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

In this paper, we report the synthesis, phase formation and basic characterization of polycrystalline Bi 2 Fe 4 O 9 , Bi 1.98 Ba 0.02 Fe 4 O 9 , and Bi 1.98 Ce 0.02 Fe 4 O 9 sample prepared through sol-gel route. Crystal structure of the as prepared samples have been characterized by X-ray diffraction and Raman scattering spectroscopy. All the obtained XRD peaks could be indexed to orthorhombic Pbam structure and revealed the formation of Bi 2 Fe 4 O 9 . Raman spectrum identies A g , B 2g and B 3g active optical phonon modes. The size and morphology of the nanoparticles have been analyzed using scanning electron microscopy (SEM). Dielectric constant (e') decreases with increase in frequency and it is constant at higher frequencies region and it can be explained on the basis of ionic conduction phenomenon in the low frequency region. The value of e' for all Bi 2 Fe 4 O 9 based S1, S2, S3 and S4 samples are about 37, 75, 40 and 393, respectively at frequency 10 Hz. The observed properties signify that these materials are very useful in advanced technological and practical applications.


Introduction
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-lm, etc) [1]. In addition, metastable phases, which are hardly synthesized via solid-state route, often crystallize in ceramics prepared by Sol-gel route [2,3]. Therefore, the processing has been studied intensively in order to produce the sophisticated functional materials. Crystal structure of Bi 2 Fe 4 O 9 is orthorhombic (Pbam) and belongs to family of mullite-type crystal structures [4,5]. A unit cell of Bi 2 Fe 4 O 9 contains two formula units with evenly Fe ions distributed between octahedral (FeO 4 ) and tetrahedral (FeO 4 ) sites. In addition, Bi 3+ ions are surrounded by eight oxygen ions. An unexpected multiferroic effect, which is observed as a coexistence of antiferromagnetism and ferroelectric polarization, was reported in Bi 2 Fe 4 O 9 , attributed to frustrated spin system coupled with phonons [6]. The perovskite BiFeO 3 (BFO), which is both FE (T CE =1103 K) and antiferromagnetic (AFM) (T N =640 K), is one of the well-known multiferroics. However, it is di cult 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 Bi 2 Fe 4 O 9 , Bi 46 Fe 2 O 72 , and Bi 25 FeO 40 . [7,8]. Among the impurity phases, Bi 2 Fe 4 O 9 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 [9][10][11].
Very recently, bulk Bi 2 Fe 4 O 9 were synthesized by solid state reaction and the multiferroic properties have been studied, displaying FE hysteresis loops at T = 250 K and AFM ordering T N =260 K [12]. This shows that Bi 2 Fe 4 O 9 is one of the promising multiferroic materials. Whereas, impurity phases particularly Fe 2 O 3 were existed in the samples, which could affect the magnetic and FE properties of polycrystalline Bi 2 Fe 4 O 9 ceramics. 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 in uenced by an order of magnitude of conductivity and is mostly dependent on preparation method and sintering conditions [7].
In this paper, we have synthesized the pristine Bi 2 Fe 4 O 9 and Ce 3+ and Ba 2+ ion modi ed Bi 2 Fe 4 O 9 to improve its structural and physical properties. Individually, both of the Ce 3+ and Ba 2+ ions were doped at Bi-site in Bi 2 Fe 4 O 9 . In order to obtain the proper phase of ceramic we examine the phase formation and physical properties, the glassy precursor was subjected to different heat-treatment at different temperatures for different time duration. Despite the evident importance of Bi 2 Fe 4 O 9 as a functional material, very few reports have appeared. Here, we report the phase formation, physical properties and vibrational properties of polycrystalline Bi 2 Fe 4 O 9 ceramic synthesized by a sol-gel route. Precursors of bismuth nitrate and iron nitrate are taken in speci c ratio (molar ratio). Appropriate deionized water was added to dissolve and solution was then stirred on magnetic stirrer. While stirring citric acid was introduced as a complexant and pH value (6-7) is maintained by adding ammonia [12]. Temperature is supplied to form gel up to 100 °C and after that xero-gel was completely burnt to form ashes. Then calcination is done at 650 °C for 2 h. For the same powder we did second calcination again at 850 °C for 2 h and observe the changes. In another heating treatment the calcination was done at 800 °C and then after grinding again calcined at 850 °C. Obtained powder was then ground in agate mortar and pestle. Finally, the pellets were sintered at temperature 850 °C for 8 hrs, respectively.

