Showing papers in "Phase Transitions in 1989"

Ferroelectric properties of vinylidene fluoride copolymers

Abstract: Ferroelectric properties of copolymers of vinylidene fluoride with trifluoroethylene and tetrafluoroethylene are described with special interest in their polarization reversal and phase transition behavior. The ferroelectric phase consists of all-trans molecules packed in a parallel fashion while molecules adopt irregular TT, TG, T[Gbar] conformations in the paraelectric phase. In the ferroelectric phase, polarization reversal occurs at very high fields (> 100 MV/m) as a result of eventual 180° rotations of individual chain molecules around their axes. The switching time ranges from sec to nsec depending upon the strength of the applied field according to an exponential law with a particularly large activation field (∼ 1 GV/m). The value of the observed remnant polarization is consistent with prediction from a simple dipole sum implying a minor contribution from the Coulomb interaction. The ferroelectric-to-paraelectric transition appears most clearly for copolymers containing 50-80 mol% vinylide...

Structural phase transition in ferroelectric fluorine polymers: X-ray diffraction and infrared/Raman spectroscopic study

Abstract: The structural phase transition has been investigated by X-ray diffraction and infrared and Raman spectroscopic measurements for ferroelectric fluorine polymers, including poly(vinylidene fluoride) and its copolymers with trifluoroethylene or tetrafluoroethylene. One of the most characteristic features of this ferroelectric transition is the large conformational change of the molecular chains between the trans and gauche rotational isomers, quite different from the structural change observed generally in the usual ionic ferroelectric materials. The crystallization and transition behaviors depend sensitively on the monomer composition in the copolymers as well as on the sample preparation conditions. The roles of the optic and acoustic phonons in the ferroelectric phase transition have been discussed based on the temperature dependences of the far-infrared spectra and the ultrasonic velocity.

Magnetoelectric effect and low temperature transition of PbFe0.5Nb0.5O3 single crystal

Abstract: Magnetic and magnetoelectric measurements were made on a single crystal of ferroelectric PbFe0.5Nb0.5O3. It was revealed that PbFe0.5Nb0.5O3 has a weak ferromagnetic moment and linear magnetoelectric effect below a transition point T m c of approximately 9 K. The magnetic class below and above T m c was determined to be m' and ml', respectively. A possible model of the transition which explains this symmetry change is proposed.

About the origin of mica polytypes

Abstract: The crystal growth aspect of mica polytypism is reviewed with emphasis on the relationships between the growth mechanisms of the basal faces and the ordering of the unit modules of the mica structure. On synthetic micas, it is shown that the most common short-period polytypes (basic structures) and the disordered layer-stacking sequences as well originate during the early 3D-nucleation and 2D-layer growth mechanisms. During those stages, basic structures may be randomly faulted to various extents. Subsequently, spiral growth may take place and originate long-period (or complex) polytypes. Supported by such observations on synthetic as well as natural micas, the principles of the perfect-matrix and faulted-matrix models of the screw dislocation theory or polytypism are remembered and shown to be powerful in explaining most of the complex polytypes of the micas. On natural biotite, original data are given about the very nature of stacking disorder and also about weak but significant differences bet...

Global hysteresis in ferroelectrics with incommensurate phases

Abstract: A review of the basic experimental results connected with the formation of long-living metastable states in the incommensurate phases of ferroelectric crystals and related phenomena is given. The interpretation and appropriate physical mechanisms underlying the global hysteresis phenomena associated with these states are considered.

Introduction: Early studies in piezoelectricity, pyroelectricity, and ferroelectricity in polymers

Abstract: Studies on piezoelectricity and pyroelectricity in polymers were initiated in materials of biological origin. A variety of polysaccharides, proteins and DNA were found to exhibit piezolectricity. Synthetic polymers such as polypeptides and optically-active polymers were also found to be piezoelectric. The piezolectricity and pyroelectricity in bone and tendon aroused interests in orthopaedists and led to studies on the electrical stimulation of osteogenesis. The discovery of large piezoelectricity in poled polyvinylidene fluoride opened a new field of research towards ferroelectric polymers. The Curie temperature was confirmed in the copolymers of vinylidene fluoride and trifluoroethylene. The characteristic changes of molecular conformation and associated crystalline structure were revealed at the temperature range of the phase transition. Piezoelectric and ferroelectric-like properties were found in the copolymers of vinylidenecyanide and vinylacetate, which are amorphous and transparent.

