**4. Synthesis of perovskites**

When production of superior ceramic powders for advanced technology the ceramics industries are developing into one of the most important and quickly

expanding sectors. Particularly, there will be a lot of interest in the creation of fine ceramic powders with exceptional and uncommon qualities. Regarding the structural and physico-chemical characteristics of perovskite structured materials, the synthesis processes are crucial. A crucial duty is sample synthesis, which can be accomplished via a variety of synthesis approaches [56]. Perovskites can be prepared in various forms like nanocrystalline [57], bulk [58], thin films [32], nanowires [59], nanotubes [60], nanocubes [61], nanorods [62] etc. forms depending on its applications using particular bottom-up and top down approach. There are several methods to synthesize perovskite materials in different forms such as chemical co-precipitation [63], microemulsion [64], hydrothermal [65], solvothermal [66], microwave irradiation [67], spray-pyrolysis [68], chemical vapor deposition [69] etc. The sol-gel auto combustion approach for bulk and nanoparticles of the BaTiO3 solid is highlighted here with a brief explanation of the solid-state reaction.

#### **4.1 Solid state reaction method**

The majority of prior research on perovskite materials concentrated on their solidstate reaction-produced structure and physical characteristics. The most popular technique for creating polycrystalline bulk solids from a variety of solid starting materials is the solid-state reaction approach, sometimes referred to as the ceramic method. The solid state reaction pathway offers a wide variety of raw materials, including oxides, carbonates, etc. At room temperature, solids do not react with one another; therefore, it is required to heat them to much higher temperatures, typically between 800 and 2000°C, for the reaction to take place at an acceptable rate. Consequently, in this strategy, both the thermodynamic and kinetic elements are crucial [70]. Precursor materials BaCO3 and TiO2 were combined to create bulk BaTiO3, and the mixture was then pulverized with a mortar and pestle to reduce the size of the particle sizes and increase the surface area exposed to the reaction. The crushed powder was calcined in furnace over 900°C to form the necessary product. Different wet chemical techniques can be used to overcome the limitations of the traditional solid state reaction approach, including high sintering temperature, secondary phase formation, poor ingredient dispersion, high porosity, and big particle size [71]. Due to its higher surface-to-volume ratio and quantum confinement effects, nanomaterials have different properties than those of bulk materials [72]. It is a widely acknowledged fact that the properties of a bulk material radically alter as it gets closer to the nanoscale. These characteristics are related to the size, shape, and distribution of the particles inside the materials, which in turn are dependent upon the synthesis process.

#### **4.2 Sol-gel auto combustion method**

Another name for the sol-gel auto combustion process is low-temperature selfcombustion, commonly known as auto-ignition, self-propagation, nitrate-citrate combustion, gel-thermal breakdown technique, etc. [73]. This method uses a sol-gel procedure to create a gel out of an aqueous solution of the necessary metal salts (often nitrates or acetates) and organic fuel. The gel is then ignited to cause combustion, producing a fluffy, volumous end product with a sizable surface area. Fuel is utilized in this procedure as a complexant to create a homogenous precursor called xerogel [74]. In comparison towards other synthesis techniques, a sol-gel auto combustion approach has unquestionable advantages since it can precisely manage the composition, purity, least particle aggregation, homogeneity at the microscopic scale, and

#### *Perovskite Structured Materials: Synthesis, Structure, Physical Properties… DOI: http://dx.doi.org/10.5772/intechopen.106252*

