Phase Transition Entropy Research Articles

Investigation of Interaction of Alloy Y2MgNi9 with Hydrogen and Ammonia

The alloy consisting of 90% of phase Y2MgNi9 (structural type PuNi3) and 10% of phase YNi5 was prepared. The interaction of alloy with hydrogen and ammonia at different temperatures was investigated. In system Y2MgNi9–H2 at temperatures from –30 to +10 °C the isotherms of hydrogen sorption and desorption were constructed, the formation of two hydride phases was established and the changes in enthalpy and entropy of phase transitions were calculated. The phase transformations at treatment of the alloy Y2MgNi9 with ammonia at temperatures of 200–400 °C were discovered.

FT-IR Study on Liquid Crystal Phase Transitions of Thermotropic Hydrogen-Bonded Cubic Mesogenes, 1,2-Bis(4'-n-alkoxybenzoyl)hydrazines (BABH-n) and 4'-n-Alkoxy-3'-nitrobiphenyl-4-carboxlic acid (ANBC-n): Spectroscopic Evidence for Quasibinary Picture Model.

Changes in intermolecular interactions and molecular geometry for two kinds of thermotropic cubic mesogenes, 4'-n-alkoxy-3'-nitrobiphenyl-4-carboxlic acid (denoted as ANBC-n, where n represents the number of carbon atoms in the alkoxy group) and 1,2-bis(4'-n-alkoxybenzoyl)hydrazines (BABH-n), at liquid crystal (LC) phase transitions were revealed utilizing the frequency shifts in Fourier transform infrared (FT-IR) bands as a guide. The ANBC-n and BABH-n form two kinds of bicontinuous cubic (Cub(bi)), Ia3d and Im3m types, and smectic LC phases depending on the length of the alkyl chain and temperature. In the present work, two kinds of phase transitions, i.e., smectic C ↔ Ia3d-Cub(bi) phase transition for the ANBC-16 and BABH-9 and Ia3d-Cub(bi) ↔ Im3m-Cub(bi) phase transition for the BABH-13 and BABH-16, were examined, and the experimental result was compared to the entropy changes predicted by the quasibinary picture model. In this model, it is postulated that the basic units in the BABH-n and ANBC-n, i.e., the "chain" and "core", would contribute to the phase transition entropy in different ways. A conclusion of the FT-IR result shows the adequacy of this model for the behavior of the alkyl chain. On the other hand, the FT-IR result suggested that entropy changes for the "core" predicted by this model are not directly related to changes in the intermolecular interactions between the aromatic cores of the LC molecules at the phase transitions.

Structural transformation between two cubic phases of (NH4)3SnF7

Abstract Reinvestigations of the room temperature structure of ammonium heptafluorostannate (NH4)3SnF7 = (NH4)2SnF6·NH4F = (NH4)3[SnF6]F by both powder and single crystal X-ray diffractions have revealed that its real symmetry is Pa−3 (Z = 8) instead of Pm−3m (Z = 1) suggested earlier. Polarizing optical observations, heat capacity, X-ray powder, and single crystal measurements were performed in a wide temperature range (100–420 K). A reversible structural phase transition of the first order between two cubic modifications Pa−3 ↔ Pm−3m was found at about T0 = 360 K. The structural models associated with partially disordered and totally ordered high and low temperature phases, respectively, comply with a large value of phase transition entropy.

A phenomenological description of the entropy and specific heat anomalies in lawsonite near phase transitions

The theoretical temperature dependences of the entropy and specific heat near the phase transitions in the mineral lawsonite were calculated using the phenomenological model for describing a sequence of these transitions. The theoretical curves qualitatively agree with the experimental data.

