Polar Microregions Research Articles

Dielectric relaxation behavior of ceramics in the Pb(Zn 1/3 Nb 2/3 )O 3 -BaTiO 3 -PbTiO 3 system

The dielectric properties of ceramics in Pb(Zn1/3Nb2/3)O3-BaTiO3-PbTiO3 system were characterized using dielectric-temperature spectra. A spontaneous (zero field) relaxor-normal ferroelectric transition was observed for tetragonal-rich compositions. A significant hysteresis effect accompanied by this transition, similar to first-order phase transition of normal ferroelectrics. This behavior was different from that of other relaxors, in which such transitions occurred only under a biased dc field. This observation was explained in terms of a thermally driven transformation from an ensemble of polar microregions to normal long-range ferroelectric state (micro-macro domain transition), which was attributed to the internal field resulting from the tetragonal strain.

Relaxor-like behaviour of pyrochlores

Dielectric dispersion and Raman scattering studies of Cd2Nb2O7 pyrochlore single crystals in the ferroelectric phase (T < Tc= 196 K < Ts = 205 K) are presented. Below Tc, the behaviour of normalized ε (T) and ε (T) with decreasing frequency is typical of that for relaxors and gives evidence for an existence of disordered polar microregions. The thermal hysteresis near the secondorder phase transitions at Ts and Tc is due to ‘overheated’ ferroelectric state. The slow relaxation processes developing on a long-time scale below Tc are ascribed to quasi-nonergodicity of the system long before a glassy state appears (below Tg ≈ 18 K). The intensity of the Raman-active hard modes arising below Tc changes on cooling towards Tg and on thermal scanning within the ferroelectric phase. These facts imply that the modes are involved in the relaxation processes caused by disordering the system. The relaxor-like behaviour of the system at Tg < T < Tc is ascribed to a dynamical disorder in alignement of Cd-O(7th) dipoles due to fast reorientational motions of Cd ions over the six equivalent off-center positions and to appearance of the short-range orientational order in microregions below Tc. The change in intensity of the Raman-active hard modes of Cd2Ta2O7 in the same temperature region as for Cd2Nb2O7 does not exclude an existence of slow relaxation processes and disordered state in this pyrochlore as well.

Direct observation of ferroelectric domains in MOCVD-grown PbTiO3 thin films by means of transmission electron microscopy

Transmission electron microscopic observations of domain structures have been made on the PbTiO3 ferroelectric thin films prepared on (100) NaCl by metalorganic chemical vapor deposition (MOCVD). Both single-domain and alternating 90[ddot] a-a and a-c domain configurations were observed in the as-deposited thin films. 180 domain boundaries were also detected in the films which can compensate the polarization charges in grain boundaries. Domain formation in closely-packed polycrystalline thin films was different from that in free crystallites without boundary clamping. Domain width was found to decrease with decreasing grain size and domain splitting cannot be observed in ultrafine grains less than about 40 nm. An interesting phenomenon of polar microregions was also observed and attributed to the effect of surface electric field arising from electron irradiation during the observations.

The effect of ferroelastic coupling in controlling the abnormal aging behavior in lead magnesium niobate-lead titanate relaxor ferroelectrics

The abnormal aging behavior, i.e., severe aging in the electric field induced piezoelectric coefficient while very weak dielectric aging, observed in the relaxor ferroelectric lead magnesium niobate-lead titanate (PMN-PT) ceramics under a dc electric bias field, can be significantly reduced by hot isostatic pressing (HIP) treatment on presintered samples. The aging can also be reduced by doping suitable amounts of either La (donor) or Mn (acceptor). We suggest that the reduction in the aging is due to the introduction of additional random fields into the material, which reduces the probability of the growth of micropolar regions into metastable and/or stable macropolar domains. The abnormal aging behavior and the effectiveness of HIP in reducing it indicate the importance of the elastic energy in controlling the aging and relaxor behavior in PMN-PT relaxor ferroelectrics.

