Investigation of Structural, Electrical, and Vibrational Properties of Bi1.98A0.02Fe4O9 (A = Ba, Ce) Multiferroic Ceramics

Kumar, Ashwini; Sharma, Poorva; Bhardwaj, Nikhil; Tang, Jingyou; Tan, Guolong

doi:https://doi.org/10.1155/2021/5597415

Advances in Materials Science and Engineering

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Abstract Introduction Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

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Advances in Multiferroic Materials: Bulk, Nano, and Thin Films

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Research Article | Open Access

Volume 2021 | Article ID 5597415 | https://doi.org/10.1155/2021/5597415

Investigation of Structural, Electrical, and Vibrational Properties of Bi_1.98A_0.02Fe₄O₉ (A = Ba, Ce) Multiferroic Ceramics

Ashwini Kumar,¹Poorva Sharma,¹Nikhil Bhardwaj,²Jingyou Tang,¹and Guolong Tan^1,3

Academic Editor: Francisco Javier Fernández Fernández

Received31 Jan 2021

Revised01 Jul 2021

Accepted05 Jul 2021

Published15 Jul 2021

Abstract

In this paper, we report the synthesis, phase formation, and basic characterization of polycrystalline Bi₂Fe₄O₉, Bi_1.98Ba_0.02Fe₄O₉, and Bi_1.98Ce_0.02Fe₄O₉ samples prepared by the sol-gel technique. The crystal structure of the prepared samples has been characterized by means of X-ray diffraction and Raman scattering spectroscopy. All the obtained XRD peaks can be indexed to the orthorhombic Pbam structure and reveal the formation of Bi₂Fe₄O₉. The Raman spectrum identifies A_g, B_2g, and B_3g active optical phonon modes. The crystallite size and morphology of the nanoparticles have been analyzed using scanning electron microscope (SEM). Dielectric constant (ε′) decreases as the frequency increases, and it is constant at the higher frequency region which can be explained based on the ionic conduction phenomenon in the low frequency region. The ε′ values of Bi₂Fe₄O₉(650–850), Bi₂Fe₄O₉(800–850), Bi_1.98Ba_0.02Fe₄O₉, and Bi_1.98Ce_0.02Fe₄O₉ samples at 10 Hz frequency are about 37, 75, 90, and 393, respectively. The observed properties signify that these materials are very useful in advanced technological and practical applications.

1. Introduction

In the past few years, there has been great attention in bismuth ferrite Bi₂Fe₄O₉ because of its potential applications in information and technology applications, digital memory, catalytic as well as in gas sensing [1, 2]. Bi₂Fe₄O₉ has an orthorhombic (space group Pbam) structure and belongs to family of mullite-type crystal structures [3, 4]. A unit cell of Bi₂Fe₄O₉ contains two formula units with evenly Fe ions distributed between octahedral (FeO₄) and tetrahedral (FeO₄) sites. In addition, Bi³⁺ ions are surrounded by eight oxygen ions. The perovskite BiFeO₃ (BFO), which is both FE (T_CE = 1103 K) and antiferromagnetic (AFM) (T_N = 640 K), is one of the well-known multiferroics [5]. However, it is difficult to obtain phase-pure BFO avoiding the formation of second phases during the conventional synthesis process. Various impurity phases have been reported to occur, such as Bi₂Fe₄O₉, Bi₄₆Fe₂O₇₂, and Bi₂₅FeO₄₀ [6, 7]. Among the impurity phases, Bi₂Fe₄O₉ is a well-known material, which has been extensively studied over the past several decades for various functional applications such as a semiconductor gas sensor and as a catalyst for ammonia oxidation [8, 9]. An unexpected multiferroic effect, which is observed as a coexistence of antiferromagnetism and ferroelectric polarization, was reported in Bi₂Fe₄O₉, attributed to the frustrated spin system coupled with phonons [10]. In the past, bulk Bi₂Fe₄O₉ were synthesized by solid-state reaction, and the multiferroic properties have been studied, displaying ferroelectric hysteresis loops at T = 250 K and antiferromagnetic (AFM) ordering T_N = 260 K [11]. It is also claimed that Bi₂Fe₄O₉ is one of the promising multiferroic materials. However, the evidence for ferroelectricity is not very strong. Few authors reported hysteresis data [5, 12, 13] in which the P-E field curves tend to give oval rather than ferroic-shaped hysteresis loops. Bi₂Fe₄O₉ with a band gap of about 1.53 eV presents catalytic activity under visible light irradiation [14, 15].

