Nicholas V. Morozovsky

Surface and finite size effects impact on the phase diagrams, polar, and dielectric properties of (Sr,Bi)Ta2O9 ferroelectric nanoparticles

In the framework of the thermodynamic approach Landau-Ginzburg-Devonshire (LGD) combined with the equations of electrostatics, we investigated the effect of polarization surface screening on finite size effects of the phase diagrams, polar, and dielectric properties of ferroelectric nanoparticles of different shapes. We obtained and analyzed the analytical results for the dependences of the ferroelectric phase transition temperature, critical size, spontaneous polarization, and thermodynamic coercive field on the shape and size of the nanoparticles. The pronounced size effect of these characteristics on the scaling parameter, the ratio of the particle characteristic size to the length of the surface screening, was revealed. Also our modeling predicts a significant impact of the flexo-chemical effect (that is a joint action of flexoelectric effect and chemical pressure) on the temperature of phase transition, polar, and dielectric properties of nanoparticles when their chemical composition deviates from th...

Nonlinear space charge dynamics in mixed ionic-electronic conductors: Resistive switching and ferroelectric-like hysteresis of electromechanical response

We performed self-consistent modelling of nonlinear electrotransport and electromechanical response of thin films of mixed ionic-electronic conductors (MIEC) allowing for steric effects of mobile charged defects (ions, protons, or vacancies), electron degeneration, and Vegard stresses. We establish correlations between the features of the nonlinear space-charge dynamics, current-voltage, and bending-voltage curves for different types of the film electrodes. A pronounced ferroelectric-like hysteresis of the bending-voltage loops and current maxima on the double hysteresis current-voltage loops appear for the electron-transport electrodes. The double hysteresis loop with pronounced humps indicates a memristor-type resistive switching. The switching occurs due to the strong nonlinear coupling between the electronic and ionic subsystems. A sharp meta-stable maximum of the electron density appears near one open electrode and moves to another one during the periodic change of applied voltage. Our results can explain the nonlinear nature and correlation of electrical and mechanical memory effects in thin MIEC films. The analytical expression proving that the electrically induced bending of MIEC films can be detected by interferometric methods is derived.

Ferroionic states in ferroelectric thin films

The electric coupling between surface ions and bulk ferroelectricity gives rise to a continuum of mixed states in ferroelectric thin films, exquisitely sensitive to temperature and external factors, such as applied voltage and oxygen pressure. Here we develop the comprehensive analytical description of these coupled ferroelectric and ionic (ferroionic) states by combining the Ginzburg-Landau-Devonshire description of the ferroelectric properties of the film with Langmuir adsorption model for the electrochemical reaction at the film surface. We explore the thermodynamic and kinetic characteristics of the ferroionic states as a function of temperature, film thickness, and external electric potential. These studies provide a new insight into mesoscopic properties of ferroelectric thin films, whose surface is exposed to chemical environment as screening charges supplier.

Pyroelectricity: some new research and application aspects

Attention is concentrated on the original results of investigations of pyroelectric effect and related phenomena in various ferroelectric materials (FEM) used for creation of pyroelectric convertors for various purposes including single and multielements pyroelectric detectors of radiation (PDR) of plane and cavity types. Methods of pyroelectric certification developed for both simultaneous determination of pyroelectric, piezoelectric, dielectric and thermal parameters of pyroactive FEM and express selection of sensitive elements of PDR and pyrovidicons in conditions of mass production are considered. The results of amplitude- phase-frequency analysis of dynamic pyroelectric response under near-surface pyroelectric probing of multilayer contact systems based on wide used traditional dielectric FEM, on photosensitive FEM with semiconductive properties and ionic conductivity, namely ferroelectric-semiconductors and ferroelectric-ionics and also polymeric FEM of PVDF-type are presented. The applications of a number of effects connected with high pyroactivity, thermo-, photo- and electrical controllability characteristic for such FEM are discussed. The examples of practical realization of new principles under development of pyro- and photoelectric convertors, pyroelectric devices and units of electronic support are given. The peculiarities of PDR operation in pyroelectric current and voltage mode are discussed.