sintering temperature [75]. Here, a straightforward procedure for producing nanocrystalline BaTiO3 material is described. Selecting the proper complexant additives is crucial for the production of homogeneous phases. Metal particles of varying sizes can be successfully chelated by complexant agents or fuel, which helps to maintain the specific precipitation needed to maintain compositional homogeneity among the constituents. The complexant substances act as reductants and are oxidized by nitrate ions to produce fuel. According to a literature review, many types of complexant agents are utilized to create perovskite materials. Most commonly used complexants are citric acid (C6H8O7), glycine (NH2CH2COOH), urea (CO(NH2)2), ethylene glycol (CH2OH)2, dextrose (C6H12O6), acetic acid (CH3COOH), ascorbic acid (C6H8O6), propionic acid (CH3CH2COOH) etc. [76]. Here, tetra butyl titanate (Ti(OC4H9)4) or titanium isopropoxide Ti[OCH(CH3)2]4 can be chosen as a precursor (for Ti4+ ions) due to the fact that it is a transition metal alkoxide. It is very reactive because it contains strongly electronegative groups that keep the metal in the highest oxidation state and enable nucleophilic attack on the metal. These alkoxide precursors are highly electrophilic, making them less resistant to condensation, hydrolysis, and other nucleophilic processes. Additionally, the group R from Ti(OR)4 influences the gel's shape (size and surface area of the crystallites) and crystallization behavior. Controlling the condensation path and polymer development requires chemical modification of the transition metal alkoxide with chelating ligands. Tetra butyl titanate was chelated with ethanol (1:2) to produce a highly condensed product and to aid in the gelification process. A unidimensional polymer is produced when too much water is introduced to (Ti(OC4H9)4)-ethanol, which causes the O-R ligands to be hydrolyzed preferentially [77]. The metal nitrate to citric acid molar concentration was taken to be 1:5 in accordance with the rules of propellant chemistry, which take into account the oxidizing and reducing valencies of various components. The total valence of the precursors was used for this Ba(NO3)2 = 10, C6H8O7 = +18 and Ti(OC4H9)4 = +96. Tetra butyl titanate solution was initially added to a citric acid aqueous solution with a pH of 8, which was then brought to the desired level by adding the necessary amount of ammonia. A yellowish translucent liquid that is designated as solution "A" was produced after being agitated at 80°C for 1 hour. Inorganic compounds (in this case, barium nitrate) were simultaneously dissolved in distilled water while being continuously stirred; the resulting solution, designated as solution "B," is the result. Then, mixtures of solutions "A" and "B" were added. Ammonia was used to bring the pH level to 7 and maintain it there until a translucent liquid was obtained. The solution's viscosity progressively rose after 3 hours of nonstop stirring, after which a stable translucent sol formed. The creation of the gel is started by continuous heating to 110° C. Viscous gel becomes dry gel when heated and constantly stirred. It was discovered that the nitrate-citrate gel's combustion process was autocatalytic, and experimental results revealed that the dried gel made of metal nitrates and citric acid exhibited selfpropagating combustion behavior. The entire combustion process was completed in a matter of minutes. To produce the nanocrystalline powders, the resulting powders were dried, crushed, and annealed at 900°C for 5 hours in a muffle furnace. **Figure 4** shows the process for making BaTiO3 nanoceramics using the sol-gel method.

### **5. Unique properties of perovskites**

The ceramic materials with the general chemical formula ABO3 made of perovskite are utilized as high-value materials in a variety of engineering and technological

**Figure 4.** *The scheme of synthesis of BaTiO3 nanoceramics.*

applications. Because of the non-stoichiometry of the cations and/or anions, the distortion of the cation configuration, and the mixed valence and valence mixture electronic structure, perovskite-type structures have functional features. Furthermore, it is known that the majority of the naturally occurring metallic elements are stable in perovskite-type oxide structures, and the ability to synthesize multicomponent perovskites by partially substituting cations in positions A and B results in a variety of complicated kinds. These traits are what give perovskites their unique qualities. These include ferroelectric, thermoelectric, multiferroic,

superconductive, dielectric, optical, and many more unique properties. A few of them are described in depth here [78–80].

