**1.1 The basic structure of perovskites**

**Figure 1** depicts the ideal perovskite structure. In the ideal crystal structure of perovskite with general formula ABX3; where "A" and "B" are generally metal cations and "X" is an oxide or halide like Cl, Br, I, etc., "A" can be Ca, K, Na, Pb, Sr, and other rare-earth metals which occupy the 12-fold coordinated sites between the octahedra. "X" forms the BX6 octahedra where B located at the center of octahedra. Perovskite can be described as consisting of corner-sharing [BX6] octahedra with the A-cation occupying the 12-fold coordination site formed in the middle of the cube of eight such octahedra. In an ideal cubic unit cell of perovskite, Wyckoff positions for A- ion is at cube-corner positions (0, 0, 0); ion B sites at body center position (1/2, 1/2, 1/2) and ion X sits at face-centered positions (1/2, 1/2, 0). **Figure 2** shows the elements which can be in A-site, B-site from the periodic table.

In ideal perovskite such as SrTiO3 [3], CsSnBr3 [4], etc., there is no such distortion in the unit cell. There are many different types of lattice distortions that can happen due to the flexibility of bond angles within the ideal perovskite structure [5].


The different physical properties (mainly electronic, magnetic, dielectric, and piezoelectric properties) of perovskite materials are crucially dependent on these distortions. The distortion as a consequence of cationic substitution can be used to fine-tune physical properties exhibited by perovskite.

#### **Figure 1.**

*The ideal structure for perovskite; blue balls represent the A-site, yellow ball shows the B-site, and magenta balls showing the position of X anion (face center position) [1].*

**31**

equation.

**Figure 2.**

radius for X-anion.

**1.2 Why lead-free?**

*1.2.1 Toxicity effects of lead*

*Lead-Free Perovskite Nanocomposites: An Aspect for Environmental Application*

In the case of perovskite structure (closed packed), A-cation must fit among four BX6 octahedra. Each A-cation is surrounded by 12 nearest X-anions (12 fold coordination). Therefore A-cations have limited space to accommodate itself in the interstitial position. In the case of ideal perovskite structure, the cell axis (a) is geometrically related to the ionic radii (, , and ) as described in the following

*A map of the elements in the periodic table which can occupy the A, B, and/or X sites [2].*

The ratio of the two expressions for the cell length is called Goldschmidt's tolerance factor (*t*) and it allows us for evaluating the degree of distortion in the unit

> ( ) ( ) <sup>+</sup> <sup>=</sup> <sup>2</sup> <sup>+</sup> *A X B X*

where is the radius of A-cation is, is the radius for B-cation and is the

Lead (and its oxide form) is highly hazardous and its harmfulness is further improved due to its volatilization at high temperature mainly during calcination and sintering causing environmental pollution during different sample preparation techniques [7]. According to the European Union (EU), hazardous substances like

The main indications of lead poisoning are tiredness, muscles and joints pain, abdominal uneasiness, etc. Sometimes the deposition of lead sulfide

cell. The expression for Goldschmidt's tolerance factor [6] is as follows.

lead and other heavy metals is planning to strictly prohibit [8, 9].

*a rr rr* = += + 2 2 ( *AX BX* ) ( ) (1)

*r r <sup>t</sup> r r* (2)

*DOI: http://dx.doi.org/10.5772/intechopen.93052*

*Lead-Free Perovskite Nanocomposites: An Aspect for Environmental Application DOI: http://dx.doi.org/10.5772/intechopen.93052*

**Figure 2.** *A map of the elements in the periodic table which can occupy the A, B, and/or X sites [2].*

In the case of perovskite structure (closed packed), A-cation must fit among four BX6 octahedra. Each A-cation is surrounded by 12 nearest X-anions (12 fold coordination). Therefore A-cations have limited space to accommodate itself in the interstitial position. In the case of ideal perovskite structure, the cell axis (a) is geometrically related to the ionic radii (, , and ) as described in the following equation.

$$a = \sqrt{2} \left( r\_{\lambda} + r\_{\chi} \right) = 2 \left( r\_{\mathcal{B}} + r\_{\chi} \right) \tag{1}$$

The ratio of the two expressions for the cell length is called Goldschmidt's tolerance factor (*t*) and it allows us for evaluating the degree of distortion in the unit cell. The expression for Goldschmidt's tolerance factor [6] is as follows.

$$t = \frac{\left(r\_{\mathcal{A}} + r\_{\mathcal{X}}\right)}{\sqrt{2}\left(r\_{\mathcal{B}} + r\_{\mathcal{X}}\right)}\tag{2}$$

where is the radius of A-cation is, is the radius for B-cation and is the radius for X-anion.

### **1.2 Why lead-free?**

Lead (and its oxide form) is highly hazardous and its harmfulness is further improved due to its volatilization at high temperature mainly during calcination and sintering causing environmental pollution during different sample preparation techniques [7]. According to the European Union (EU), hazardous substances like lead and other heavy metals is planning to strictly prohibit [8, 9].

#### *1.2.1 Toxicity effects of lead*

The main indications of lead poisoning are tiredness, muscles and joints pain, abdominal uneasiness, etc. Sometimes the deposition of lead sulfide

*Perovskite and Piezoelectric Materials*

structure [5].