Polarization reversal in organic-inorganic ferroelectric composites: Modeling and experiment

Tailoring of ferroelectric properties of copolymer P(VDF-TrFE) by incorporation of ceramic inclusions in the polymer matrix is promising for advanced applications in sensorics. We have observed experimentally that in composites of P(VDF-TrFE) with barium-doped lead zirconate titanate (BPZT), the remanent polarization increases, while the coercive field substantially decreases in comparison with the pure polymer samples. Results of simulation in framework of the modified Weiss model have shown that the changes of the hysteresis loops characteristics are due to increase of the dielectric susceptibility of the composite as compared to pure PVDF-TrFE. This originates from the strong dispersion of the mean field constant α, which describes the feedback of the polarization on the electric field at the location of the dipoles and is closely related with the local increase of composite susceptibility in the vicinity the BPZT inclusions. This phenomenon effectively becomes macro-scale due to the long-range nature of the inhomogeneous elastic and electric fields occurring at the interfaces between the matrix and inclusions.

Low-Temperature Pyroelectric Phenomena in Lithium Niobate Single Crystals

The results of studying low temperature pyroelectric phenomena in lithium niobate LiNbO3 single crystals are presented. The temperature dependences of pyroelectric response Uπ(T), dielectric permittivity ϵ(T), changes of electrical charge qe(T), thermograms of thermostimulated light emission IL(T) and non-stationary currents JN(T) during heating-cooling runs were investigated in the range of 4, 2 ≤ T ≤ 300 K. In the vicinity of T* ≈ 200 K we observed the smooth Uπ(T) break, step-like peculiarities of ϵ(T) and the changes of IL(T) and JN(T) thermograms parameters. Near Ts ≈ 25 K between the low-temperature maximum of Uπ(T) at 60 K and double 180°-change of Uπ phase at 11 K and 6 K is observed the Uπ(T) break, IL(T) maximum, anomaly of JN(T) and change of sign of qe(T). The double change of the sign of pyroelectric coefficient is explained in the frame of the microscopic model with account of the defect-subsystem.

Low-temperature pyroelectric and luminescence phenomena in Sn2P2S6 single crystals

Abstract The complex investigation of temperature changes of pyroelectric, dielectric, photo- and thermoluminescence characteristics of Sn2P2S6 single crystals was performed in the temperature range from 4.2 to 300 K. The correlation in low-temperature behaviour of investigated characteristics is revealed. The view of temperature dependences of pyroelectric response and dielectric permittivity is interpreted in the framework of glass-like behaviour model.

Effect of surface ionic screening on the polarization reversal scenario in ferroelectric thin films: Crossover from ferroionic to antiferroionic states

Nonlinear electrostatic interaction between the surface ions of electrochemical nature and ferroelectric dipoles gives rise to the coupled ferroionic states in nanoscale ferroelectrics. Here, we investigate the role of the surface ion formation energy on the polarization states and its reversal mechanisms, domain structure, and corresponding phase diagrams of ferroelectric thin films. Using 3D finite element modeling, we analyze the distribution and hysteresis loops of ferroelectric polarization and ionic charge, and the dynamics of the domain states. These calculations performed over large parameter space delineate the regions of single- and polydomain ferroelectric, ferroionic, antiferroionic, and nonferroelectric states as a function of surface ion formation energy, film thickness, applied voltage, and temperature. We further map the analytical theory for 1D systems onto an effective Landau-Ginzburg free energy and establish the correspondence between the 3D numerical and 1D analytical results. This approach allows us to perform an overview of the ferroionic system phase diagrams and explore the specifics of polarization reversal and domain evolution phenomena.

Peculiarities of dynamic pyroelectric response of spin-coated corona-charged P(VDF/TrFE) polymer films

Dynamic pyroelectric response of spin coated corona charged P(VDF/TrFE) copolymer films and commercial P(VDF/TrFE) films has been studied in a wide range of frequencies of radiation flux modulation. Investigation has been concentrated on the films with non-uniform distribution of pyroactivity through the film thickness. A low frequency pyroelectric resonance discovered earlier in commercially available PVDF films of various production has not been observed in the spin coated corona charged films and was weak pronounced in the commercial P(VDF/TrFE) copolymer films from Piezotech. Results are explained with the assumption of significant role of prior stressed state in the polymer films for occurrence of pyroelectric resonance phenomena. Advantages of spin coated corona charged films for pyroelectric applications are discussed.

Experimental and theoretical investigations of the pyroelectric effect in ferroelectric crystals at low temperatures

Abstract The temperature dependence of the low-temperature pyroelectric response, Uπ, has been investigated by a dynamic method for a number of ferroelectric single crystals. The main features of the behaviour of Uπ, (T) are explained in the framework of the proposed microscopic model. Good agreement between theoretical results and experimental data for the uniaxial displacive-type ferroelectric Sn2P2S6 is obtained. In addition, some possible applications, based on the effect of the pyroelectric coefficient sign change, are proposed.