#### **5.1 Dielectric properties**

Materials that allow electro-static fields to last a long time are known as dielectric or electrical insulating materials [81]. The basic electrical properties of conductive materials and dielectric materials sharply diverge because the dielectric materials offer a very high resistance to the channel of electric current under the influence of the applied direct-current voltage. Capacitors frequently have layers of these materials added to them to improve performance; the term "dielectric" specifically refers to this use [82]. Ferromagnetic or high dielectric permittivity materials are crucial for the production of electroceramics in engineering and electronics. BaTiO3 and KNbO3 are two examples of perovskites that have underwent substantial research in the past [35, 83]. The ratio of a substance's permittivity to the permittivity of empty space is known as the dielectric constant. A significant dielectric constant (ε), which is a highly nonlinear and anisotropic incident, is based on the collective polar deflections of the metal ions with respect to the oxygen sublattice [84]. A soft-mode model typically describes the phase shift that results in ferroelectricity [85]. Many variety of careers have been taken from the structurally straightforward BaTiO3 by the solid coordination compound Pb(Zr,Ti)O3 to various unique families of materials in order to adjust the dielectric and mechanical properties [86]. These approaches specifically account for the fact the perovskites' adaptability to chemical alteration and docility [87]. The relaxor ferroelectric is one of them. It is undoubtedly based on a multi-element substituted lead titanate (PbTiO3) with the chemical formula A(B<sup>0</sup> <sup>B</sup>″)O3 with random metal cation occupants at the A and B sites with various valence states and ionic radii [88, 89]. Large dielectric constants, a clear frequency dispersion, and temperature-dependent dielectric constant variations are all characteristics of relaxor ferroelectrics. For temperatures above the glass transition, these effects result from slow relaxation processes [90]. The effects are based on electrical inhomogeneities and the presence of polar nano-regions, and their length scales for changing composition and spontaneous polarization are 2–5 nm. The lattice component of the response is thought to constitute a local softening of the transverse-optical phonon branch that prevents long-wavelength (q = 0) phonon propagation. It is remarkable to notice that for such small length scales, the basic limit, the superpara-electric state, has not yet been attained [91]. PZT and PMN are two general examples of relaxor ferroelectrics [92]. Two common examples of relaxor ferroelectrics are PZT and PMN (eg. SrTiO3) [93]. Perovskites, such as BaTiO3, are one of the potential possibilities for usage in dynamic random access memory (DRAM) and tunable microwave devices due to their high dielectric constant and low dielectric loss [94, 95].

#### **5.2 Superconductivity**

When cooled below a specific critical temperature, certain materials exhibit the phenomena of superconductivity, which results in exactly zero electrical resistance and the expulsion of magnetic flux fields [96]. A wide family of materials with a variety of crucial physical characteristics is known as oxide perovskites. Notably, this form of perovskite structure offers a superb structural foundation for the presence of superconductivity. The most well-known examples of superconducting perovskites are high *Tc* copper oxides, although there are other others as well [97]. Many superconducting materials have historically come from intermetallic compounds, however perovskite oxides have recently overshadowed their existence. Sweedler et al. looked at the tungsten bronzes'superconductivity [98]. Bronzes made of cesium, sodium, potassium, rubidium, and tungsten were discovered to be superconductors. Three samples of decreased strontium titanate were examined by Schooley and colleagues to measure superconducting transitions [99]. The reduced crystals were made by heating for a lengthy time in a vacuum between 10�<sup>5</sup> and 10�<sup>7</sup> mm Hg. The transitions happened at respective temperatures of 0.25 K and 0.28 K. Additionally, superconductivity has been discovered in the systems' decreased phases Ba*x*Sr1-*x*TiO3 and CaySr1-yTiO3 when *x* ≤ 0.1 and y ≤ 0.3 [100]. Superconductors classified as "Type 2″ are made of metallic compounds and alloys (except for the elements vanadium, technetium and niobium). This Type 2 group includes the recently found superconducting "perovskites" metal-oxide ceramics, which typically have a ratio of 2 metal atoms to every 3 oxygen atoms. They outperform Type 1 superconductors in terms of transition temperature Tc by a mechanism that is yet not fully understood [101]. The discovery that Ba(Pb,Bi)O3 had a *Tc* of 13 K in 1973 by a DuPont research team led to the development of the first of the oxide superconductors [102].

#### **5.3 Ferroelectricity**

*"You may say anything you like but, we all are made up of ferroelectrics".* (B. T. Matthias).