Effect of lamellarity and size on calorimetric phase transitions in single component phosphatidylcholine vesicles

Nano-differential scanning calorimetry (nano-DSC) is a powerful tool in the investigation of unilamellar (small unilamellar, SUVs, or large unilamellar, LUVs) vesicles, as well as lipids on supported bilayers, since it measures the main gel-to-liquid phase transition temperature (Tm), enthalpies and entropies. In order to assign these transitions in single component systems, where Tm often occurred as a doublet, nano-DSC, dynamic light scattering and cryo-transmission electron microscopy (cryo-TEM) data were compared. The two Tms were not attributable to decoupled phase transitions between the two leaflets of the bilayer, i.e. nano-DSC measurements were not able to distinguish between the outer and inner leaflets of the vesicle bilayers. Instead, the two Tms were attributed to mixtures of oligolamellar and unilamellar vesicles, as confirmed by cryo-TEM images. Tm for the oligolamellar vesicles was assigned to the peak closest to that of the parent multilamellar vesicle (MLV) peak. The other transition was higher than that of the parent MLVs for 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and increased in temperature as the vesicle size decreased, while it was lower in temperature than that of the parent MLVs for 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and decreased as the vesicle size decreased. These subtle shifts arose due to small differences in the values of ΔH and ΔS, since Tm is determined by their ratio (ΔH/ΔS). It was not possible to completely eliminate oligolamellar structures for MLVs extruded with the 200nm pore size filter, even after 120 passes, while these structures were eliminated for MLVs extruded through the 50nm pore size filter.

Synthesis and characterisation of imidazolium-based ionic liquid crystals bearing a cholesteryl mesogenic group

A series of cholesteryl-containing imidazolium chlorides and imidazolium tetrachloroaluminates were synthesised, and the chemical structure, liquid crystalline behaviour and ionic conductivity were characterised by several technical methods. Whereas the imidazolium chlorides show chiral smectic A (SmA*) phase on heating and cooling cycles, the imidazolium tetrachloroaluminates display chiral nematic (N*) phase, which is uncommon for ionic liquid crystals (ILCs). The imidazolium chlorides display similar phase transition temperature and entropy, indicating the cholesteryl component influence predominately on the phase transition rather than the different alkyl substituent groups. The imidazolium tetrachloroaluminates show lower melting point temperatures and lower clear point temperature than the imidazolium chlorides. The mesophases exist at rather moderate temperatures. Non-mesomorphic imidazolium tetrachloroaluminate(III) salts with short alkyl substituents have been known for a long time, and the synthesised imidazolium tetrachloroaluminates are the first examples of tetrahalogenoaluminate(III)-containing ILCs. For the imidazolium tetrachloroaluminates, imidazolium cations combine loosely with AlCl4− ions because AlCl4− ions are large and occupy more space in spite of the hydrogen bond and electrostatic attraction interaction, indicating that the layer structure can be destroyed easily to form N* phase on heating.

Effect of Al content on the order of phase transition and magnetic entropy change in LaFe11Co0.8(Si1−xAlx)1.2 alloys

The comparative studies of the effect of partial substitution of Al by Si on the structure and magnetic entropy change in LaFe11Co0.8(Si1−xAlx)1.2 alloys produced by arc-melting followed by long time annealing at 1323K were performed. The X-ray diffraction analysis revealed almost single phase composition of annealed samples. The high intensity synchrotron radiation was used for studying the thermal evolution of lattice constant of the La(Fe,Co,Si)13 phase. Furthermore, calculations of temperature dependences of Landau coefficients were used to reveal changes in the character of phase transformation at around the Curie temperature.

Cholesteryl-containing ionic liquid crystals composed of alkylimidazolium cations and different anions

