Features Of Phase Diagram Research Articles

Nucleation in a Potts lattice gas model of crystallization from solution

Nucleation from solution is important in many pharmaceutical crystallization, biomineralization, material synthesis, and self-assembly processes. Simulation methodology has progressed rapidly for studies of nucleation in pure component and implicit solvent systems; however little progress has been made in the simulation of explicit solvent systems. The impasse stems from the inability of rare events simulation methodology to be combined with simulation techniques which maintain a constant chemical potential driving force (supersaturation) for nucleation. We present a Potts lattice gas (PLG) to aid in the development of new simulation strategies for nucleation from solution. The PLG captures common crystallization phase diagram features such as a eutectic point and solute/solvent melting points. Simulations of the PLG below the bulk solute melting temperature reveal a competition between amorphous and crystalline nuclei. As the temperature is increased toward the bulk melting temperature, the nucleation pathway changes from a one step crystalline nucleation pathway to a two step pathway, where an amorphous nucleus forms and then crystallizes. We explain these results in terms of classical nucleation theory with different size-dependant chemical potentials for the amorphous and crystalline nucleation pathways. The two step pathway may be particularly important when crystallization is favored only at postcritical sizes.

Liquid Crystal Phase Transitions in Systems of Colloidal Platelets with Bimodal Shape Distribution

We have studied a system of polydisperse, charged colloidal gibbsite platelets with a bimodal distribution in the particle aspect ratio. We observe a density inversion of the coexisting isotropic and nematic phases as well as a three-phase equilibrium involving a lower density nematic phase, an isotropic phase of intermediate density, and a higher density columnar phase. To relate these phenomena to the bimodality of the shape distribution, we have calculated the liquid crystal phase behavior of binary mixtures of thick and thin hard platelets for various thickness ratios. The predictions are based on the Onsager-Parsons theory for the isotropic-nematic (I-N) transition combined with a modified Lennard-Jones-Devonshire cell theory for the columnar (C) state. For sufficiently large thickness ratios, the phase diagram features an I-N density inversion and triphasic I-N-C equilibrium, in agreement with experiment. The density inversion can be attributed to a marked shape fractionation among the coexisting phases with the thick species accumulating in the isotropic phase. At high concentrations, the theory predicts a coexistence between two columnar phases with distinctly different concentrations. In experiment, however, the demixing transition is pre-empted by a transition to a kinetically arrested, glassy state with structural features resembling a columnar phase.

Phase diagram features and solidification behaviour of CoCu 2O 3 at elevated oxygen pressure

A unique behaviour of the phase CoCu 2O 3 was found both from CALPHAD calculations and directional solidification experiments. For elevated oxygen partial pressure the solidification mode changed from double-peritectic to a congruent melting behaviour with respect to the metals (Cu, Co) and to the oxygen content. This transition predicted by the phase diagram calculations was confirmed by microstructure and phase analyses of samples solidified at oxygen pressures up to 60 bar. A DTA analysis has verified basic features of the phase diagram at normal pressures.

Spin-one Heisenberg antiferromagnetic chain with exchange and single-ion anisotropies

Using density-matrix renormalization group calculations, ground-state properties of the spin-1 Heisenberg chain with exchange and single-ion anisotropies in an external field are studied. Our findings confirm and refine recent numerical and analytic results by Sengupta and Batista [Phys. Rev. Lett. 99, 217205 (2007)] on the same model. In particular, we present evidence for two types of biconical (or supersolid) and for two types of spin-flop (or superfluid) structures for chains of finite length. Basic features of the quantum phase diagram may be interpreted qualitatively in the framework of classical spin models.