High pressure phase transitions and variation of elastic constants of some II-VI semiconductors

Abstract: This paper reports an investigation of the pressure-induced phase transitions and dependence of elastic constants of ZnS, ZnSe and ZnTe on pressure using a three-body-potential (TBP) approach. The phase-transition pressures and associated volume collapses obtained from this approach show a reasonably good agreement with experimental data. The variations of elastic constants and their combinations with pressure follow a systematic trend, identical to that observed in other semiconductors of the zincblende structure family. It is found that this TBP model has a promise to predict the phase-transition pressure and the pressure variation of elastic constants of other semiconductors as well.

Phase transitions and electrical properties of Cs1−x M x HSO4 “glassy crystals” and solid solutions

Abstract: At high temperature many compounds containing isolated tetrahedra show transitions to a state with a strong orientational disorder. At the same time, a large translational disorder associated with superionic conductivity can occur. For MHSO4 acid sulphates the large size and the weak charge of ions facilitate the disorder. A partial substitution of Cs+ by Li+, K+ or Rb+ spreads out the ionic/superionic conductor transition bringing together the properties of the high and low-temperature phases. Melting-annealing cycles of mixed compound lead to a state exhibiting a DSC trace of a glassy material. Attempts were made to understand the nature of the disorder and the relationship to the electrical properties using calorimetry, X-Ray diffraction, vibrational spectroscopy and impedance spectroscopy.

Order parameters of some homologue cybotactic nematics from X-ray diffraction measurements

Abstract: The hexyl, octyl and decyl members of the homologous series bis-(4′-n-alkoxybenzal)-2-chloro-1,4-phenylinediamine have been studied by X-ray diffraction at different temperatures under various magnetic fields. The intermolecular distance the tilt angle, the orientational distribution function and the orientational order parameters have been calculated. The intermolecular distance, tilt angle and the ‘degree of ordering’ is found to depend on magnetic field strength. Order parameters 〈P 2〉 and 〈P 4〉 are found to be slightly lower for BHeCP, more or less equal for BOCP, and slightly higher for BDeCP than those predicted by mean field theory. 〈P 2〉 values for BHeCP snd BOCP agree well with the values obtained from refractive-index measurements, but for BDeCP our 〈P 2〉 values are higher.

Low-temperature phase transition in Ba2TiGe2O8

Abstract: Previous work on Ba2TiGe2O8 crystals has shown an unusual low-temperature (∼ 223 K on cooling, ∼ 273 K on heating) phase transition. Precession x-ray photographs on Ba2TiGe2O8 single crystals show an incommensurate modulation along b∗, and, for the first time, also along a∗. Single crystal intensity data confirm the average structure in space group Cmm2. There is positional disorder in the pyrogermanate groups, and this is the probable cause for the modulated structure. The low-temperature phase transition is proposed to be a lock-in transition, with the modulation along a∗ locking in at a value of 1/3. Several properties, as well as other unusual features of the low-temperature phase transition, are discussed in light of the proposed lock-in transition. Domain studies show that the ferroelastic domains are unstable in the low-temperature phase.

Magnetostriction and piezomagnetism of noncollinear antiferromagnet Mn3NiN

Abstract: Magnetoelastic effects in the three-sublattice noncollinear antiferromagnet Mn3NiN are studied theoretically. Shear deformations are shown to give rise in the crystal to a piezomagnetic moment of an exchange nature. The magnitude of the piezomagnetic moment is comparable with the weak ferromagnet moment detected experimentally in Mn3NiN. The absence of critical softening of sound waves near spinorientational phase transitions is demonstrated. It is proved that the experimentally observed temperature hysteresis at a first-order phase transition from the para- to antiferromagnet state is due to exchange magnetostriction.