Ferroelectricity is a phenomena in which the introduction of an external electric field induces a spontaneous electric polarization in some materials [103]. Various crystals, including quartz, tourmaline, and Rochelle salt, exhibit piezoelectricity, which was discovered by brothers Pierre and Paul-Jacques Curie in 1880 [104, 105]. This discovery inspired subsequent research in the topic of piezoelectrics, most notably Erwin Schrodinger's work [106, 107]. Erwin Schrodinger originally used "ferroelektrisch" or "ferroelectricity" in 1912 [108]. Joseph Valasek is credited with finding ferroelectricity for his systematic investigation of the magnetic characteristics of ferromagnetic and the dielectric properties of rochelle salt, which he presented at the American Physical Society's annual conference in Washington on April 23, 1920 [106]. The early 1940s saw the discovery of ferroelectricity in materials based on perovskite, such as barium titanate (BaTiO3), which was a significant advance in the field of ferroelectric research [109, 110]. There is a lot of interest in various forms of ferroelectrics as a result of the finding of ferroelectricity in BaTiO3. This discovery opened up new application possibilities for ferroelectric materials [105, 111]. Ven'skev and Zhdanov identified the perovskite family's distribution of ferroelectrics (FE) and antiferroelectrics (AFE) [112]. The FEs cover the entire perovskite range 0.78 ≤ t <sup>0</sup> ≤ 1.05. The AFEs are found to have a restricted distribution (0.78 ≤ t <sup>0</sup> ≤ 1.0). In addition to classifying ferroic behavior in perovskites using the tolerance factor (t<sup>0</sup> ), Halliyal and Shrout discovered that graphing t against the average electronegativity as indicated by [112];

$$
\overline{\chi} = (\chi\_{AO} + \chi\_{BO})/\mathfrak{Z} \tag{2}
$$

*Perovskite Structured Materials: Synthesis, Structure, Physical Properties… DOI: http://dx.doi.org/10.5772/intechopen.106252*

where the electronegativity differences between the A and B cations and oxygen are denoted by χAO and χBO, respectively. The endurance of the crystalline phase for a variety of simple and complicated perovskites was discovered. Perovskite compounds with low t<sup>0</sup> extended to develop pyrochlore phase(s), and the perovskite phase was stabilized by solid solutions in which t<sup>0</sup> was raised [113]. The dielectric constant of ferroelectric materials is about two orders of magnitude more than that of regular dielectric. A well-known ferroelectric substance with a relative dielectric constant of above 2000 is BaTiO3. Ferroelectric materials must include permanent electric dipoles; one of the most common ferroelectrics, barium titanate, is explained for its creation [114]. The spontaneous polarization is a consequence of the positioning of the Ba2+, Ti4+, and O2� ions within the unit cell, as represented in **Figure 5a**.

The O2� anions are at the face centres, the Ti4+ ion is in the octahedral void at the body centre, and the Ba2+ ions are in the body corners. Only one of the four octahedral voids in the unit cell is filled, which matches the chemical formula of one titanium for every four species of the other kinds: one barium plus three oxygen. Barium titanate is a cubic crystal with the ion positions mentioned above 120°C [115]. The positive and negative charge centres coincide in this instance, and a spontaneous dipole moment is absent. The Ti4+ ion shifts to one side of the body centre if the crystal is cooled below the Curie temperature of 120°C, as indicated by the dotted line in **Figure 5**'s front view (b). A displacement of the nearby oxygen anions also occurs. At normal temperature, the crystal changes from its cubic phase to its tetragonal phase. The tetragonal cell's c/a ratio is almost 1.012. Local dipoles are formed all around the crystal as a result of the positive and negative charge centres no longer lining up. A significant amount of polarization occurs in the solid as a result of the alignment of the dipoles of nearby unit cells. Even when the dipoles of nearby unit cells are aligned, a BaTiO3 crystal typically displays no net polarization at ambient temperature in the absence of an external field. This can be understood by visualizing the behavior of ferroelectric domains in a manner similar to that of ferromagnetic domains. When an electric field is applied, the domains have a tendency to line up in the field's direction, and we notice all the hysteresis loop phenomena, including domain rotation and domain growth. A ferroelectric hysteresis loop is shown in

#### **Figure 5.**

*BaTiO3 crystal unit cell in (a), with the Ti4+ ion and O2 ions migrating out from the centre as indicated by the arrows in (b).*

#### **Figure 6.**

*The ferroelectric material's polarization (P) vs. applied field (E) hysteresis loop.*

**Figure 6**. By extrapolating the linear area of the curve backwards to zero electric field, the spontaneous polarization Ps is obtained [116, 117].

