V. V. Gladkii

The kinetic characteristics of polarization of relaxor ferroelectrics

SBN crystals doped with rare-earth metal ions were studied to show that relaxor ferroelectrics had pronounced anomalies, which manifested themselves by the noncoincidence of the trajectories of the first several cycles of dielectric hysteresis loops, the absence of an unambiguous coercive field, and other special features of the kinetics of polarization. These anomalies were related to structural disorder of the crystals and a random internal electric field distribution and could only be observed in constant and quasi-static electric fields. A phenomenological analysis of the thermal activation stages of polarization relaxation was performed. The spectra of the energy distribution of potential barriers were reproduced in the approximation of their independence. Electric conductivity was shown to play an important role in the formation of giant barriers.

Slow polarization processes in relaxor ferroelectrics

The general features of the slow polarization kinetics in relaxor ferroelectrics, observed and studied by us in the specific example of crystals of barium-strontium niobate (SBN) solid solutions of various compositions and of lead magnesium niobate (PMN), are considered. The dielectric hysteresis loops and spectra of the polarization relaxation time distribution in quasi-static and static electric fields were found to reveal characteristic anomalies related to a random internal electric field in the bulk of a relaxor ferroelectric. Such a field caused by structural disorder accounts for the anomalously broad spectrum of potential barriers for domain walls. The part played by free charge carriers in the formation of giant barriers is demonstrated. The paper presents some quantitative data characterizing the specific features of the structure and physical properties of the relaxors.

Polarization anomalies in a relaxor ferroelectric

With a strontium barium niobate crystal used as an example, it is experimentally demonstrated that relaxor ferroelectrics exhibit pronounced polarization anomalies, which manifest themselves in the difference between the trajectories of the dielectric hysteresis observed in several repeated cycles of a varying external electric field. These anomalies originate from the nonuniformity of the relaxor composition, local symmetry lowering, local internal electric field, and a wide distribution of potential barriers in energy for the polarized regions. The anomalies can be observed only in slowly varying (quasistatic) or constant electric fields.

Polarization and depolarization of relaxor ferroelectric strontium barium niobate

The anomalies of polarization (mismatch of hysteresis loops for several first repolarization cycles, the absence of specific coercive field, etc.) in quasi-static and dc electric fields have been found in the strontium barium niobate relaxor ferroelectric. The anomalies are associated with a highly inhomogeneous structure of the crystal, which is considered a clearly defined nonergodic system with a random distribution of strong local internal fields. The energy distributions of potential barriers for the polarization and the depolarization are obtained at different electric field strengths and temperatures.

Phase transitions in Cs2HgCl4 crystals

Abstract Dielectric, piezodielectric and pyroelectric properties of Cs2HgCl4 were studied. Seven temperature anomalies of the properties attributed to structural phase transitions are found. Spontaneous polarization is shown to arise in two temperature regions. Some memory effects specific for incommensurate structures are revealed in one of the phases.

Specific features in the ferroelectric properties of Sr0.75Ba0.25Nb2O6 crystals

The dielectric properties of strontium-barium niobate crystals with a strontium concentration of 0.75, which corresponds to the most smeared ferroelectric phase transition, were measured. The dielectric hysteresis loops and the polarization relaxation are shown to exhibit clearly pronounced features characteristic of crystals with structural disorder and a broad distribution of a random internal electric field. The measurements were performed in dc, slowly varying (quasi-static), and pulsed electric fields. The experimental data obtained are used to construct the distribution function for the coercive field in the crystal bulk.

Polarization relaxation in a ferroelectric crystal having different states of the domain structure and surface

Slow polarization relaxation of a ferroelectric in a weak electric field is investigated in triglycine sulfate crystals with different states of the domain structure and surface. An automated setup is described that allows one to register relaxation with record accuracy. It is shown that under conditions of little change in the “degree of metastability” of the structure, the variation of the polarization ΔP with time t follows the law ΔP=C/(1+t/ta)n in all cases, where the parameters C, a, and n depend on the state of the structure and the surface. We present a phenomenological analysis of the experimental data based on the assumption that the nuclei are independent and contribute additively to the total polarization of the crystal, and construct spectra of the energy barriers of the domain walls. Aspects of the transformation of the spectra with variation of the nature of the domain structure, depth of surface relief, and magnitude of the external electric field are elucidated.

Dielectric Hysteresis Loops of Photosensitive Relaxor Ferroelectrics

We have studied the effects of illumination by a photoactive light on the polarization switching of strontium-barium niobate crystals in quasi-static periodic electric field. The illumination results in an increase of the polarization amplitude and of the most probable coercive field. The origin of this phenomenon is discussed in terms of screening of depolarizing fields and of internal random fields presenting within the bulk of a relaxor ferroelectric with photoinduced free charge carriers.

Suppression of ferroelectricity by low uniaxial stresses in some crystals with successive phase transitions

Abstract Study of disappearance of ferroelectricity on compressing the tetramethylammonium tetrachlorozincate, tetrachlorocobaltate and thiourea crystals by uniaxial stresses [sgrave] is presented. The phase [sgrave], T - diagrams are plotted, the critical values of [sgrave] being anomalously small. The effect is shown to be essentially anisotro-pic and nonlinear. The data published recently are shortly reviewed and some new results are presented.

Peculiarities of dielectric properties of ferroelectrics in the region of incommensurate phase

Abstract Investigations of the specific dielectric properties of incommensurate phase in ferroelectrics are briefly reviewed. A quantative analysis of the experimental data is made and as the result an electrical extra inhomogeneity is supposed to arise in a real crystal sample at the transition to incommensurate phase.