First-principle calculation for the high-temperature diffusion coefficients of small pairs: the H–Ar Case

Argon has been widely used as a diluent for high-temperature reacting flow experiments. Numerical simulation of such process requires accurate diffusion coefficients for the H–Ar pair. All available potential energy functions for the H–Ar system have been empirically extrapolated in the repulsive region and are probably not reliable for prediction of H–Ar binary diffusion coefficient at high temperatures. We perform calculations using the restricted coupled cluster theory with single and double excitation (plus triple corrections) [RCCSD(T)] and suitable basis sets to obtain accurate potential energies. The calculated potential energy function is corrected for basis set superposition error and is validated against molecular beam scattering data. Comparisons with previous literature potential functions are also made. Using the Chapman–Enskog theory we carried out first-principle calculation of high-temperature diffusion coefficients by direct numerical integration of the collision integrals using the RCCSD(T) potential function. The computed diffusion coefficients are validated against available experimental data. Comparisons are also made with results obtained from transport compilations and packages commonly used in combustion simulation.     

The NaNO3–KNO3 phase diagram

Many papers have been published in relation to the NaNO₃–KNO₃ phase diagram determination in the last 160 years. These papers fall in two categories: (1) the solid–liquid equilibrium is assumed to be of the eutectic type, and (2) the solid–liquid equilibrium is considered as a loop with a minimum. The discordance between the two views is related to the slow transition kinetics that complicate the assessment of thermal ‘fluctuations’, and also to the appearance of a metastable form of potassium nitrate. The main result of this paper is the experimental phase diagram constructed with new experimental data so that we can assure that the second option is correct. This phase diagram is defined by a eutectoid invariant, an asymmetric immiscibility gap and a continuous solid solution with a minimum of melting point. Additionally, the ABθ model simulates correctly the experimental piece of evidence.     

Simulation of the (p,T) phase diagram of the temperature-driven metamagnet α-FeRh

We perform finite-pressure Monte Carlo simulations of an effective spin-analogous model with coupled magnetic and lattice degrees of freedom, which has previously been proposed in order to explain the anomalous temperature driven metamagnetic phase transition in α-FeRh. The results are in reasonable agreement with the experimental (p,?T) phase diagram. The critical behaviour of the system along the transition lines is analysed in detail.     

The two-dimensional alternative binary L-J system: liquid——gas phase diagram

A two-dimensional (2D) binary system without considering the　Lennard-Jones (L-J) potential has been studied by using the　Collins model. In this paper, we introduce the L-J potential　into the 2D binary system and consider the existence of the　holes that are called the ``molecular fraction\". The liquid--gas　phase diagram of the 2D alternative binary L-J system is obtained. The　results are quite analogous to the behaviour of 3D substances.     

Generic phase diagram of Nd2−xCexCuO4

We investigated the generic phase diagram of the electron doped superconductor, Nd_2?xCe_xCuO₄, using films prepared by metal organic decomposition. After careful oxygen reduction treatment to remove interstitial O_ap atoms, we found that the T_c increases monotonically from 24 K to 29 K with decreasing x from 0.15 to 0.00, demonstrating a quite different phase diagram from the previous bulk one. Based on the discussion on the tremendous influence of O_ap “impurities” on superconductivity in T′ cuprates, we conclude that our result represents the generic phase diagram for oxygen-stoichiometric Nd_2?xCe_xCuO₄.     

Reexamining the phase diagram of the Gay—Berne fluid

This work reexamines and updates earlier investigations on the phase behaviour of the Gay-Berne liquid crystal model, concentrating on the effect of varying temperature. Constant volume and constant pressure Monte Carlo simulations are combined for systems consisting of N = 500 molecules along different isotherms over the reduced temperature range 0.60 ≤ T ≤ 1.25. As in previous simulation studies of the model, the study identifies nematic and smectic B phases on compressing the isotropic fluid, the particular phase sequence depending on temperature. The nematic phase is found to be stable with respect to the isotropic phase for reduced temperatures T ≥ 0.75. In the temperature range 0.75 ≤ T ≤ 1.25, the phase boundaries of the isotropic-nematic transition are obtained by computing the Helmholtz free energy of both phases from thermodynamic integration. From the simulation data, the relative volume change at the isotropic-nematic transition is about 2%, and this value appears to be rather insensitive to changes in temperature. On compressing the nematic phase, the Gay-Berne fluid undergoes a strong first-order transition to the smectic B phase. This transition is studied by using constant pressure simulation, and the coexistence properties are estimated from the limits of mechanical stability of the nematic phase. Larger relative volume changes are found at the transition than those suggested by previous studies, with typical values increasing up to 10.5% as the temperature is decreased. The results are consistent with the existence of strong coupling between nematic and smectic order parameters. For temperatures T ≤ 0.70 the nematic phase is no longer stable, and the phase sequence isotropic-smectic B is observed. Therefore, the Gay-Berne model exhibits an isotropic-nematic-smectic B triple point. Extrapolating the present simulation data, this triple point is located approximately at reduced temperature T_INB ? 0.70 and reduced pressure P_INB ? 1.825.     

