Paraelectric-ferroelectric interface dynamics induced by latent heat transfer and irreversibility of ferroelectric phase transitions

The temperature gradients that arise in the paraelectric-ferroelectric interface dynamics induced by the latent heat transfer are studied from the point of view that a ferroelectric phase transition is a stationary, thermal-electric coupled transport process. The local entropy production is derived for a ferroelectric phase transition system from the Gibbs equation. Three types of regions in the system are described well by using the Onsager relations and the principle of minimum entropy production. The theoretical results coincides with the experimental ones.

Combined effect of the transition layer and interfacial coupling on the properties of ferroelectric bilayer film

Within the framework of modified Ginzburg Landau-Devonshire phenomenological theory, a ferroelectric bilayer film with a transition layer within each constituent film and an interfacial coupling between two materials has been studied. Properties including the Curie temperature and the spontaneous polarization of a bilayer film composed of two equally thick ferroelectric constituent films are discussed. The results show that the combined effect of the transition layer and the interracial coupling plays an important role in explaining the interesting behaviour of ferroelectric multilayer structures consisting of two ferroelectrie materials.

Dynamic phase transition of ferroelectric nanotube described by a spin-1/2 transverse Ising model

The dynamic phase transition properties for ferroelectric nanotube under a spin-1/2 transverse Ising model are studied under the effective field theory(EFT)with correlations.The temperature effects on the pseudo-spin systems are unveiled in three-dimensional(3-D)and two-dimensional(2-D)phase diagrams.Moreover,the dynamic behaviors of exchange interactions on the 3-D and 2-D phase transitions under high temperature are exhibited.The results present that it is hard to obtain pure ferroelectric phase under high temperature;that is,the vibration of orderly pseudo-spins cannot be eliminated completely.

Shape-induced phase transition of vortex domain structures in ferroelectric nanodots and their controllability by electrical and mechanical loads

Shape-induced phase transition of vortex domain structures （VDSs） in BaTiO3 （BT） nanodots under open circuit boundary condition have been investigated using an effective Hamiltonian method. Our calculation indicates the tetragonal VDS missing in cubic BT nanodots can be induced by varying the shape of a nanodot from cube to platelet. Interestingly, a novel VDS is found in BT nanoplatelets in our simulations. Further investigation shows that it is a result of compromise between the ground state and the symmetry of the shape of the nanodot. Furthermore, based on the novel VDS, routes of controlling VDSs governed by homogeneous electric field and uniform stress are discussed. In particular, our results show the possibility of designing multi-states devices based on a single VDS. ~ 2017 The Authors. Published by Elsevier Ltd on behalf of The Chinese Society of Theoretical and Applied Mechanics.

Critical exponents of ferroelectric transitions in modulated SrTiO_3:Consequences of quantum fluctuations and quenched disorder

The ferroelectric transitions of several SrTiO3-based ferroelectrics are investigated experimentally and theoretically, with special attention to the critical scaling exponents associated with the phase transitions, in order to understand the competition among quantum fluctuations （QFs）, quenched disorder, and ferroelectric ordering. Two representative systems with sufficiently strong QFs and quenched disorders in competition with the ferroelectric ordering are investigated. We start from non-stoichiometric SrTiO3（STO） with the Sr/Ti ratio deviating slightly from one, which is believed to maintain strong QFs. Then, we address Ba/Ca co-doped Sr1-x（Ca0.6389Ba0.3611）xTiO3（SCBT） with the averaged Sr-site ionic radius identical to the Sr2＋ ionic radius, which is believed to offer remarkable quenched disorder associated with the Sr-site ionic mismatch. The critical exponents associated with polarization P and dielectric susceptibility ε, respectively, as functions of temperature T close to the critical point Tc, are evaluated. It is revealed that both non-stoichiometric SrTiO3 and SCBT exhibit much bigger critical exponents than the Landau mean-field theory predictions. These critical exponents then decrease gradually with increasing doping level or deviation of Sr/Ti ratio from one. A transverse Ising model applicable to the Sr-site doped STO （e.g., Sr1-xCaxTiO3） at low level is used to explain the observed experimental data. It is suggested that the serious deviation of these critical exponents from the Landau theory predictions in these STO-based systems is ascribed to the significant QFs and quenched disorder by partially suppressing the long-range spatial correlation of electric dipoles around the transitions. The present work thus sheds light on our understanding of the critical behaviors of ferroelectric transitions in STO in the presence of quantum fluctuations and quenched disorder, whose effects have been demonstrated to be remarkable.

