Microscopic Phase Segregation Research Articles

Cosolvent polarity dependence of solution structure in [BMIM] [PF6] + acetonitrile/1, 4-dioxane/hexane binary mixtures: Insights from composition dependent Voronoi polyhedra analyses, iso-surfaces and radial distribution functions

We investigate in this paper, via molecular dynamics simulations, the composition dependent solution structures in the binary mixtures composed of the ionic liquid (IL), 1-butyl-3methyl-imidazolium hexafluorophosphate, ([BMIM] [PF6]), and three different co-solvents of comparable sizes but with differing polarities. These cosolvents are acetonitrile (dipolar), 1,4-dioxane (quadrupolar) and hexane (non-polar). Five different IL mole fractions for the binary mixtures (FIL = 0.90, 0.75, 0.50, 0.25, 0.10) along with the neat solvents have been studied. The cosolvent polarity and concentration dependencies have been assessed through the examination of the composition dependent spatial and radial distribution functions, coordination numbers and the Voronoi polyhedra analyses. The solution structure is found to be dominated by the cosolvent clustering at high IL concentrations, and segregation of the IL into polar and non-polar domains in dilute IL solutions, their extent being strongly dependent upon cosolvent polarity. The aggregation behavior of the IL in the non-polar and dipolar binary mixtures is explained in terms of the Debye screening length (DSL). Such an explanation, however, fails to explain the comparable solubility of 1,4-dioxane in this IL. Isodensity surfaces constructed from the simulated spatial distribution functions show a dramatic cosolvent polarity dependence for the binary solution structure and reflects that microscopic phase segregation is the most pronounced for IL + hexane binary mixtures. The Voronoi polyhedra (VP) analyses indicate broader void and neck distributions upon dilution of the IL by cosolvents, suggesting increased heterogeneity in the solution structure of the binary mixtures. The severe phase segregation observed for [BMIM] [PF6] + hexane mixtures at the low IL concentrations did not allow similar VP analyses for this binary mixture for all the mixture compositions considered.

Antiferromagnetic metal phases in double perovskites having microscopic phase segregation due to strong antisite defect concentration

Recently an antiferromagnetic metal phase has been proposed in double perovskites materials like Sr2FeMoO6 (SFMO), when electron doped. This material has been found to change from half-metallic ferromagnet to a novel antiferromagnetic metal (AFM) upon La-overdoping. The original proposition of such an AFM phase was made for ordered samples, but the experimental realization of La-overdoped SFMO has been found to contain a substantial fraction of antisite defects. A microscopic chemical phase segregation into alternate Fe and Mo rich regions was observed. In this paper we propose a possible scenario in which an antiferromagnetic metal phase can still be stabilized even in presence of such strong antisite defect concentration and phase segregation, by a novel kinetic energy-driven mechanism. Our results thus provide a plausible explanation to the experimental observations in the La-overdoped regime. Antisite regions can thus give rise to antiferromagnetic metallic phases, although the metal is low-dimensional.

Probe Dependent Solvation Dynamics Study in a Microscopically Immiscible Dimethyl Sulfoxide-Glycerol Binary Solvent.

Excited state dipole solvation of three coumarin dyes with different hydrophobicities was studied in DMSO-glycerol binary solvent. The solvation times obtained from the three dyes are remarkably different. The highly hydrophilic dye coumarin 343 (C343) exhibits the slowest solvation time (>12 ns) among all the dyes we used. This is in contrast to the most hydrophobic dye coumarin 153 (C153), where the solvated state is reached just within ∼104 ps. However, the moderately hydrophobic dye coumarin 480 (C480) demonstrates an intermediate (∼396 ps) solvation time. Unprecedented slowdown of solvation time of C343 is probably due to the slow diffusion of solvent molecules in the glycerol-rich first solvation shell followed by hydrogen bond rearrangements around the solute dipole. On the other hand, fast solvation of hydrophobic dye C153 is most likely caused by the fast reorganization dynamics of hydrophobic -CH3 groups of DMSO or the carbon backbone of the glycerol molecule around the solute dipole. Interestingly, a remarkable probe dependency in solvation dynamics was not observed in the case of DMSO-water binary solvent or in a neat solvent isopropanol. Probe dependent solvation in a DMSO-glycerol mixture is attributed to the microscopic phase segregation and different locations of coumarin dyes within this binary solvent.

Transport properties and phase stability of mixed conducting oxide membranes

At high strontium doping levels, perovskite oxides containing iron have suitable stability and transport properties for use as oxide ion transport membranes. In our studies of these materials, we have investigated the pO 2 and temperature dependence of the conductivity and non-stoichiometry of La 1− x Sr x Fe 1− y M y O 3− δ (M = Cr, Ti) by using electrochemical cells and the thermal expansion by dilatometry. Non-equilibrium behavior is observed in both the chemical expansion data and also in the conductivity and stoichiometry and suggests the occurrence of microscopic phase segregation on reduction. Analysis of the microstructure of quenched samples confirms the occurrence of local phase separation. Bulk diffusion and surface exchange coefficients under near-gradientless conditions have been determined by the electrical conductivity relaxation (ECR) technique and by isotope exchange depth profiling (IEDP). Evaluation of transport under a chemical gradient was accomplished by transient isotopic tracing of operating membranes. The isotope transients ( 16O 2– 18O 2) were performed on tubular membranes operating at steady state at temperatures between 1023 K and 1173 K and allow an unambiguous separation of surface and bulk resistances to oxygen permeation under steady state conditions, a separation not possible by permeation measurements alone.

