Possibility Of Phase Separation Research Articles

Ca0.85Sm0.15MnO3:A mixed antiferromagnet with unusual properties

We investigated electrical and magnetic properties of the electron-doped manganites Ca1-xSmxMnO3 (0<x<0.2). Our results indicate the possibility of phase separation in electron doped manganites. The compounds with x = 0.13-0.15 show unusual difference between zero-field cooled and field-cooled resistivities which are not observed in lower compositions. Our results suggest that electronic phase separation alone is insufficient to understand the origin of the resistivity irreversibilities.

Elastic Constants of Binary Liquid Crystalline Mixtures

Abstract Microscopic expressions for the elastic constants of binary liquid crystalline mixtures composed of short rigid uniaxial molecules are derived in the thermodynamic limit at small distorsions and a small density. Uniaxial and biaxial nematic phases are considered. The expressions involve the one-particle distribution functions and the potential energy of two-body short-range interactions. The theory is used to calculate the phase diagram of a mixture of rigid prolate and oblate molecules. The concentration dependence of the order parameters and the elastic constants are obtained. The possibility of phase separation is not investigated.

Electron Self-Exchange in Redox Polymers. 2. A Study of Ion Aggregation and Intermolecular Interactions

The diffusion−migration equation (derived in part 1) is subjected to the analysis of steady-state current−potential responses and the effects of intermolecular interaction between neighboring particles and ion association between the electroactive and electroinactive species on maximal limiting current and the corresponding redox composition are studied both in the presence and absence of site diluent. The variation of the apparent diffusion coefficient with molar fraction of redox active species is analyzed as a function of the ion pairing constant and interaction energetics. The steady-state analysis suggests that the transport equation is valid only for a small region of interaction energy and breaks down when its value exceeds the order of kBT. The possibility of phase separation in redox polymers is also discussed.

Stability of22×22oxygen ordered superstructures inYBa2Cu3O6+x

We have compared the ground-state energy of several observed or proposed ``$2\sqrt{2}\ifmmode\times\else\texttimes\fi{}2\sqrt{2}$ oxygen ordered superstructures'' [``herringbone'' structures (HS's)], with those of ``chain superstructures'' (CS's) (in which the O atoms of the basal plane are ordered in chains), for different compositions $x$ in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}.$ The model Hamiltonian contains (i) the Madelung energy, (ii) a term linear in the difference between Cu and O hole occupancies which controls charge transfer, and (iii) covalency effects based on known results for $t\ensuremath{-}J$ models in one and two dimensions. The optimum distribution of charge is determined, minimizing the total energy, and depends on two parameters which are determined from known results for $x=1$ and $x=0.5$. We obtain that on the O lean side, only CS's are stable, while for $x=7/8$, a HS with regularly spaced O vacancies added to the $x=1$ structure is more stable than the corresponding CS for the same $x$. We find that the detailed positions of the atoms in the structure and long-range Coulomb interactions are crucial for the electronic structure, the mechanism of charge transfer, the stability of the different phases, and the possibility of phase separation.

Effects of sugars and polymers on crystallization of poly(ethylene glycol) in frozen solutions: phase separation between incompatible polymers.

This study examined the effect of third components (low-molecular-weight saccharides and polymers) on the crystallization of poly(ethylene) glycol (PEG) in frozen solutions, focusing on the relationship between their crystallization-inhibiting ability and molecular compatibility. Effects of sugars and polymers on the crystallization of PEG 3000 in frozen solution were monitored by differential scanning calorimetry (DSC). Pulsed-NMR was employed to monitor the molecular mobility of water and solutes in the frozen solutions. Miscibility between PEG and third components in aqueous solution was estimated from the lowering of cloud point of PEG 20,000. Thermal analysis of frozen solutions containing some non-crystallizing solutes was used to examine the possibility of phase separation in frozen solutions. Some sugars and polymers inhibited the crystallization of PEG and formed practically stable amorphous phases among ice crystals. The mobility of solute molecules in the amorphous phase increased above the softening temperature of maximally concentrated solutions (Ts), whereas that of water molecules appeared at a lower temperature. Mono- and disaccharides that are relatively less miscible with PEG in solution inhibit PEG crystallization to a lesser degree. Two Ts regions were observed in frozen solutions containing both polyvinylpyrrolidone (PVP) and dextran, at much lower concentrations than those causing aqueous two-phase separation at ambient temperatures. Ice crystallization raises the concentration of solutes in the remaining solution, which can lead to phase separation in the amorphous phase. Molecular compatibility between components is an important factor determining their propensity to phase separate and crystallize.

