Dielectric Response Of Relaxor Ferroelectrics Research Articles

Nonlinear dielectric susceptibility in relaxor ferroelectrics Pb(Zn1/3Nb2/3)O3-PbTiO3

ABSTRACTThe temperature dependence of the third-order nonlinear susceptibility ϵ3 in relaxor ferroelectrics (1-x)Pb(Zn1/3Nb2/3)O3-8%PbTiO3 was measured in the vicinity of critical endpoint, TCEP (172°C). It was found that ϵ3 takes positive values near TCEP and changes its sign to negative values at about 200°C. The nonlinear dielectric response of relaxor ferroelectrics was discussed on the basis of the Landau-Ginzburg-Devonshire free energy density.

Non-linear dielectric properties in based-PMN relaxor ferroelectrics

The investigation of the non-linear dielectric response in ferroelectric materials has become one of the most important issues in the field of ferroelectricity due to its technological and scientific interest. Rather, from the practical point of view the understanding of the non-linear dielectric properties is essential to improve the performance of ferroelectric multilayer capacitors and actuators devices, which commonly operate at high field levels. On the other hand, the non-linear (NL) dielectric response to large electric fields has been revealed as a powerful technique to investigate the physical origin of the dielectric relaxor state. In this work, low frequency dielectric measurements were performed in 0.9[Pb(Mg 1/3Nb 3/2)O 3]–0.1PbTiO 3 (PMN–PT) relaxor ceramics in a wide frequency and temperature range. The non-linear dielectric properties were investigated by using the measurements of the dielectric permittivity of the PMN–PT as a function of the dc “bias” driving field. The obtained results were analyzed within the framework of the current models for the dielectric response of relaxor ferroelectrics.

Infrared dielectric response of relaxor ferroelectrics

In the article we discuss the dielectric response of relaxor-type complex perovskites in the phonon-frequency region on the basis of a rich collection of experimental data accumulated in our laboratory by numerous infrared spectroscopy investigations over the past decade. The effect of the cation occupational disorder and of the nanoscopic inhomogeneous polarisation on the infrared response is considered in the framework of the factor-group symmetry analysis and the effective-medium approach. Polar mode assignment and comparison of the mode parameters of different materials is facilitated by systematic evaluation of mode-plasma frequencies. Whenever possible, the results are presented in form of tables convenient for further analysis or by comparison with results of other techniques, including ab-initio calculations. Infrared properties of other non-perovskite relaxors are briefly mentioned.

Low-Temperature Dielectric Response of Relaxor Ferroelectrics and Related Disordered Materials

This paper reviews the characteristic dielectric dispersion of relaxor ferroelectrics PbMg⅓Nb2/3O3, and (Pb1- x La x )(Zr y Ti1-y )1-x/4O3 and ferroelectric BiScO3-PbTiO3 solid solutions in the frequency range of 100 Hz–2 THz. In relaxors the main dielectric dispersion at high temperatures appears in the microwave and mm-wave region, which softens and broadens on cooling. Distribution of relaxation times below room temperature is very broad and below freezing temperature it results in nearly frequency independent dielectric loss in the entire available frequency range. The broad distribution of relaxation times stems from structural inhomogeneities which cause random fields and have influence on the distribution of activation energies for hopping of disordered ions. Landau-type thermodynamic model was proposed for explanation of the distribution of relaxation times. Besides the frequency independent dielectric loss, additional dielectric relaxation of not thermally activated origin was observed in BiScO3-64%PbTiO3 below 1 GHz.

Dielectric response of relaxor ferroelectrics in the time-domain

Dielectric response of relaxor ferroelectrics was analyzed in the time-domain by evaluating the relaxation-time (τ) distribution of Pb(Mg1/3Nb2/3)O3-10 mol % PbTiO3 using the method of Tikhonov regularization. The distribution function g(τ) became exponentially broad and flat as temperature approached a certain critical point (∼230 K), indicating the advent of the frozen polar domains with infinitely long τ in a matrix of the thermally activated dynamic polar domains. The dielectric response of the high-temperature relaxation part was then separately estimated with the help of the two-states-polarization model. The estimated g(τ) of the relaxation part showed logarithmic Gaussian behavior, which manifested an ergodicity in the thermally activated flips of dynamic polarizations at high temperatures. In contrast to this, a resonance kernel was postulated to describe the dielectric response of the frozen polar domains at lower temperatures.

Effect of defect-induced internal field on the aging of relaxors

The effect of a defect-induced internal field on the dielectric response of relaxor ferroelectrics is investigated using a Monte Carlo simulation. It was observed that only at a small temperature range near the temperature of the dielectric maximum does the susceptibility decrease markedly due to the internal field. This temperature range increases with enhancing internal field. We found that the susceptibility is almost independent of the internal field width at low internal field width, and then decreases linearly with enhancing internal field width. This dependence of the susceptibility on the internal field width is very similar to the relation of the dielectric constant with logarithmic aging time, which probably suggests a linear dependence of the internal field on the logarithm of the aging time. The frequency dependence of the susceptibility aging is sensitive to the temperature. With increasing temperature, the curve of the susceptibility change against logarithmic frequency varies from concave to approximately linear, and then to convex, which is in agreement with the recent aging rate measurement.