Photoinduced charge effects in

Photoinduced effects were studied in , making use of birefringence and second-harmonic generation as probes of structural modifications induced by illumination at 2.4 eV. It is shown that photoinduced charges have no direct influence on the size and orientational freedom of polar microregions. Quenching of the polar order by illumination is only observed in the presence of an external field and it is consistent with the build-up of a depoling field, owing to space-charge formation. Light of energy below 2 eV is not effective by itself, but it can be if the sample is pre-sensitized with photons of higher energy.

Dynamics of nanodomains (clusters) and dielectric relaxation in disordered ferroelectrics

Dielectric relaxation in the disordered ferroelectric-type materials of different kind is under consideration. We defined the common features of the dielectric spectra and phase transitions of this materials and propose to consider observed phenomena from common position as an intrinsic feature of disordered ferroelectric materials of different kind. Dielectric relaxation, phase transition diffusing and peculiarities of dielectric spectra are a consequence of frequency and temperature dependent dipole dynamics of the polar phase microregions (clusters) and interphase boundaries. The evidence of the electrical heterogeneity of some disordered ferroelectric materials was obtained by the Scanning Force Microscopy method with simultaneous measurement of the local surface potential distribution.

Dielectric response process in relaxor ferroelectrics

The dielectric constant ε*(ω,T)=ε′(ω,T)+iε″(ω,T) of relaxor ferroelectrics is simulated by the Monte Carlo method. It is proved that the polar microregions switch among different polarization directions as a whole under the electrical field, giving rise to the high dielectric constant. The relaxor characteristics are caused by the randomly distributed interactions between the polar microregions. Factors which influence the dielectric response process are discussed.

Low‐Temperature Observation of Relaxor Ferroelectric Domains in Lead Zinc Niobate

Domain reorientation in single crystals of lead zinc niobatelead titanate solid solutions was examined, because the reorientation contributes to the electrically controlled change of shape and change of response in piezoelectric transducers and actuators. An optical microscope technique was used to explore the buildup of macropolar domains from micropolar regions in relaxor compositions. Poorly defined “ambiguous” spindlelike domains changed to distinct lamellar domains as PbTiO3 content was increased. The domain walls in Pb(Zn1/3Nb2/3)O3‐rich samples moved with a wavelike motion. The motion ceased or “froze‐in” below –130° and –30°C for field‐biased and nonbiased samples, respectively. The domains were observed at various temperatures from 300° to –185°C and electric fields up to ±10kV/cm.

Dynamics of the freezing process in relaxor ferroelectrics.

The dynamics of the freezing process in relaxor ferroelectrics are investigated by Monte Carlo computer simulation. The concepts of ``frozen dipole'' and ``slow dipole'' are proposed according to the flipping frequency during the observation time of simulation. The deviation from Curie-Weiss behavior and the frequency dispersion in relaxor ferroelectrics have been proved to be caused by the increasing proportion of slow dipoles. The simulation shows that the external field reduces the dielectric constant and increases the temperature of dielectric maximum, which agrees with the experiment results. The polarization behavior with various electrical and thermal histories and starting from different initial states is also simulated and a systematic explanation based on the bifurcation tree of components in phase space is given. It is proposed that the relaxor characteristics are caused by a gradual freezing process of polar microregions, which arises from the inhomogeneous crystal structure of this kind of material.

Distribution of relaxation times in perovskite-type relaxor ferroelectrics

The distribution of relaxation times in perovskite-type relaxor ferroelectrics is studied by computer simulation. It has been proved that the spectra move to longer relaxation times with the decreasing of temperature and the increasing of the external field. The frequency dispersion of relaxors is explained by a gradual freezing process of polar microregions. The relationship between the time dependence of the polarization and the relaxation time distribution at various temperatures and external fields has been investigated. The macroscopic polarization is suggested to be sustained mainly by frozen dipoles during the observation time.

