Hong Gui

Dielectric response process in relaxor ferroelectrics

The dielectric constant e*(ω,T)=e′(ω,T)+ie″(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.

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.

Spontaneous polarization in lead magnesium niobate relaxor

Abstract A theoretical model is proposed to calculate the spontaneous polarization in PMN relaxor. It is supposed that the Nb ions in the ordered microregions displace cooperatively due to the structure homogeneity, giving rise to the polarization below Td (600 K). The results agree with the polarization deduced from the electro-optic and electrostrictive effects. The ordered microregions have been proved to be the locations of the polar clusters at Tf ≪ T < Td , where Tf is the freezing temperature (near 210 K). The spontaneous polarization process in the ordered microregions is suggested to be similar to that of normal ferroelectrics. The dipole glass model of relaxors is discussed.

Possible order - disorder phase transitions in and complex perovskites

The stabilities of different kinds of ordered structure in complex perovskites have been studied by the eight-point cluster variation method (CVM). The grand potentials of these structures have been calculated and compared with that of the disordered state. It has been proved that the -type ordered structure will be stable in the system when the nearest-neighbour interaction is much stronger than other longer-distance interactions. The - and -type ordered structures result from the next-nearest- and the third-neighbour interactions respectively. The grand potentials of the - and -type ordered structures reach minima at the composition x = 0.5, whereas the grand potential of one of the -type ordered structures reaches its minima at around x = 0.25 and x = 0.75. The formation of different kinds of ordered structure in real material systems has been discussed.

A-site cation ordering and the glassy polarization behavior of lanthanum-modified lead zirconate titanate

The microstructure and dielectric behavior of lanthanum-modified lead zirconate titanate with 9 at.% lanthanum and a Zr/Ti ratio of 65/35 (PLZT 9/65/35) were investigated. By analyzing the frequency dependence of the temperature of the dielectric maximum (Tm) using the Volgel-Fulcher relationship, an activation energy and freezing temperature of 0.0393 ev and 347.4 K respectively, were obtained, showing that the polarization behavior of the sample bears resemblance to spin and dipolar glasses. Both selected-area diffraction (SAD) and high-resolution electron microscopy (HREM) reveal that {h+1/2, k+1/2, 0}-type superlattice exists in the sample. The lattice images of the ordered and the disordered regions were shown directly by HREM. The size of the ordered domains is from 10-20 nm. An ordered model with body-centered cubic superstructure on A-site cations was proposed. Based on the A-site cations ordering, the dielectric polarization behavior in different temperature range was discussed.

Phase transition of ergodic space shrinking in succession and relaxor ferroelectrics

Abstract A special phase transition, the phase transition of ergodic space shrinking in succession (abbreviated as ES3 phase transition), is proposed according to the freezing process of the ordered microregion dipoles in relaxors. Its features are discussed. The special characteristics of relaxors are explained from the viewpoint of the ES3 phase transition.

Possible order–disorder phase diagrams in (Ax′A1−x″)BO3 and A(Bx′B1−x″)O3 complex perovskites

The structural phase diagrams of complex perovskites have been studied within the mean-field approximation. Various phase diagrams with possible topological structures of the 1/2{111}-, 1/2{110}-, and 1/2{100}-type ordered structures are obtained. The 1/2{111}- and 1/2{100}-type ordered structures have similar phase diagrams and their ordering transitions are both of the second kind, while the 1/2{110}-type ordered structure has a more complex phase diagram and its ordering transition is of the first kind. In the two-phase region of the phase diagram, the ordered phase’s composition deviates from that of the chemical formula towards the completely ordering composition which is 0.5 for the 1/2{111}-, 1/2{100}-type ordered structures and 0.25, 0.50, 0.75 for the 1/2{110}-type ordered structures.

Dielectric mechanism in complex perovskite

After analyzing the characteristics of interactions between the polar microregions in the complex perovskite relaxor ferroelectrics, a physical model is built up in which the effective interaction energy parameters have a Gaussian distribution. The dielectric mechanism of the materials is studied by using the Monte Carlo computer simulation. The concepts of “frozen dipole” and “slow dipole” are proposed according to the distribution of relaxation time of dipole flipping, the diffuse phase transition (DPT), frequency dispersion and polarization behavior, and other relaxor properties are explained systematically. It is proposed that the relaxor characteristics are caused by the ergodic space shrinking in succession, which arises from the inhomogeneous crystal structure of this kind of material.

Theoretical study of polarization behaviors in complex perovskite-type relaxor ferroelectrics

Abstract Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China The polarization behaviors of complex perovskite-type relaxor ferroelectrics are simulated by Monte Carlo method. The results show that the remanent polarization collapses near the freezing temperature Tf , at which the dipoles begin to be frozen over the observation time of simulation. The polarizations of slow heating process show strong dependence on the initial state. The slow cooling processes are not influenced by the initial state when cooling from T ≫ Tf . But if a poled system is cooled from T < Tf , the remanent polarization corresponding to the starting temperature remains unchanged through the process. It is proposed that the polarization behaviors are caused by a successive bifurcation of components in phase space of relaxors.