Magnetic field-temperature phase diagram of the organic conductor α-(BEDT-TTF)2KHg(SCN)4

We present systematic magnetic torque studies of the “magnetic field-temperature” phase diagram of the layered organic conductor α-(BEDT-TTF)₂KHg(SCN)₄ at fields nearly perpendicular and nearly parallel to the highly conducting plane. The shape of the phase diagram is compared to that predicted for a charge-density-wave system in a broad field range.     

The Tomonaga–Luttinger liquid with quantum impurity revisited: Critical line and phase diagram

We revisit the $(1+1)$ dimensional field theoretical model, which describes the Tomonaga–Luttinger liquid (TLL), interacting with a static impurity at the origin of the half line. Applying the Fermi–Bose equivalence and finite conformal transformations only, we map the model onto the Schmid model. Some details of the bosonization procedure have been given. The critical line and the phase diagram of the model follow from the renormalization group analysis of the Schmid model. The obtained critical line of the model is a hyperbola in the parameter space of the two couplings of the TLL.     

Ground-state phase diagram of the 1-D Holstein–Hubbard model

The Holstein–Hubbard model is investigated in one-dimension at half filling employing a series of unitary transformations taking into account the coherence and correlation of phonons. To treat the phonon subsystem more accurately a new squeezing transformation is introduced to incorporate the electron-density-dependent onsite phonon correlations to lower the energy further. The effective electronic Hamiltonian is next obtained by averaging the transformed Hamiltonian with respect to the zero-phonon state and the resulting effective electronic Hamiltonian is solved exactly using the method of Bethe ansatz. Finally the ground state is obtained by minimizing the energy with respect to all the variational parameters. The present method gives better results for the ground state energy of the system and also suggests the existence of a wider intermediate metallic phase at the charge-density-wave–spin-density-wave crossover region, which was first predicted by Takada and Chatterjee and later supported by Krishna and Chatterjee.     

On the phase diagram of a two-dimensional electron–hole system

The phase diagram for a system of spatially separated electrons and holes in coupled quantum wells or graphene double layers is studied in the framework of a BCS-like mean-field approach and a Landau expansion in terms of the pairing order parameter. We find a second order transition between an electron–hole plasma and a BCS phase, as well as a first-order transition between the BCS phase and a bosonic Mott phase of tightly bound electron–hole pairs without phase coherence. The electron–hole plasma exists at low and at high densities for weak interaction, the BCS phase at moderate density and the Mott phase at high density and strong interaction.     

The Fe–Fe2P phase diagram at 6 GPa

Here, we report experimental results on melting and subsolidus phase relations in the Fe–Fe₂P system at 6?GPa and 900–1600°C. The system has two P-bearing compounds: Fe₃P and Fe₂P. X-ray diffraction patterns of these compounds correspond to schreibersite and barringerite, respectively. The Fe–Fe₃P eutectic appears at 1075°C and 16?mol% P. Schreibersite (Fe₃P) melts incongruently at 1250°C to produce barringerite (Fe₂P) and liquid containing 23?mol% P. Barringerite (Fe₂P) melts congruently at 1575°C. Maximum solid solution of P in metallic iron at 6?GPa is 5?mol%. As temperature increases to 1600°C, the P solubility in the metallic iron decreases to 0.5?mol%, whereas the P content in coexisting liquid decreases to 3?mol%. The composition of quenched phases from Fe–P melt coincides with the compositions of equilibrium phases at corresponding temperature. Consequently, the composition of quenched products of Fe-P melts in meteorites can be used for reconstruction of P–T conditions of their crystallization under ambient or low pressures or during shock melting by impact collisions.     

The low temperature magnetic phase diagram of EuxSr1−xTiO3

Specific heat and magnetization measurements demonstrate that the antiferromagnetic (AFM) phase transition at T _N  = 5.7 K of EuTiO₃ is rapidly suppressed with Sr doping in Eu _x Sr_1?x TiO₃. Close to x = 0.25, T _N  = 0 K and AFM order vanishes. Above this critical concentration a finite transition temperature to an AFM phase is observed. The exchange couplings are derived as a function of x and the corresponding low temperature phase diagram is presented.     