Phase boundaries in NaNbO3 single crystals

Abstract: Orientation of phase boundaries has been studied for high-temperature phase transitions in NaNbO3 single crystals. Experimentally determined angles formed by the normal to the interface with the axes of the system associated with the initial cubic phase were ascertained to be in good agreement with calculated values based on the Roitburd theory. Confirmation was also found for the conclusion drawn from this theory, i.e. that if the specific strain of transition is not an invariant plane strain, then the twinning process accompanying this transition is its invariable characteristic.

Martensitic transformation: Phenomenology and the origin of the two-way memory effect

Abstract: Primary characteristics of martensitic transformation and their relation to the one-way memory effect and the influence of the stresses are first reviewed. Afterwards, the various training processes are described and analysed from a phenomenological point of view. The physical origin of the two-way memory effect (TWME) is then discussed: it is shown that the structural defects, such as oriented dislocations created during training, are probably responsible for the TWME. The possible mechanisms for the role played by these defects are also reviewed and discussed.

Pressure effect on phase transition in ferroelectic polymers

Abstract: Pressure effects of phase transition behaviour in two kinds of ferroelectric polymers of poly(vinylidene fluoride), PVDF, and copolymers of vinylidene fluoride and trifluoroethylene, (VDF/TrFE), are discussed. In the case of PVDF, several high-pressure treatments including a high-pressure annealing and a uniaxial compression were shown to induce a crystal transformation from a non-polar Form II crystal to a polar Form I crystal, which has ferroelectric characteristics and high piezoelectric activity. In addition, substantial pressure effects on ferroelectric phase transition points as well as crystal structures were observed for (VDF/TrFE) copolymers with different VDF contents. The most significant pressure effects were observed for copolymer samples with unstable ferroelectric structures at atmospheric pressure. From high-pressure X-ray and Raman scattering studies, these pressure effects were suggested to originate from the pressure-induced conformational transition from gauche to trans in the...

Defects, phase transitions and dynamical disorder in superionic protonic conductors H3OUO2PO4 · 3H2O and CsHSO4

Abstract: Specific properties of the hydrogen bond and protons appear to be responsible for the formation of the quasi-liquid state of conducting ions and the resulting superionic behaviour. This state is reached by successive phase transitions involving the mobile species and their interactions with a more or less rigid framework. H3OUO2PO4 · 3H2O (HUP) and CsHSO4 can be considered as models of wet and dry superionic conduction, respectively. Interactions between static effects and dynamical disorder as well as the coupling between sublattices are discussed in relation to results obtained by calorimetry, impedance spectroscopy (up to 10 GHz), vibrational spectroscopy and quasi-elastic neutron scattering.

Thermal and dielectric investigations of the curie transition in Poly(vinylidene fluoride—trifluoroethylene) copolymers

Abstract: The ferroelectric-to-paraelectric phase transition in poly(vinylidene fluoride-trifluoroethylene) copolymers has been investigated by differential scanning calorimetry and dielectric techniques. The transition appears to be very dependent on the crystallization kinetics and thermal history. The anomalies accompanying the transition show that the structural transformations are complex. In particular, the 30 mol % TrFE sample presents the most remarkable effects. The results are discussed in terms of the existence of a possible intermediate crystalline phase. The role of defects and of the anchored amorphous phase have also been considered.

Cu chemistry of YBa2Cu3O6+x investigated by nuclear quadrupole resonance

Abstract: The nuclear quadrupole resonance (NQR) technique has been utilized to characterize the local oxygen coordination of inequivalent Cu sites in YBa2Cu3O6+x(0 ≤ x ≤ 091) Essentially, four distinct NQR lines which correspond to 2, 3,4 oxygen coordinated Cu sites in the Cu-O chains and 5 oxygen coordinated Cu sites in the Cu-O planes have been observed The zero-field NQR frequencies of these are centered at about 301, 240, 220 and 315 MHz for 63Cu, respectively For the antiferromagnetic ordered state (x ≤ 03), antiferromagnetic nuclear resonance (AFNR) has been observed at 90 MHz with quadrupole splittings associated with the moment-bearing Cu sites in the Cu-O planes The relative intensities of these resonance lines depend on the oxygen content, and this gives us a microscopic understanding of the Cu chemistry of this system

