Thermodynamics and kinetics of switching of ferroelastic ferroelectrics at its initial stage

Thermodynamics and kinetics of switching of ferroelastic ferroelectrics (FFs) are studied at the initial (weak-metastability) stage. It is shown that in the switching of a uniaxial FF, the role of the electric field and mechanical stress simultaneously applied to the FF is analogous to that of supersaturation or supercooling of solutions and melts in the process of a conventional phase transition. The dependence of the critical size of a domain on the strength of the switching field is derived. In the space of domain sizes, the dependence of the steady flux of repolarized and redeformed nuclei on the applied field is determined and the time required for this steady flux to set in (latent time) and the time during which this flux is steady are estimated.     

Polarization switching kinetics in ferroelectrics

The switching kinetics in ferroelectrics in the bulk polarization switching stage and in the final stage of the process are studied. Consideration is given to the specific case of switching of intrinsic ferroelectrics with 180° domains. A complete system of equations describing the switching processes and taking into account the change in repolarization in the course of a phase transformation is derived. The solution of this system is found. All the main characteristics of the switching process are calculated; namely, the evolution of the domain size distribution function is revealed and the time dependences of the domain density and flux are determined. An expression describing the variation in repolarization with time is obtained. The mechanisms of domain growth are studied. An equation for calculating the switching current and its variation with time is derived. A method is proposed for determining a number of constants for ferroelectric crystals by studying the switching current evolution.     

Polarization switching kinetics in ferroelectrics in the region of strong metastability

The thermodynamics and kinetics of polarization switching in ferroelectrics are studied in the framework of the field theory in the vicinity of the critical point of first-order phase transitions. The study is exemplified by the switching of intrinsic ferroelectrics with 180° domains. An expression describing the dependence of the domain critical size on the switching field is derived. The switching process is studied at high switching fields. Relationships for calculating the field dependence of the number of switched domains are obtained.     

Thermodynamics and kinetics of the initial stages of polarization switching in ferroelectrics

The thermodynamics and kinetics of polarization switching in ferroelectrics are studied in the specific case of switching in intrinsic ferroelectrics with 180° domains. The initial stage of the switching in the region of weak metastability is analyzed. An expression relating the critical domain size to the switching field is derived. An equation describing the evolution of the size distribution function of the switched domains is obtained. Expressions for calculating the number of polarization switching nuclei as a function of the switching field are derived.     

The ferroelastic transition of betaine borate

The ferroelastic transition of betaine borate at about 142 K and the nonlinear temperature dependence of the corresponding critical elastic modulus are related to the softening of optical phonons. The transition has to be considered a ferroelastic one of the optic type, i.e. pseudoproper one, therefore. It is shown by Raman spectroscopic results that the transition is induced by a strong bilinear coupling between a homogeneous strain and at least two optic lattice modes which are of the same symmetry as the strain. Optic and dielectric, investigations support the second-order character of and the classical behaviour at this transition, which have been reported in the literature. Differences and similarities to the ferroelastic transition of betaine fumarate are discussed.     

The ferroelastic transition of betaine borate

The ferroelastic transition of betaine borate at about 142 K and the nonlinear temperature dependence of the corresponding critical elastic modulus are related to the softening of optical phonons. The transition has to be considered a ferroelastic one of the optic type, i.e. pseudoproper one, therefore. It is shown by Raman spectroscopic results that the transition is induced by a strong bilinear coupling between a homogeneous strain and at least two optic lattice modes which are of the same symmetry as the strain. Optic and dielectric, investigations support the second-order character of and the classical behaviour at this transition, which have been reported in the literature. Differences and similarities to the ferroelastic transition of betaine fumarate are discussed.     

Quasi-two-dimensional solitons in ferroelectrics

The production and dynamics of nonlinear localized acoustic formations in ferroelectrics with the relaxation absorption are investigated. These formations are interpreted as algebraic solitons (lumps). It is shown that they are stable for transverse perturbations. Volgograd State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 14–18, July, 2000.     

Multicritical behavior in ferroelectrics

Solid state systems exhibit besides usual second order phase transitions a rich variety of multicritical phenomena like Lifshitz points (or lines), tricritical points (or lines) and even tricritical Lifshitz points. Realizations of such points are numerous and were also verified in the family of ferroelectrics of the type (Pb_ySn_1y)₂P₂(Se_xS_1-x)₆. A review of the critical behavior at such points is presented here. Because of the importance of the uniaxial dipolar interaction in ferroelectrics the critical behavior is different from systems with short range interaction only. Moreover the coupling to the elastic degrees of freedom may not be neglected, and leads under certain conditions to a critical temperature dependence in certain elastic constants. Crossover phenomena, which are expected in the experimental accessible region of experiments are also considered.     