Isotopic effect in the solid–liquid phase diagram of quantum clusters

Applying the Fourier path integral formalism to the isothermal-isobaric ensemble, the solid–liquid transition for 13-atom pure Lennard–Jones clusters was characterized. The masses of the clusters were taken as the masses of hydrogen, deuterium and tritium, hence isotopic effects of quantum clusters were considered. The parallel tempering Monte Carlo algorithm was used to solve all multidimensional integral in the FPI method. The volume of the system was defined with respect to the centroids of the quantum particles and a variable constraining potential was used to restrict undesirable thermodynamic events. The maximum value of the constant pressure heat capacity at a given temperature was used to identify the melting temperature. Pressure versus temperature phase diagrams were constructed for these systems with and without the inclusion of quantum effects. A significant difference in the melting temperature was encountered for the different isotopes due to quantum contribution.     

Phase stability of α-, γ-, and ε-Ce: DFT+DMFT study

We present the total energy calculation of α-, γ-, and ε-Ce phases in the frame of GGA, GGA+U, and DFT+DMFT methods. It has been shown that taken into account of Coulomb correlations in the frame of dynamical mean-field theory allows reproducing the phase diagram of Ce in correct way. Equilibrium volume calculated within the DFT+DMFT method for face-centered cubic (fcc) structure agrees with experimental Ce-γ cell volume. With temperature decrease energy minimum shifts toward α cell volume. Moreover, the DFT+DMFT total energy for body-centered tetragonal (bct) structure becomes smaller than for fcc one with increase in pressure in agreement with experimental phase diagram. Importance of accounting of Coulomb correlation in the frame of DMFT is discussed.     

The phase diagram and phase transition of the V2O3−V2O5, system

The phase relation in the V₂O₃−V₂O₅ system and the homogeneity range of each Magnéli phases were re-examined by magnetic and differential thermal analyses, in addition to the usual X-ray analyses. In particular, this report aimed to clarify the nature of the phase transition in the homologous series V_nO_2n−1. The Mössbauer effect measurements revealed that the phase transformation in V₄O₇ and V₅O₉ is a transition from paramagnetic to paramagnetic, namely merely a crystallographic transformation. The origin of phase transition of vanadium oxides including VO₂ and V₂O₃ is discussed on the basis of the d-electron pairing model proposed by Goodenough.     

Calculation of the structure properties and P–T phase diagram of hafnium

Abstract

The full-potential linear muffin-tin orbitals (FP-LMTO) method is used to calculate the total energy and equilibrium lattice properties for the observed phases of Hf. The temperature and pressure dependences of the Gibbs energy are found for these structures within the Debye model. A quantitative agreement with the experimental points of the P-T phase diagram is obtained.     

Calculation of the phase diagram of the TiZr alloy and investigation of the tendency toward separation of the ω phase

The electronic, structural, and thermodynamic properties of the TiZr equiatomic alloy have been calculated in terms of the electron density functional theory and Debye-Grüneisen model. The calculated lattice parameters a and c/a agree well with experimental data for the α, ω, and β phases. It has been shown that the ω phase is stable at atmospheric pressure and low temperatures, and it remains energetically more favorable up to T = 600 K. In the temperature range 600 K < T < 900 K, the α phase becomes stable, and above 900 K, the β phase of the TiZr alloy is stable. The phase diagram constructed in this study agrees qualitatively with the available experimental data. A tendency toward separation of the TiZr equiatomic alloy in the ω phase has been analyzed. It has been demonstrated that, in the ground state, the TiZr equiatomic alloy in the ω phase exhibits a tendency toward ordering rather than toward phase separation.     

Detecting a first-order transition in the QCD phase diagram with baryon–baryon correlations

We suggest baryon–baryon correlations as an experimentally accessible signature for a first-order phase transition between a baryon-rich phase, like quarkyonic, and a baryon-suppressed hadronic phase in the QCD phase diagram. We examine the consequences of baryon-rich bubble formation in an expanding medium and show how the two-particle correlations vary in the transverse and longitudinal direction depending on the strength of the radial flow, the bubble temperature, and the time when the baryons are emitted.     

Random-bond and random-anisotropy effects in the phase diagram of the Blume–Capel model

We report on Monte Carlo studies of the influence of quenched randomness on the phase diagram of the three-dimensional (3D) Blume–Capel model. The randomness is supposed to act either on the exchange coupling constants (bond randomness) or on the anisotropy distribution. With increasing disorder, first-order phase transitions are shown to change into second-order phase transitions. The trajectory of the tricritical point in the phase space as a function of disorder is presented. We have also calculated critical exponents at some points in the second-order phase region which show a change of universality class in agreement with the Harris criterion.