**1. Introduction**

Multiferroic materials exhibit more than one primary ferroic order parameters (i.e. ferroelectricity, ferroelasticity and ferromagnetism) in same phase which was first proposed by Schmid in 1994 [1]. In recent years, there has been a strong interest in systems that exhibit convergence between magnetic degrees of freedom, electronic degrees of freedom, and orbital degrees of freedom. Perovskite based oxides have attracted much attention due to their interesting structural, magnetic, optical and electronic properties [2]. A large number of publications have been devoted to multiferroic materials working with theory, experimentation and application features. Bismuth-based complex oxides (Bi2Fe4O9) with mullite-type structure, as an important active material, has a wide application prospect in the fields of magnetic recording media, sensor, magnetoresistive devices, solid oxide fuel cell, scintillators and photocatalyst [3–7].

The crystallographic structure of Bi2Fe4O9 is orthorhombic with space group *Pbam*, No. 55, which belongs to the mullite-type crystal structure family [8, 9].

A unit cell of Bi2Fe4O9 consists of two formula units with an equal distribution of Fe ions between the edge-sharing octahedral (FeO6) and corner-sharing tetrahedral (FeO4) positions with Bi3+ ions are surrounded by eight oxygen atoms. Bulk Bi2Fe4O9 synthesized by solid state reaction exhibiting an antiferromagnetic (AFM) ordering at *T*N = 260 K and ferroelectric (FE) hysteresis loops at *T* = 250 K, which indicates that Bi2Fe4O9 is a promising multiferroic material [9, 10]. An unexpected multiferroic effect, which was observed as a coexistence of AFM and FE polarization, was reported in Bi2Fe4O9, attributed to frustrated spin system coupled with phonons [10]. Low electrical conductivity in ferrites is useful for inductor, transformer cores and in switch mode power supplies. On the other hand, studies of electric and dielectric properties are also equally important from both fundamental and application point of view. Dielectric and magnetic behavior of ferrites is greatly influenced by an order of magnitude of conductivity and is mostly dependent on preparation method and sintering conditions [11].

Although, due to search of new multifunctional materials, the recent work carried out is the very important and needed [12–15]. Rao et al. reported the multifunctional properties of mullite-type structured Nd-doped Bi2Fe4O9 and the spinorbital coupling by D-M interactions enhances the ferromagnetic (FM) behavior of the Nd-doping Bi2Fe4O9 [12]. Ameer et al. studied the structural, electronic, and magnetic properties of Bi2Fe4O9 with different magnetic ordering using the projector augmented wave (PAW) method based on density functional theory (DFT). They proposed that the FM Bi2Fe4O9 is a semiconductor with an indirect optical bandgap of 1.732 eV and the exchange mechanism started to work, resulting in the exchange splitting in Bi2Fe4O9, while the antiferromagnetic (AFM) Bi2Fe4O9 is a multiband semiconductor without splitting of the majority and minority spin states [13]. In another study, the researchers believed that Zn substitution in Bi2Fe4O9 would induce *p*-type conductivity, suggesting that 3*d* transition metal ions doping in Bi2Fe4O9 provides the capabilities to develop low-bandgap, heterojunction-based optoelectronic devices [14]. In addition, Pooladi et al. studied the Bi2Fe4−xMnxO9 (0.0 ≤ *x* ≤ 1.0) nanoparticles synthesized by reverse chemical co-precipitation method. With increase in Mn concentration, the coercivity of the nanoparticles enhances significantly and the saturation magnetization decreased. Also, the Mn substitution at Fe site in Bi2Fe4−xMnxO9 increases the dielectric constant [15]. These types of structures and materials are interested due to their structural, magnetic properties and the relationship between orbital, spin and charge degrees of freedom.

Various chemical methods such as solid-state reaction route, chemical co-precipitation, sol-gel and hydrothermal have been used to produce Bi2Fe4O9 [14–16]. The properties of materials are highly dependent on structural, microstructural properties and methods of synthesis. In this regard, it is of interest to develop controlled methods for making materials in oxide forms for further functional applications. Thermal heating in the oxygen atmosphere at high temperatures contributes to the oxidation process and formation of oxide forms, which has a significant impact on physical, chemical and magnetic properties of compounds [17–21]. Zdorovets et al. reported the systematic study of the effect of thermal annealing on changes in the structural properties and phase compositions of metallic cobalt based nanostructures [17]. Rusakov et al. described the effect of thermal annealing on structural and magnetic characteristics, as well as phase transformations in Fe▬Ni/ Fe▬Ni▬O nanoparticles. They found that the initial nanoparticles were a three-phase system consisting of Fe▬Ni▬O oxide with spinel structure and a Fe▬Ni alloy with face-centered and body-centered cubic lattices. As a result of thermal annealing, the decrease in the Fe▬Ni phase is associated with the subsequent ordering of the Fe▬Ni▬O phase with a decrease in the crystal lattice parameter and an increase in the degree of crystallinity [19]. If annealing is carried out in air, the phase transition