Molecular‐dynamics simulation of YBa2Cu3O7 at high temperatures

Abstract: Molecular-dynamics simulation of YBa2Cu3O7 is carried out to study the oxygen-atom distribution at high temperatures as the system undergoes the orthorhombic-to-tetragonal phase transition. While at the transition the one-dimensional arrangement of oxygen atoms in the Cul(0,0,0)-O4(0, 1/2, 0) chains begins to disorder, at temperatures above the transition the simulation also indicates the presence of a few vacancies at the O1(0, 0, z) sites. At still higher temperatures vacancies occur at all the oxygen sites. The simulation utilises an interatomic potential based on an unscreened rigid-ion model which was earlier found to be useful in understanding the structure, phonon density of states, and related properties. The thermal expansion, as obtained from the zero-pressure simulation, is in reasonable agreement with the observations of Jorgensen et al. on the oxygen-deficient system. The melting temperature obtained from the simulation is also in fair agreement with experiment.

Martensitic transformation and soft modes in KAlF4

Abstract: The structural phase transition observed at −13°C in KAlF4 is shown to be martensitic. The results of an investigation of the phonon spectrum by inelastic neutron scattering are reported. It is shown that the transition is preceded by the softening of a flat phonon branch. A model is proposed to explain how such a softening is related to the martensitic transition.

Polar regions in the paraelectric phase in a PbZr0.992Ti0.008O3 single crystal

Abstract: Anomalous behaviour of e and tan δ in the paraelectric phase was found for PbZr0.992Ti0.008O3 single crystal. In the vicinity of temperatures 280–290°C (T c = 236°C) the pyroelectric effect was also observed. Using the data obtained, the existence of an additional polarization (polar regions oriented in an external electric field) in the paraelectric phase is discussed.

A priori theory of incommensurate behavior in Rb2ZnCl4

Abstract: We have performed a study of the potential energy, normal modes, static relaxation, and molecular dynamics of Rb2ZnCl4 using ab initio Gordon-Kim interionic potentials for intermolecular and interionic potentials. We find that the Pnam–Pna21 transition in this, and almost certainly many isomorphous systems, is entropy driven and shows no soft mode behavior. This is shown to arise from the presence in this system of a large equipotential volume of phase space between the Pnam and the tripled Pna21 low temperature structure. This region arises from a major lattice instability involving large rotations and displacements and having approximately six-fold screw symmetry along the a axis. This concealed imperfect symmetry which we call latent symmetry is the basic cause of the existence of a near-tripled incommensurate phase over a range of 113 K (302 K–189 K) between the Pnam and Pna21 phases. In this region the helicity is incommensurate with the lattice, being largely determined by the relief of int...

Crystal growth and anisotropic properties of high-T c oxide superconductors

Abstract: (La1-xSrx)2CuO4(0 ≤ x ≤ 0.2), Ba2YCu3O7 and Bi-Sr-Ca-Cu-O single crystals have been grown from a non-stoichiometric solution with excess CuO. The upper critical magnetic field H c2 is estimated from the resistive transition curves as a function of temperature and field direction. Large anisotropy in H c2 between the c-axis and the a-b plane is a common feature for these Cu-based high-T c oxide superconductors. The anisotropy factor is 5.6-14.8 for (La1-xSrx)2CuO4(x = 0.07), 6.1 for Ba2YCu3O7, and 35-53 for Bi-Sr-Ca-Cu-O. The larger the crystallographic anisotropy is, the larger is the anisotropy in upper critical magnetic field. These results suggest that a two-dimensional electronic structure is a characteristic of high-T c oxide superconductors.

A point‐defect model of structural phase transitions in the ferroelastic superconductor Y-Ba-Cu-O

Abstract: A point-defect model of structural phase transitions in YBa2Cu3O6 + x is proposed. The model treats the basal-plane oxygen atoms as point defects. The treatment parallels the work of Alefeld and coworkers on ferroelasticity due to hydrogen in metals. The model predicts that the tetragonal-to-orthorhombic phase transition should be accompanied by precipitation into regions of high and low oxygen density, as well as high and low oxygen ordering. This is in agreement with experiment, as well as with the predictions of Khachaturyan's concentration-wave model. A new feature of the present model is the prediction of two characteristic temperatures related, respectively, to Snoek-type and Gorsky-type diffusive relaxations. Practical implications of the high diffusivity of the basal-plane oxygens include the easy fabricability of a bicrystal of Y-Ba-Cu-O such that the two components of the bicrystal are rotated by 90° with respect to each other, and are separated by a non-superconducting layer. Such a bi...