Sol-gel processing, which gives a polycrystalline material, can control particle size of the crystallized phase and can eliminate any pores. We can also design the shape/form of precursor (bulk, thin-film, and so on) [16]. In addition, metastable phases, which are hardly synthesized via solid-state route, often crystallize in ceramics prepared by sol-gel route or hydrothermal method [17, 18]. Therefore, the processing has been studied intensively in order to produce the sophisticated functional materials. In recent times, Mohapatra et al. studied Ho³⁺ doped Bi₂Fe₄O₉samples and reported that Bi/Fe-site substitution in Bi₂Fe₄O₉ strongly affects its magnetic properties with the observation of an increase in the Neel temperature accompanied by the enhanced magnetodielectric coupling as compared with pure Bi₂Fe₄O₉ [19]. Verseils et al. investigated the magnetic and crystallographic transitions as well as spin-lattice coupling in the Cairo pentagonal magnet Bi₂Fe₄O₉ through infrared synchrotron-based spectroscopy as a function of temperature (20–300 K) and pressure (0–15.5 GPa) [20]. Liang et al. studied the structural, magnetic, and electrical properties of Ba-modified Bi₂Fe₄O₉ samples which were prepared by sol-gel method. They observed the significant enhancement in remnant magnetization, saturation magnetization, dielectric constant as well as conductivity due to substitution of Ba content in Bi₂Fe₄O₉ [21]. In an another recent work, Bi₂Fe₄O₉ and Gd-doped Bi₂Fe₄O₉ samples were synthesized by solid-state mechanochemical ball milling technique, and it revealed the remarkable changes in magnetic and dielectric properties for Gd-doped Bi₂Fe₄O₉ samples due to electron-hole hopping mechanism and spin-orbital coupling through D-M interaction. In addition, they also found that the Gd-doped Bi₂Fe₄O₉ samples had much higher photocatalytic degradation of MB dye than the undoped Bi₂Fe₄O₉ [22]. From the point of theory, Ameer et al. used the first-principle calculations to study the effect of 3d transition metal (TM) ions (Sc, Ti, V, Cr, Mn, Co, Ni, Cu, and Zn) on the structural, electronic, and magnetic properties of ferromagnetic Bi₂Fe₄O₉ and provided a comprehensive understanding of the possible effects of 3d TM dopants on Bi₂Fe₄O₉ [23].

Studies of electric and dielectric properties are 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 the preparation method and sintering conditions [5, 24]. Sol-gel technique to prepare samples has been recognized as one of the most important synthetic methods, with the advantages of low cost, simple process, and controllable morphology. Therefore, the method of obtaining fine Bi₂Fe₄O₉ powder by the sol-gel method has become the goal of many researchers in this field. Therefore, in this paper, we have synthesized the pristine Bi₂Fe₄O₉ and Ba²⁺and Ce³⁺ doped Bi₂Fe₄O₉by sol-gel technique to study their structural and physical properties. In order to obtain a suitable ceramic phase, we examined the phase formation and physical properties and the glassy precursor was subjected to different heat-treatments at different temperatures and for different durations. Despite the evident importance of Bi₂Fe₄O₉ as a functional material, very few reports have appeared. Here, we report the phase formation, physical properties, and vibrational properties of polycrystalline Bi₂Fe₄O₉ ceramic synthesized.