Optical Properties and Model of Relaxor Ferroelectrics

Phenomenological approach to a relaxor theory is developed and some experi-mental results in the support of this theory are received. The optical absorption spectrum, luminescent emission and photoconductivity are investigated in the wide temperature region in PbMg 1/3 Nb 2/3 O 3 . On the basis of this data the density of states on the impurity band and the velocity of charge carrier localization are found. The probability distribution of relaxation time, g (ln( ))dln( ), is found analytically as the function of the density of states in the impurity polaron band. The temperature dependences of dielectric response of relaxor ferroelectrics are described in the framework of the Landau-Ginzburg-Devonshire theory of phase transitions. The properties of local centers and polaron localization dynamics are studied.

Charge Localization in the Model of Relaxor Ferroelectrics

Phenomenological approach to a relaxor theory is developed and some experimental results in the support of this theory are received. The optical absorption spectrum, luminescent emission and photoconductivity are investigated in the wide temperature region in PbMg1/3Nb2/3O3. On the basis of this data the density of states on the impurity band and the velocity of charge carrier localization are found. The probability distribution of relaxation time, g(ln(τ))dln(τ), is found analytically as the function of the density of states in the impurity polaron band. The temperature dependences of dielectric response of relaxor ferroelectrics are described in the framework of the Landau-Ginzburg-Devonshire theory of phase transitions. We show that an additional condition for the relaxor behavior to appear is the coincidence of the phase transition temperature with the temperature region of thermo-localization of charges on the defects. The properties of local centers and polaron localization dynamics are studied.

Localized Charges and Model of Relaxor Ferroelectrics

Phenomenological approach to a relaxor theory is developed and some experimental results in the support of this theory are received. The optical absorption spectrum, luminescent emission and photoconductivity are investigated in the wide temperature region in PbM g 1/3 Nb 2/3 O 3 . On the basis of this data the density of states on the impurity band and the velocity of charge carrier localization are found. The probability distribution of relaxation time, g (ln( )) d ln( ), is found analytically as the function of the density of states in the impurity polaron band. The temperature dependences of dielectric response of relaxor ferroelectrics are described in the framework of the Landau-Ginzburg-Devonshire theory of phase transitions. The properties of local centers and polaron localization dynamics are studied.

Nonlinear dielectric response of relaxor ferroelectrics

This fact neces-sitates a search for alternative mechanisms of nonlinear re-sponse not studied so far, and calls for a refinement andgeneralization of the static and dynamic SRBRF model.The mechanism of nonlinear response originally proposedwithin the SRBRF model in Ref. 5 ~to referred to here asmechanism I! has been derived under the assumption thatpolar clusters are embedded in a rigid lattice; the nonlinearityeffects are thus entirely due to the intrinsic nonlinearity ofthe rigid SRBRF model. A possible alternative mechanism,characteristic of the generalized SRBRF model and exploredin the present work, is based on the fact that polar clusterscan be displaced from their equilibrium positions by the ap-plied electric field; this then leads to the field modulation ofthe intercluster coupling ~mechanism II!. A similar idea wasused in connection with the field-temperature (E,T) phasediagram of PLZT,

Phenomenological model of dynamic nonlinear response of relaxor ferroelectrics

A phenomenological model was proposed which describes frequency dispersion of nonlinear dielectric response of relaxor ferroelectrics (relaxors) as a result of dispersion of their linear dielectric permittivity. The model was applied to Pb(Mg(1/3)Nb(2/3))O3 (PMN) relaxor. It provided a good qualitative description of temperature and frequency dependence of the third harmonics of PMN. Analysis within the model yielded a frequency independent nonlinear coefficient corresponding to static nonlinear dielectric susceptibility. The model explained the recently reported for PMN data on the Vogel-Fulcher law for frequency dependence of the temperature at which the third harmonics passes a maximum.

Nonlinear dielectric response of relaxor ferroelectrics

The calculations of nonlinear dielectric susceptibility on external dc electric field is performed in the random field theory framework. The equations which describe the dependence of order parameter and dielectric susceptibility on temperature, dc electric field, frequency, parameters of host lattice and random field sources are obtained. The numerical solutions of these equations for several random field sources concentrations and parameters have shown that nonlinear dielectric susceptibility at low frequencies smaller than linear one in dipole glass state, meanwhile in mixed ferroglass phase nonlinear part can be both negative and positive. No critical divergency of nonlinear susceptibility at freezing temperature was obtained in accordance with experiment. This proves that contrary to conventional spin glasses dipole glasses in relaxors are methastable states with long (up to infinity) relaxation times. The comparison of calculated and observed nonlinear susceptibility is carried out.

A new framework for understanding relaxor ferroelectrics

A new theoretical framework is presented for understanding the dielectric response of relaxor ferroelectrics. The theory is developed by reference to the dielectric response of lead magnesium niobate, a well-known relaxor material. The existence is proposed of a ferroelectric network linking niobiumoxygen coordination octahedra, where the temperature at which a given octahedron becomes decoupled from this network depends on its local coordination by other niobium-oxygen octahedra. p p ]This dependence permits the definition of a Curie point distribution function, which expresses the number of octahedra being decoupled from the network at a given temperature, as a function of temperature. The real part of the permittivity, ϵ r ', is simulated by convoluting the Curie point distribution function with a simple dielectric response function, φ E ( T), and an explanation is given for the frequency dependence of ϵ r '. The imaginary part of the permittivity, ϵ r '', is interpreted by regarding the decoupling of NbO 6 octahedra from the ferroelectric network as a relaxation process. The role of ferroelectric microregions and microregion walls is discussed, and the contribution of the theoretical framework to the future development and exploitation of relaxor materials is assessed.