The hysteresis loops of a ferroelectric bilayer film with surface transition layers

Based on the transverse Ising model in the framework of the mean field approximation, this paper discusses a ferroelectric bilayer film with the surface transition layers within each constituent slab and an antiferroelectric interracial coupling between two slabs. The hysteresis loop of a bilayer film is investigated. The results show that the surface transition layer in a ferroelectric bilayer film plays a significant role in realizing the multiple-state memory.

Study of Dielectric and Mechanical Spectra on Point Defects and Phase Transition in Ferroelectric Ceramics

Ferroelectric materials have enormous potential applications in advanced techniques. However, there are still many problems in its practical application. Dielectric and mechanical (internal friction) measurements are very sensitive to phase transitions, relaxation process of point defects, domain walls and their mobility, which have severe effect on ferroelectric properties. These make them become very good means to investigate substantial information on structural features and to explore the fundamental principles in ferroelectric materials and their applications. In this paper, the dielectric and internal friction measurement were used to investigate the behaviors for point defects and phase transition in ferroelectric ceramics such as Bi_ 4-x La_ x Ti_ 3 O_ 12 , Bi_ 4 Ti_ 3-y Nb_ y O_ 12 , SrBi_ 2 Ti_ 2 O_ 9 , PbZr_ x Ti_ 1-x O_ 3 ,_ PMN-PT. They were used to clarify the mechanism for some ferroelectric behaviors.

Low Temperature Phase Transition of an Orthorhombic Tungsten Bronze Ferroelectric Sr_(2-x)Ca_(x)NaNb_(5)O_(15)

The dielectric and pyroelectric properties of a new ferroelectric crystal with orthorhombic tungsten bronze structure Sr_(2-x)Ca_(x)NaNb_(5)O_(15) have been investigated in the temperature range of 300 to 10 K.Based on anomalies in the properties and by use of Curie's principle of symmetry,a ferroelectric-ferroelectric phase transition between mm2 and m is found to occur at about 75K.On cooling through the transition,the spontaneous polarization deviates by about two degrees from the two-fold axis[001]to[h0l]direction in the mirror plane.

THE APPLICATION OF FE1-FE2 PHASE TRANSITION IN Nb DOPED(95/5)FERROELECTRIC CERAMICS TO HEAT-ELECTRIC ENERGY CONVERSION

Formerly,direct conversion of heat to electricity was carried out by making use of ferroelectric to paraelectric(FE-P)transition.In this paper a new method utilizing pyroelectric effect between two ferroelectric phases(FEI-FE2)is suggested and the Nb doped PZT(95/5)ceramic materials are used as actuating media.The intrinsic efficiency as well as the output electric power in energy conversion is estimated in terms of the charge-voltage behaviour during phase transition.Two methods of the energy conversion concerned are compared.

Study of Phase Transition Properties in Epitaxial Ferroelectric Film

Based on the transverse Ising model and using decoupling approximation to the Fermi-type Green＇s function, we study the phase transition properties of the epitaxial ferroeleetric film with one substrate. A general recursive equation of the ferroelectric thin film with two n-layer materials is obtained, which enables us to study the phase transition properties for any number layers for epitaxial ferroelectric thin film. With the help of this equation, we analyze the effect of the exchange interaction and the transverse field in the phase diagram, the influence to the polarizations and Curie temperature numerically. The results show that epitaxial ferroelectric film are able to induce a strong increase or decrease of Curie temperature to different exchange interactions and transverse fields within the epitaxial film layers. The theoretical results are in reasonable accordance with experimental data of different ferroelectric thin film.

Strain-induced insulator–metal transition in ferroelectric BaTiO_3(001) surface:First-principles study

The electronic properties of TiO2-terminated BaTiO3（001） surface subjected to biaxial strain have been studied using first-principles calculations based on density functional theory. The Ti ions are always inward shifted either at compressive or tension strains, while the inward shift of the Ba ions occurs only for high compressive strain, implying an enhanced electric dipole moment in the case of high compressive strain. In particular, an insulator–metal transition is predicted at a compressive biaxial strain of 0.0475. These changes present a very interesting possibility for engineering the electronic properties of ferroelectric BaTiO3（001） surface.

