Phase separation of binary systems

In this paper, three physical predictions on the phase separation of binary systems are derived based on a dynamic transition theory developed recently by the authors. First, the order of phase transitions is precisely determined by the sign of a nondimensional parameter K such that if K>0, the transition is first order with latent heat and if K<0, the transition is second order. Here the parameter K is defined in terms of the coefficients in the quadratic and cubic nonlinear terms of the Cahn-Hilliard equation and the typical length scale of the container. Second, a phase diagram is derived, characterizing the order of phase transitions, and leading in particular to a prediction that there is only a second-order transition for molar fraction near 1/2. This is different from the prediction made by the classical phase diagram. Third, a TL-phase diagram is derived, characterizing the regions of both homogeneous and separation phases and their transitions.     

Cerium influence on the spin reorientation in Er2−xCexFe14B evidenced by Mössbauer and DSC techniques

Polycrystalline Er_2−xCe_xFe₁₄B (x=0.25, 0.5, 1.0) compounds have been studied by ⁵⁷Fe Mössbauer spectroscopy and by differential scanning calorimetry (DSC) in the temperature range 80–470 K. The spin reorientation phenomenon has been studied extensively by narrow step temperature scanning in the vicinity of the spin reorientation temperature. It was found that in the region of transition, each Mössbauer subspectrum splits into two Zeeman sextets, which are characterised by different hyperfine magnetic fields and quadrupole splittings. A consistent way of describing the Mössbauer spectra in the reorientation region, below and above, was proposed. The composition and temperature dependencies of hyperfine interaction parameters and subspectra contributions were derived from experimental spectra.The endothermic DSC peaks were observed for all studied compounds, which correspond to the transition from basal plane to axial easy magnetisation direction on increasing the temperature. The spin reorientation temperatures and the enthalpies of transitions were established from DSC data. The spin arrangement diagram was constructed and the spin reorientation temperatures obtained by the two methods were compared.     

Analysis of phase transition behavior of BaCeO3 with thermal analyses and high temperature X-ray diffraction

Structural phase transitions in BaCeO₃ have been investigated with combination of differential scanning calorimetry (DSC), dilatometry and high temperature X-ray diffraction with high sensitivity and resolution. In DSC curve at heating procedures, baseline shift, endothermic peak and another baseline shift were observed at 260 °C, 385 °C and 895 °C, respectively. From DSC curve at cooling procedure, it was revealed that all the baseline shifts and peak were reversible. No hysteresis was observed in the both baseline shifts indicating second order phase transition at 260 °C and 895 °C with variation of specific heat capacity, ΔC_p, of 10 J/mol K and 7 J/mol K, respectively; whereas the order of the phase transition at 385 °C was revealed to be the first since hysteresis was detected around 370–385 °C. Variation of enthalpy, ΔH, at the phase transition was 45 J/mol. High temperature X-ray diffraction measurements have revealed that the crystal structure of BaCeO₃ changes from primitive orthorhombic perovskite through body-centered one, rhombohedral distorted one to cubic one around 280 °C, 400 °C and 900 °C, showing correspondence with DSC curves. Dependence of molar volume on temperature estimated from high temperature X-ray diffraction showed agreement with thermal expansion behavior observed with dilatometry.     

Positron annihilation studies of a conjugated polymer

Lineshapes of the Doppler broadened annihilation radiation (DBAR) of a conjugated polymer polydiacetylene, 4-butoxycarbonylmethylurethane (P-4BCMU), in its amorphous state, were measured over the temperature range of 30 to 190°C. A differential-scanning-calorimeter (DSC) was used to correlate the transition temperatures. The DBAR lineshapes show a distinct change during the structural transitions of the polymer. A two states trapping model is used to qualitatively describe the temperature dependence of the lineshape. The nature of formation of the trapped states of positrons above T_c, through the interplay of electronic structure/conformation change, is discussed.     

The multilayer melting transition in methane adsorbed on graphite

High resolution heat capacity measurements of multilayer methane adsorbed on graphite are presented and analyzed. The evidence indicates the presence of two wetting transitions: a first-order dewetting transition at T_w = 90.48 K, and a continuous wetting transition at the triple point, T_t = 90.66 K. This behavior is to be expected in connection with the melting transition in any system where both solid and liquid wet the surface. Heat capacity measurements can provide a valuable diagnostic tool for the wetting behavior of films too thick to be investigated by other means. In the thin film limit, we find that the latent heat of melting vanishes at about 4 layers.     

Phase transition in thin epitaxial ferroelectric films as derived from thermal measurements

The temperature dependence of the heat capacity of thin epitaxial films BaTiO₃/MgO is studied by the dynamic 3ω method in the thickness range 50–500 nm. It is revealed that the heat capacity exhibits diffuse anomalies due to phase transitions. The temperature of the ferroelectric phase transition T _C increases with decreasing film thickness. The reasons for the strong diffuseness of the transition and the nonlinear dependence of the transition temperature on the film thickness are discussed.     