Relaxor-normal ferroelectric transition in tetragonal-rich field of Pb(Ni1/3Nb2/3)O3-PbTiO3-PbZrO3 system

The possibility of the existence of a spontaneous relaxor-normal ferroelectric transition was proposed and examined using Pb(Ni1/3Nb2/3)O3-PbTiO3-PbZrO3 (PNN-PT-PZ) pseudoternary system across the morphotropic phase boundary (MPB). On cooling, the PNN-PT-PZ system in the tetragonal-rich side of the MPB underwent a spontaneous transition from a relaxor to a normal ferroelectric state. This observation was explained in terms of a thermally driven transformation from an ensemble of polar microregions to normal long-range ferroelectric domains (micro-macrodomain switching). The relaxor-normal ferroelectric transition was further correlated with the rhombohedral-tetragonal first-order phase transition. Increasing the relative content of PbZrO3 enhanced dielectric dispersion characteristics but weakened the extent of relaxor-normal transition. This observation was explained by the enhanced 1:1 Nb-Zr short-range ordering and the breaking of the translational invariance of the ferroelectrically active BO6 octahedra with increasing content of PbZrO3.

Relation between deformation and polarization in the region of diffused phase transition of plzt 8/65/35

Abstract The deformation as a function of polarization was measured in PLZT 8-10/65/35 in the region of diffused phase transition. A different behaviour of deformation in the region of double hysteresis loops was observed corresponding to ferroelectric and nonpolar phases as well as to the field induced phase transition region. In the ferroelectric phase the results are described by electrostriction. The decrease of volume as well as small longitudinal deformations at the field induced transition to ferroelectric state are explained by large remanent deformations, existing in the nonpolar phase. Since description of deformation in the framework of electrostriction cannot be motivated in the nonpolar phase, the influence of reorientation of polar microregions on deformation is discussed.

Dielectric studies of psnt single crystals

Abstract The paper reports results of dielectric studies of Pb(Sc0.5Nb0.2Ta0.3)O3 single crystals (Nb and Ta ions having the same effective radius) as grown, and thermally treated. Dielectric anomaly at 317 K can be ascribed to the ferroelectric paraelectric phase transition, whereas the disordered phase of the mixed crystal exhibits dielectric anomaly at ∼ 306 K. The existence of polar microregions in a wide temperature range above the Curie point was observed by us directly in dielectric measurements and the activation energy of 0.49 eV can be ascribed to the polar clusters.

Mechanism of Diffuse Phase Transition in Relaxor Ferroelectrics

Dielectric properties of lead magnesium niobate (PMN) and Ta-bearing strontium barium niobate (SBNT) were measured as a function of temperature, and the crystal structure of PMN was refined by the Rietveld analysis method. The results of structure refinement indicate that the volume of polar microregions (PMR) increases with decreasing temperature. The dielectric properties of PMN and SBNT are well explained by an advanced theory of dielectric dispersion. The anomalous behavior in dielectric permittivity (ε) observed on the low temperature side of T m, the temperature of ε maximum, is explained by simple dielectric relaxation, while on the high-temperature side of T m is explained by the volume increase of PMR. It is concluded that these is no phase transition around T m, and the diffuse phase transition is an overlapping phenomenon of volume increase of PMR, freezing process of fluctuating dipoles in PMR and dielectric dispersion around the measuring frequrncy. A model of diffuse phase transition and relaxor ferroelectrics is proposed. The difference between normal ferroelectrics and relaxor ferroelectrics is discussed from the viewpoints of spreading of soft-mode phonons and disorder in the crystals.

Dielectric properties of MnO2doped Pb(Zn1/3Nb2/3)O3-BaTiO3ceramics

Abstract Abstract Dielectric, pyroelectric and piezoelectric properties of MnO2 doped 0.85PbPb(Zn1/3Nb2/3)O3-0.15BaTiO3(0.85PZN-0.15BT)ceramics were investigated. The dielectric constant frequency dispersion of MnO2 doped 0.85PZN-0.15BT ceramics weakens gradually with increasing of MnO2 while the diffuse phase transition still persists. After MnO2 doping, the dielectric constant decreases, average phase transition temperature moves toward higher temperature, the aging rate of dielectric constant around room temperature becomes pronounced, and the temperature dependence of dielectric constant becomes saddle-like with weaker frequency dispersion. The results can be explained using a model with distributed polar micro-regions of various size.

