**3. Materials aspects**

Currently, there are several classes of relaxor ferroelectrics available based on different compositions. However, the existing relaxor ferroelectrics are further modified with suitable foreign elements to tune the physical properties. On the basis of material aspect, the relaxor ferroelectrics are classified into two categories as lead-based and lead-free relaxor ferroelectrics. In the present scenario, the leadbased RFEs are dominated extensively on the commercial market due to their superior dielectric, ferroelectric and piezoelectric properties. However, the restriction of lead-based compounds due to their environmental issue (toxic in nature) leads to an increase in the demand for lead-free relaxor ferroelectrics. Therefore, lead-based and lead-free relaxor ferroelectrics are briefly discussed [38, 39]. In addition, the concept of morphotropic phase boundary has been discussed to understand the formation of various solid solutions.

#### **3.1 Concept of morphotropic phase boundary**

This term is very common in the field of complex ferroelectric solid solutions. Basically, the term 'morphotropic' referred to the crystal phase transition due to the change in the composition and which is different from the polymorphic phase boundary (PPB) [40]. PPB is generally referred to as a change in phase transition with respect to the temperature. Morphotropic phase boundary represents the phase transition between rhombohedral and tetragonal ferroelectric phases with respect to the variation of composition or pressure. In the vicinity of MPB, the materials exhibit maximum dielectric and piezoelectric properties due to the presence of structural inhomogeneity [41]. The most common and well-known ferroelectric material, i.e., Lead Zirconate Titanate [Pb(Zr0.52Ti0.48)TiO3/PZT] lies near morphotropic phase boundary (MPB), as shown in **Figure 7**. Basically, PZT solid solution is the competing of coexistence phases, i.e., PbZrO3 with rhombohedral symmetry (*R3c*) and PbTiO3 with tetragonal symmetry (*P4mm*) as shown in **Figure 7**. However, the origin of extraordinary physical properties near MPB is still a matter of debate. In recent years, researchers have been drawn attention towards preparing solid solutions near MPB of different ferroelectric oxides [41]. The MPB composition of PZT is associated with the 14 possible polarization axis (i.e., 6 from tetragonal and 8 from the rhombohedral), which leads to reduce the energy barrier (i.e., Landau free energy) of the crystal system and, subsequently enhanced the piezoelectric, dielectric, and ferroelectric properties. The dielectric, polarizability, and electromechanical coefficients can be further increased by modifying MPB composition with different suitable acceptors or donor dopants [41].

#### **3.2 Lead-based relaxor ferroelectrics**

The most widely used ferroelectric materials for the different applications are derived from the solid solution of (x) PbTiO3-(1-x) PbZrO3. The lead-based

**59**

acceptor types (K<sup>+</sup>

**Figure 7.**

*[41] (open access).*

, Na<sup>+</sup>

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

solid solutions exhibit different characteristics properties ranging from normal ferroelectric-relaxor- antiferroelectric. Therefore, the lead-based relaxor ferroelectrics are quite important for the modern device industries. To date, the PZT is considered a vastly used piezoelectric material. The relaxor properties in PZT have been successfully developed by incorporating foreign elements in A/B sites with isovalent and aliovalent dopants (donor and acceptor dopants). Some of the dopants which have been used in the PZT crystal system are as follows: Isovalent types (Ba2+, Sr2+ for Pb2+ site & Sn4+ for Zr4+ and Ti4+ sites), Donor types (La3+, Nd3+, Sb3+ for Pb2+ sites & Nb5+, Ta5+, Sb5+, W6+ for Zr4+ and Ti4+ sites) and

*Lead Zirconate Titanate (PZT) solid solution near morphotropic phase boundary (MPB) composition between PbZrO3 with rhombohedral symmetry (R3c) and PbTiO3 tetragonal symmetry (P4mm). Adapted from ref.* 

sites). Other than PZT, the lead-based relaxor ferroelectrics are Pb(Mg1/3Nb2/3) O3(PMN) or Pb(Sc1/2Ta1/2)O3(PST), Pb1-*x*Lax(Zr1-*y*Tiy)1-x*/*4O3(PLZT),Pb(Zn1*/* 3Nb2*/*3)O3(PZN) Pb(Mg1*/*3Ta2*/*3)O3(PMT), Pb(Sc1*/*2Nb1*/*2)O3(PSN),Pb(In1*/*2Nb1*/*2)