2. Experimental Details

2.1. Synthesis

Bi₂Fe₄O₉, Bi_1.98Ba_0.02Fe₄O₉, and Bi_1.98Ce_0.02Fe₄O₉ ceramics have been successfully synthesized by sol-gel route. All the chemicals are analytical grade and used without further purification. High purity nitrates such as Bi(NO₃)₃.5H₂O and Fe(NO₃)₃.9H₂O were carefully weighed and stoichiometrically mixed. It utilizes multifunctional organic acids capable of chelating metal ions into stable complexes. Suitable metal salts are introduced into the ethylene glycol, citric acid, which is added in large excess to form metal citrates. Bi₂Fe₄O₉: The precursors of Bi(NO₃)₃.5H₂O and Fe(NO₃)₃.9H₂O are taken in a ratio (molar ratio) of 1:1. For 10 gm of sample, we weighed 12.3553 g Bi(NO₃)₃.5H₂O and 20.5805 g Fe(NO₃)₃.9H₂O precursors. Appropriate amount of deionized water is added to dissolve, and then the solution is stirred on a magnetic stirrer. While stirring, citric acids (C₆H₈O₇) are introduced in 1.5 : 1 M ratio with respect to the metal nitrates as a complexing agent and maintained the pH value (6-7) of mixed solution by adding ammonia [11]. Temperature was supplied to allow the gel to form up to a temperature of 100°C, and the xero-gel was completely burned into ash. It was then calcined at 650°C for 2 hours. For the same powder, we carried out the second calcination at 850°C for 2 hours and observed its changes. In another heating treatment, the calcination was done at 800°C and then calcined at 850°C again after grinding. The powder obtained was then ground in agate mortar and pestle. Finally, powder products were reground and pressed into the pellets and then sintered in a muffle furnace at temperature 850°C for 8 hours. Ba and Ce-doped Bi₂Fe₄O₉: Nitrate precursors of Bi, Fe, Ba, and Ce (i.e., Bi(NO₃)₃.5H₂O and Fe(NO₃)₃.9H₂O, Ba(NO₃)₂, and Ce(NO₃)₃.6H₂O) are taken in 1 : 1 molar ratio. Appropriate deionized water was added to dissolve, and solution was then stirred on magnetic stirrer. During stirring, we have added the citric acid (C₆H₈O₇) in 1.5 : 1 M ratio with respect to the metal nitrates as complexant, and the solution was adjusted to a pH value ∼6-7 by addition of ammonia. Temperature was supplied to form gel up to 100°C and after that xero-gel was completely burned to form ash. First calcination was done at 650°C, and second calcination was done at 850°C with intermediate grinding. Finally, the pellets were sintered in a muffle furnace at the temperature of 850°C for 8 hours.

2.2. Experimental Techniques

In order to study the crystal structure and phase formation, we obtained the X-ray diffraction (XRD) pattern for the powder samples using a Bruker D8 Advance X-ray diffractometer with a step size of 0.02° and CuKα (1.5406 Å) radiation. The surface morphology of the prepared samples (pellet form) was investigated using a JEOL, JSM-5600 scanning electron microscope. Further, the particle sizes were estimated from SEM micrographs using ImageJ software. Raman measurements on as synthesized sample were 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 1 Hz–10 MHz on the Novocontrol alpha-ANB impedance analyzer at room temperature. The dielectric measurements were performed on the circular pellet (10 mm diameter and 1 mm thickness) samples with silver paint coated on two sides as the electrodes. The real (ε′) and imaginary (ε″) parts of the complex dielectric constant were calculated from raw data and the pertinent sample dimensions as follows:where C is the capacitance, d is the thickness of the pellet, A is the area of sintered pellet, and ε₀ is the permittivity of vacuum.