Raman scattering of ferroelectric Sr1-xCaxTiO3, x=0.007

First-order Raman scattering of A1g and Eg modes originating from the antiferrodistortive F2u soft mode confirms the cubic-tetragonal (Oh-D4h) phase transition in Sr0.993Ca0.007TiO3, at T0=125 K. Within the cubic and the tetragonal phase first-order Raman scattering of the soft ferroelectric F1u mode (TO1) and of the hard TO2 and TO4 optic modes evidences the existence of Ca2+-centred polar microregions. Their size increases rapidly on cooling towards the ferroelectric ordering temperature, Tc=18 K. Splitting and incomplete softening of the F1u components is consistent with an order-disorder transition into an orthorhombic low-temperature phase with C2v symmetry.

Origin of F spots and stress sensitivity in lanthanum lead zirconate titanate

In this study a systematic microstructural characterization of La-modified lead zirconate titanate was carried out by hot- and cold-stage transmission electron microscopy and selected area electron diffraction. The existence of 1/2[111] superlattice reflections (F spots) became evident near 300 °C on cooling. The intensity of the F spots was found to increase with decreasing temperature, approaching a near constant intensity below 100 °C. In conjunction with compositional studies, it was proposed that the F spots arise from a doubling of the unit cell associated with a tilting of oxygen octahedra. The presence of F spots at temperatures approaching the onset of local polarization was then interpreted as the existence of a hierarchy of domain states. This hierarchy was proposed to consist of an average-cubic macroscopic structure containing rhombohedral micropolar regions on the scale of 50–100 Å that contain ferroelastic microtwins on the scale of several tens of angstroms.

Dielectric properties of tetragonal lanthanum modified lead zirconate titanate ceramics

Tetragonal La-modified lead zirconate titanate ceramics with a Zr/Ti ratio of 40/60 (x/40/60) with La contents (x) between 0 and 21 at. % were prepared by the mixed oxide method and were subsequently characterized using dielectric spectroscopy. Small La contents tended to diffuse the dielectric response in the temperature region around the phase transition, however, no distinct changes were observed in the paraelectric region. Compositions with intermediate La contents showed a thermally driven (spontaneous) transformation from a relaxor to a normal ferroelectric, i.e., micro-macro domain switching. Materials with a high La content exhibited typical relaxor ferroelectric behavior. The relative thermodynamic stability of the ensemble of micropolar regions is then discussed in reference to that of the long-range ferroelectric state.

Polar fluctuations and first-order Raman scattering in highly polarizable KTaO3 crystals with off-center Li and Nb ions.

Relatively low concentrations of randomly substituted off-center ions are known to induce ferroelectric order in highly polarizable crystals. In the case of potassium tantalate (${\mathrm{KTaO}}_{3}$) doped with lithium or niobium, the results of first-order Raman scattering from two hard phonon modes (${\mathrm{TO}}_{2}$ and ${\mathrm{TO}}_{3}$) show that polar microregions are formed at relatively high temperatures. At lower temperatures, and above a certain critical dopant concentration, the Raman results are consistent with the occurrence of a structural phase transition. These results can be reasonably explained by using a random-molecular-field theory that takes into account the indirect dipolar interactions mediated by the soft lattice. This approach accounts for the characteristic asymmetry of the ${\mathrm{TO}}_{2}$ Raman scattering detected at high temperatures and for the subsequent increase in the integrated scattering intensity with decreasing temperature. An analysis of the Raman results provides a means for determining the size of the polar microregions formed in the highly polarizable ${\mathrm{KTaO}}_{3}$ lattice. The good agreement obtained between the theoretical and experimental results provides support to the physical concepts embodied in the present theoretical approach to describing the onset of order in these systems.

PmP113. Frequency dependence of the piezoelectric d31coefficient as a function of ceramic tetragonality

Abstract Measurements have been made of the complex piezoelectric coefficient (d31), by a quasistatic method, so as to determine its real and imaginary components as a function of frequency (f = 10−3 - 103 Hz) and temperature. Results obtained with Ca-modified lead titanate and lead lanthanum titanate-zirconate ceramic samples reveal the existence of relaxations at a very low frequency for the former and at frequencies of tens of hertzs for the latter. Tetragonal distortion and polar microregions could be considered as the respective causes of such relaxation processes.