The Morphology of Blends of Linear and Branched Polyethylenes by Small-Angle Neutron and X-Ray Scattering

ABSTRACTThe solid-state morphology and liquid-state homogeneity of blends of high-density polyethylene (HDPE) and low-density polyethylene (LDPE) were investigated by small-angle neutron and x-ray scattering (SANS and SAXS). The solid state morphology was investigated as a function of composition and cooling rate from the melt. After slow cooling, the evidence indicated that the mixtures were either completely (HDPE-rich blends) or almost completely (LDPE-rich blends) phase separated into separate HDPE and LDPE lamellae over the whole compositional range. In contrast, for rapidly quenched blends the components are extensively co-crystallized for all concentrations, though the SANS data indicated that the branched component had a tendency to be preferentially located in the inter-lamellar regions. In the liquid state, the blends were homogenous at all compositions, showing that the solid state morphology is not determined by the melt structure, but is a function of the crystallization kinetics. Further evidence for blend homogeneity in the liquid is presented. In particular we examine the hypothesis that a phase separated mixture might give a scattering pattern similar to a homogenous blend if the domain sizes were larger that the maximum spatial resolution of the SANS experiment (D &gt; 2π/Qmin ~ 2000Å). In this scenario, the differential scattering cross section dΣ/dΩ(Q) ~ Q-2, though phase separation decreases the cross section in this Q-range with respect to the homogenous blend. For HDPE/LDPE blends in the melt, this decrease in intensity was not observed, thus ruling out the possibility of phase separation.

Equilibrium density of an electron-hole liquid in a strong magnetic field: A possibility of phase separation.

The ground state of a model electron-hole liquid (EHL) in a strong magnetic field is investigated. In the regime when only a few Landau subbands are occupied the complex behavior of the equilibrium density leads to a possibility of phase separation. At certain magnetic-field strengths the EHL ground-state energy plotted as a function of the density has two local minima corresponding to two phases with different numbers of Landau subbands occupied. As a result the equilibrium density of EHL will have strong discontinuities as a function of the magnetic field, occurring whenever the relative strength of those minima changes. In a nonequilibrium system, such as a real electron-hole droplet, both phases may be present simultaneously. We discuss several luminescence and infrared absorption experiments, and suggest that they indicate such a phase separation.

Grand-canonical simulations of solvated ideal fermions. Evidence for phase separation

A novel scheme to perform finite-temperature grand-canonical simulations of ideal fermions in arbitrary external potentials is introduced. This scheme is based on the evaluation of the grand-canonical function of lattice fermions. As an application, we present results on the phase behavior of a mixture of fermions and hard spheres. A simple analytic model of solvated fermions in a hard-sphere fluid is also studied. We address here the possibility of phase separation between a pure delocalized phase of fermions and a homogeneous solution of solvated fermions. These calculations indicate that the homogeneous phase is expected to be stable only at low fermion concentration and low thermal wavelengths. The fermion simulations indicate that such phase separation is a likely scenario.

Light scattering and thermodynamic phase behavior of the system 11S globulin-κ-carrageenan-water

The thermodynamic incompatibility of 11S globulin ( Vicia faba) and the sodium form of k-carrageenan has been studied by measurements of the phase behavior at moderate to high concentrations and by light scattering on dilute solutions below the separation threshold. The two approaches provide consistent information. The second virial coefficient obtained by light scattering for the different types of pair interactions between macromolecules [protein-protein ( A 22), polysaccharide-polysaccharide ( A 44) and protein-polysaccharide ( A 24)] indicates the possibility of phase separation ( A 24) due to mutual exclusion forces which exist in the solution for these biopolymer pairs. The nature (but not magnitude) of these forces is independent of the conformation of κ-carrageenan. The temperature dependence of A 24 suggests entropie and enthalpic contributions of comparable magnitude. The general character of the water distribution between coexisting phases in this system is consistent with the relative hydrophilicities of the two polymers. Helical conformations of κ-carrageenan give rise to increased thermodynamic incompatibility presumably because of increased excluded volume effects.

On the possibility of phase separation in the extended Hubbard model

Abstract The one-band extended Hubbard model on the square lattice in the strong interaction limit is investigated using an exact diagonalization technique. The main emphasis was on the possibility of a phase separation. It is shown that the instability of the system against separation into a no-hole antiferromagnetic phase and an hole-rich phase that takes place at low hole concentrations is effectively destroyed through the inclusion of a moderate interatomic Coulomb interaction. In the two-phase region, i.e. at small interaction energies, a negative hole-binding energy indicates that the hole-rich phase is build up by hole-pairs. We comment on the relevance of our results for the description of the copper-oxide superconductors.