For the creation of sensors, capacitors, memory devices, etc., ferroelectric properties are exploited. Ferroelectric materials have a non-linear feature that tunable capacitors take advantage of to tune the capacitance. Two electrodes and a layer of ferroelectric material sandwich each other in the ferroelectric capacitor. Comparatively speaking to non-tunable dielectric capacitors, ferroelectric capacitors are both tunable and very compact in size. Ferroelectric materials' hysteresis during spontaneous polarization can be used to create ferroelectric RAM and RFID cards. Input devices in ultrasonic imagers, infrared cameras, fire sensors, motion detectors, etc., use ferroelectric material.

#### **5.4 Piezoelectricity**

The capacity of some materials to produce an electric charge in response to applied mechanical stress is known as piezoelectricity. The Curie brothers, Pierre and Jacques, discovered the piezoelectric effect in 1880 [118]. They discovered that certain crystals became electrically polarized when subjected to mechanical strain, and the amount of polarization was proportional to the applied strain. The Curies also found that when subjected to an electric field, the same materials distorted. The inverse piezoelectric effect has been coined to describe this [119]. **Figure 7a** and **b** show, respectively, the piezoelectric effect and the inverse piezoelectric effect.

By interacting with one another, electric dipoles spontaneously align in ferroelectric materials, whereas in piezoelectric materials, an additional force is needed. In light of this, all ferroelectrics are also piezoelectrics, but not vice versa [120]. The piezoelectric ceramics, of which PZT is an example, are a significant group of piezoelectric materials in addition to the crystals already described [121]. These are perovskitebased polycrystalline ferroelectric materials, which have tetragonal/rhombohedral

*Perovskite Structured Materials: Synthesis, Structure, Physical Properties… DOI: http://dx.doi.org/10.5772/intechopen.106252*

**Figure 7.**

*(a) Piezoelectric effect and (b) Inverse piezoelectric effect.*

crystal structures that resemble cubic shapes. They possess the common formula A<sup>2</sup> + B4+O2� 3, where A stands for a big divalent metal ion, like lead or barium, and B stands for a tetravalent metal ion, like titanium or zirconium.

#### *5.4.1 Mathematical modeling*

Piezoelectricity is a result of combining Hook's law with the electrical behavior of materials [122]:

$$D = \epsilon E \tag{3}$$

$$\mathcal{S} = \mathfrak{s}T \tag{4}$$

where, *D*: electric displacement, *ε*: permittivity, *E*: electric field strength, *S*: strain, *s*: compliance, and *T*: stress.

The coupled strain-voltage equation:

*S* ¼ *s ET* <sup>þ</sup> *<sup>d</sup><sup>t</sup> E* converse piezoelectric effect (5)

$$D = \epsilon^T E + dT \text{ direct piezeelectric effect} \tag{6}$$

$$D\_{\vec{\imath}j,k} = d\mathbb{S}\_{\vec{\imath}j} \tag{7}$$

$$D\_{\vec{\eta},k} = \frac{\partial \mathcal{S}\_{\vec{\eta}}}{\partial E\_k} \text{ piezoelectric coefficient} \tag{8}$$

Natural materials including quartz, cane sugar, collagen, topaz, dna, rochelle salt, wood, and tendon are examples of piezoelectric materials. Man-made crystals and ceramics include langasite, gallium orthophosphate (GaPO4), and quartz analogous crystals (La3Ga5SiO14), quartz analogic crystal, barium titanate, lead zirconate titanate etc. Acuosto-optic modulators, valves, high voltage and power sources, cigarette lighters, energy harvesting, AC voltage multipliers, piezoelectric motors, actuators, loudspeakers, pressure sensors, force sensors, strain gauges, microphones, pick-ups, actuators, and many other applications use piezoelectric materials.

