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

Abstract

This work reports the structure, electrical and magnetic properties of the orthorhombic (Pbam) structured bulk Bi2Fe4O9 synthesized by the solid-state reaction process. Bi2Fe4O9 has been studied using several experimental techniques such as X-ray diffraction, scanning electron microscopy, Raman spectroscopy, dielectric spectroscopy, and magnetometry. Rietveld-refined X-ray diffraction data and Raman spectroscopy results clearly reveal the formation of Bi2Fe4O9 perovskite structure and all the peaks of Bi2Fe4O9 perfectly indexed in the orthorhombic (Pbam) structure. It has been established that the Raman spectrum identified Ag, B2g, and B3g active optical phonon modes, and that the Raman peak at 470 cm−1 may have a magnetic origin. As a result, the coexistence of weak ferromagnetic and antiferromagnetic orders in Bi2Fe4O9 ceramic was established. The remanent magnetization (2Mr) and coercivity (2Hc) are 8.74 × 10−4 emu/g and 478.8 Oe, respectively. We report a remarkable multiferroic effects in polycrystalline Bi2Fe4O9 ceramic. These characteristics make this material very useful in technology and practical applications.

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, 4, 5, 6, 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, 13, 14, 15]. Rao et al. reported the multifunctional properties of mullite-type structured Nd-doped Bi₂Fe₄O₉ and the spin-orbital 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 3d 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 Bi₂Fe_4−xMn_xO₉ (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 Bi₂Fe_4−xMn_xO₉ 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, 15, 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, 18, 19, 20, 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 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.

2. Experimental details

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].

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

3. Results and discussion

3.1 Crystal structure analysis

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 #01-74-1098 (Bi₂Fe₄O₉) [24]. In order to further confirm structural data, Rietveld refinement of the XRD pattern for Bi₂Fe₄O₉ sample was performed using FullPROF program and shown in Figure 1(b). The composition of phase and its concentration in the structure have been determined using the Rietveld method, which is based on the estimation of the diffraction peak area and the analysis of their contributions to the entire X-ray diffraction. It should be noted here that XRD pattern having a small secondary phase peaks corresponding to the Fe₂O₃ and its phase concentration is less than 2%, which does not affect the measured properties of studied ceramic. The XRD pattern of parent Bi₂Fe₄O₉ was refined with orthorhombic (Pbam) structure with lattice parameters a = 7.941(4) Å, b = 8.420(4) Å and c = 5.986(4) Å. The obtained lattice parameters are consistent with earlier reported data [25]. The Rietveld-refined calculated parameters of Bi₂Fe₄O₉ are documented in Table 1. We have illustrated structural parameters for Bi₂Fe₄O₉ ceramic, and also identify the residuals for weighted pattern R_wp, the expected weighted profile factor R_exp, and goodness of fit χ². The selected bond lengths and bond angles are mentioned in Table 1. The average value of the Bi▬O bond is 2.482 Å. The generated orthorhombic structure of Bi₂Fe₄O₉ ceramic is depicted in Figure 1(c). In the crystal structure, chains of FeO₆ octahedra parallel to the c axis are connected via FeO₄ tetrahedra alternating with bismuth atoms along the c axis.

Figure 1.

(a) Measured powder X-ray diffraction pattern for Bi₂Fe₄O₉ ceramic at room temperature, (b) Rietveld-refined XRD pattern for Bi₂Fe₄O₉, and (c) symmetric representation of the unit cell for Bi₂Fe₄O₉ ceramic viewed in abc and ab planes.

a = 7.941(4) Å	b = 8.420(4) Å		c = 5.986(4) Å	V = 400.31(2) Å^3
Atoms	x	y	z	R-values
Bi	0.3230	0.1745	0.0000	Rwp = 7.36%
Fe1	0.0000	0.0000	0.2582	Rexp = 3.41%
Fe2	0.1465	0.3360	0.5000	Rp = 4.61%
O1	0.3485	0.4292	0.0000	χ^2 = 4.65%
O2	0.3671	0.4047	0.5000	GoF = 2.2
O3	0.1312	0.2054	0.2413
O4	0.0000	0.5000	0.5000
Bond type	Bond length (Å)		Bond type	Bond angle (°)
Bi▬O1	2.153		O1▬Bi▬O1	151.93
Bi▬O3	3.017		O3▬Bi▬O3	86.06
Fe1▬O1	2.047		Bi▬O1▬Bi	141.13
Fe1▬O2	1.962		O2▬Fe2▬O3	103.98
Fe1▬O3	2.022		O3▬Fe2▬O3	108.93
Fe2▬O1	3.485		O3▬Fe2▬O4	113.59
Fe2▬O2	1.846		Fe1▬O1▬Fe1	98.02
Fe2▬O3	1.901		Fe1▬O2▬Fe1	94.99
Fe2▬O4	1.805		Fe1▬O2▬Fe2	129.72
			Fe1▬O3▬Fe2	119.09
			Fe2▬O4▬Fe2	172.00

Table 1.