Effect of a weak external electric field on the kinetics of the ordering of ferroelectrics upon first-order phase transitions

The kinetics of the formation and growth of 180° domains in a weak quasi-stationary external electric field has been considered in the framework of the phenomenological Ginzburg–Landau model using the example of sodium nitrite (NaNO₂) crystals that undergo a first-order ferroelectric phase transition of the order–disorder type. The influence of the rate and temperature of quenching, as well as the strength of an external electric field, on the subsequent evolution of the system toward the thermodynamic equilibrium state has been analyzed. It has been shown that, by varying a weak external electric field applied to the ferroelectric crystal after quenching, it is possible to obtain both single-domain and multi-domain ordered structures. It has been established that the formation of nonequilibrium (“virtual”) multi-domain structures of the asymmetric type is possible for particular strengths of the electric field applied to the ferroelectric after quenching. A similar effect can be achieved by varying the depth of quenching of the sample. It has been found that, if the size of the order parameter inhomogeneities formed at the stage of quenching does not exceed a critical value, they can be reoriented partially or completely into domains of opposite sign. For this purpose, the relaxation after quenching should be performed in an external electric field of the appropriate sign.     

铁电体的光伏效应

介绍了铁电光伏效应的发展历史和现状,通过与传统半导体p-n结光伏器件比较,旨在阐述铁电光伏器件非比寻常的优点和重要的应用前景.铁电光伏效应分为体光伏效应和反常光伏效应,多种物理机制已被发现,无疑为铁电光伏效应的提高指明了方向.还对钙钛矿氧化物、卤化物和双钙钛矿结构氧化物等铁电体中的光伏效应进行了阐述,讨论了通过引入新的自由度实现多功能性光伏器件的可能性.     

Anisotropic models of polymer ferroelectrics

The phase transitions to an ordered state in three-dimensional polymer systems that consist of flexible and rigid segments with intrachain and interchain orientational-deformational dipole interactions are considered. The statistic properties of the proposed models correspond to the Gaussian and spherical approximations that are used to describe the behavior of anisotropic Heisenberg ferroelectrics and ferromagnets. In the three-dimensional models under consideration, there exists a critical point T _c where a secondorder phase transition from an isotropic state to a state with long-range orientational order occurs. The laws of variation in the temperature T _c as a function of anisotropy of the interactions are established by analytical methods. The temperature dependences of the long-range dipole order parameter for a given chain bending are calculated and the results are compared with the experimental data obtained by the piezoelectric pressure step (PPS) method for thick ferroelectric PVDF and P(VDF-TrFE) polymer films.     

Dielectric properties of microcomposite ferroelectrics

The dielectric response in two-phase composites of ferroelectric and dielectric materials was studied within the quasistatic approximation. Using different effective medium theories (Maxwell-Garnet theory, the effective medium approximation and the Bergman representation) the dielectric function was represented as a distribution of polar modes. One of the main results concerns the existence of modes due to geometric resonances which among other effects can lead to new low-frequency absorption peaks near the percolation threshold of the ferroelectric material. These peaks appear always higher than the transverse polar mode frequencies of the bulk components but below their corresponding longitudinal mode frequencies. The soft mode with renormalized mode strength but unchanged frequency exists only in the composite with percolated ferroelectric clusters. When the ferroelectric clusters are only finite the soft mode does not soften completely.     

Effect of stress on the ferroelastic phase-transition temperature

On the basis of the expression giving the dependence of the free energy of ferroelastics on temperature, spontaneous strain u, and stress σ, it is shown that the ferroelastic phase-transition temperature changes proportionally to σ^2/3.     

铁电体电特性测量实验

利用电滞回线发生器,通过信号测量电路及计算机接口技术,描绘电滞回线,计算出待测铁电体(压电陶瓷片)的电特性参量.     

Soft acoustic mode in ferroelastic lanthanum pentaphosphate

The temperature dependence of the C₅₅ elastic constant of LaP₅O₁₄ has been investigated, using Brillouin scattering, between room-temperature and 225°C, across the 126°C ferroelastic phase transition of this material. C₅₅ is found to vanish at the transitión. Physical implications of this result are discussed.