Ba and Ce doped Bi 2 Fe 4 O 9
Nitrates of Bi, Fe, Ba and Ce are taken in speci c molar ratio. Appropriate deionized water was added to dissolve and solution was then stirred on magnetic stirrer. While stirring citric acid was introduced as a complexant and pH value (6-7) is maintained by adding ammonia. Temperature was supplied to form gel up to 100 °C and after that xero-gel was completely burnt to form ashes. First calcination was done at 650 °C and second calcination was done at 850 °C with intermediate grinding. Finally, the pellets were sintered at temperature 850 °C for 8 hrs.

Experimental techniques
X-ray diffraction (XRD) pattern were taken using CuKα1 radiation (λ = 1.5406 Å) of a Bruker D8 Advance X-ray diffractometer. The sample morphology was investigated with a 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.

Crystal structure analysis
Room temperature XRD pattern of Bi 2 Fe 4 O 9 and Ba and Ce-doped Bi 2 Fe 4 O 9 samples as prepared by Solgel route is shown in Fig. 1(a-d) [13] with no trace of other impurity phases within the limit of instrument sensitivity. These result con rms that phase-pure Bi 2 Fe 4 O 9 can be synthesized by sol-gel route. From Fig. 1(a, b), we can observe the broading of the peaks in (Bi 2 Fe 4 O 9 )650-850 sample due to low calcination temperature. Whereas, due to high calcination temperature, proper crystalline phase has been observerd in ( XRD for alkaline earth metal Ba + 2 ion and rare earth Ce + 3 ion doped Bi 2 Fe 4 O 9 ceramics are shown in Fig. 1(c, d). For the doped samples the diffraction peaks are well indexed with the orthorhombic structure of Bi 2 Fe 4 O 9 (space group: Pbam), matches well with (JCPDS: 25-0090). As witnessed from the grain size is estimated to be about 65 nm. As we can see in the SEM images, the particles of all these four samples are agglomerated and little porous in nature. Therefore, we can say that the particle size increses with increasing the calcination temperature as we can see in SEM image of (Bi 2 Fe 4 O 9 )800-850 sample. The typical SEM images reveals that microstructures are dense comprising of non-uniform grains with varying particles size indicating polycrystalline nature of as prepared samples. The SEM image also showed some poros among the loosely connected grains in the sample.

Raman scattring analysis
Raman spectrum of Bi 2 Fe 4 O 9 at room temperature is depicted in Fig. 3(a-d). The Raman active modes of the structure can be summarized using the irreducible representation 12A g + 12B1 g + 9B2 g + 9 B3 g , which is employed to describe Raman modes of orthorhombic (Pbam space group) [13]. In the measured Raman spectra of (Bi It would be more practical to study the magnetic excitations in Bi 2 Fe 4 O 9 under the assumption that they involve two-magnon processes, like in the wellknown cases of ferrites [14] or cuprates [12,15]. At higher frequency (> 250 cm − 1 ), it is unlikely the magnetic-order-induced bands correspond to one-magnon excitations but in rare-earth orthoferrites (RFeO 3 ; R = Dy, Ho, Er, Sm etc.) have frequencies below 25 cm − 1 for comparison the zone-center magnons [16].

Dielectric response
The real part of permittivity (ε') as a function of frequency of Bi 2 Fe 4 O 9 based ceramics at room temperature is shown in Fig. 4(a-d) Fig. 4 we have found that the value of dielectric constant in the whole frequency range (100 Hz-1 MHz) is nearly constant in the prepared ceramics. This result appears to be consistent with our previous empirical analysis using the Maxwell-Wagner model with thermal activation across multiple band gaps in isolated impurities [17]. The dielectric loss (tanδ) as a function of frequency of Bi 2 Fe 4 O 9 based ceramics at room temperature is shown in Fig. 5. We have observed that the value of dielectric loss is low for (Bi 2 Fe 4 O 9 )650-850 among the entire prepared sample, dielectric relaxation are possible in all prepared sample.

Conclusions
In conclusion, the polycrystalline samples of (Bi