3. Results and Discussion

3.1. Crystal Structure Analysis

Figure 1 shows the X-ray powder diffraction (XRD) pattern of pristine Bi₂Fe₄O₉, Ba, and Ce-doped Bi₂Fe₄O₉ samples carried out at room temperature. As we mentioned before, the Bi₂Fe₄O₉ samples were calcined at different temperatures and are further designated as Bi₂Fe₄O₉(650–850) for Bi₂Fe₄O₉ calined at 650°C and 850°C and Bi₂Fe₄O₉(800–850) for Bi₂Fe₄O₉ sample calcined at 800°C and 850°C. Finally, these both samples have been sintered at 850°C for 8 hours. For the samples Bi₂Fe₄O₉(650–850) and Bi₂Fe₄O₉(800–850), the diffraction peaks are well indexed with the orthorhombic structure of Bi₂Fe₄O₉ (space group: Pbam) and match well with JCPDS card No. 74–1098 [25]. These results confirm that phase-pure Bi₂Fe₄O₉ can be synthesized by sol-gel route. The XRD peak close to 11° is of bismuth due to the presence of Bi(NO₃)₃.5H₂O [26]. From (a) and (b) in Figure 1, we can observe the broading of the peaks in Bi₂Fe₄O₉(650–850) sample due to low calcination temperature, whereas due to high calcination temperature, proper crystalline phase has been observed in Bi₂Fe₄O₉(800–850) sample. Due to calcination temperature variation, double peaks transform into a single peak from sample Bi₂Fe₄O₉(650–850) to Bi₂Fe₄O₉(800–850). Furthermore, the refined lattice parameters of sample Bi₂Fe₄O₉(650–850) are a = 7.94 Å, b = 8.40 Å, and c = 5.92 Å and a = 7.95 Å, b = 8.45 Å, and c = 5.94 Å for Bi₂Fe₄O₉(800–850), revealing the slight expansion in both the ab plane and the c axis with doping contents increasing. In addition, with the increase in calcination temperature, there is a limited intensity increasing and narrowing of the diffraction peaks, indicative of better crystallization and the increase in crystalline sizes. Using the Debye Scherrer formula, the average crystalline size can be estimated to be about 52 nm, 80 nm for Bi₂Fe₄O₉(650–850) and Bi₂Fe₄O₉(800–850).

XRD for alkaline earth metal Ba⁺² ion and rare earth Ce⁺³ ion-doped Bi₂Fe₄O₉ ceramics are shown in (c) and (d) of Figure 1. For the doped samples, the diffraction peaks are well indexed with the orthorhombic structure of Bi₂Fe₄O₉ (space group: Pbam) and matche well with (JCPDS: 25–0090). As witnessed from (c) and (d) in Figure 1 in the doped Bi₂Fe₄O₉samples, there is a shift in lower theta value due to ionic radius mismatch of dopent (ionic radius of Ba = 1.35 and Ce = 1.01) with Bi⁺³ (0.96) ion. The ionic radius of Ba and Ce ions is greater than that of Bi ion; that is why we are getting a shifting at lower theta value as compared with parent Bi₂Fe₄O₉ ceramic. The lattice parameters were obtained using indexing such as a = 7.101 Å, b = 8.50 Å, and c = 5.94 Å for Bi_1.98Ba_0.02Fe₄O₉ and a = 7.99 Å, b = 8.48 Å, and c = 5.93 Å for Bi_1.98Ce_0.02Fe₄O₉ sample. The crystalline size for Ce⁺³ and Ba⁺² ions-doped ceramics is found to be 62 and 65 nm, respectively.

3.2. Microstructure Analysis

As the dielectric, optical, and magnetic properties are highly dependent on composition and microstructure acquired by the samples, we used the scanning electron microscope (SEM) to study the surface morphological and microstructural characteristics of pristine and doped Bi₂Fe₄O₉ compounds. Figures 2–2 show typical SEM images of Bi₂Fe₄O₉(650–850), Bi₂Fe₄O₉(800–850), Bi_1.98Ba_0.02Fe₄O₉, and Bi_1.98Ce_0.02Fe₄O₉ samples, respectively. Compared with the pristine Bi₂Fe₄O₉ sample, the grains of the doped Bi₂Fe₄O₉ sample are randomly oriented and have the smaller grain size. We determined average grain size using ImageJ Software and found to be nearly 0.8–1.2 micrometer. As we can see in the SEM images, the micrograph revealed that Bi₂Fe₄O₉ materials mostly consist of nonuniform grains and each grain is interconnected edge by edge. Distribution of grains is homogeneous and agglomerations of the particles. Samples seem to be porous in nature, which is expected to highly influence the dielectric properties. Therefore, we can say that the particle size increases with increase in the calcination temperature as we can see in SEM image of Bi₂Fe₄O₉(800–850) sample. The typical SEM images reveal that microstructures comprise of nonuniform grains with varying particles size indicating polycrystalline nature of as-prepared samples [27].