O3(PIN), Pb(Fe1*/*2Nb1*/*2)O3(PFN), Pb(Fe2*/*3W1*/*3)O3(PFW) and the solid solutions:(1-*x*)Pb(Mg1*/*3Nb2*/*3)O3-*x*PbTiO3(PMN-PT) and (1-*x*)Pb(Zn1*/*3Nb2*/*3) O3-*x*PbTiO3 (PZN-PT) [42–45]. Therefore, few recent experimental results of lead based ferroelectrics have been explained here to understand the formation and dynamic of PNRs. As per the literature, (1-x) [Pb(Mg1/3Nb2/3)O3]-(x) [PbTiO3] solid solution exhibit normal ferroelectric properties near MPB (x = 0.30 to 0.35) [46]. Also, the structural fluctuation from rhombohedral to tetragonal through intermediate phases (i.e. monoclinic/orthorhombic/triclinic) has been observed with respect to x vary from 0.13 to 0.30. The relaxor properties in above solid solution have been developed (refer **Figures 5** and 7 [46]) with substitution of optimum mole fraction (4%) of Sr2+ in place of Pb2+. The formation of compositional fluctuation leads to form short-range order PNRs and, subsequently, reduces the remanent polarization and coercive field and the appearance of diffuse phase transition. The presence of PNRs significantly enhanced the dispersive nature of dielectric permittivity with frequency [46].

for Pb2+ & Fe3+, Al3+, Sc3+, In3+, Cr3+ for Zr4+ and Ti4+

**Figure 7.**

*Multifunctional Ferroelectric Materials*

**3. Materials aspects**

formation of various solid solutions.

**3.2 Lead-based relaxor ferroelectrics**

**3.1 Concept of morphotropic phase boundary**

(*T*m*-T*A) and dielectric constant ( ) *r A*

frequency. Hence, the DPT behavior of RFEs represents in terms of temperature

for Ba (Ti0.8Sn0.2)O3 relaxor ferroelectric [37]. Similarly, Lorentz type relationship in temperature-dependent dielectric permittivity in Ba (ZrxTi1-x)O3 solid solutions, PbMg1/3Nb2/3O3 relaxor with diffuse phase transition has been reported by S. Ke *et* 

Currently, there are several classes of relaxor ferroelectrics available based on different compositions. However, the existing relaxor ferroelectrics are further modified with suitable foreign elements to tune the physical properties. On the basis of material aspect, the relaxor ferroelectrics are classified into two categories as lead-based and lead-free relaxor ferroelectrics. In the present scenario, the leadbased RFEs are dominated extensively on the commercial market due to their superior dielectric, ferroelectric and piezoelectric properties. However, the restriction of lead-based compounds due to their environmental issue (toxic in nature) leads to an increase in the demand for lead-free relaxor ferroelectrics. Therefore, lead-based and lead-free relaxor ferroelectrics are briefly discussed [38, 39]. In addition, the concept of morphotropic phase boundary has been discussed to understand the

This term is very common in the field of complex ferroelectric solid solutions. Basically, the term 'morphotropic' referred to the crystal phase transition due to the change in the composition and which is different from the polymorphic phase boundary (PPB) [40]. PPB is generally referred to as a change in phase transition with respect to the temperature. Morphotropic phase boundary represents the phase transition between rhombohedral and tetragonal ferroelectric phases with respect to the variation of composition or pressure. In the vicinity of MPB, the materials exhibit maximum dielectric and piezoelectric properties due to the presence of structural inhomogeneity [41]. The most common and well-known ferroelectric material, i.e., Lead Zirconate Titanate [Pb(Zr0.52Ti0.48)TiO3/PZT] lies near morphotropic phase boundary (MPB), as shown in **Figure 7**. Basically, PZT solid solution is the competing of coexistence phases, i.e., PbZrO3 with rhombohedral symmetry (*R3c*) and PbTiO3 with tetragonal symmetry (*P4mm*) as shown in **Figure 7**. However, the origin of extraordinary physical properties near MPB is still a matter of debate. In recent years, researchers have been drawn attention towards preparing solid solutions near MPB of different ferroelectric oxides [41]. The MPB composition of PZT is associated with the 14 possible polarization axis (i.e., 6 from tetragonal and 8 from the rhombohedral), which leads to reduce the energy barrier (i.e., Landau free energy) of the crystal system and, subsequently enhanced the piezoelectric, dielectric, and ferroelectric properties. The dielectric, polarizability, and electromechanical coefficients can be further increased by modifying MPB

