Search results for "Phase Transition"
showing 10 items of 1281 documents
Structural characterization, thermal, dielectric, vibrational properties and molecular dynamics of (C5H5NH)3BiCl6
2009
Abstract (C5H5NH)3BiCl6 crystallizes at room temperature in the triclinic space group P 1 ¯ . The crystal is built up of the separated BiCl 6 3 - octahedral anions and pyridinium cations. Differential scanning calorimetry (DSC) and dilatometric measurements disclose structural phase transition of first-order type at 265/302 K (cooling/heating). The dielectric investigations reveal a step-wise change of the electric permittivity at Tc characteristic of the crystals with the high-temperature plastic-like phase. Temperature-dependent infrared spectra for polycrystalline samples have been taken between 20 and 306 K to clarify the cation dynamics contribution to the mechanism of the phase transi…
Structure, ferroelasticity and Goldilocks zone phase transitions in C3H5N2Al(SO4)2·6H2O
2021
The single crystal growth and sequence of reversible phase transition are described for C3H5N2Al(SO4)2·6H2O. Thermal and structural analyses combined with dielectric studies and optical observations revealed the structural phase transition at T 1 = 339/340 K (I↔II) and T 2 = 347/348 K (II↔III) on heating and cooling, respectively. Both phase transitions are of the first-order type. The symmetry changes from monoclinic to trigonal phase. At 293 K, the large crystals are usually divided into numerous domains of the ferroelastic type that disappear above T 1 on heating and reappear below T 1 on cooling. The domain structure pattern is characteristic for the transition between trigonal and mono…
Uniaxial Pressure Effect on Electric Properties of PSN and 0.95PSN-0.05PLuN Ceramics
2006
The dielectric, pyroelectric and hysteresis loops measurements of PSN and 0.95PSN-0.05PLuN ceramics stress-free and under the stress (0–1.5 kbar) have been carried out. It was shown that axial pressure seems to change the partially ordered state to a disordered one, shifts the phase transformation and decreases the thermal hysteresis of the dielectric permittivity and the field-polarization hysteresis. These effects can be connected with change in domain structure, inter-ionic distances and defect density under the action of uniaxial pressure. The thermodynamic parameters of the phase transition were also determined.
Structure and dielectric properties of Na<inf>1/2</inf>Bi<inf>1/2</inf>TiO<inf>3</inf>-BaTiO<inf>3</inf&…
2012
Dielectric properties, polarization and x-ray diffraction are studied for (1−x)Na 1/2 Bi 1/2 TiO 3 -xBaTiO 3 solid solutions in a concentration range above the morphotropic phase boundary. It is showed that all the studied compositions have tetragonal structure with maximal tetragonality slightly above the morphotropic phase boundary. Ferroelectric relaxor behavior is observed in a wide concentration range of BaTiO 3 . Stability of the relaxor state decreases with increasing BaTiO 3 concentration, but only for compositions with low Na 1/2 Bi 1/2 TiO 3 content the normal ferroelectric-paraelectric phase transition, which is characteristic to pure BaTiO 3 , occurs. Mechanisms, which influence…
A new insight into the isotropic–nematic phase transition in lyotropic solutions of semiflexible polymers: density-functional theory tested by molecu…
2016
Semiflexible polymers in solution are studied for a wide range of both contour length L and persistence length lp as a function of monomer concentration under good solvent conditions. Both density-functional theory (DFT) and molecular dynamics (MD) simulation methods are used, and a very good agreement between both techniques is observed for rather stiff polymers. Evidence for a new mechanism of order parameter fluctuations in the nematic phase is presented, namely collective deformations of bundles of wormlike chains twisted around each other, and the typical wavelengths and amplitudes of these modes are estimated. These long wavelength fluctuations cause a reduction of the order parameter…
Bending or buckling: Compression-induced phase transition in a semi-flexible polymer brush
2013
Molecular-dynamics simulations are presented for systems of densely grafted semiflexible macromolecules grafted to a planar non-adsorbing substrate, studying the case where the persistence length of the polymers is of the same order as their contour length so that the polymer brush may exhibit nematic order. We focus our attention on the case where the first bond must orient perpendicularly to the substrate (so the structure resembles a "Fakir's bed" for short chains and a "polymer bristle" for longer chains). When such layers are exposed to uniform compression, the pressure vs. distance relationship exhibits two stages: i) for very small compression the chains exhibit "buckling" yet mainta…
Monte Carlo Simulations of Semi-Flexible Polymers
2005
We present Monte Carlo simulations on the phase behavior of semiflexible macromolecules. For a single chain this question is of biophysical interest given the fact that long and stiff DNA chains are typically folded up into very tight compartments. So one can ask the question how the state diagram of a semiflexible chain differs from the coilglobule behavior of a flexible macromolecule. Another effect connected with rigidity of the chains is their tendency to aggregate and form nematically ordered structures. As a consequence one has two competing phase transitions: a gas-liquid and an isotropic-nematic transition potentially giving rise to a complicated phase diagram.
Determination of trace amounts of β tegafur in commercial α tegafur by powder X-ray diffractometric analysis.
2011
Abstract Objectives The main objective of this work was to develop a suitable analytical technique for determining trace amounts of the thermodynamically stable solid form in bulk samples of metastable form, to a sensitivity of 0.005%–1.0%. Tegafur (5-fluoro-1-(tetrahydro-2-furyl)-uracil) α and β crystalline forms were used as a model for this problem. Methods The trace content of the thermodynamically stable β polymorphic form in tegafur samples was increased by promoting phase transition from the bulk of thermodynamically metastable α form to β form, and achieving sufficient β form content for a quantitative powder X-ray diffractometry (PXRD) analysis. The phase transition was stimulated …
High-pressure, high-temperature phase diagram of InSe: A comprehensive study of the electronic and structural properties of the monoclinic phase of I…
2006
We report on an investigation of the high-pressure, high-temperature phase diagram of InSe. We optically observed the phase transition from the rhombohedral polytype (InSe-I) to the monoclinic phase (InSe-II) and determined the phase boundary up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. High-pressure resistivity measurements were performed to complement the optical measurements. Monoclinic and cubic InSe (InSe-III) were observed to be metastable around $14.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $420\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and evidence suggesting the existence of an as yet unidentified new high-pressure and high-temperature phase was found. By means of optical ab…
Structure and Properties of Novel Pb(B’1/2Nb1/2)O3-PbTiO3 Binary Systems with High Piezoelectric Coupling
2000
The (l-x)Pb(Lu1/2Nb1/2)O3-xPbTiO3 (PLuNT) and (l-x)Pb(Er1/2Nb1/2)O3-xPbTiO3 (PErNT) binary systems have been originally synthesized. Pure lutecium niobate (PLuN) (x = 0) has a pronounced long-range order in the B-sublattice and an antiferroelectric to paraelectric phase transition at ~258°C. The phase structure of the PLuNT system, at room temperature, changes from a pseudomonoclinic (psd-M, space group Bmm2) to tetragonal (T, space group P4mm). The pseudomonoclinic phase extends over the 0 ≤ x ≤ 0.38 interval within which the monoclinic angle β proceeds a minimum near to 90° at x≅0.2. The morphotropic region covers over the interval x = 0.38–0.49, the concentration ratio psd-M:T≅l (the mor…