Phase Transitions In Ferroelectric Crystals Research Articles

Quantum lattice model with local multi-well potentials: Riemannian geometric interpretation for the phase transitions in ferroelectric crystals

Geometrical aspects of quantum lattice model with the local anharmonic potentials are presented for the case of deformed ferroelectric lattice. A metric is defined in a two-dimensional phase space of the dipole ordering or polarization ( η) vs. volume deformation (u). Based on the metric components, an expression for the thermodynamic Ricci curvature scalar (R) is derived in terms of the known equilibrium values of η and u introduced by Velychko and Stasyuk (2019). As an example, the calculated curvature in the ferroelectric phase of Sn2P2S6 crystal demonstrates negative value while positive curvature in the paraelectric phase is obtained. The presence of anomalies of R in the ferroelectric phase transition regime of the first- and second-order as well as the tricritical point is observed.

Phase-field method and Materials Genome Initiative (MGI)

Predicting and controlling the microstructure evolution within a material are considered as the “holy grail” of materials science and engineering. Many important engineering materials are designed by controlling their phase transformations and microstructure evolution. Examples include the improvement of mechanical properties through solid-state precipitation reactions in Ni-based superalloys and age-hardened Al alloys, the useful dielectric properties and electro-mechanical coupling effects by manipulating the phase transitions in ferroelectric crystals, and the memory effect of shape-memory alloys by utilizing martensitic transformations. Phase-field method has become the method of choice for modeling three-dimensional microstructure evolution for a wide variety of materials processes. This short article briefly discusses the potential roles that phase-field method can play in the Materials Genome Initiative.

Polarization rotation and phase transition in [100]-oriented PZN-PT single crystals determined by the electro-crystalline anisotropy

We have investigated the polarization rotation and phase transition in [100]-oriented PZN-4.5%PT and PZN-8%PT single crystals on the basis of the single domain polarization rotation model. The dependence of both the polarization and the electrostrictive responses in ferroelectric crystals on the field at various temperatures has been numerically obtained. The numerical results have shown that both the polarization rotation and the phase transition are in nature determined by the electro-crystalline anisotropy constants. These constants are dependent on the composition of PZN-PT solution and ferroelectric properties of its components. Without considering the piezoelectric effect, the numerical results are in agreement with the experimental data. This paper is useful for better understanding of the polarization switching and the phase transition in ferroelectric crystals.

Structural phase transitions in ferroelectric crystals and thin films studied by VUV spectroscopic ellipsometry with synchrotron radiation

Results of thermo-optical investigations for lead germanate (Pb5Ge3O11), potassium dihydrogen phosphate (KH2PO4) ferroelectric crystals, and lead titanate (PbTiO3) thin films expitaxially deposited on (001) SrTiO3 substrate are presented in this article. The measurements were performed using VUV spectroscopic ellipsometry with synchrotron radiation in the temperature range comprising structural phase transitions (PTs) in the materials under study. Distinct anomalies in the temperature dependencies of complex dielectric permittivity and the intensity of reflected synchrotron light are found in the vicinity of ferroelectric-to-paraelectric PT point and discussed. Different temperature points of the characteristic anomalies at the PT of the real and imaginary components of the pseudo-dielectric function are observed for Pb5Ge3O11 and KH2PO4. It is crucial that anomalies in the measured temperature dependences for PbTiO3 thin films on SrTiO3 substrates are observed at the temperature near 105 K, corresponding to the PT in the SrTiO3 single crystal.

Cellular Complexes for Thermodynamic Potential of Ferroelectrics

The topological method of investigation of regularities at phase transitions in ferroelectric crystals and the nonlinear phenomena, based on representations about cellular complexes of system of special points of non-equilibrium thermodynamic potential have been stated. Full set of possible cellular complexes for crystals of cubic structure, for which thermodynamic potential can be presented as polynomial of the sixth degree, is constructed.

Topological Method of Research of Nonlinear Properties of Ferroelectrics

The obvious topological method of investigation of phase transitions in ferroelectric crystals and nonlinear phenomena is proposed. This method is based on representations about cellular complexes of system of singular points of thermodynamic potential. A full set of cellular complexes for a barium titanate is constructed. Regularities for the phase transitions induced by an external electric field are found. Opportunities of use of the method for research of structure and phase transitions in ferroelectric domain have been shown.