Heterogeneous chemical equilibria in multicomponent mixtures

An analysis of the geometry of the free energy surface for multicomponent mixtures is discussed for conditions where there may be chemical reactions taking place in the mixture, as well as the possibility of phase separation. The latter is shown to take place in a direction orthogonal to the reaction subspace, and can therefore take place only if there are such directions. It is shown that such directions exist unless the brute chemical formulae of all components are multiple of each other. The analysis is generalized to the case of continuous mixtures, where the number of components, of independent reactions, and of possible phases all approach infinity.

Nonideality and Nonlinearity in Reaction‐Diffusion Plasmas

AbstractReaction and diffusion processes in dense plasmas are investigated on the basis of quantum statistical theory.Many particle effects lead to nonlinear expressions for reaction and diffusion coefficients.In the case of ambipolar diffusion and isobarity the plasma is described by only one reaction‐diffusion equation. Travelling front solutions and droplet formation are investigated in the approximation of constant diffusion. The influence of nonlinearity of the diffusion on these solutions as well as the possibility of phase separation via spinodal decompoisition and the formation of stable droplets are discussed.

Theory of bipolaron lattices in quasi-one-dimensional conducting polymers.

We present a theory of the bipolaron lattice in the Peierls-distorted system with nondegenerate ground states. First, we obtain an exact solution for the order parameter of a bipolaron lattice in terms of the Weierstrass \ensuremath{\zeta} function for the continuum model of Brazovskii and Kirova. Making use of the exact solution, we determine the complete band structure of a bipolaron lattice. In addition to the conduction and valence bands, we find two bipolaron bands symmetrically located about the middle of the Peierls energy gap. We also calculate the electronic density of states and the energy of formation of a bipolaron lattice as a function of the bipolaron density ${n}_{b}$. The chemical potential of a bipolaron determines the domain of stability of the lattice as a function of the confinement parameter \ensuremath{\gamma}. In the weak-coupling limit (infinite momentum cutoff \ensuremath{\Lambda}) the bipolaron-bipolaron interaction decays as a repulsive exponential while for finite \ensuremath{\Lambda} it decays inversely proportional to the distance and is attractive. This yields the possibility of phase separation in doped quasi-one-dimensional conducting polymers with nondegenerate ground states. For higher dopant concentrations we describe a possible first-order phase transition to a metallic phase consisting of an array of polaronlike distortions from a lattice of charged bipolarons. As well, the optical absorption spectrum of a bipolaron lattice is briefly discussed. Our results apply to cis-polyacetylene as well as to a class of heterocyclic compound polymers, namely, polypyrroles, polythiophenes, and their derivatives. Finally, we make contact with recent experiments which study the evolution of ordered phases during the electrochemical doping of polyparaphenylene by alkali atoms and indicate the possibility of a bipolaron lattice.

Limited solubility of iron in the Sun's interior

STRIPPED iron nuclei in a hydrogen plasma under central solar conditions, according to the classical Debye–Huckel model, would undergo phase separation for concentrations well below the cosmic abundance value. The higher concentration corrections, needed to characterise the iron-rich phase, lead to enhanced solubility for a simplified model where the electrons form a uniform background. Support for an iron-rich phase coalescing in the solar interior requires more accurate treatment of bound and partially bound electrons in such a mixture. The results of the Debye–Huckel model where the electrons are treated discretely and as a continuum, are reported here and support the possibility of phase separation. The physical cause of that phase separation is simply that the potential energy is lower in the separated phases than in the mixture because the local charge neutralisation is much better satisfied in the two separated phases.

Phase behaviour of fischer‐tropsch wax fractions under high pressure

AbstractThe phase behaviour of five distillation fractions of a Fischer‐Tropsch wax were investigated at pressures up to 40 kbar by means of differential thermal analysis. The approximate constancy in the number of significant components (i.e. greater than 5% by wt.) in each wax fraction is probably responsible for them appearing to behave as a series. Similarity of the phase diagrams of the individual wax fractions gave rise to the construction of a ‘master’ phase diagram. A possible new high‐pressure phase was detected in the heaviest fraction, but the possibility of phase separation cannot be excluded.

Particle packing in latex films

Combination of light scattering and electron microscopy for characterization of packing texture in solid films of polyvinyl acetate is described. Evidence is presented for a sedimentation caused particle size gradient from top to bottom in a film of a large particle latex. The influence of the polymeric stabilizer on the optical heterogeneity of latex films is discussed in terms of a continuous matrix in which latex particles are imbedded. The possibility of phase separation according to particle size is presented.