#### **5.5 Magneto-electronic correlations**

In perovskites containing transition metal ions with open *3d* electron shells, magnetism or different orbital (electronic) ordering phenomena are seen. When compared to other electronic states, these 3*d* states have a larger ratio of the Coulomb repulsion energy *Ud* to the bandwidth W, which gives them a more localized character and a propensity for insulating states or metal-insulator transitions [123]. Due to the overlap of the different wave functions, these electrons hop and super-exchange via oxygen sites. Thus, non-stoichiometry and the [BO6] octahedra's tilt or distortion have a significant impact on the characteristics and phase diagrams of a perovskite. Additional factors include charge doping, charge/orbital inhomogeneous states that cause enormous response, such as to external magnetic fields, and order/disorder processes of the orbital part of the 3D wave function [23]. But before taking into account such effects, the electronic structure, the number of *3d* electrons, the Hund's Rule coupling, the crystalline electric field or Jahn-Teller splitting of the *3d* electronic configuration a hierarchy of energies that describes the properties of the system.

#### **5.6 Multiferroicity**

*"Materials should exist, which can be polarized by a magnetic field and magnetized via an electric field." P. Curie.* A remarkable class of materials displaying simultaneous ferromagnetic, ferroelectric, and ferroelastic ordering is represented by multiferroics. H. Schmid coined the term "multiferroic" in 1994 [124]. Multiferroic materials with the corresponding properties are shown in **Figure 8**.

These materials are unique in that they have the capacity to simultaneously use both their magnetization and polarization states, a huge potential that would make them excellent candidates for next-generation sensors and memory technology [125, 126]. Numerous multiferroics, such as rare-earth manganites and ferrites, are transition metal oxides with perovskite crystal structure (e.g. HoMn2O5, TbMnO3, LuFe2O4) [127, 128]. Ba2CoGe2O7, Ca2CoSi2O7, TbFe3(BO3)4, CoCr2O4, FeCr2O4, MnCr2O4 NdFe3(BO3)4 are those substances that exhibit multiferroicity even at ambient temperature [129–132]. Bismuth ferrite (BiFeO3), a rhombohedrally distorted

**Figure 8.** *Multiferroic materials.*

*Perovskite Structured Materials: Synthesis, Structure, Physical Properties… DOI: http://dx.doi.org/10.5772/intechopen.106252*

**Figure 9.**

*The prerequisites for ferromagnetism and ferroelectricity (polarization) (unpaired electron spin motion).*

perovskite, is one of the many multiferroics that have been studied, and it is receiving constant attention because it exhibits both ferroelectric order and anti-ferromagnetic order over a wide temperature range that is significantly above room temperature [133]. The majority of ferromagnetic materials are typically metals, therefore a ferroelectric material must be an insulator. As a result, the simultaneous occurrence of ferroelectric and ferromagnetic ordering is constrained by the lack of ferromagnetic insulators. Multiferroics are still uncommon even when antiferromagnetic systems are taken into account. A structural deformation from the high symmetry phase, which removes the centre of inversion and enables an electric polarization as seen in **Figure 9**, is the traditional prerequisite for ferroelectricity. Ferroelectricity and ferromagnetism exhibit synchronous time and spatial inversion breaking in **Figure 9**. Unpaired d electrons are necessary for any type of magnetic ordering, however ferroelectric materials like typical perovskite oxides lack this property (ABO3) have a *d<sup>0</sup>* configuration on the small B cation. The tendency for the tiny cation to generate a distortion that removes the centre of symmetry is significantly suppressed if the d shell is only partially populated. With some unpaired electrons in the *d* orbitals, magnetoelectric multiferroic materials should therefore exhibit some deformation in their crystal structure. Recently, it was discovered that magnetic spin ordering can create ferroelectricity even in the absence of any structural distortion. As a result, there are now a lot more ferroic materials that could be used [134]. Experimental and theoretical research both point to a new era in the attainability of multiferroicity in transition metals doped BaTiO3 [135]. Extrinsic and intrinsic dopants like excess oxygen vacancies and transition metal cations can be added to these materials to improve their chemical and physical properties [136].