composition with different suitable acceptors or donor dopants [41].

The most widely used ferroelectric materials for the different applications are derived from the solid solution of (x) PbTiO3-(1-x) PbZrO3. The lead-based

− . Lei *et al*. have reported that the Lorentz

ε

*<sup>r</sup>* ~ *T* curve

ε ε

formula is well fitted in both lower and higher temperature regions in

*al*. A typical plot of Lorentz formula is shown in **Figure 6**.

**58**

*Lead Zirconate Titanate (PZT) solid solution near morphotropic phase boundary (MPB) composition between PbZrO3 with rhombohedral symmetry (R3c) and PbTiO3 tetragonal symmetry (P4mm). Adapted from ref. [41] (open access).*

solid solutions exhibit different characteristics properties ranging from normal ferroelectric-relaxor- antiferroelectric. Therefore, the lead-based relaxor ferroelectrics are quite important for the modern device industries. To date, the PZT is considered a vastly used piezoelectric material. The relaxor properties in PZT have been successfully developed by incorporating foreign elements in A/B sites with isovalent and aliovalent dopants (donor and acceptor dopants). Some of the dopants which have been used in the PZT crystal system are as follows: Isovalent types (Ba2+, Sr2+ for Pb2+ site & Sn4+ for Zr4+ and Ti4+ sites), Donor types (La3+, Nd3+, Sb3+ for Pb2+ sites & Nb5+, Ta5+, Sb5+, W6+ for Zr4+ and Ti4+ sites) and acceptor types (K<sup>+</sup> , Na<sup>+</sup> for Pb2+ & Fe3+, Al3+, Sc3+, In3+, Cr3+ for Zr4+ and Ti4+ sites). Other than PZT, the lead-based relaxor ferroelectrics are Pb(Mg1/3Nb2/3) O3(PMN) or Pb(Sc1/2Ta1/2)O3(PST), Pb1-*x*Lax(Zr1-*y*Tiy)1-x*/*4O3(PLZT),Pb(Zn1*/* 3Nb2*/*3)O3(PZN) Pb(Mg1*/*3Ta2*/*3)O3(PMT), Pb(Sc1*/*2Nb1*/*2)O3(PSN),Pb(In1*/*2Nb1*/*2) O3(PIN), Pb(Fe1*/*2Nb1*/*2)O3(PFN), Pb(Fe2*/*3W1*/*3)O3(PFW) and the solid solutions:(1-*x*)Pb(Mg1*/*3Nb2*/*3)O3-*x*PbTiO3(PMN-PT) and (1-*x*)Pb(Zn1*/*3Nb2*/*3) O3-*x*PbTiO3 (PZN-PT) [42–45]. Therefore, few recent experimental results of lead based ferroelectrics have been explained here to understand the formation and dynamic of PNRs. As per the literature, (1-x) [Pb(Mg1/3Nb2/3)O3]-(x) [PbTiO3] solid solution exhibit normal ferroelectric properties near MPB (x = 0.30 to 0.35) [46]. Also, the structural fluctuation from rhombohedral to tetragonal through intermediate phases (i.e. monoclinic/orthorhombic/triclinic) has been observed with respect to x vary from 0.13 to 0.30. The relaxor properties in above solid solution have been developed (refer **Figures 5** and 7 [46]) with substitution of optimum mole fraction (4%) of Sr2+ in place of Pb2+. The formation of compositional fluctuation leads to form short-range order PNRs and, subsequently, reduces the remanent polarization and coercive field and the appearance of diffuse phase transition. The presence of PNRs significantly enhanced the dispersive nature of dielectric permittivity with frequency [46].