Ferroelectricity and Large Rashba Splitting in Two-Dimensional Tellurium

Two-dimensional(2D)ferroelectric(FE)systems are promising candidates for non-volatile nanodevices.Previous studies mainly focused on 2D compounds.Though counter-intuitive,here we propose several new phases of tellurium with(anti)ferroelectricity.Two-dimensional films can be viewed as a collection of one-dimensional chains,and lone-pair instability is responsible for the(anti)ferroelectricity.The total polarization is determined to be 0.34×10^(-10)C/m for the FE ground state.Due to the local polarization field in the FE film,we show a large Rashba splitting(α_(R)~2 eV·?)with nonzero spin Hall conductivity for experimental detection.Furthermore,a dipole-like distribution of Berry curvature is verified,which may facilitate a nonlinear Hall effect.Because Rashba-splitting/Berry-curvature distributions are fully coupled with a polarization field,they can be reversed through FE phase transition.Our results not only broaden the elemental FE materials,but also shed light on their intriguing transport phenomena.

Dehydration-triggered structural phase transition-associated ferroelectricity in a hybrid perovskite-type crystal

Since the appearance of Rochelle salt,ferroelectrics have received extensive attention from researchers due to they are playing an important role in sensors,memories,mechanical actuation,and so on.In recent years,with the rapid development of molecular ferroelectrics,high-performance molecular ferroelectrics have become effective complement to inorganic ferroelectrics.However,compared with inorganic ferroelectrics,the family of molecular ferroelectrics is relatively scarce,and exploring highperformance ferroelectric materials through new synthesis strategies has become the trend of molecular ferroelectrics.Here,we successfully transformed non-polar material 1(2-H_(2)PCA)_(2)(H_(2)O)CdCl_(6)(2-H_(2)PCA=2-picolylamine cation)into polar material 2(2-H_(2)PCA)2CdCl_(6)by single-crystal to single-crystal transformation(SCSCT).Meanwhile,2 exhibits clear ferroelectricity with a high-temperature Tc of 378 K,a Ps of 1.18μC/cm^(2)at 300 K.This work not only realizes the purpose of synthesizing ferroelectrics by forming polar structures by SCSCT,but also realizes the reversibility of SCSCT,which provides ideas for the construction and exploration of new molecular ferroelectrics.

Energy storage optimization of ferroelectric ceramics during phase-transition process of amorphous/nanocrystalline and polycrystalline by using a phaseffeld model for dielectric breakdown

Ferroelectric ceramics have the potential to be widely applied in the modern industry and military power systems due to their ultrafast charging/discharging speed and high energy density.Considering the structural design and electrical properties of ferroelectric capacitor,it is still a challenge to ffnd out the optimal energy storage of ferroelectric ceramics during the phase-transition process of amorphous/nanocrystalline and polycrystalline.In this work,a ffnite element model suitable for the multiphase ceramic system is constructed based on the phase ffeld breakdown theory.The nonlinear coupling relationship of multiple physical ffelds between multiphase ceramics was taken into account in this model.The basic structures of multiphase ceramics are generated by using the Voronoi diagram construction method.The speciffed structure of multiphase ceramics in the phase-transition process of amorphous/nanocrystalline and polycrystalline was further obtained through the grain boundary diffusion equation.The simulation results show that the multiphase ceramics have an optimal energy storage in the process of amorphous polycrystalline transformation,and the energy storage density reaches the maximum when the crystallinity is 13.96%and the volume fraction of grain is 2.08%.It provides a research plan and idea for revealing the correlation between microstructure and breakdown characteristics of multiphase ceramics.This simulation model realizes the nonlinear coupling of the multiphase ceramic mesoscopic structure and the phase ffeld breakdown.It provides a reference scheme for the structural design and performance optimization of ferroelectric ceramics.

Hybrid improper ferroelectricity and phase transition behavior of Li_(2)Nd_(2)Ti_(3)O_(10)ceramics with A-site ordered triple-layer Ruddlesden-Popper structure

Hybrid improper ferroelectricity has been extensively reported through theoretical prediction and experimental investigation in the oxides with Ruddlesden-Popper(R-P)structures.However,the experimentally reported ferroelectric materials based on triple-layer R-P structures are rare,and the weak ferroelectricity impedes its practical application.In the present work,the single-phase Li_(2)Nd_(2)Ti_(3)O_(10)ceramics with A-site cation ordered triple-layer R-P structure was obtained by spark plasma sintering technique,and the crystal structure of P2_(1)ab symmetry was revealed by neutron powder diffraction and transmission electron microscope analysis.Furthermore,the switchable ferroelectricity in Li_(2)Nd_(2)Ti_(3)O_(10)ceramics has been observed at room temperature,and the saturated polarization-electric field hysteresis loop was obtained with P_(r)=0.4μC/cm^(2)under the electric field of 250 kV/cm.The phase transition behavior of Li_(2)Nd_(2)Ti_(3)O_(10)oxides was revealed by the temperature-dependence Raman and dielectric spectra combined with the piezoelectric property.

