**1. Introduction**

The origin of ferroelectricity is linked with Rochelle salt and discovered by Joseph Valasek (1897–1993) during the measurement of polarization in Rochelle salt by the applied electric field in 1920 [1]. The fundamental origin of ferroelectricity lies in the response of order parameter (electric dipole) with respect to the applied electric field. However, the field of ferroelectricity remained silent till 1940 [1]. One of the major turning points in ferroelectricity came into the picture around 1940 after the discovery of unusual behavior in dielectric properties of mixed oxides, which are crystallized to perovskite structure [1]. After that, ferroelectricity was intensively focused by the scientific community in all over the world. The perovskite compounds exhibit the ABX3 structure, where A and B are the cations having different charges and ionic radii. "X" refers to an anion that bonded with both cations. In general, X is often Oxygen (O2−) but also other ions such as sulfides, nitrides, and halides can be considered [2]. However, the perovskite compounds with the general formula ABO3 create a distinguish place in ferroelectricity compared to others [2]. Currently different groups of perovskite compounds available in the market such as A2BO4-layered perovskites (ex: Sr2RuO4, K2NiF4), A2BB'O6-double perovskites (ex: Ba2TiRuO6) and A2A'B2B'O9-triple perovskite (ex: La2SrCo2FeO9), etc. [2]. Hence, ferroelectrics are fascinating groups of materials, which have extensively attracted the fundamental understanding of its complex physical properties and emerging device applications (i.e., sensors, actuators, ferroelectric random access memories,

etc.) [3]. The presence of spontaneous polarization (*P*) in ferroelectric material exhibits unique behavior in the presence of external stimuli such as electric field (*E*), temperature (*T*), and stress (*σ*). Nowadays, large numbers of reports are available on the ferroelectric properties of various kinds of materials and their possible applications in modern technologies [3]. Out of the several compounds, Lead Zirconate Titanate (PbZr0.48Ti0.52O3/PZT) is one of the most well-known ferroelectric materials with superior ferroelectric, dielectric, and piezoelectric properties [4]. For a layman's understanding, ferroelectric materials are those which exhibit a high dielectric constant. Furthermore, normal ferroelectrics are characterized by the temperaturedependent maximum dielectric constant near ferroelectric to paraelectric phase transition temperature (*T*c) and, also known as Curie's temperature [4]. Some of the basic characteristics of normal ferroelectric are the non-dispersive nature of transition temperature and follow the first-order phase transition. The temperaturedependent dielectric constant of a typical regular ferroelectric ceramic is shown in **Figure 1** [5]. Also, it is well known that the fundamental physics behind the normal ferroelectric lies in the presence of long-range order parameters (electric dipoles) in ferroelectric domains [6].

As per the literature, PZT exhibits the normal ferroelectric in nature with the above-mentioned properties and, well understood experimentally as well as theoretically. However, the spatial substitution of a foreign element such as lanthanum (La3+) in PZT shows the intriguing behavior in terms of dielectric, ferroelectric and piezoelectric properties with respect to frequency, temperature, and electric field, which are different from the normal ferroelectric [7, 8]. The extraordinary properties of modified PZT are related to a special group of ferroelectric materials, which is known as relaxor ferroelectric after the name by the scientist Cross in 1987 [9]. He proposed few characteristic properties for the material to be relaxor ferroelectric, as discussed in the later section. Currently, large numbers of materials with relaxor ferroelectric behavior are available in various forms of crystal structures such as perovskite, layer perovskite, tungsten bronze structure, etc. However, the exact origin of extraordinary properties of relaxor ferroelectrics is still a matter of investigation. A series of explanations have been reported to explain the origin of relaxor behavior by using various models such as

#### **Figure 1.**

*Temperature-dependent dielectric constant and loss of a typical normal ferroelectric ceramic (PZT ceramic). It is adapted from Ref. [5] (open access).*

**51**

*Relaxor Ferroelectric Oxides: Concept to Applications DOI: http://dx.doi.org/10.5772/intechopen.96185*

been discussed briefly in the present chapter.

assigned few basic properties as follows [9].

reduction of remanent polarization.

strong frequency dependence.

gions (PNRs).

mum dielectric temperature (*T*m).

the Curie temperature for normal ferroelectric.

electrics at above the Curie temperature (*T*c).

physical properties, as discussed below.

1.The temperature-dependent dielectric permittivity (

up to a certain extend [10].

**2. Relaxor ferroelectrics**

diffuse phase transition model, dipolar glass model, random field model, super paraelectric model, and so on [10]. In contrast to normal ferroelectrics, the origin of relaxor ferroelectric has been correlated with the presence of compositional fluctuation induced polar nanoregions (PNRs) and tried to explain its properties

Due to its interesting physical properties, relaxor ferroelectrics can have possible applications in portable electronics, medical devices, pulse power devices, electric vehicles, advanced storage materials, and so on. Therefore, the different aspects from its origin to possible technological applications for future advancement have

From the earlier discussion (introduction section), the relaxor ferroelectrics show the abnormal behavior as compared to normal ferroelectric in terms of dielectric, piezoelectric, ferroelectric properties and, subsequently, received much attention by the scientific community. To distinguish a relaxor ferroelectric, Cross

smeared maximum, which is known as a diffuse phase transition.

2.The depolarization (*T*d) is defined as temperature corresponds to the steepest

3.The temperatures (*T*m) correspond to maximum dielectric permittivity exhibit

4.There is no macroscopic symmetry breaking (structural changes) near-maxi-

5.The Curie–Weiss (C-W) law does not follow the temperature-dependent dielectric permittivity near *T*m (dielectric maxima temperature). However, Curie–Weiss law is well fitted above the *T*C for normal ferroelectric. Here, *T*c is

6.Exhibits slim/constricted ferroelectric hysteresis loop due to the presence of nanosize and randomly oriented polar islands known as polar nanore-

7.Existence of polar nano regions at well above the dielectric maxima temperature '*T*m' up to Burn temperature (*T*B), whereas absences in the normal ferro-

The temperature-dependent dielectric permittivity of a typical relaxor ferroelectric has been shown in **Figure 2** to visualize the different characteristic temperatures. As per the earlier reports, the interesting properties of relaxor ferroelectric are basically due to the presence of unique polar structure in nanometer size (polar nanoregions: PNRs) along with their response towards the external stimuli [11]. Therefore, it is necessary to understand the origin of PNRs and their effects on the

ε

′ ) exhibits a broad and

*Relaxor Ferroelectric Oxides: Concept to Applications DOI: http://dx.doi.org/10.5772/intechopen.96185*

diffuse phase transition model, dipolar glass model, random field model, super paraelectric model, and so on [10]. In contrast to normal ferroelectrics, the origin of relaxor ferroelectric has been correlated with the presence of compositional fluctuation induced polar nanoregions (PNRs) and tried to explain its properties up to a certain extend [10].

Due to its interesting physical properties, relaxor ferroelectrics can have possible applications in portable electronics, medical devices, pulse power devices, electric vehicles, advanced storage materials, and so on. Therefore, the different aspects from its origin to possible technological applications for future advancement have been discussed briefly in the present chapter.