#### **3.3 Lead-free relaxor ferroelectrics**

Although lead-based relaxor ferroelectrics are dominating in the electronic markets, lead-free ceramics have been focused intensively for last few years due to the restriction of hazardous substances such as lead, lead oxide and heavy metals. There is no equivalent alternative as compared to lead-based compounds, particularly PZT based relaxor ferroelectrics till now. However, certain lead-free relaxor ferroelectric groups with a perovskite crystal structure are impressed by the current researchers for their enhanced physical properties in terms of dielectric, ferroelectric, and piezoelectric properties. Those relaxor ferroelectrics are typical classified as follows: (a) barium titanate (BaTiO3/BTO) based-, (b) potassium sodium niobate (K0.5Na0.5NbO3/KNN) based-, (c) bismuth sodium titanate (Bi0.5Na0.5TiO3/BNT) based- and (d) bismuth layer structured ferroelectrics (BLSFs) [47].

#### *3.3.1 BTO based relaxor ferroelectrics*

At room temperature, BTO exhibits stable electrical properties (dielectric and ferroelectric), good electrochemical coupling (*k*33 ~ 0.50), high-quality factor, low dielectric loss, but limited by low *T*c (120 °C–135 °C) and *d*33 (~190 pC/N). Also, it follows the subsequent structural phase transitions from cubic (>120 °C–135 °C)-tetragonal (120 °C to 20 °C)-orthorhombic (20 °C to −80 °C)-rhombohedral (<−80 °C). In general, BaTiO3 exhibits normal ferroelectric and follows the Curie–Weiss law at ferroelectric to the paraelectric phase transition. The BaTiO3-BaSnO3 solid solution was the first BTO based compound in which relaxor ferroelectric behavior was observed. After that, the relaxor behavior in BTO has been developed by designing the A and B-sites with incorporation of both heterovalent and isovalent ionic substitutions. Currently, there are several modified BTO ceramics available with diffuse phase transition. The available BTO based relaxor ferroelectric systems are BaTiO3-CaTiO3, BaTiO3-BaZrO3- CaTiO3 [*d*33 ~ 620pC/N for Ba0.85Ca0.15Ti0.90Zr0.10O3], BaTiO3-BiFeO3-Bi(Mg0.5Ti0.5) O3, BaTi0.8Sn0.2O3, Ba(Ti0.94Sn0.03Zr0.03)O3 BaTiO3–La(Mg0.5Ti0.5)O3, BiTiO3-(x) Bi(Mg2/3Nb1/3)O3 and so on [47–49].

#### *3.3.2 KNN based ceramic system*

The KNN system is one of the most promising lead-free alternatives due to its high *T*c (~410 °C), high *P*r (~33μcm−2), and large *K*p (~0.454). Basically, KNN is the solid solution of two perovskite compounds, i.e., KNbO3 (orthorhombic: ferroelectric) and NaNbO3 (orthorhombic: antiferroelectric). In general, the KNN forms the morphotropic phase boundary as similar to PZT [47]. It exhibits moderate dielectric, ferroelectric and piezoelectric properties as compared to PZT. Similar to BTO, the relaxor behavior of KNN has been developed by introducing other elements through interrupting the long rang polar ordering and, forms the PNRs as evidenced by several experimental results. Some of the KNN based relaxor ferroelectrics with physical properties are (K0.48Na0.535)0.942Li0.058NbO3 [*d*33 ~ 314, *K*33 ~ 41,*T*c ~ 490 °C], (K0.44Na0.52Li0.04) (Na0.86Ta0.10Sb0.04)O3 [*d*33 ~ 416pC/N],0.96(K0.5Na0.5)0.95Li0.05Nb1-xSbxO3– 0.04BaZrO3, 0.5wt%Mn-KNN (*T*c ~ 416 °C, *d*33 ~ 350 pCN−1), (Na0.44K0.515Li0.045) Nb0.915 Sb0.045Ta0.05O3(*d*33 ~ 390pC/N, *T*c ~ 320 °C *K*33 ~ 0.49), 0.96(K0.4Na0.6) (Nb0.96Sb0.04)O3–0.04Bi0.5K0.5Zr0.9Sn0.1O3 (*d*33 ~ 460pC/N, *T*c ~ 250 °C, *K*33 ~ 0.47),(Na0.5K0.5)0.975Li0.025Nb0.76 Sb0.06Ta0.18O3 (*d*33 ~ 352, *T*c ~ 200 °C, *K*33 ~ 0.47) [48, 50].