Inclusions of magmatic fluids: P-V-T-X properties of aqueous salt solutions of various types and petrological implications

The P-V-T-X properties of H2O-salt systems are compared depending on the solubility coefficient of compounds contained in these systems and the presence or absence of critical phenomena in the saturated solutions. Data on synthetic and natural inclusions captured in minerals at elevated temperatures and pressures and employed to discuss the principal features of phase diagrams of the H2O-NaCl system (type I) and H2O-NaF system (type II or P-Q type). It is demonstrated how characteristics of magmatic fluids of various types are manifested during the development of miarolitic pegmatites (Malkhan field in Transbaikalia) and during the crystallization of F-rich ongonitic melts (Ary-Bulak Massif in eastern Transbaikalia). Characteristics of solutions and gas-rich (gaseous) fluid inclusions in quartz phenocrysts from porphyritic ongonites (disappearance of the liquid regardless of its density and the overall salinity near the critical point of water, distinctive features of the dissolution of the crystalline phase, and the ability of the inclusions to withstand heating to 1400°C without decrepitation), and the richness of the fluid-magmatic system as a whole in F suggest that the ongonite melt crystallized in the presence of low-density NaF-bearing fluids of the P-Q type with a minor admixture of chlorides. It is important to identify the type of solutions in the fluid inclusions, because without knowing this type, it is impossible to accurately enough calculate the pressure at the temperatures of inclusion capture. For example, the unwarranted classification of solutions of type II (P-Q) in inclusions with the chloride system results in a significant overestimation of the calculated fluid pressures. A technique is proposed for studying the high-temperature immiscibility region in P-Q systems based on data obtained on gaseous fluid inclusions.

The special features of liquid-vapor phase diagrams and an analysis of the forms of azeotropy rules

The special features of heterogeneous phase equilibria in three-component mixtures were studied. In particular, new equations for relations between azeotropes and points corresponding to the pure components were obtained. The use of the Zharov equation for the passage from concentration simplexes to concentration complexes was shown to lead to serious errors in the determination of the nonlocal features of phase diagrams.

Thermodynamic Description of the Mg-Mn, Al-Mn and Mg-Al-Mn Systems Using the Modified Quasichemical Model for the Liquid Phases

A self-consistent thermodynamic model of the Mg-Mn, Al-Mn and Mg-Al-Mn systems has been developed. The major difference between this work and the already existing assessments of these systems is the application of the modified quasichemical model for the liquid phase in each system while most of the existing descriptions use the random mixing model. In the absence of key data for the Mg-Mn system, the calculated thermodynamic properties from the model have been found comparable to other similar systems and the estimated critical temperature of the Mg-Mn liquid miscibility gap using the available empirical equation has been found to be in acceptable agreement with the calculated value. A comparison between the current work and the most recent work on the Al-Mn system that uses the same model for the liquid phase reveals that better agreement with the experimental data with less number of model parameters has been achieved in the current work. Kohler symmetric extrapolation model with only one ternary interaction parameter has been used to calculate the ternary Mg-Al-Mn system. The thermodynamic description of the Mg-Al-Mn system has been verified by extensive comparison with the available experimental data from numerous independent experiments. The model can satisfactorily reproduce all the invariant points and the key phase diagram and thermodynamic features of the ternary as well as the constituent binary systems.

Columnar phases of discotic spherocylinders

The liquid crystal phase diagram of the discotic hard spherocylinder fluid is investigated by Monte Carlo simulations. Thickness-to-diameter aspect ratios within L/D=0.2-0.5 are considered. Three distinct columnar phases are found, namely, a hexatic interdigitated phase (D(hi)), a hexatic ordered phase (D(ho)), both with long-range spatial correlations, and a hexatic disordered phase (D(hd)), in which the columns become fluidlike. Local domains of stacked particles are also observed in the isotropic phase. The stability of the D(ho) and D(hd) phases is favored with increasing anisotropy of the particle shape. As a consequence, the packing fraction versus the aspect ratio representation of the phase diagram features D(ho)-D(hd)-I and D(hi)-D(ho)-I triple points. The study involved the development of an efficient algorithm to compute the shortest distance between two oblate spherocylinder particles. The study provides a general coarse-grain methodology to explore discotic behavior, with fundamental advantages against alternative molecular models.

Structural complexity in monodisperse systems of isotropic particles

Abstract It has recently been shown that identical, isotropic particles can form complex crystals and quasicrystals. In order to understand the relation between the particle interaction and the structure, which it stabilizes, the phase behavior of a class of two-scale potentials is studied. In two dimensions, the phase diagram features many phases previously observed in experiment and simulation. The three-dimensional system includes the sigma phase with 30 particles per unit cell, not grown in simulations before, and an amorphous state, which we found impossible to crystallize in molecular dynamics. We suggest that the appearance of structural complexity in monodisperse systems is related to competing nearest neighbor distances and discuss implications of our result for the self-assembly of macromolecules.

