Search results for "Phase Transition"
showing 10 items of 1281 documents
Double-well thermodynamic potentials and spinodal curves: how real are they?
2007
The concept of double-well thermodynamic potentials, ubiquitous since the van der Waals description of the vapour-to-liquid transition and the Landau theory of phase transitions, is critically re-examined. Particular emphasis is put on the extent to which spinodal curves (separating ‘metastable’ from ‘unstable’ states) are meaningful. It is argued that in full thermodynamic equilibrium spinodals are well-defined when one either considers finite subsystems of an infinitely large system, or systems with all linear dimensions finite. Evidence is given that in a finite (cubic) d-dimensional box the spinodals correspond (in a fluid) to the rounded ‘droplet evaporation’ or ‘bubble condensation’ t…
Time scale of protein aggregation dictated by liquid-liquid demixing
2003
The growing impact of protein aggregation pathologies, together with the current high need for extensive information on protein structures are focusing much interest on the physics underlying the nucleation and growth of protein aggregates and crystals. Sickle Cell Hemoglobin (HbS), a point-mutant form of normal human Hemoglobin (HbA), is the first recognized and best-studied case of pathologically aggregating protein. Here we reanalyze kinetic data on nucleation of deoxy-HbS aggregates by referring them to the (concentration-dependent) temperature Ts characterizing the occurrence of the phase transition of liquid-liquid demixing (LLD) of the solution. In this way, and by appropriate scalin…
Copolymer Melts in Disordered Media
1996
The symmetric AB block copolymer melt in a gel matrix with preferential adsorption of A monomers on the gel gives an example of a random-field system, which is described near the point of the microphase separation transition by the random field Landau-Brazovskii Hamiltonian. By using the technique of the 2-nd Legendre transform, the phase diagram of the system is calculated. We found that the preferential adsorption of the copolymer on the gel results in two effects: a) It decreases the temperature of the first order phase transition between disordered and ordered phase. b) There exists a region on the phase diagram at some small but finite value of the adsorption energy in which the replic…
Classification theory for anequilibrium phase transitions
1993
The paper introduces a classification of phase transitions in which each transition is characterized through its generalized order and a slowly varying function. This characterization is shown to be applicable in statistical mechanics as well as in thermodynamics albeit for different mathematical reasons. By introducing the block ensemble limit the statistical classification is based on the theory of stable laws from probability theory. The block ensemble limit combines scaling limit and thermodynamic limit. The thermodynamic classification on the other hand is based on generalizing Ehrenfest's traditional classification scheme. Both schemes imply the validity of scaling at phase transition…
High-pressure- and low-temperature-induced changes in [(CH3)2NH(CH2)2NH3][SbCl5].
2006
The structure of N,N-dimethylethylenediammonium pentachloroantimonate(III), [(CH3)2NH(CH2)2NH3][SbCl5], NNDP, was investigated at 100 and 15 K at ambient pressure, as well as at pressures up to 4.00 GPa at room temperature in the diamond-anvil cell. The stable structure at low temperatures and low pressures consists of isolated [SbCl5]2- anions and [(CH3)2NH(CH2)2NH3]2+ cations. The inorganic anions have a distorted square pyramidal geometry. They are arranged in linear chains parallel to the c axis. In contrast to the low-temperature studies, where no phase transition was detected, pressure induces a P2(1)/c --P2(1)/n phase transition between 0.55 and 1.00 GPa, accompanied by a doubling of…
Unprecedented multi-stable spin crossover molecular material with two thermal memory channels.
2013
et al.
Relaxor Properties of Barium-Strontium Niobate Ceramics
2011
Dielectric properties of Sr0.75Ba0.25Nb2O6 (SBN-75) ceramics in the range of low and infra-low frequencies are reported. The values of ϵ′ and, in particular of ϵ″, in ceramics are found to be much smaller compared with SBN-75 single crystals below the temperature (Тm) of the ϵ′(T) maximum a sharp increase of the values of dielectric parameters observed above Тm being also different from their thermal behaviour in single crystals. The broadening of phase transition and dependence of the response on sample history are similar to the features observed in other relaxor materials.
Raman Studies of Structural Phase Transitions in Perovskite Ferroelectric Sodium Niobate Solid Solutions
2008
Raman spectra are sensitive to interactions between structural units and, consequently, may reflect fine rearrangements of the units of cation sublattice occuring at compositional changes affecting the phase equlibrium and ferroelectric properties of the crystal. Obtaining Raman spectra show the information on disordering of structural units depending on composition of the cation sublattice in real Li x Na 1 − x Ta y Nb 1 − y O 3 ferroelectrics. On the basis of experimental studies shown, that not only the frequency but also intensity of some “rigid” bands may serve as the function of order parameter of phase transition in Landau's theory of the second order phase transition.
PHASE TRANSITIONS IN HALOGENOANTIMONATE(V) CRYSTALS: [N(CH3)4]SbCl6 AND [N(C2H5)4]SbCl6
1998
Abstract New crystals of the family of alkylammonium chloroantimonates(V), [N(CH 3 ) 4 ]SbCl 6 and [N(C 2 H 5 ) 4 ]SbCl 6 , have been grown and systematically studied. X-ray studies show that [N(C 2 H 5 ) 4 ]SbCl 6 is composed of isolated cations and ideal SbCl 6 − octahedra. Differential scanning calorimetry, dilatometric and dielectric measurements reveal structural phase transitions in [N(CH 3 ) 4 ]SbCl 6 and [N(C 2 H 5 ) 4 ]SbCl 6 of first order at 236 and 345 K, respectively. The measurements of 1 H-NMR spin-lattice relaxation times, T 1 , show that mechanism of the phase transitions in [N(CH 3 ) 4 ]SbCl 6 and [N(C 2 H 5 ) 4 ]SbCl 6 is due to the reorientations of the tetraalkylammoniu…
Molecular Motion, Defect Structures and Phase Transitions in Oligomer Crystals
1983
Defect structures in the high temperature modifications of oligomer crystals can generally be discribed as a superposition of chain rotations, longitudinal shifts of the molecules and the occurrence of conformational defects. Applying a suitable combination of experimental techniques it is possible to discriminate between the different components of disorder. This is demonstrated in a discussion of results obtained on n-alkanes, cycloalkanes and perfluoro-n-alkanes.