**61**

following relation [52].

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

BNT is one of the promising lead-free materials to compete with PZT for actuator applications. It exhibits relaxor ferroelectric properties with relatively large

Recently, BLSFs are considered as lead-free relaxor ferroelectrics due to its excel-

Nb5+, Ta5+, W6+, Mo6+, etc. The number "m" (=1, 2, 3, 4, and 5) is the number of BO6

low εr, and decent aging resistance. The above mentioned various classes of relaxor ferroelectrics exhibit different unique relaxor behavior depending upon the formation of PNRs due to the compositional fluctuation in the crystallographic sites.

The relaxor ferroelectrics can have wide range of technological applications due to its intriguing physical properties in terms of dielectric, ferroelectric and piezoelectric. As per the earlier discussion, the fundamental origin of unusual behavior in RFEs is mainly due to the presence of polar nanoregions (PNRs). In this section, few important applications of RFEs in modern technologies based on the specific physical property have been briefly discussed for the scientific community. In addition,

i.Currently, the electrical energy storage systems (EESSs) with high energy density and power density are the essential components for the various types of electronics. Out of several EESSs, dielectric capacitors (DCs) are widely used for delivering energy due to its high power density (PD). Power density (*P*) is defined as the amount of energy delivered by the device per unit time

*We <sup>P</sup>*

Where, *We* is the energy storage by the device, *t* and *V* are the time and volume, respectively. To obtain a high power density, it is essential to increase the energy storage density of the device. The energy storage density of DCs directly related to the dielectric displacement (*D*) and external applied electric field (*E*) by the

the systematic approaches have been formulated for individual properties.

per unit volume. It can be defined as; [52]

, Pb2+, Ba2+, La3+, Bi3+, Ce3+, etc. and B-site could be Ti4+,

2− perovskite blocks. BLSFs exhibit high *T*c, low tanδ,

*tV* <sup>=</sup> (7)

high Curie temperature (~320 °C). It follows the character of an ergodic relaxor with room temperature rhombohedral crystal symmetry. The physical properties of several BNT based binary and solid ternary solutions near MPB composition along with the substitution of various cations have been reported; such as BNT-ATiO3 (A = Ca2+, Sr2+, Ba2+, and Pb2+), BNT-KNbO3, BNT-Bi0.5Li0.5TiO3, BNT-Bi0.5K0.5TiO3(BNT-BKT), BNT-K0.5Na0.5NbO3(BNT-KNN), BNT-BKT-KNN, BNT-

BT-KNN, BNT-BKT-BiFeO3, BNT-BKT-BaTiO3-SrTiO3 and so on [50, 51].

with A-site occupy by mono-, di- or trivalent ions, B-site occupies by tetra-, penta- or hexavalent ions with appropriate size [47]. The possible A-site elements

lent fatigue properties. The general formula of BLSFs is (Bi2O3)

), large coercivity (*E*c ~ 73 kV/cm), and

2+ (Am-1BmO3m + 1)

2−

*3.3.3 BNT based ceramic systems*

could be K+

, Na+

**4. Possible applications**

octahedra in the (Am-1BmO3m + 1)

, Ca+

, Sr.+

remanent polarizations (*P*r ~ 38 μC/cm<sup>2</sup>

*3.3.4 Bismuth layer structured ferroelectrics (BLSFs)*