Cholesteryl-containing ionic liquid crystals (ILCs) 1-cholesteryloxycarbonylmethyl(propyl)-3-methyl(butyl)imidazolium chlorides ([Ca-Me-Im]Cl, [Ca-Bu-Im]Cl, [Cb-Me-Im]Cl and [Cb-Bu-Im]Cl) and corresponding imidazolium tetrachloroaluminates ([Ca-Me-Im]AlCl4, [Ca-Bu-Im]AlCl4, [Cb-Me-Im]AlCl4 and [Cb-Bu-Im]AlCl4) were synthesized in this work, and the chemical structure, LC behavior and ionic conductivity of all these ILCs were characterized by several technical methods. The imidazolium-based salts with Cl− ions showed chiral smectic A (SA*) phase on both heating and cooling cycles, while the tetrachloroaluminates exhibited chiral nematic (N*) phase. The mesophase was confirmed by characteristic LC textures observed by polarizing optical microscopy and typical diffractogram obtained by X-ray diffraction measurements. The samples with similar cholesteryl-linkage component showed similar phase transition temperature and entropy, indicating the cholesteryl component influence predominately on the phase transition rather than alkyl substituents on the imidazole ring. The imidazolium tetrachloroaluminates display relatively low phase transition temperature compared with the precursor chlorides. The functional difference in LC behavior and ionic conductivity were discussed by investigated the structural difference between the Cl−-containing and AlCl4—containing materials. The imidazolium chlorides exhibited layer structure both in crystal and mesophase states, and should be organized with a ‘head-to-tail’ organization to form interdigitated monolayer structures due to the tight ion pairs. But the imidazolium tetrachloroaluminates displayed layer structure only in crystal phase, and should be organized in ‘head-to-head’ arrangements form bilayer structures due to loose combination of ion pairs despite of hydrogen-bond and electrostatic attraction interaction.

Generalized Landauer bound as a universal thermodynamic entropy in continuous phase transitions.

The classic Landauer bound can be lowered when erasure errors are permitted. Here we point out that continuous phase transitions characterized by an order parameter can also be viewed as information erasure by resetting a certain number of bits to a standard value. The information-theoretic expression for the generalized Landauer bound in terms of error probability implies thus a universal form for the thermodynamic entropy in the partially ordered phase. We explicitly show that the thermodynamic entropy as a function of interaction parameters and temperature is identical to the information-theoretic expression in terms of error probability alone in the specific example of the Hopfield neural network model of associative memory, a distributed information-processing system of many interacting stochastic bits. In this framework the Landauer bound sets a lower limit for the work associated with "remembering" rather than "forgetting."

Phase- and glass-transition phenomena due to the same configurational order–disorder mechanism in crystalline racemic sec-butylcyclohexane

We report molar heat capacities for racemic sec-butylcyclohexane (s-BCH) measured by the adiabatic calorimetric method within the temperature range from 14 to 200 K. In the crystalline state, we identified the presence of a second-order phase transition and a glass transition phenomenon, both originating from the same configurational degree of freedom. The phase and glass transition temperatures Ttrs and Tg were determined to be (136.7 ± 1.0) and (100 ± 1) K, respectively. The entropy of phase transition was estimated to be ΔtrsSm = 1.4 J K−1 mol−1 ≈ (1/4) R ln 2. The phase transition was judged to be of an order–disorder type on the basis of the fact that the glass transition occurred in the low-temperature heat-capacity tail. The entropy was interpreted to suggest that four molecules in the crystalline state constitute a unit for producing two distinct configurations in the coexistence of (d)- and (l)-s-BCH.

Molar heat capacity and thermodynamic properties of N-alklypyridinium hexafluorophosphate salts, [Cnpy][PF6] (n = 2, 3, 5)

Molar heat capacities (Cp,m) of the ionic liquids (ILs) N-alkylpyridinium hexafluorophosphate [Cnpy][PF6] (n=2, 3, 5) were measured with a high-precision automated adiabatic calorimeter (AC). The melting temperatures were determined. The molar enthalpy and entropy of phase transition were calculated according to the data obtained of the molar heat capacities by the empirical equation. The polynomial equations of molar heat capacity as a function of the temperature were fitted before and after the phase transition for each IL compounds. The thermodynamic functions (HT−H298.15) and (ST−S298.15) were derived from the molar heat capacity values over the temperature range from (80 to 400)K with an interval of 5K.

Enthalpy of SmAlO3 in the range 472–2252 K

The enthalpy increment of SmAlO3 is measured for the first time using high-temperature drop calorimetry in the temperature range 472–2252 K. The enthalpy values are used to determine other thermodynamic functions, such as heat capacity, entropy, and reduced Gibbs energy. Phase transitions from orthorhombic to trigonal structure and from trigonal to cubic structure are observed at 1055 and 2103 K, respectively. The phase transition at 1055 K is studied by differential thermal analysis. The enthalpies and entropies of phase transitions are determined.