On the Quaternary System Ti-Fe-Ni-Al

The homogeneity ranges of the Laves phases and phase relations concerning the Laves phases in the quaternary system Ti-Fe-Ni-Al at 900 °C were defined by x-ray powder diffraction (XPD) data and electron probe microanalysis (EPMA). Although at higher temperatures the Laves phase forms a continuous solid solution, two separate homogeneity fields of TiFe2-based (denoted by λFe) and Ti(TiNiAl)2-based (denoted by λNi) Laves phases appear at 900 °C. The relative locations of Laves phases, G phase, Heusler phase, and CsCl-type phase as well as the associated tie-tetrahedra were experimentally established in the quaternary for 900 °C and presented in three-dimensional (3D) view. Furthermore, a partial isothermal section TiFe2-TiAl2-TiNi2 was constructed, and a connectivity scheme, derived for equilibria involving Laves phases in the Ti-Fe-Ni-Al quaternary system at 900 °C was derived. As a characteristic feature of the quaternary phase diagram, the solid solubility of fourth elements in both the TiFe2-based and Ti(NiAl)2-based Laves phases is limited at 900 °C and is dependent on the ternary Laves phase composition. A maximum solubility of about 8 at.% Ni is reached for composition Ti33.3Fe33.3Al33.4. Structural details have been evaluated from powder x-ray and neutron diffraction data for (i) the Ti-Fe-Ni ternary and the Ti-Fe-Ni-Al quaternary Laves phases (MgZn2-type, space group: P63/mmc) and (ii) the quaternary G phase. Atom site occupation behavior for all phases from the quaternary system corresponds to that of the ternary systems. For the quaternary Laves phase, Ti occupies the 4f site and additional Ti (for compositions higher than 33.3 at.%Ti) preferably enters the 6h site. Aluminum and (Fe,Ni) share the 6h and the 2a sites. The compositional dependence of unit cell dimensions, atomic coordinates, and interatomic distances for the Laves phases from the quaternary system is discussed. For the quaternary cubic G phase, a centrosymmetric as well as a noncentrosymmetric variety was observed depending on the composition: from combined x-ray and neutron powder diffraction measurements Ti33.33Fe13.33Ni10.67Al42.67 was found to adopt the lower symmetry with space group \( F\bar 43m \).

Combined ab initio/CALPHAD modeling of fcc-based phase-equilibria in the Ir–Nb system

A preliminary thermodynamic assessment of the Ir–Nb system, one of the key binary systems of the Ir-based refractory superalloys, has been performed by combining ab initio calculations and the CALPHAD (CALculation of PHAse Diagrams) technique. The ground-state formation enthalpies have been calculated by the full-potential linearized augmented plane wave method. The free energies at finite temperatures have been estimated using the cluster variation method, where the effective cluster interaction energies have been extracted from the formation enthalpies by the cluster expansion method. The liquid and A1 phases are modeled as substitutional solutions. The L1 0 and L1 2 phases are described using the four-sublattice model with the formula (Ir,Nb) 1/4(Ir,Nb) 1/4(Ir,Nb) 1/4(Ir,Nb) 1/4, while other solid phases are not considered in the present assessment. The obtained parameter set reproduces well the characteristic features of the experimental phase diagram and thermodynamic quantities.

Transport through constricted quantum Hall edge systems: Beyond the quantum point contact

Motivated by surprises in recent experimental findings, we study transport in a model of a quantum Hall edge system with a gate-voltage controlled constriction. A finite backscattered current at finite edge bias is explained from a Landauer-Buttiker analysis as arising from the splitting of edge current caused by the difference in the filling fractions of the bulk $({\ensuremath{\nu}}_{1})$ and constriction $({\ensuremath{\nu}}_{2})$ quantum Hall fluid regions. We develop a hydrodynamic theory for bosonic edge modes inspired by this model. The constriction region splits the incident long-wavelength chiral edge density-wave excitations among the transmitting and reflecting edge states encircling it. The competition between two interedge tunneling processes taking place inside the constriction, related by a quasiparticle-quasihole (qp-qh) symmetry, is accounted for by computing the boundary theories of the system. This competition is found to determine the strong coupling configuration of the system. A separatrix of qp-qh symmetric gapless critical states is found to lie between the relevant renormalization group (RG) flows to a metallic and an insulating configuration of the constriction system. This constitutes an interesting generalization of the Kane-Fisher quantum impurity model. The features of the RG phase diagram are also confirmed by computing various correlators and chiral linear conductances of the system. In this way, our results find excellent agreement with many recent puzzling experimental results for the cases of ${\ensuremath{\nu}}_{1}=1∕3$ and 1. We also discuss and make predictions for the case of a constriction system with ${\ensuremath{\nu}}_{2}=5∕2$.