Quantum Effects of Phase Transitions in Ferroelectric Crystals—Experiments and Models

From structural data obtained experimentally, some microscopic parameters can be estimated unambiguously. By using these parameters, a quantum Ising model can explain the isotope effect in SrTiO 3 and KH 2 PO 4 . The ferroelectric transition in 18 O isotope crystal of SrTiO 3 is successfully explained only by a mass effect in quantum state, which demonstrates importance of quantum effects in ferroelectrics.

Topological method of analysis in the thermodynamics of ferroelectrics

The topological method for studying phase transitions in ferroelectric crystals is described, which makes it possible to clearly represent the regularities of change in different macroscopic characteristics of a crystal in the presence of a cascade of phase transitions.

Optical second harmonic generation study of the ferroelectric phase transition in crystals and nanocrystals of lithium heptagermanate

The D2h-C2v ferroelectric phase transition in crystals of lithium heptagermanate Li2Ge7O15 was revealed by the optical second harmonic generation technique. The phase transition temperature Tc was observed to shift from 10 to 6°C when the samples were doped with chromium (0.05%). No second harmonic signal was detected in the ferroelectric phase of nanocrystalline Li2Ge7O15 samples.

Acoustic Properties of Glycine Phosphite Crystals with an Admixture of Glycine Phosphate

Acoustic and dielectric anomalies in the region of the ferroelectric phase transition in crystals of glycine phosphite (GPI) with a 2 mol % admixture of glycine phosphate (GP) are studied. The acoustic anomalies were found to differ strongly from those observed in nominally pure glycine phosphite crystals. A theoretical analysis of the acoustic and dielectric properties of the crystals was carried out within the model of a pseudoproper ferroelectric phase transition. It is shown that the acoustic anomalies, as well as the temperature dependences of the dielectric constant (for various external electric fields) and pyroelectric current observed in the vicinity of the phase transition in GPI-GP crystals, can be adequately described when the macroscopic polarization present in these crystals above the phase transition temperature is taken into account. The thermodynamic-potential parameters describing electrostriction and the biquadratic relation between the polarization and strain turned out to be close to those characterizing a nominally pure GPI crystal. An irreversible phase transition was observed to occur in GPI-GP crystals at T = 240 K, i.e., above the ferroelectric phase transition temperature.

The Phase Transitions in Ferroelectric Crystals of A 2 MX 4 and AMX 3 Type

Investigations of characteristics the ferroelectric crystals of A 2 MX 4 and AMX 3 type with incommensurate phases are very interesting both from a scientific and practical point of view. Some aspects of behaviour phase transitions of these crystals are discussed. Special attention is paid to the temperature dependence of series of different type phase transitions. Analysis of the literature data and results of our research let us to consider potential possibilities of practical applications of these materials for optoelectronic devices. Observed temperature hysteresis of birefringence and absorption coefficient in incommensurate phases are especially important for application as optical memories.

The Phase Transitions in Ferroelectric Crystals of A 2 MX 4 and AMX 3 Type

Investigations of characteristics the ferroelectric crystals of A 2 MX 4 and AMX 3 type with incommensurate phases are very interesting both from a scientific and practical point of view. Some aspects of behaviour phase transitions of these crystals are discussed. Special attention is paid to the temperature dependence of series of different type phase transitions. Analysis of the literature data and results of our research let us to consider potential possibilities of practical applications of these materials for optoelectronic devices. Observed temperature hysteresis of birefringence and absorption coefficient in incommensurate phases are especially important for application as optical memories.

Order-disorder model of phase transitions in the dmagas-dmaals family crystals

A four-state model is proposed for the description of the sequence of phase transitions in ferroelectric crystals of DMAGaS and DMAAlS. The ordering processes in the subsystems of the DMA groups are considered to be the main reason for such transformations. The interaction between the groups in their various orientational states is taken into account in the dipole-dipole approximation. The obtained in the mean field approximation thermodynamic characteristics of the model (spontaneous polarization, occupancy of orientational states, dielectric susceptibility, phase diagrams) are in good agreement with experimental data. The experimental fact of ferroelectric phase suppression under hydrostatic pressure is explained under the assumption that pressure changes not only the interaction constants but also the difference between the energies of the various orientational states of DMA groups.