3.3. Raman Scattring Analysis

The Raman spectrum of Bi₂Fe₄O₉ at room temperature is depicted in Figure 3. The Raman active modes of the structure can be summarized using the irreducible representation 12A + 12B1v9B2 + 9 B3, which is employed to describe Raman modes of orthorhombic (Pbam space group) [25]. In the measured Raman spectra of Bi₂Fe₄O₉(650–850), the A_g modes existed at 83, 95, 115, 122, 139, 222, 289, and 607 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 472 cm⁻¹ 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. Similarly, for Bi₂Fe₄O₉(800–850), the obtained Raman A_g modes are at 91, 206, 282, 326, 365, 430, 554, and 640 cm⁻¹. We can analyze that there are number of modes decreasing due to increase in calcination temperature. The A_g modes for Bi_1.98Ba_0.02Fe₄O₉ are as follows (53, 93, 119, 183, 209, 313, 446, and 558 cm⁻¹). Similarly, for Bi_1.98Ce_0.02Fe₄O₉, the A_g modes are at 53, 68, 118, 183, 206, 308, 442, and 560 cm⁻¹. 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 [28] or cuprates [11, 29]. At higher frequency (>250 cm⁻¹), it is unlikely that the magnetic-order-induced bands correspond to one-magnon excitations but rare-earth orthoferrites (RFeO₃; R = Dy, Ho, Er, Sm, and so on) have frequencies below 25 cm⁻¹ for comparison of the zone-center magnons [30].

3.4. Dielectric Response

Figure 4 illustrates the frequency dependence of dielectric constant (ε′) for Bi₂Fe₄O₉ ceramics in the frequency range of 1 Hz to 10 MHz with room temperature. As observed from Figure 4, the dielectric constant (ε′) decreases with increase in frequency and almost constant at high frequency due to dielectric relaxations in all prepared samples. The observed high value of dielectric constant and dielectric loss at low frequencies indicates the presence of significant dc conductivity, which may be due to space charge polarization that originated from oxygen vacancies, bismuth vacancies, and so on [31]. In general, the dipolar, electronic, ionic, and interfacial polarizations have effect on the dielectric constant of any material. At low frequencies, dipolar polarization and interfacial polarization are effective for the dielectric constant. However, at higher frequencies, the electronic polarization is effective and the dipolar contribution becomes insignificant. The decrease in dielectric constant with increased frequency could be explained based on the phenomenon of dipole relaxation. When the sample is placed under the field, charge carriers move freely within the crystal grains but contain at the boundary and temporarily stop until they pass the boundary. This will result in the formation of potential barrier across the sample and gets polarized. Therefore, it was found that the dielectric constant decreases with increasing frequency [32]. At higher frequencies, the decrease in the dielectric constant is obvious due to lagging behind of applied field. Beyond the certain frequencies, dipoles do not respond to the applied field and thus remain nearly constant.