Role of quasiparticles in universal low-temperature properties of [formula omitted

We demonstrate that the main universal features of the low temperature magnetic field-temperature experimental phase diagram of CeCoIn 5 and other heavy-fermion metals can be well explained within the concept of quasiparticles and fermion condensation quantum phase transition. We analyze dynamic conductance recently obtained in measurements on CeCoIn 5 and show that the particle–hole symmetry is violated in this metal making dynamic conductance asymmetric as a function of applied voltage V .

Formation of quasicrystals and metallic glasses in relation to icosahedral clusters

It has been widely accepted that quasicrystals and at least some metallic glasses are built up with icosahedral clusters. Information about the cluster structures can be obtained from crystalline counterparts. In this paper, we will describe the formation rules of bulk metallic glasses, originally developed for quasicrystals, by combining cluster structures with phase diagram features. We will introduce the e/a-constant and e/a-variant criteria for ternary systems, and e/a-constant and atomic size constant criteria for quaternary systems. We will show how the glass forming composition optimization is realized by applying these rules in the Zr–Al–Ni and Zr–Al–Ni–Cu systems. In both systems the optimized glass-forming composition is related to a common binary icosahedral cluster Zr9Ni4 derived from the fcc Zr2Ni phase. A novel route to reach amorphous forming composition is also attempted by mixing Al-based quasicrystal-forming compositions with Zr.

Influence of oxygen ordering and charge imbalance on the existence of the 60-K plateau in the YBa 2Cu 3O 6+ y phase diagram

We study a “phase only” model of YBa 2Cu 3O 6+ y , which captures characteristic energy scales present in YBCO and allows for strong anisotropy within basal planes to investigate the influence of oxygen ordering and charge imbalance on the critical temperature T c( y). We describe the high-temperature superconducting system in terms of collective variables utilizing a three-dimensional semi-microscopic XY model with two-component vectors that involve phase variables and adjustable parameters representing microscopic phase stiffnesses. With the help of the spherical closure relation and the transfer matrix method we solve the XY model and determine T c for chosen system parameters and discuss a possible origin of characteristic feature of YBCO phase diagram: the 60-K plateau. We find it unlikely that the plateau is a result of oxygen ordering within Cu–O chains, however can be caused by a charge redistribution among CuO 2 planes and apical sites.

Phase separation and the phase diagram of cuprate superconductors

We show that the main features of the cuprate superconductor phase diagram can bederived considering the disorder as a key property of these materials. Our basic point isthat the high pseudogap line is an onset of phase separation which generatescompounds made up of regions with distinct doping levels. We calculate howthis continuous temperature dependent phase separation process occurs in highcritical temperature superconductors (HTSC) using the Cahn–Hilliard approach,originally applied to study alloys. Since the level of phase separation varies fordifferent cuprates, it is possible that different systems with average doping levelpm exhibit different degrees of charge and spin segregation. Calculations on inhomogeneouscharge distributions in the form of stripes on finite clusters performed bythe Bogoliubov–de Gennes superconducting approach yield good agreementbetween the onset of local pairing amplitude and the pseudogap temperatureT*(pm). Assuming that the local order parameters generated at these clusters havetheir phase locked, we can follow how the superconducting phase develops atTc(pm) by possible percolation or Josephson coupling.