Defects and phase transformation in TGSe and DTGSe crystals

The specific heats of TGSe and DTGSe crystals, pure and γ-irradiated, were measured. The influence of radiation defects on the phase transition in ferroelectric crystals was studied by calorimetric experiments. The Landau free energy expansion coefficients were measured for various doses of γ-irradiation (0 < D < 5 MR). The order of the phase transition changed from the first to the second through the tricritical point in DTGSe crystal.

Dielectric properties of single crystals of lead barium scandoniobate solid solutions

It has been established that the concentration dependence of the temperature T m of maximum permittivity e corresponding to the ferroelectric phase transition in crystals of Pb1− xBaxSc0.5Nb0.5O3 solid solutions does not saturate in the range 0<x<0.6, unlike in the ceramic. However, the broadening of the maximum of e is similar in both crystals and ceramics. The frequency dependence of T m is described by the Vogel-Fulcher law, which can relate the strong broadening of the phase transition in solid solutions to the appearance of properties typical of dipole glasses.

Order-disorder versus soft mode behaviour of the ferroelectric phase transition in

This paper explains the coexistence of the displacive and the order-disorder features of the ferroelectric phase transition in crystals. Both have been observed in experiments. The height of the potential barrier hindering the order parameter fluctuations, estimated from experimental data, shows that the phase transition in is actually very close to the theoretical case of the order-disorder versus displacive crossover. Moreover, previously performed model calculations can be used for the analysis of the temperature dependence of dielectric susceptibility and other physical properties which do not obey the predictions of standard Landau theory.

Temperature-composition-degree of compositional ordering phase diagram of lead scandoniobate-scandotantalate solid solutions

It was established that the temperature of the ferroelectric phase transition in crystals of compositionally ordered solid solutions of lead scandoniobate—scandotantalate with high and low degrees of ordering S depends linearly on the composition x. These data were used to calculate the T-x-S phase diagram of these solid solutions for the first time.

Nanosecond dynamics of phase transitions in chromophoredoped ferroelectric crystals

The microscopic dynamics of phase transitions in ferroelectric crystals are studied using time-resolved fluorescence spectroscopy. Fluorescence decay times of the molecular probe tryptophan, embedded in the ferroelectric crystals potassium dihydrogen phosphate (KDP) and triglycine sulfate (TGS), are measured over a large temperature range. The dependence of the lifetime data on temperature were different between the two crystals. The magnitude of the fluorescence lifetime shows a rapid change around the phase transition temperature in KDP, while the changes in the lifetime data of the TGS crystal have been smooth. The data indicate that the order of the phase transition in solids (first or second) can be recovered using this technique. Since fluorescence lifetime is related to the dynamic interactions between the chromophore and the environment, insight into the emergence of macroscopic phase transition behavior from microscopic fluctuations near the phase transition region may be realized.

Experimental evidence for a continuous phase transition in a multidimensional ferroelectric

Two phase transitions in ferroelectric crystals of pyridinium tetrafluoroborate ${[\mathrm{C}}_{5}{\mathrm{H}}_{6}{\mathrm{N}]}^{+}{\mathrm{BF}}_{4}^{\mathrm{\ensuremath{-}}}$ have been characterized by x-ray- and neutron-diffraction studies and measurements of permittivity, pyroeffect, and spontaneous polarization. At ${T}_{1}=238.7\mathrm{K}$ the crystals transform from $R3\ifmmode\bar\else\textasciimacron\fi{}m$ to $C2$ symmetry, and at ${T}_{2}=204\mathrm{K}$ to space group $P2.$ The ions are orientationally disordered in the prototype phase, and the ferroelectricity below ${T}_{1}$ is induced by the onset of ordering of the ions. The spontaneous polarization of $1.5\ensuremath{\mu}{\mathrm{C}/\mathrm{c}\mathrm{m}}^{2}$ was found from pyroelectric-effect measurements. The anisotropy measurements of permittivity indicate clearly that the phase transition at ${T}_{1}$ is continuous, which is the unique case corroborating Landau's approach of deducing the character of phase transitions from symmetry considerations for multidimensional ferroelectrics.

A bibliographical review of the high pressure studies of ferroelectric phase transitions

A brief review of the various high pressure studies on the phase transitions in ferroelectric crystals carried out over the past two decades is presented. The theoretical framework in which high pressure studies have to be viewed is given. The pressure data of ferroelectric crystals are organized in the form of tables. High pressure spectroscopic and structural studies are included. The review contains a bibliography of over two hundred references.