Rietveld-refined room temperature structural parameters, important bond lengths, and bond angles for Bi₂Fe₄O₉ ceramic from XRD.

The symmetric pseudo-Voigt functions are used to calculate the degree of crystallinity based on the estimation of the diffraction width and shapes. We have measured the full width half maxima (FWHM) of the recorded diffraction lines, which allowed us to characterize the perfection of the crystal structure and evaluate the degree of crystallinity [21]. The value of % of crystallinity for Bi₂Fe₄O₉ = 81.3% was calculated using the formula:

$$\mathit{Crystallinity}=\frac{\mathit{Area}\mathit{of}\mathit{Crystalline}\mathit{peaks}}{\mathit{Area}\mathit{ofallpeaks}\left(i.\mathit{eCrystalline}+\mathit{Amorphous}\right)}\times 100$$E1

The distortion of the crystal lattice (ρ_dil), which characterizes the number of defective or porous inclusions in the structure as a result of external influences, was found according to formula:

$${\rho }_{dil}=\left(1-\frac{\rho }{{\rho }_0}\right)\times 100\%$$E2

Here, ρ₀ is the density of the reference sample taken from the JCPDS #01-74-1098 database, ρ is the calculated density of the sample. Calculated value of distortion of the crystal lattice is ρ_dil = 3.1%.

We used the Williamson-Hall plot to observed the effect of the phase composition on distortions and deformation of the crystal structure of Bi₂Fe₄O₉ ceramic, based on estimating the angular dependence of the full width at half maximum (FWHM) of diffraction lines (Figure 2). We obtained the strain (ε) value for Bi₂Fe₄O₉ ceramic is 0.00412 ± 0.0016. Thermal heating at high temperature helps to reduce the distortion value [19].

Figure 2.

Williamson-Hall plot for Bi₂Fe₄O₉ ceramic.

3.2 SEM analysis

The surface morphological and microstructural properties of Bi₂Fe₄O₉ compound was investigated using scanning electron microscopy (SEM). Figure 3 (upper part) shows the SEM micrograph of Bi₂Fe₄O₉ thermally sintered at 850°C for 10 hours. Typical SEM image shows that microstructures comprising of non-uniform distribution of grains with an estimated average grain size of 1.5 μm indicating polycrystalline nature. Even though the SEM image shows that there are some pores between loosely connected grains in the sample. The surface area of a catalyst is a key aspect to determine the adsorption capacity of reactants on the catalyst surface [26]. We have measured the active surface area using a Brunauer-Emmett-Teller (BET) measurement system at 77 K through nitrogen adsorption-desorption isotherm method. The BET active surface area of Bi₂Fe₄O₉ is 1.2 m²/g, which is in good agreement with the values reported in the literature [27]. In order to obtain photocatalytic efficiency, it is necessary to increase the specific surface area by doping or reducing grain sizes. In addition, we have measured the material’s apparent density which is defined as the mass per unit volume of the material in absolute dense condition [28]. The obtained density of the present calcined Bi₂Fe₄O₉ ceramic is 6.51 g/cm³ which match well with the density for the Bi₂Fe₄O₉ (ρ = 6.48 g/cm³) from reference file: JCPDS card number 74-1098.

Figure 3.

(Upper) Scanning electron microscope images of Bi₂Fe₄O₉ and (lower) Raman spectra for Bi₂Fe₄O₉ mullite-type ceramic carried out at room temperature.

3.3 Raman scattering analysis

Raman scattering spectroscopy has been extensively utilized to study the crystal lattice vibrations. Raman scattering spectroscopy would also offer a distinctive potential as a sensitive probe for the spin dynamics and studying the effect of magnetic ordering. Raman spectrum of Bi₂Fe₄O₉ at room temperature is depicted in lower part of Figure 3. The Raman active modes of the structure can be summarized using the irreducible representation 12A_g + 12B1_g + 9B2_g + 9B3_g, which is employed to describe Raman modes of orthorhombic (Pbam space group) [9]. In the measured Raman spectra all are the A_g modes (85, 93, 114, 210, 227, 280, 331, 366, 436, 465, 561, and 648 cm⁻¹) accept modes in attendance at 164 (B_2g) and 189 (B_3g) cm⁻¹. The agreement between experimental and predicted values is relatively good for the all frequency modes, dominated by Bi vibrations. The Raman peak centered at 470 cm⁻¹ is might be attributed to magnetic ordering effect on phonon line width consistent with earlier observation of bands at ~260 and 472 cm⁻¹ due to magnon scattering [9]. It would be more practical to study the magnetic excitations in Bi₂Fe₄O₉ under the assumption that they involve two-magnon processes, like in the well-known cases of ferrites [29] or cuprates [30, 31]. At higher frequency (>250 cm⁻¹), it is unlikely the magnetic-order-induced bands correspond to one-magnon excitations but in rare-earth orthoferrites (RFeO₃; R = Dy, Ho, Er, Sm, etc.) have frequencies below 25 cm⁻¹ for comparison the zone-center magnons [32].

