Dielectric Relaxation in Relaxor Ferroelectrics

Abstract

In this review the dielectric properties of relaxor ferroelectrics are discussed and compared with the properties of normal dielectrics and ferroelectrics. We try to draw a general picture of dielectric relaxation starting from a textbook review of the underlying concepts and pay attention to common behavior of relaxors rather than to the features observed in specific materials. We hope that this general approach is beneficial to those physicists, chemists, material scientists and device engineers who deal with relaxors. Based on the analysis of dielectric properties, a comprehensive definition of relaxors is proposed: relaxors are defined as ferroelectrics in which the maximum in the temperature dependence of static susceptibility occurs within the temperature range of dielectric relaxation, but does not coincide with the temperature of singularity of relaxation time or soft mode frequency.

Effect of cobalt doping on the dielectric response of Ba0.95Pb0.05TiO3 ceramics

“…The phase transition from the rhombohedral to the orthorhombic structure apparent as ε″(f, T) anomaly at~160 K for Ba 0.95 Pb 0.05 TiO 3 disappears for cobalt doped ceramic and in the same temperature range relaxor-like relaxation processes are observed. In the case of BPTC(0.1 %) samples the maximum values of the imaginary part of dielectric permittivity ε″ max decrease with increasing frequency in the window 100 Hz≤f ≤6 kHz whereas at higher frequencies an increase in ε″ max is observed and at 100 kHz its maximum appears at T ε″max ≈203 K. According to the theory of the dielectric response of ferroelectric relaxors [49,50], which considers the behavior of ε′ max and ε″ max with increasing frequency, i) in the case when ε′ max decreases and ε″ max increases with increasing frequency the main contribution to the dielectric response is due to flipping of the dipole moments between allowed directions in the polar nanoregions (PNRs), ii) whereas a decrease in ε′ max and ε″ max with increasing frequency is related to the contribution of fluctuations of the boundaries of PNRs. Thus we would like to relate the dielectric response observed in the frequency range 100 Hz-6 kHz to be due mainly to the contribution of fluctuations of PNR boundaries, whereas that apparent in the frequency range 10 kHz-100 kHz to the response of the dipole moments in the dynamic polar nanoregions.…”

“…The activation energy of free oxygen vacancy migration in ferroelectric perovskite oxides has been reported to be of 0.8-1 eV [51,52] thus we can relate the high-temperature electric conduction determined for BPT and BPTC(1 %) ceramics to the process of oxygen vacancies migration. As the distribution of the oxygen vacancies in the lattice has been considered in the tetragonal phase of the ferroelectric perovskites only [26,50,52] it is hard to discuss the lowering of the activation energy for BPTC(0.1 %) since the energy landscape of oxygen and probably also Pb vacancies is uncertain.…”

Electrocaloric Cooling - A New Application of Relaxor Ferroelectrics

“…Relaxors are a class of disordered crystals possessing peculiar structure and properties [13]. They exhibit frequency dispersion, i.e., the temperature T m of the dielectric constant maximum increases with increase of frequency.…”

“…The huge dielectric response and the existence of a maximum of ε(T ) is attributed to the peculiarities of the PNR behavior. Above the temperature of maximum dielectric permittivity up to a temperature T * where static PNRs disappear [14], relaxors provide a large and reversible polarization change due to contributions to polarization not present in ordinary ferroelectrics [13]. This is just required for a large EC response.…”

“…This is just required for a large EC response. In this temperature region, the dielectric response is dominated by the static dielectric susceptibility which is given by the empirical formula [13] …”