Detailed infrared spectroscopic study of thermal transitions in new hybrid [(CH3)2CHNH3]4Cd3Cl10 crystal

Phase transitions of tetra(isopropylammonium)decachlorotricadmate(II) [(CH₃)₂CHNH₃]₄Cd₃Cl₁₀ crystal have been studied by infrared, far infrared and Raman measurements in wide temperature range, between 11 K and 388 K. The temperature changes of wavenumber, center of gravity, width and intensity of the bands were analyzed to clarify cationic and anionic contributions to the phase transitions mechanism. The results of investigation showed earlier by differential scanning calorimetry (DSC), thermal expansion and dielectric measurements clearly confirmed the sequence of phase transitions at T₁=353 K, T₂=294 K and T₃=260 K. The current results derived from DSC and infrared measurements revealed additional phase transition at T₄=120 K.     

Phase transitions in molecular crystals of <Emphasis Type="Italic">n</Emphasis>-alkane alcohols

A comparative analysis of the phase transitions in molecular crystals of n-alkane alcohols with different lengths of the (C _n H_{2n + 2}OH) chain was performed using differential scanning calorimetry (DSC). Investigation of the temperature hysteresis made it possible to reveal a number of new effects associated with the specific features of the first-order phase transitions. A quantitative analysis of the temperature dependence of the heat capacity was carried out in the framework of the theory of diffuse (Λ-shaped) first-order phase transitions.     

Franck-Condon factors and absolute transition probabilities for the if (B3Π0+-X1Σ+) transition

Improved spectroscopic constants have been used to calculate Rydberg-Klein-Rees (RKR) potentials and Franck-Condon factors for the IF(B³Π₀₊-X¹Σ⁺) transition. The Franck-Condon factors are generally in good agreement with previously calculated values, but differ by as much as 30% for transitions from higher levels of the B-state. Several experimentally measured relative transition moment functions have been evaluated and the best scaled, so that the total transition probability calculated for each B-state vibrational level, A(v'), matched measured values. The scaled function was then used to calculate individual transition probabilities, A(v',v), for the vibronic transitions.     

Phase transitions in the two-dimensional ferro- and antiferromagnetic potts models on a triangular lattice

The phase transitions in the two-dimensional ferro- and antiferromagnetic Potts models with q = 3 states of spin on a triangular lattice are studied using cluster algorithms and the classical Monte Carlo method. Systems with linear sizes L = 20–120 are considered. The method of fourth-order Binder cumulants and histogram analysis are used to discover that a second-order phase transition occurs in the ferromagnetic Potts model and a first-order phase transition takes place in the antiferromagnetic Potts model. The static critical indices of heat capacity (α), magnetic susceptibility (γ), magnetization (β), and correlation radius index (ν) are calculated for the ferromagnetic Potts model using the finite-size scaling theory.     

Drug–DNA interaction: A theoretical study on the binding of thionine with DNAs of varying base composition

The recent studies carried out on the binding of small molecule to deoxyribonucleic acids suggested that the intercalation of a tricyclic heteroaromatic molecule, thionine, with natural DNAs provided thermal stabilization to the DNA complex. In the present study, we reported theoretical analysis of thionine binding with natural DNAs of varying base composition by using an amended Zimm and Bragg theory, to explain the melting behaviour and heat capacity of DNAs with and without thionine binding. We used experimental models of Paul et al. for implementing this study (Paul et al., 2010). The sharpness of transition has been examined in terms of half width and sensitivity parameter (ΔH/σ). The results of theoretical analysis concluded that the various parameters such as heat capacity curve, transition profile, half widths and sharpness of the transition are in good agreement with the experimental measurements for binding of thionine determined through DSC. The theoretical analysis proposed in this study, therefore, may be useful to understand interaction of small molecules with deoxyribonucleic acids. This approach may also be applied to design DNA binding therapeutic molecules and in the process of drug formulation and development.     

Determination of kinetics parameters of glass transition in glassy Se and glassy Se98M2 alloys using DSC technique

Glassy Se and Se₉₈M₂ (M = Ag, Cd, Zn) alloys are obtained by the melt quenching technique. Differential Scanning Calorimetry (DSC) technique (under non-isothermal conditions) has been applied to see the effects of Ag, Cd, and Zn additives on the glass transition kinetics of Se-rich glassy alloys at different heating rates. The variation of glass transition temperature, T _g with the heating rate, β has been used to investigate the glass transition kinetics. The values of various kinetic parameters such as glass transition temperature, activation energy of glass transition, overall mean bond energy 〈E〉, heat of atomization H _S , bond strength (Se–M) have also been calculated.     