Magnetic Phase Transition and Magnetic Proporties of LaFe<sub>11.6</sub>Si<sub>1.4</sub>B<sub>x</sub> Compounds

The magnetic phase transition and magnetic entropy change of melt-spun LaFe11.6Si1.4Bx (x=0.0, 0.3, 0.5, 0.7) ribbons were investigated by measuring the magnetization as the function of temperature. Comparing with LaFe11.6Si1.4, the NaZn13-type structure of LaFe11.6Si1.4Bx did not change after introducing B atoms, while the amount of α-Fe phase in the ribbons significantly decreased. The Curie temperature slightly changes after the addition of B. The LaFe11.6Si1.4Bx (x=0.3, 0.5) exhibits first-order magnetic transition and large magnetic entropy change as observed in LaFe11.6Si1.4 compound.

Heat Capacity and Thermodynamic Property Studies of Diethyl Acetamidomalonate (C9H15NO5)

The heat capacity of diethyl acetamidomalonate (DEAM) has been studied using an adiabatic calorimeter in the temperature range from (78 to 395) K. No thermal anomalies were observed in temperature regions from (78 to 344) K and from (375 to 395) K, where the heat capacity data were fit to two polynomial equations. A solid–liquid phase transition was detected in temperatures from (344 to 375) K. The phase transition temperature, enthalpy, entropy and mole fraction purity of this compound were determined to be 368.72 K, 34.3 kJ·mol–1, 93.0 J·K–1·mol–1 and 0.9997(mole fraction), respectively, based on the experimental data in the phase transition region. The thermodynamic functions of DEAM relative to the reference temperature of 298.15 K were derived based on the fitted heat capacity data in the entire experimental region.

Thermodynamic and aromaticity studies for the assessment of the halogen⋯cyano interactions on Iodobenzonitrile

The standard (p°=0.1MPa) molar enthalpies of formation, in the gaseous phase, of the 2-, 3- and 4-iodobenzonitrile isomers were derived from the combination of the corresponding standard molar enthalpies of formation, in the condensed phase, at T=298.15K, and the standard molar enthalpies of sublimation, at the same temperature, calculated respectively from the standard molar energies of combustion in oxygen, measured by rotating-bomb calorimetry, and from the vapour-pressure study of the referred compounds, measured by mass-loss Knudsen effusion technique. The strength of the halogen-halogen and the halogen-cyano intermolecular interactions, in the crystal, are evaluated by the enthalpies and entropies of phase transition of the iodobenzonitrile derived from mass-loss Knudsen technique and differential scanning calorimetry measurements and compared with those reported to fluorobenzonitrile and bromobenzonitrile isomers. The computational calculations complement the experimental work, using different aromaticity criteria (HOMA, NICS, Shannom Aromaticity, PDI and ATI) for the analysis of the electronic behaviour of each iodobenzonitrile isomer.

Structural, thermal, and magnetic properties of MnFePSiGe compounds prepared by spark plasma sintering method

Crystal structure, and thermal and magnetic properties have been studied for bulk MnFePSiGe compounds with different silicon contents prepared by the spark plasma sintering method. The MnFePSiGe compounds crystallize in a hexagonal Fe2P-type structure without obvious secondary phase. The compounds undergo a ferromagnetic to paramagnetic first-order transition near their Curie temperature (TC). With increase of Si content, both TC and magnetic entropy change (−ΔSM) of the compounds rise remarkably, while the thermal hysteresis (Thys) remains almost invariant. For MnFeP0.63Si0.26Ge0.11 compound, the TC and −ΔSM for a magnetic field change from 0 to 2T are 318K and 27.9J/KgK, respectively, and its phase transition entropy change (ΔS) obtained from DSC is 32.3J/KgK.

