembrane

Proton NMR was employed as a probe for the collective hydrocarbon chain dynamics in decylammonium chloride (C10H21NH3Cl), a model biomembrane undergoing an irreversible structural phase transition sequence. Our rotating frame spin-lattice relaxation measurements revealed a low-frequency critical collective chain dynamics in the kHz regime, which is associated with the interdigitated to noninterdigitated chain configurational phase transition.

Asymmetric kinks: stabilization by entropic forces.

Asymmetric kinks bridging two adjacent potential valleys of equal depth but different curvature are unstable against phonon modes. When coupled to a heat bath, a kink-bearing string tends to cross over into the shallower valley; kinks are thus predicted to drift in the appropriate direction with velocity proportional to the temperature, in close agreement with numerical simulation. When contrasted by a mechanical bias, these entropic forces give rise to a rich phenomenology that includes configurational phase transitions, double-kink dissociation, and noise-directed signal transmission.