Dynamical phase separation under laser scanning

Dynamical patterns of microscopic phase segregation under linear scanning of the focus of IR laser were investigated in a homogeneous oil/water mixture. When the scanning frequency is low, the generated oil droplet follows the motion of the laser focus. When the frequency is high, the generated oil droplet is located at a certain position. The mechanism of these characteristic trapping patterns is discussed in terms of the effective potential by the focused laser beam, which depends on the scan rate and the mode of scanning.

Field effect on phase segregation in the electron-doped mixed-valence manganites near a structural instability

The rich and peculiar macro- and microscopic phenomenology observed in the electron-doped region of the Sm-Ca phase diagram of manganites can be found in the unique and fascinating compound ${\mathrm{Sm}}_{0.15}{\mathrm{Ca}}_{0.85}{\mathrm{MnO}}_{3}.$ We report the existence of nano- and microscopic phase segregation above and below ${T}_{N}\ensuremath{\approx}112\mathrm{K}.$ The very different nature of the coexisting phases and the field and temperature dependencies are explained considering the interplay of magnetic and structural correlations, which give rise to different structural and magnetic ground states, close in energies $(P\ensuremath{-}Pnma,$ $G+F\ensuremath{-}Pnma,$ $G\ensuremath{-}Pnma,$ ${C\ensuremath{-}P2}_{1}/m).$

Kinetics of gelatin transitions with phase separation: T-jump and step-wise DSC study

The isothermal gelation (or melting) of gelatin after fast cooling (or heating) steps is studied by using high sensitivity differential scanning micro-calorimetry, in order to determine the dependence of the kinetic and thermodynamic parameters upon changes in composition and in temperature. The calorimetric heat flow curves, obtained according to defined temperature profiles, have been fitted with exponential functions (simple exponentials or stretched exponentials for the step-wise and for T-jump experiments, respectively). The gelation process of gelatin alone for t<300 min shows that the characteristic time τ and the fractional exponent are β very sensitive to the concentration of gelatin chains and to the microscopic phase segregation due to the presence of another polymeric component.

Neutral-ionic phase separation and one-dimensional ferroelectricity in organic relaxors

Microscopic phase segregation by chemical doping and resultant anomalous dielectric response have been investigated for tetrathiafulvalene-p-chloranil complex doped with trichloro-p-benzoquinone ( QCl3). Beyond a critical QCl3 content, the system shows a behavior of relaxor ferroelectrics, as characterized by strong frequency dispersion and a rounded peak shape of gigantic dielectric susceptibility. The relaxor phase arises from one-dimensional ferroelectricity, as evidenced by a diffuse x-ray scattering, in which interchain ferroelectric coupling associated with neutral-ionic transition is interrupted by impurity-generated neutral microclusters.

Phase Segregation Dynamics in Particle Systems with Long Range Interactions II: Interface Motion

We study properties of the solutions of a family of second-order integrodifferential equations, which describe the large scale dynamics of a class of microscopic phase segregation models with particle conserving dynamics. We first establish existence and uniqueness as well as some properties of the instantonic solutions. Then we concentrate on formal asymptotic (sharp interface) limits. We argue that the obtained interface evolution laws (a Stefan-like problem and the Mullins--Sekerka solidification model) coincide with the ones which can be obtained in the analogous limits from the Cahn--Hilliard equation, the fourth-order PDE which is the standard macroscopic model for phase segregation with one conservation law.

Interaction of superconducting Y1Ba2Cu3O7−x targets with 1.064 μm Q-switched Nd:yttrium–aluminum–garnet laser

Bulk superconducting targets of Y1Ba2Cu3O7−x were prepared and characterized for resistance, structure, morphology, and composition. The targets were irradiated by a Q-switched 1.064 μm wavelength Nd:yttrium–aluminum–garnet (YAG) laser having 20 ns pulse duration at 2.0 J/cm2 (shot) energy density. A cumulative laser exposure of the target surface results in the formation of microscopic conical columnar structures aligned parallel to the incident laser beam. Evolution of these structures in the present case is attributed to the longer penetration depth of optical radiation and localized material removal by microexplosions beneath the surface. Energy dispersive analysis by an x ray of the individual column shows microscopic phase segregation and erosion accompanied by shadowing as the primary cause of the cone formation process.

A 31P Nuclear Magnetic Resonance study of InP, GaP and InGaP

The applicability of 31P nuclear magnetic resonance (NMR) to the study of spinodal decomposition of the pseudo-binary alloys of the type In x Ga( 1− x .AS y P( 1− y is investigated. In particular, this study considers the detection of microscopic phase-segregation in In x Ga( 1− x .AS y P via the chemical shift of the 31P NMR spectra. This appeared to be a promising approach since the difference between the isotropie chemical shifts of the 31P NMR spectra of InP and GaP is reported in the literature to be over 60 ppm. Based on this relatively large separation between the peaks, it appeared possible to determine the distribution of Ga and In from the distribution of isotropic chemical shifts in the 31P spectrum of In x Ga( 1− x P. However, more accurate measurements of the chemical shifts of InP, GaP and In 0.9Ga 0.1P reveal that the spectrum of each compound is centered in the narrow range of 146 to 149 ppm, relative to 85% H 3PO 4. Consequently, in the presence of large dipolar broadening, it is not possible to calculate the local distribution of Ga and In from the 31P chemical shift data.