The value of dielectric constant (ε′) for all Bi₂Fe₄O₉-based sintered samples at frequency of 10 Hz has been found to be 37, 75, 90, and 393 for Bi₂Fe₄O₉(650–850), Bi₂Fe₄O₉(800–850), Bi_1.98Ba_0.02Fe₄O₉, and Bi_1.98Ce_0.02Fe₄O₉, respectively, whereas the value of dielectric constant (ε′) at higher frequency of 1 MHz has been found to be 11.68, 12.90, 5.06, and 5.36 for Bi₂Fe₄O₉(650–850), Bi₂Fe₄O₉(800–850), Bi_1.98Ba_0.02Fe₄O₉, and Bi_1.98Ce_0.02Fe₄O₉ samples, respectively. As shown in Figure 4, dielectric constant ε′ relatively increases with increasing sintering temperature confirming that it exhibits space charge polarization, and it can be explained with the help of the Maxwell–Wagner effect. These results appear to be consistent with previous empirical analysis using the Maxwell–Wagner model with thermal activation across multiple band gaps in isolated impurities [33, 34]. Apart from this, it is observed that the dielectric constant shows enhancement after Ba and Ce substitution in Bi₂Fe₄O₉ at lower frequencies as compared with pristine Bi₂Fe₄O₉. This increase may be ascribed to large number of defects due to Ba and Ce doping in Bi₂Fe₄O₉ lattice. However, at higher frequencies, the dielectric constant shows reduction in the value and is obvious due to lagging behind the applied field. It can be noticed that Ba and Ce additions to Bi₂Fe₄O₉ lattice help to improve the dielectric constant. Figure 5 represents the dielectric loss (tanδ) as a function of frequency of Bi₂Fe₄O₉-based ceramics at room temperature. Furthermore, Ba and Ce-doped Bi₂Fe₄O₉ exhibited higher value of dielectric constant and dielectric loss as compared with pristine Bi₂Fe₄O₉ due to large off-center displacement of Fe³⁺ ions in octahedral and reduced particle size [35]. As the hopping process of conducting electrons and holes increases, the band gap decreases and the conductivity increases [36]. We have observed that the value of dielectric loss is low for Bi₂Fe₄O₉(650–850) among the entire prepared sample, and dielectric relaxation is possible in all prepared samples.

4. Conclusions

In conclusion, the polycrystalline samples of Bi₂Fe₄O₉(650–850), Bi₂Fe₄O₉(800–850), Bi_1.98Ba_0.02Fe₄O₉, and Bi_1.98Ce_0.02Fe₄O₉ samples were successfully prepared by sol-gel route. By the indexing of X-ray diffraction data, the formation of Bi₂Fe₄O₉ phase having orthorhombic structure with Pbam space group is confirmed. There is no evidence for structural change in the prepared samples. Raman spectra reveal that the presence of magnon peak is only in Bi₂Fe₄O₉(650–850) sample. The value of ε′ for all Bi₂Fe₄O₉-based Bi₂Fe₄O₉(650–850), Bi₂Fe₄O₉(800–850), Bi_1.98Ba_0.02Fe₄O₉, and Bi_1.98Ce_0.02Fe₄O₉ samples is about 37, 75, 90, and 393, respectively, at frequency of 10 Hz. At higher frequency of 1 MHz, the values of ε′ are 11.68, 12.90, 5.06, and 5.36 for Bi₂Fe₄O₉(650–850), Bi₂Fe₄O₉(800–850), Bi_1.98Ba_0.02Fe₄O₉, and Bi_1.98Ce_0.02Fe₄O₉ samples, respectively. Dielectric constant (ε′) decreases with increasing frequency and becomes almost constant at high frequency region. The value of dielectric loss is low for Bi₂Fe₄O₉(650–850) among the entire prepared sample, and dielectric relaxation is possible in all as-prepared samples. The decrease in dielectric constant with increased frequency could be explained on the basis of the ionic conduction phenomenon in the low frequency region. These properties make this material very useful in technological and practical applications.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest regarding the publication of this study.

Acknowledgments

The authors acknowledge the financial support from National Natural Science Foundation of China under grant no. 11774276. Kumar and Sharma acknowledge the Luzhou Vocational and Technical College for providing them start-up research grants and support and also acknowledge the Luzhou Talent Work Leading Group Office for job-incentive and resettlement subsidies for high-level talents. The authors are grateful to Dr. S. Satapathy for their long-term collaboration and numerous fruitful discussions.

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Copyright © 2021 Ashwini Kumar et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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