Atomistic simulation of a superionic transition in UO2

The results of the atomistic simulation of a superionic transition and melting of uranium dioxide are presented. The temperature dependences of the concentration of defects in the oxygen sublattice and changes in the heat capacity and isothermal compressibility upon the superionic transition are calculated. It is shown that the curve of the superionic transition in the PT diagram can be described by the Ehrenfest’s equation. The possibility of describing the superionic transition within the framework of the theory of second- order phase transitions is discussed. Based on the results obtained, it is considered that this structural transformation can occur in other materials.     

Strong coupling analysis of the SO(3) lattice gauge theory at different space-time dimensionalities

High order strong coupling series for free energy, internal energy and specific heat are constructed to study the SO(3) lattice gauge theory at space-time dimensionalities from d = 2 to 7. Padé approximants are used to look for singularities. First-order phase transitions for d = 4, 5, 6 and 7 are clearly detected. A second-order phase transition seems to be present in the three-dimensional case.     

Raman scattering and X-ray diffraction study of structural phase transitions in the perovskite-type layer compound (C3H7NH3)2CdCl4

The structural phase transitions of (C₃H₇NH₃)₂CdCl₄ (PACC) have been studied by means of Raman scattering. X-ray diffraction and DSC measurements It is shown that the order-disorder phase transition Abma ? Pbca occurs at 156 K and not at 183 K as previously proposed by Chapuis (Acta CrystB34, 1506 (1978)) Apparently, the transition at 183 K does not change the Abma space group; it is suggested that it could be related to the occurrence of an incommensurate phase Another structural transformation of PACC is detected at 114 K, but no conclusion can be given as far as the structure of the low-temperature phase is not determined.     

Phase transition of the solid lithium ion conductor with the spinel structure: Li2−2xM1+xCl4 (M=Mg,Mn)

The phase transition of the high lithium ion conductor Li_2−2xM_1+xCl₄ (M=Mg, Mn) has been examined by means of high temperature X-ray diffraction analysis and differential scanning calorimetry (DSC). In Li₂MgCl₄, two broad endothermic peaks in DSC curves were observed in the temperature ranges of 490–640 K and of 640–820 K. The former corresponds to the transition from the low to high conducting state (T₁), the latter to that from the spinel to the defect NaCl-type structure (T₂). The DSC curves of Li₂MnCl₄ showed three broad endothermic peaks at 450–620 K, 620–710 K, and 710–750 K. The former peak corresponds to the phase transition from the low to high conducting state (T₁), the latter two peaks correspond to those to the defect NaCl-type structure. In the nonstoichiometric spinel, Li_2−2xM_1+xCl₄ (M= Mg, Mn), the enthalpy change (ΔH) of the phase transition T₁ decreased with increasing x. The phase transitions have been discussed by considering the neutron diffraction data.     

The rotational spectrum of methoxyethyne: Centrifugal distortion and internal rotation analysis

In the rotational spectrum of methoxyethyne 173 new transitions (J ≤ 30) have been measured between 150 and 240 GHz. In the centimeter range 25 new transitions (J ≤ 11) of the first excited torsional state have also been assigned. An overall fit of the measurements using a structure relaxation model has allowed us to accurately determine the internal rotation parameters. For the A substate effective rotational parameters are given which allow the calculation of transition frequencies of possible astrophysical interest.     

Ionic conductivity of Li2NaK(SO4)2

A DSC study of Li₂NaK(SO₄)₂ in the range 300∽925 K has revealed that the compound undergoes phase transitions at 654, 682, 778 and 866 K, before melting congruently at 900 K. The five phase which are observed are designated I, II, III, IV and V with increasing order of temperature. A comparison of the enthalpy of melting with the transition enthalpies suggests that the phases IV and V are plastic phases. The ionic conductivity was measured in the range 584∽895 K by an ac impedance technique. The phase IV showed the smallest activation energy as determined from the temperature dependence of the conductivity (E_a=0.82 eV). The highest conductivity (1.0 Ω⁻¹ cm⁻¹) was observed in phase V at 895 K.     

First-Order Phase Transformation at Constant Volume: A Continuous Transition?

We describe a first-order phase transition of a simple system in a process where the volume is kept constant. We show that, unlike what happens when the pressure is constant, (i) the transformation extends over a finite temperature (and pressure) range, (ii) each and every extensive potential (internal energy U, enthalpy H, Helmholtz energy F, and Gibbs energy G), and the entropy S is continuous across the transition, and (iii) the constant-volume heat capacity does not diverge during the transition and only exhibits discrete jumps. These non-intuitive results highlight the importance of controlling the correct variables in order to distinguish between continuous and discontinuous transitions. We apply our results to describe the transition between ice VI and liquid water using thermodynamic information available in the literature and also to show that a first-order phase transition driven in isochoric condition can be used as the operating principle of a mechanical actuator.