**1.1 The basic structure of perovskites**

**Figure 1** depicts the ideal perovskite structure. In the ideal crystal structure of perovskite with general formula ABX3; where "A" and "B" are generally metal cations and "X" is an oxide or halide like Cl, Br, I, etc., "A" can be Ca, K, Na, Pb, Sr, and other rare-earth metals which occupy the 12-fold coordinated sites between the octahedra. "X" forms the BX6 octahedra where B located at the center of octahedra. Perovskite can be described as consisting of corner-sharing [BX6] octahedra with the A-cation occupying the 12-fold coordination site formed in the middle of the cube of eight such octahedra. In an ideal cubic unit cell of perovskite, Wyckoff positions for A- ion is at cube-corner positions (0, 0, 0); ion B sites at body center position (1/2, 1/2, 1/2) and ion X sits at face-centered positions (1/2, 1/2, 0). **Figure 2**

shows the elements which can be in A-site, B-site from the periodic table. In ideal perovskite such as SrTiO3 [3], CsSnBr3 [4], etc., there is no such distortion in the unit cell. There are many different types of lattice distortions that can happen due to the flexibility of bond angles within the ideal perovskite

ii.Off-center displacement of B-cation in BX6 octahedra, this one of the causes

iii.So-called tilting in octahedra framework, usually occurring as a result of too

The different physical properties (mainly electronic, magnetic, dielectric, and piezoelectric properties) of perovskite materials are crucially dependent on these distortions. The distortion as a consequence of cationic substitution can be used to

*The ideal structure for perovskite; blue balls represent the A-site, yellow ball shows the B-site, and magenta* 

iv.Ordering of more than one type of cations A or B, or of vacancies.

v.Ordering of more than one kind of anions X, or of vacancies.

i.Distortion in BX6 octahedra, by the Jahn-Teller effect.

for ferroelectricity in these type of materials.

small A-cations at cuboctahedral site.

fine-tune physical properties exhibited by perovskite.

*balls showing the position of X anion (face center position) [1].*

**30**

**Figure 1.**

can be found out in the dental margin of the gums of the patients having poor dental hygiene. Lead harming has been considered as a health hazard, for its bad effects on neurological and cerebral development [10–12]. The main route of absorption in adults is the respiratory region where 30–70% of inhaled lead (typically the inorganic form like oxides and salts) goes into the cardiovascular system. The maximum tolerance of lead in blood ranges from 1.45 to 2.4 mol L<sup>−</sup><sup>1</sup> (30–50 g 100 mL<sup>−</sup><sup>1</sup> ) with a provision of 6 monthly observations [13]. Basically, lead has few significant biochemical properties that give toxic effects on the human biological system. (i) As lead is electropositive in nature, it shows a very high affinity for the enzymes, which are necessary for the synthesis of hemoglobin. (ii) The divalent lead behaves similarly to calcium preventing mitochondrial oxidative phosphorylation as a result intelligence quotient (IQ ) got reducing. (iii) The transcription of DNA can also disturb by lead by interacting with binding protein and nucleic acids [14, 15]. **Figure 3** illustrated the adverse effect of lead on the human body.

Bearing in mind the hazardous effect of Pb in Pb-based compounds, the research communities focused on designing the materials which are basically Pb-free. Hence, this chapter concludes with some Pb-free perovskite-type materials for the environmental application point of view.

**33**

**Figure 4.**

*Diagram illustrates the working principle of SOFC [19].*

*Lead-Free Perovskite Nanocomposites: An Aspect for Environmental Application*

**2. Application of Pb-free perovskites from a different aspect** 

Recently, the inorganic perovskite-type of oxide nanomaterials have been widely

at 500 and 600°C, respectively [20].

at 800°C. This anode mate-

applied in the processing of chemically modified electrodes [16, 17]. They have acknowledged considerable attention in the last few decades because of their catalytic activity in diverse processes like purification of waste gas and catalytic combustion. In the fuel cell, there is a direct conversion of chemical energy into electrical energy similar to a battery. These are attractive because of their great efficiency, low emission, almost zero pollution (basically noise pollution). The solid oxide fuel cells (SOFCs) have come into the picture as effective substitutions to the combustion engines due to their prospective to minimize the environmental impact of the use of conventional fossil fuels. Perovskite oxides exhibited attractive properties like a high electrical and ionic conductivity similar to that of metals and the perfect mix of these two types [18]. This mixed conduction properties of perovskite oxides are advantageous for electrochemical reaction. The working principle of a SOFC is depicted by **Figure 4** [19]. The perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ used as an effective cathode for intermediary SOFC reported by Shao and Haile. This cathode unveiled the maxi-

The combination of single and double perovskite oxide Ba0.5Sr0.5(Co0.7Fe0.3)0.6875W0. 3125O3−δ (B-SCFW) was investigated by Shin et al. [21] for self-assembled perovskite composites for SOFC. In contrast, Goodenough reported that the double perovskite Sr2MgMnMoO6-δ can act as an anode material for SOFC with dry methane as the fuel

rial exhibited long-term stability and having oxygen insufficiency, as well as some good environmental effects like tolerance to sulfur, stability in reducing atmosphere [22]. **Table 1** enlisted with some perovskites used as anode and cathode for SOFCs.

*DOI: http://dx.doi.org/10.5772/intechopen.93052*

**2.1 Perovskites as solid oxide fuel cells**

mum power density of 402 and 1010 mW cm<sup>−</sup><sup>2</sup>

and it shows maximum power density of 438 mW cm<sup>−</sup><sup>2</sup>

**of environmental**

**Figure 3.** *Schematic diagram for toxic effect of lead on human body.*