The electrical or magnetic characteristics of perovskites with transition metal ions (TM) doped on the B site are incredibly diverse and intriguing. This variety is more closely linked to the intricate role that transition metal ions play in certain coordinations with oxygen or halides than it is to the chemical flexibility of these compounds [137]. Although unfilled *3d* electron shells of the TM are typically associated with magnetism and electronic correlations, filled *3d* electron shells are associated with strong dielectric characteristics. Due to the limited number of low-symmetry magnetic point groups that permit spontaneous polarization, multiferroicity, the coexistence of spontaneous ferroelectric and ferromagnetic moments, is a rare phenomenon [138]. One of two categories can be used to group all multiferroic materials. Multiferroics of type I and type II. At high temperatures, Type I go through a structural, nonpolar-to-polar phase transition that, in most cases, involves the breaking of inversion symmetry and results in ferroelectricity. At lower temperatures, the emergence of magnetic order takes place below a different phase transition. The main order parameter for type II is the staggered antiferromagnetic) magnetization. Below a certain temperature, magnetic ordering transforms the symmetry group from a nonpolar parent phase to a polar magnetic phase. This results in inappropriate ferroelectricity. Additionally, magneto-structural coupling to the crystal structure creates an electrically polar state. However, polar non-centrosymmetric magnetic structures typically originate from the complicated magnetic ordering of geometrically frustrated states or from competing interactions in this situation, where the magnetic and ferroelectric order parameters are intimately related [139].

#### **5.7 Optical properties**

Perovskites have emerged as a revolutionary class of materials having excellent optical and photoluminescence properties. W.J. Merz studied the optical properties of single domain crystals of BaTiO3 at various temperatures [140]. The crystal's refractive index was almost constant at 2.4 between 20 and 90 degrees Celsius and peaked at 2.46 at 120 degrees. BaTiO3's index of refraction was also measured by W.N. Lawless and R.C. De Vries at 5893 in the temperature range of 20–105°C; above Curie point, the index increased 1.3 percent to 2.398 and stayed constant to 160°C [141]. It was discovered that the single BaTiO3 crystal, which is 0.25 mm thick, transmits between 0.5 μ and 6 μ. A weak absorption band was identified near 8, and complete absorption was found for wavelengths longer than 11 μ. Noland reported on the optical characteristics of single crystals of strontium titanate created during flame fusion [142]. The wavelength range where the optical coefficient was obtained was 0.20–17 μ. From 0.55 μ to 5 μ, a transmission of more than 70% was seen. These crystals have an index of refraction of 2.407 at 5893 Å, a dielectric constant of 310, and a loss tangent of 0.00025. Linz and Herrington reported the optical density of CaTiO3 [143]. With the exception of the absorptions being shifted to shorter wavelengths, the absorption properties are remarkably comparable to those of SrTiO3 crystals. High temperature infrared windows have been considered for BaTiO3 and SrTiO3. SrTiO3 is regarded as a superior material for infrared detectors that are optically submerged. The detector-lens combinations are frequently chilled to liquid N2 and solid CO2 temperatures to boost sensitivity. Geusic et al. evaluated the electro-optic characteristics of K(Ta0.65Nb0.35)O3, BaTiO3 and SrTiO3 in the paraelectric phase [144]. When the distortions of the optical indicatrix are described in terms of the induced polarization, the electro-optic coefficients of these perovskites are almost constant with temperature and from material to material. These experiments also demonstrated the K(Ta0.65Nb0.35)O3's strong electro-optic action at ambient temperature. There has been a lot of interest in materials that can be applied using lasers in recent years. Perovskite laser host materials are widely used. The most often used ion for insertion into somewhat large crystallographic locations seems to be Nd3+. However, compensatory ions are necessary in these substitutions, barring the usage of LaF3 as a host. Without compensating ions, divalent Tm2+ and Dy2+ can be substituted in CaF2, but they are very unstable. Cr3+ turned shown to be the best replacement for Al3+ at its crystallographic positions. Recent studies have increasingly concentrated on the luminous characteristics of rare earth ion doped perovskite-type oxides. Oxygen phosphors of the perovskite type are exceedingly stable and can consistently function in a variety of conditions. [145–147]. Additionally, Perovskite-type oxide phosphors have been discovered to be a likely contender in field emission display (FED) and plasma