Quasiperiodicity in irrational interfaces

Abstract: Recent work on quasiperiodicity in irrational grain boundaries is reviewed and generalized to heterophase interfaces. The concept of local isomorphism is shown to be very important at irrational interfaces, replacing translational invariance at periodic interfaces. Symmetry related variants of irrational interfaces are enumerated and line defects separating energetically degenerate domains of irrational interfaces are characterised by products of space group operations, one from each crystal. The symmetry operations relating the variants belong to a six dimensional crystal and the variants are locally isomorphic. A framework is described for discussing the symmetries and long range order at interfaces regardless of whether they are rational or irrational.

Coating of superconducting.123 compound on silver substrate by electrophoretic technique

Abstract: Superconducting 123 powders have been deposited on silver metal wire and foil by an electrophoretic deposition technique, followed by appropriate heat treatment, to obtain superconducting wires and tapes. At the present stage the critical current density is rather low (85 A/cm2), however there remains a considerable scope for improvement.

The study of modulated structures, mixed layer polytypes and 1-D quasi-crystals by means of electron microscopy and electron diffraction

Abstract: The characteristic features of electron diffraction patterns due to modulated structures of various types are discussed and observations illustrating the various types of patterns are presented. The use of imaging techniques is illustrated as well.

X-Ray kinetic study of glassy crystal formation in adamantane derivatives: TTT curves and crystal size effect

Abstract: The kinetics of polymorphic solid-state transformation in mixed adamantane compounds (CN1−x Cl x ADM: x = 0 and x = 0.25) have been studied by X-ray scattering. The classical form of the time-temperature-transformation TTT curves has been directly observed for the first time for the ordering supercooled plastic phases. For both compounds a considerable effect of crystal size on the kinetics has been observed. For x = 0.25 it leads to a continuous transition from Avrami to nucleation behaviour. These observations help us to understand the factors controlling nucleation and growth as well as to establish better operating conditions in order to form a glassy crystal.

Modulated phases in the Falicov-Kimball model

Abstract: The arrangement of localized f electrons in the ground state of the Falicov-Kimball model is studied as a function of the energy of the f-level. For the half-filled case in one dimension, perturbation theory in the d-electron bandwidth together with numerical results reveals a sequence of transitions between long-period modulated arrangements of the f electrons. Results suggest the existence of a multiphase point at zero d-band-width and f energy, and a complete devil's staircase for the variation of f occupation as the energy of the f-level is changed when the d-bandwidth is nonzero.

The nonlinear dielectric effect in critical solutions with different types of background effects

Abstract: The nonlinear dielectric effect (NDE) has been investigated in critical solutions of nitrobenzene-heptane and veratrole-heptane. In each of these solutions the background effect is associated with a different molecular mechanism. The background effect was determined from NDE behavior in 3-component solutions. It would appear that this procedure made it possible to achieve a relatively accurate separation of the critical effect and the noncritical background effect. Values of critical effect parameters were in agreement with experimental results obtained earlier, and allowed certain discrepancies between theory and experiment to be confirmed.

The influence of L-α-alanine admixture, rejuvenation process, and electric field on the electrical conductivity of TGS crystals

Abstract: The influence of L-α-alanine admixture on the electrical conductivity of TGS crystals has been studied. With increasing content of admixture, the transition temperature (Tc ) is shifted towards higher values while the electrical conductivity (σ) decreases both in the para- and ferroelectric phases. This work also presents the results of rejuvenation processes and electric field effects on the electrical conductivity of admixtured crystals (LATGS). Both Tc and σ were found to increase with increasing rejuvenation time and electric field strength, in quite good agreement with dielectric measurements on LATGS crystals.