A. A. Bokov

Hierarchical micro-/nanoscale domain structure in MC phase of (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 single crystal

A hierarchical micro-/nanoscale domain structure is revealed in MC phase of (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 single crystal near the morphotropic phase boundary by combination of analytical electron microscopy and polarized light microscopy. The hierarchical domain structure is a self-assembling of nanodomains with tetragonal structure, submicrodomains appearing as monoclinic, and microdomains exhibiting the same monoclinic state due to an average effect. The outstanding piezoelectric properties may result from the cooperative response of the hierarchical micro-/nanoscale domain structure.

Quasi-ferroelectric state in Ba(Ti1−xZrx)O3 relaxor: dielectric spectroscopy evidence

The dielectric spectroscopy of perovskite Ba(Ti0.675Zr0.325)O3 (BTZ325) relaxor ceramics is performed in a wide frequency range of 10?2?106?Hz. In contrast to other known relaxors, where the dipole dynamics is subject to non-Arrhenius slowing-down and freezing upon cooling so that the cubic ergodic relaxor phase transforms into a cluster dipolar-glass phase or a ferroelectric phase, none of these transformations are observed in BTZ325. In the course of cooling from the ergodic relaxor phase the characteristic time and the spectral width of the main relaxation process first increase rapidly in a Vogel?Fulcher manner, but then become almost temperature independent below the temperature of permittivity maximum, indicating the onset of the state which we call quasi-ferroelectric. The properties and the origin of this state are discussed.

Structural phase transition and dielectric relaxation in Pb(Zn1/3Nb2/3)O3 single crystals

The structure and the dielectric properties of Pb(Zn1/3Nb2/3)O3 (PZN) crystal have been investigated by means of high-resolution synchrotron x-ray diffraction (with an x-ray energy of 32 keV) and dielectric spectroscopy (in the frequency range 100 Hz–1 MHz). At high temperatures, the PZN crystal exhibits a cubic symmetry and polar nanoregions inherent to relaxor ferroelectrics are present, as evidenced by the single (222) Bragg peak and by the noticeable tails at the base of the peak. At low temperatures, in addition to the well-known rhombohedral phase, another low-symmetry, probably monoclinic, phase is found. The two phases coexist in the form of mesoscopic domains. The ferroelectric phase transition is diffuse and observed between 325 and 390 K, where the concentration of the low-temperature phases gradually increases and the cubic phase disappears upon cooling. However, no dielectric anomalies can be detected in the temperature range of the diffuse phase transition. The temperature dependence of the dielectric constant shows a maximum at higher temperature (Tm = 417–429 K, depending on frequency) with the typical relaxor dispersion at T < Tm and the frequency dependence of Tm fitted to the Vogel–Fulcher relation. Application of an electric field upon cooling from the cubic phase or poling the crystal in the ferroelectric phase gives rise to a sharp anomaly of the dielectric constant at T ≈ 390 K and greatly diminishes the dispersion at lower temperatures, but the dielectric relaxation process around Tm remains qualitatively unchanged. The results are discussed in the framework of the present models of relaxors and in comparison with the prototypical relaxor ferroelectric Pb(Mg1/3Nb2/3)O3.

Dielectric anomalies of the relaxor-based 0.9Pb(Mg1∕3Nb2∕3)O3‐0.1PbTiO3 single crystals

The temperature, frequency, and orientation dependence of complex electrical permittivity in 0.9Pb(Mg1∕3Nb2∕3)O3‐0.1PbTiO3 (0.9PMN-0.1PT) unpoled single crystals have been investigated in detail. The spontaneous ferroelectric-to-relaxor (FE-R) phase transition was observed in 0.9PMN-0.1PT crystals at about 280K. Interestingly, the Vogel-Fulcher-type anomaly of the dielectric constant has been obtained, with the fitted temperature Tf being very close to this FE-R phase transition temperature. A small frequency dependence of electric permittivity magnitude appears at and below 280K. The changes of the polar nanoregion subsystem and the peculiarities of the domain structure play an important role in the dielectric anomalies and the appearance of the FE-R crossover region.

Field-induced shift of morphotropic phase boundary and effect of overpoling in (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 piezocrystals

The domain structure and phase symmetry of the (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 piezoelectric crystals with a composition close to the morphotropic phase boundary (MPB) between the rhombohedral R3m and monoclinic Pm phases (x≅0.3) have been studied by polarized light microscopy. It is found that poling by an electric field parallel to the ⟨001⟩ pseudocubic direction shifts the MPB toward the lower x side. This shift is the origin of the overpoling effect which degrades the performance of the materials, e.g., the piezoelectric coefficient (d33) and dielectric constant decrease with increasing poling field. In optimally poled crystals, d33 can reach as high as 3100pC∕N.

Relaxor Behavior in Ba(Ti0.72Sn0.28)O3 Solid Solution

Dielectric properties of the Ba(Ti0.72Sn0.28)O3 solid solution ceramics are studied as a function of frequency (0.01 Hz–100 kHz) in the temperature range of 300°C to −150°C. A broad dielectric peak with strong frequency dispersion is observed. The temperatures T m of real and imaginary dielectric peaks fit well to the Vogel-Fulcher law. Both the high-temperature and low-temperature slopes of the real permittivity peak can be described by a Lorenz-type formula. The dielectric hysteresis loops are displayed at low temperatures as well as at temperatures significantly higher than T m . The Curie-Weiss law is observed at temperatures much higher than T m .

Dielectric behavior of Ba(Ti1-xZrx)O3 solid solution

The dielectric permittivity as a function of temperature at frequencies from 10−2 to 105 Hz is studied and parameterized in the perovskite Ba(Ti0.675Zr0.325)O3 ceramics. The features characteristic of relaxor ferroelectrics are found, namely, the Lorenz-type temperature dependence at high-temperature slope of the real permittivity peak, the Curie-Weiss law observed at temperatures much higher than the temperature of the peak, the Vogel–Fulcher scaling of the temperature/frequency dependencies of the permittivity. However, scaling characteristics differ from those in classical complex perovskite relaxors suggesting different origin of dielectric relaxation.

Preparation of KNbO3 Nanoparticles at Room Temperature

Using a simple solution reaction, nanoparticles of potassium niobate KNbO3 were synthesized at room temperature. Precursor solutions and as-prepared samples were characterized by UV-Vis and Nb K-edge X-ray Absorption Fine Structure (XAFS) spectroscopy. Both data showed that the niobate nanosheets consist of distorted Nb-O polyhedra with a low coordination number.