*Perovskite Structured Materials: Synthesis, Structure, Physical Properties… DOI: http://dx.doi.org/10.5772/intechopen.106252*

display panel (PDP) systems because they are sufficiently conductive to release electric charges on the phosphor particle surfaces [148]. Many perovskite-type oxide phosphors, such as A2+B4+O3 (A = Ca, Sr, Ba; B = Ti, Zr, Si, Hf, etc.) are therefore activated by rare earth ions, such as Sm3+, Tm3+, Pr3+, Eu3+, Tb3+ and so forth [149– 151] have been made, and their luminous characteristics have also been extensive examination. There aren't many research on photoluminescence (PL) in zirconates, particularly ones with visible emission regions [152]. Phosphors of rare earth ions doped perovskite type oxides, such as SrHfO3:Ce [153] and CaTiO3:Pr [154] X-ray phosphors could be used in many displays, hence current study has concentrated on their luminous qualities. Eu3+ is an effective activator ion that emits red or red-orange light in a variety of hosts, including borates [155], niobates [156] and molybdates [157]. Host BaZrO3 is now recognized as a readily available, inexpensive, and environmentally benign photoluminescence (PL) material that produces light in the visible spectrum [158]. This material is promising because of its PL feature, which makes it useful for applications including scitillators, plasma displays, solid state lightning, green photocatalysts, and field emission displays [159–161].

#### **5.8 Colossal magnetoresistance (CMR)**

Some materials have a feature called colossal magnetoresistance (CMR) that allows them to drastically alter their electrical resistance when a magnetic field is present (mostly manganese-based perovskite oxides) [162]. CMR was first identified in mixedvalence perovskite manganites in the 1950s by G. H. Jonker and J. H. van Santen [163]. Due to their rich fundamental physics and significant potential for use in spintronics devices, the discovery of the colossal magnetoresistance (CMR) influence in divalent alkaline-earth ion dopant perovskite manganites Re1-*x*Ae*x*MnO3, where Re is a trivalent rare-earth (La, Pr, Sm, etc.), has generated a great deal of interest [164–167].

Different magnetic phases, including insulating antiferromagnetic phases with multiple orbital orders and a ferromagnetic, metallic, orbitally disordered phase, are detected depending on the orbital occupancy of the manganese ions and the related orbital order. Because of the close coupling between spins and orbitals in these compounds, both degrees of freedom have ordering temperatures that are similar in magnitude. In a three-dimensional lattice, the primary spin exchange pathways pass over almost 180 TM-O-TM limits (TM-transition metal). But merely taking into account spins, low dimensionality, magnetic frustration, and quantum processes can also result in some incredibly strange phase diagrams, even ones without magnetic long range order [168]. In fact, in frustrated lattices, quantum fluctuations or a second order energy scale can frequently lift the degeneracy of the magnetic ground state [169, 170]. The nature of CMR manganites, which are strongly correlated electron systems with interactions between the lattice, spin, charge, and orbital degrees of freedom, including the double exchange interaction, Jahn-Taller effect, electronic phase separation, charge ordering, etc., is generally thought to be the root cause of the CMR effect. This theory has been extensively discussed in some review papers [171–175].

## **6. Applications of perovskites**

Calcium titanate (CaTiO3)-like perovskite materials offer fascinating and exceptional physical features that have been thoroughly investigated for use in both theoretical modeling and real-world applications. Due to their extremely stable structure, abundance of compounds, diversity of characteristics, and several useful applications, inorganic perovskite-type oxides are fascinating nanomaterials with a wide range of uses. Current applications for these solids include electronics, geophysics, astronomy, nuclear, optics, medicine, the environment, etc. [176].

Perovskites are useful for many different applications depending on the aforementioned distinctive properties, including thin film capacitors, non-volatile memories, photoelectrochemical cells, recording apps, read heads in hard discs, spintronics devices, laser applications, for windows to block high temperature infrared rays, high temperature heating applications (thermal barrier coatings), frequency filters for wireless technology, non-volatile memorabilia, and spintronics devices. **Table 1** lists additional significant uses for various perovskite-structured materials, along with the corresponding characteristics.


**Table 1.**

*Applications of perovskites along with respective properties.*