The chiral critical line ofNf= 2+1 QCD at zero and non-zero baryon density

We present numerical results for the location of the chiral critical line at finite temperature and zero and non-zero baryon density for QCD with N_f=2+1 flavours of staggered fermions on lattices with temporal extent N_t=4. For degenerate quark masses, we compare our results obtained with the exact RHMC algorithm with earlier, inexact R-algorithm results and find a reduction of 25% in the critical quark mass, for which the first order phase transition changes to a smooth crossover. Extending our analysis to non-degenerate quark masses, we map out the chiral critical line up to the neighbourhood of the physical point, which we confirm to be in the crossover region. Our data are consistent with a tricritical point at a strange quark mass of ~500 MeV. Finally, we investigate the shift of the critical line with finite baryon density, by simulating with an imaginary chemical potential for which there is no sign problem. We observe this shift to be very small or, conversely, the critical endpoint \mu^c(m_{u,d},m_s) to be extremely quark mass sensitive. Moreover, the sign of this shift is opposite to standard expectations. If confirmed on a finer lattice, it implies the absence of a critical endpoint or phase transition for chemical potentials \mu_B < 500 MeV. We thus argue that finer lattices are required to settle even the qualitative features of the QCD phase diagram.

有機伝導体における磁場誘起超伝導

In two dimensional organic conductors based on BETS molecule, where BETS is bis(ethylenedithio)tetraselenafulvalene, superconductivity is induced under very high magnetic field parallel to the conducting layers. We review the overall features of the global magnetic phase diagrams and discuss the mechanism of the field induced superconductivity in terms of the Jaccarino-Peter effect, where the exchange interaction between the conduction electron spins on the BETS molecule layers and the localized magnetic moments in the insulating layers plays a crucial role.

Medium-ranged interactions of transition-metal (3d and 4d) impurity pairs in Al and atomic structures of Al-rich Al–transition-metal alloys

We give systematic ab initio calculations for the interaction energies (from first to eighth neighbors) of impurity pairs X–X (X = Sc–Zn, Y–Cd) in Al and discuss the micromechanism of the structural stability of Al-rich AlX alloys. The calculations are based on the generalized gradient approximation in the density-functional theory and employ the all-electron full-potential Korringa–Kohn–Rostoker Green's function method for point defects, which guarantees the correct embedding of the cluster of impurities and vacancies in an otherwise perfect crystal. We show: (1) the fundamental features of phase diagrams of these alloys, such as ordering and segregation, are understood by the sign of first-neighboring X–X interaction energies; (2) the structural stability of Al-rich AlX alloys such as L1 2 (Al 3Sc and Al 3Y), DO 22 (Al 3V and Al 3Nb), and Mackay icosahedron in AlMn quasicrystal, are understood qualitatively by using the medium-ranged and oscillating X–X interactions, due to the strong sp-d (Al–X) hybridization; (3) the strength and oscillating behavior of the medium-ranged X–X interaction energies are specified very well by the d-electron numbers of X impurities.

Possible origin of60−Kplateau in theYBa2Cu3O6+yphase diagram

We study a model of YBa2Cu3O(6+y) to investigate the influence of oxygen ordering and doping imbalance on the critical temperature Tc(y) and to elucidate a possible origin of well-known feature of YBCO phase diagram: the 60-K plateau. Focusing on "phase only" description of the high-temperature superconducting system in terms of collective variables we utilize a three-dimensional semi microscopic XY model with two-component vectors that involve phase variables and adjustable parameters representing microscopic phase stiffnesses. The model captures characteristic energy scales present in YBCO and allows for strong anisotropy within basal planes to simulate oxygen ordering. Applying spherical closure relation we have solved the phase XY model with the help of transfer matrix method and calculated Tc for chosen system parameters. Furthermore, we investigate the influence of oxygen ordering and doping imbalance on the shape of YBCO phase diagram. We find it unlikely that oxygen ordering alone can be responsible for the existence of 60-K plateau. Relying on experimental data unveiling that oxygen doping of YBCO may introduce significant charge imbalance between CuO2 planes and other sites, we show that simultaneously the former are underdoped, while the latter -- strongly overdoped almost in the whole region of oxygen doping in which YBCO is superconducting. As a result, while oxygen content is increased, this provides two counter acting factors, which possibly lead to rise of 60K plateau. Additionally, our result can provide an important contribution to understanding of experimental data supporting existence of multicomponent superconductivity in YBCO.