3.4 Dielectric and P-E loop studies

The real part of permittivity (ε′) and loss tangent (tanδ) as a function of frequency of Bi₂Fe₄O₉ ceramics near at surrounding temperature is shown in Figure 4(a) and (b). The value of ε′ and tanδ for Bi₂Fe₄O₉ are about 21.57 and 0.05, respectively at frequency 10 Hz. At higher frequency (~1 MHz) the value of ε′ and tanδ are 18.59 and 0.006, respectively. Dielectric behavior (i.e. ε′ and tanδ) decreases with increase in frequency and it is constant at higher frequency region. From Figure 4(a) and (b) we have found that the value of dielectric constant in the whole frequency range (10 Hz–1 MHz) is nearly constant representing the low loss in the prepared ceramic. This result appears to be consistent with previous empirical analysis using the Maxwell-Wagner model with thermal activation across multiple band gaps in isolated impurities [15, 33]. Figure 4(c) shows the semilog plot of conductivity (σ) at room temperature with frequency. The study of the frequency dependence of the conductivity is a deep-rooted method for describing the hopping dynamics of the charge carrier. The conductivity plot exhibits both low and high frequency dispersion phenomena [34, 35, 36, 37]. The low-frequency region corresponds to the dc conductivity (σ_dc), which is due to the band conduction, and it is frequency independent. The high-frequency region corresponds to the ac conductivity (σ_ac), which is frequency dependent. To conclude, the electrical conductivity σ for Bi₂Fe₄O₉ follows the Jonscher power law [37]: σ_ac(ω) = σ_dc + A(T)ωⁿ. Here, A is the pre exponential factor and n is the power law exponent. The exponent n can have a value between 0 and 1. This parameter is frequency independent but temperature and material dependent.

Figure 4.

(a) Dielectric constant, (b) dielectric loss, and (c) frequency-dependent conductivity of Bi₂Fe₄O₉ ceramic.

Ferroelectric hysteresis P(E) loop of the Bi₂Fe₄O₉ ceramic at room temperature represented in Figure 5(a). The obtained loop indicates that there are ferroelectric properties with finite remanent polarization with the applied electric field in the prepared sample. Under an electric field of up to 10 kV/cm, the remanent polarization (2P_r) of Bi₂Fe₄O₉ was found to be 0.006 μC/cm². The observed polarization values are closely consistent with literature results [38].

Figure 5.

(a) Polarization hysteresis P-E loop and (b) field-dependent magnetic hysteresis loop of Bi₂Fe₄O₉ ceramic at room temperature.

3.5 Magnetic analysis (M-H curve)

From the measured M-H loop of Bi₂Fe₄O₉ ceramic (Figure 5(b)), the magnetic parameters we obtained are remanent magnetization (M_r = 4.37 × 10⁻⁴ emu/g), coercivity (H_c = 239.4 Oe) and saturation magnetization (M_s = 0.024 emu/g). In our Bi₂Fe₄O₉ ceramic, antiferromagnetic (AFM) and weak ferromagnetic (WFM) interactions exist simultaneously are consistent with the data reported earlier [15, 38, 39, 40]. The WFM order can be seen in the low magnetic field region. WFM order itself can be understood as a result of canted spin arrangements in two sublattices [41]. As the magnetic field increases, the ferromagnetic order saturates and the antiferromagnetic component dominates. There is even no sign of saturation. Obviously, in the prepared ceramic, M_r and M_s have achieved a non-zero value. We may note that the measured hysteresis curve confirm that the relationship between the applied magnetic field and the magnetization does not evidence of a linear behavior and shows the WFM. In the future, we can improve the magnetic and electric properties of Bi₂Fe₄O₉ ceramics with the appropriate doping or preparation techniques.

4. Conclusions

We have successfully synthesized polycrystalline Bi₂Fe₄O₉ through solid-state reaction route. X-ray diffraction pattern confirmed the formation of Bi₂Fe₄O₉ with the orthorhombic structure (space group Pbam). The obtained density of the present calcined Bi₂Fe₄O₉ ceramic is 6.51 g/cm³. The room temperature Raman peak at 470 cm⁻¹ is may be due to the magnetic origin of the material and might be related to magnon scattering. The value of the dielectric constant seems to be non-variable over the entire frequency range. It has been observed that the sample showed the presence of ferroelectricity. This study conclusively reported that a significant magnetic and ferroelectric order coexist in polycrystalline Bi₂Fe₄O₉ at room temperature. There is hysteresis in the electrical and magnetic curves, which proves the multiferroic nature of Bi₂Fe₄O₉ and making it more suitable for advanced technology and practical applications.

Acknowledgments

We acknowledge the financial support from National Natural Science Foundation of China under grant numbers 11774276 and 51074131. The authors are grateful to Dr. S. Satapathy for their long-term collaboration and numerous fruitful discussions.

Conflicts of interest

All the authors declare that they have no conflict of interest.