**5**

**3. Results and discussion**

**3.1 Crystal structure analysis**

*Structural, Electrical, and Magnetic Properties of Mullite-Type Ceramic: Bi2Fe4O9*

point defects and the introduction of oxygen at high temperature [20].

related to the structural transformation of iron oxide is in the range of 600–1000°C due to the change of thermal vibration of atoms in the lattice node, the annealing of

Although Bi2Fe4O9 has obvious importance as a functional material, there are few reports in the literature. Here, we present the structural and physical properties of bulk Bi2Fe4O9 ceramic synthesized by a solid-state reaction route. One needs detailed knowledge of the crystal structure to understand the physical properties. Therefore, we aimed to understand the crystal structure by X-ray powder diffraction followed by Rietveld refinement using FullPROF program [22]. In addition, Bi2Fe4O9 was subsequently characterized using several experimental techniques, such as Raman spectroscopy, SEM, dielectric and ferroelectric spectroscopy, and

Bulk Bi2Fe4O9 ceramic was synthesized through solid-state reaction route (SSR). The SSR is a commonly used synthesis method for obtaining polycrystalline bulk materials from solid reagents. This method provides a great deal of choices for starting materials like oxides, carbonates, etc. Since solids do not react with each other at room temperature, very high temperatures are usually employed to allow appropriate reaction to occur at a significant rate. Therefore, both thermodynamic and kinetic factors are important in SSR. In the SSR method, the solid reactants undergo a chemical reaction at high temperature in the absence of any solvent, thereby producing a stable product. High purity Bi2O3, Fe2O3 were carefully weighed and stoichiometrically mixed in an agate mortar for 5 hours. The powder was doubly thermally calcined consecutively at 650°C for 1 hour and 850°C for 6 hours with intermediate grinding in oxygen-containing medium. Finally, pellets were sintered at 850°C for 6 hours, resulting in good densification. Thermal heating (i.e. calcination and annealing) is a mean of controlling the structural changes, properties, and phase compositions [23]. In this case, introduction of oxygen leads to the formation of oxide compounds. For crystallinity and phase identification X-ray diffraction (XRD) pattern were taken using CuKα1 radiation (λ = 1.5406 Å) of a Bruker D8 Advance X-ray diffractometer. Crystal structure characterization of synthesized sample was performed by employing

Rietveld whole profile fitting method using FullPROF software [22].

*M-H* curve was performed using a Lakeshore VSM 7410 model.

The sample quality, morphology, grain distribution, density/voids in the samples were studied with scanning electron microscope (JEOL, JSM-5600). Raman measurements on as synthesized sample was carried out on Jobin-Yovn Horiba LABRAM (System HR800) spectrometer with a 632.8 nm excitation source equipped with a Peltier cooled CCD detector. Dielectric measurements were made as a function of frequency in the range of 100 Hz–1 MHz on Novocontrol alpha-ANB impedance analyzer at room temperature. Ferroelectric measurement was carried out using a ferroelectric loop tracer based on Sawyer-Tower circuit. The

The room temperature XRD pattern of bulk Bi2Fe4O9 sample is shown in **Figure 1(a)**. From the XRD pattern we can index the data in orthorhombic phase as shown in **Figure 1(a)**. The present XRD patterns matches with JCPDS

*DOI: http://dx.doi.org/10.5772/intechopen.93280*

magnetometry, which are discussed in detail.

**2. Experimental details**

*Structural, Electrical, and Magnetic Properties of Mullite-Type Ceramic: Bi2Fe4O9 DOI: http://dx.doi.org/10.5772/intechopen.93280*

related to the structural transformation of iron oxide is in the range of 600–1000°C due to the change of thermal vibration of atoms in the lattice node, the annealing of point defects and the introduction of oxygen at high temperature [20].

Although Bi2Fe4O9 has obvious importance as a functional material, there are few reports in the literature. Here, we present the structural and physical properties of bulk Bi2Fe4O9 ceramic synthesized by a solid-state reaction route. One needs detailed knowledge of the crystal structure to understand the physical properties. Therefore, we aimed to understand the crystal structure by X-ray powder diffraction followed by Rietveld refinement using FullPROF program [22]. In addition, Bi2Fe4O9 was subsequently characterized using several experimental techniques, such as Raman spectroscopy, SEM, dielectric and ferroelectric spectroscopy, and magnetometry, which are discussed in detail.