Dielectric properties and relaxor ferroelectric behavior of （1-y）Ba(Zr0.1Ti0.9)O3-yBa(Zn1/3Nb2/3)O3 ceramics

The microstructure,dielectric and ferroelectric properties of(1-y)Ba(Zr0.1Ti0.9)O3-yBa(Zn1/3Nb2/3)O3(y=0-0.05)ceramics prepared by traditional solid state method were investigated by X-ray diffractometer,scanning electron microscope,electric parameter testing system and ferroelectric tester.It is found that the barium zirconate titanate based ceramics are single-phase perovskites as y increases up to 0.05 and their average grain size decreases with the increase of y.The permittivity maximumεr,max is suppressed from 8948 to 1611 at 1 kHz with increasing y,and the ferroelectric-paraelectric phase transition temperature Tm decreases from 93 to-89℃at 1 kHz as y increases.The composition-induced diffuse phase transition is enhanced with increasingy.The relaxor-like ferroelectric behavior with a strong frequency dispersion of Tm and permittivity at T<Tm accompanied by a strong diffuse phase transition is found for the system with high y value.The remnant polarization decreases with increasing y,while the coercive field decreases remarkably and then increases with the increase of y.

Dielectric and ferroelectric properties of Sr_4CaSmTi_3Nb_7O_(30) with tetragonal tungsten bronze structure

Sr4CaSmTi3Nb7O30 ceramics are synthesized and indexed as tetragonal tungsten bronze structure. The dielectric behavior and ferroelectric nature are investigated. Three dielectric anomalies are observed. The phase transition is a displacive phase transition with some diffusive characteristics, which indicates possible compositional variations within the materials on the microscopic scale. The weak distortion disappears in cooling process for differential scanning calorimetry measurement, and the large depression of Curie-Weiss temperature TO indicates the difficulty in forming macroferroelectric domain. The ferroelectric nature in these filled tungsten bronze niobates originates from the off-center displacement of B-site cations, but they are primarily dominated by A-site cation occupation. Both the radius and the valence of A1-site cations play an important role on ferroelectric properties of the filled tungsten bronze compounds. Existence of spontaneous polarization with a remanent polarization of 0.16 μC/cm^2 a coercive field of Ec = 11.74 kV/cm confirms the room-temperature ferroelectric nature of Sr4CaSmTi3Nb7O30 ceramics.

Electric properties and phase transition behavior in lead lanthanum zirconate stannate titanate ceramics with low zirconate content

The phase transitions, dielectric properties, and polarization versus electric field （P-E） hysteresis loops of Pbo.97Lao.02（Zr0.42Sn0.58-xTix）O3 （0.13≤ x ≤0.18） （PLZST） bulk ceramics were systematically investigated. This study exhibited a sequence of phase transitions by analyzing the change of the P-E hysteresis loops with increasing temperature. The anfiferroelectric （AFE） to ferroelectric （FE） phase boundary of PLZST with the Zr content of 0.42 was found to locate at the Ti content between 0.14 and 0.15. This work is aimed to improve the ternary phase diagram of lanthanum-doped PZST with the Zr content of 0.42 and will be a good reference for seeking high energy storage density in the PLZST system with low-Zr content.

Pressure-Induced Anomalous Phase Transitions and Colossal Enhancements of Piezoelectricity in Ground-State BaTi0_(3)

Rhombohedral BaTi0_(3)under hydrostatic pressure is investigated by first principles calculations.Our results show that just like tetragonal perovskites,as pressure increases,this material first becomes para-electric at low pressures,then transfers to another ferroelectric phase at much higher pressures.We also find a giant enhancement of piezoelectricity near the phase-transition regions,due to large atomic displacements along different directions in response to the applied pressures.

Study of Defect-Layers Effect in Ferroelectric Thin Film with Transverse Ising Model

By taking into account the two-spin interaction in the transverse Ising model （TIM）, the influence of the defect layers （including JB and ΩB） on the polarization and Curie temperature are calculated numerically, within the framework of the decoupling approximation under Green＇s function. The numerical results show that the polarization and Curie temperature will both become large sensitively due to the large values of JB and the small value of ΩB of the defect layers. Meanwhile, the dependence of the crossover values of the exchange interaction JA, the transverse field ΩA of the bulk material on the exchange interaction JB and the transverse field ΩB of the defect layers are shown in 3-Dimensional （3-D） figures for the first time. Moreover, the transition features of the ferroelectric thin film with defect layers are presented.