Enhancement of the stability and temperature range of the mesophases in the binary mixtures of cholesteryl myristate and 4-n-decyloxy benzoic acid

Thermodynamical, optical, and electrical properties of the binary mixtures of cholesteryl myristate (ChM) and 4-n-decyloxybenzoic acid (DOBA) have been carried out by the differential scanning calorimeter, polarized optical microscopy (POM), and impedance spectroscopy. Through thermodynamic study, various phase transition temperatures, transition enthalpies, and transition entropies have been determined to investigate temperature range and stability of the mesophases of the mixtures. Phase diagrams in the heating and cooling cycles have been drawn for the ChM–DOBA binary system. Optical textures of different mesophases have identified with the help of POM. Dielectric permittivity has been determined in planar and homeotropic alignments of the mixtures having DOBA concentrations 30.0 and 92.3 mol %. The experimental dielectric data in the frequency range of 1 Hz to 35 MHz do not show any relaxation mode in both the alignments of these mixtures. Dielectric anisotropy has been determined for the various observed phases of the mixtures.

Hidden disorder in theα′→δtransformation of Pu-1.9 at.%Ga

Enthalpy and entropy are thermodynamic quantities critical to determining how and at what temperature a phase transition occurs. At a phase transition, the enthalpy and temperature-weighted entropy differences between two phases are equal ($\ensuremath{\Delta}H=T\ensuremath{\Delta}S$), but there are materials where this balance has not been experimentally or theoretically realized, leading to the idea of hidden order and disorder. In a Pu-1.9 at. $%$ Ga alloy, the $\ensuremath{\delta}$ phase is retained as a metastable state at room temperature, but at low temperatures, the $\ensuremath{\delta}$ phase yields to a mixed-phase microstructure of $\ensuremath{\delta}$- and ${\ensuremath{\alpha}}^{\ensuremath{'}}$-Pu. The previously measured sources of entropy associated with the ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\delta}$ transformation fail to sum to the entropy predicted theoretically. We report an experimental measurement of the entropy of the ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\delta}$ transformation that corroborates the theoretical prediction, and implies that only about 65$%$ of the entropy stabilizing the $\ensuremath{\delta}$ phase is accounted for, leaving a missing entropy of about 0.5 ${k}_{B}$/atom. Some previously proposed mechanisms for generating entropy are discussed, but none seem capable of providing the necessary disorder to stabilize the $\ensuremath{\delta}$ phase. This hidden disorder represents multiple accessible states per atom within the $\ensuremath{\delta}$ phase of Pu that may not be included in our current understanding of the properties and phase stability of $\ensuremath{\delta}$-Pu.

Phase transition line of solid molecular nitrogen into the cubic gauche-polymeric phase

The coexistence line of two high-pressure phases of solid nitrogen, molecular and polymeric cubic gauche (CG), has been predicted using two separate equations of state. The phase transition parameters, latent heat, volume, and entropy jumps, were calculated. At low temperatures, the predicted volume jump ΔV(P) is in agreement with recent experimental data; at elevated temperatures the P(T)-curve approaches the pressure maximum on the melting line.

Thermal and odd–even behaviour in a homologous series of lithium n-alkanoates

Thermotropic phase transition temperatures, enthalpies and entropies of phase changes and odd–even alternation, in a homologous series of anhydrous lithium n-alkanoates, LiC n H 2 n −1O 2 (LiC 8–19 inclusive), have been investigated by differential scanning calorimetry (d.s.c.), hot stage polarizing microscopy and solid state 13C NMR spectroscopy. The number of phases observed, between the room temperature microcrystalline solid and isotropic melt, shows a clear dependence on chain length. For LiC 8–13, only one intermediate lamellar II crystalline phase is observed. For LiC 14–19, a lamellar II and high temperature phase are evident. The high temperature phase is characterized by pre-melting and disordering of hydrocarbon chains as they change from nearly all-trans to one with increased gauche conformers. It is probably a solid rotator phase. Odd–even alternation in melting temperature, density and some thermodynamic data result from the relative distance between methyl groups, from opposite layers in a bi-layer. Molecular models indicate that the methyl groups in odd chains are more favourably orientated which lead to a more energetically favoured staggered conformer. As a consequence, the methyl groups, for odd chains, are in closer proximity than even chains. This subtle change in the molecular lattice could account for the presence of polymorphic structures on cooling from the melt.