Synthesis of single phase bismuth ferrite compound by ... - Springer Link

J Mater Sci: Mater Electron (2013) 24:2950–2955 DOI 10.1007/s10854-013-1196-0

Synthesis of single phase bismuth ferrite compound by reliable one-step method Shreeja Pillai • Deepika Bhuwal • T. Shripathi Vilas Shelke

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Received: 14 January 2013 / Accepted: 20 March 2013 / Published online: 27 March 2013 Ó Springer Science+Business Media New York 2013

Abstract We report the synthesis of polycrystalline BiFeO3 compound by solid state route and its structural and magnetic properties were investigated. The structure confirmation and phase purity of the samples were investigated by X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) techniques. No signature of thermodynamically probable impurity phases was observed in these samples. The Rietveld refined structural parameters of the XRD pattern revealed rhombohedral (R3c) structure. Scanning Electron Microscopy images show samples with larger grain size. The Raman active modes predicted by the group theory confirmed the rhombohedral symmetry. The study of the oxidation states of Bi, Fe and O through XPS analysis indicated that the sample quality is not deteriorated by vacancies or other defects formation. The absence of iron based impurity phases is confirmed by the antiferromagnetic nature of the sample obtained by magnetization measurements.

1 Introduction A specific ordering of electric and magnetic dipoles takes place through two distinct and diversified mechanisms as they follow different spatial inversion and time reversal symmetry aspects. Such materials are known as ferroelectric, ferromagnetic, etc. irrespective of their often ‘ferro-free’ S. Pillai D. Bhuwal V. Shelke (&) Novel Materials Research Laboratory, Department of Physics, Barkatullah University, Bhopal 462026, India e-mail: [email protected] T. Shripathi UGC–DAE Consortium for Scientific Research, DAVV campus, Khandwa Road, Indore 452001, India

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compositions. A concomitant presence and coupling between the so called ‘ferroic’ order parameters was observed in a new class of materials termed as multiferroics [1–3]. Among all the multiferroics known, BiFeO3 is probably the most widely studied, since it exhibits significant ferroelectric and magnetic ordering at room temperature [4, 5]. The lead free composition, high transition temperature and large polarization values were major driving force for several experimental as well as theoretical studies [4, 6] and also for invention of additional phenomena like photovoltaic effect [7], metal–insulator transition [8] etc. At room temperature bulk BiFeO3 crystallizes in a rhombohedral symmetry with R3c space group. The perovskite structured oxide shows G type antiferromagnetism with Neel temperature TN = 643 K and has a ferroelectric polarization along (111) plane with very high Curie temperature Tc = 1,103 K. BiFeO3 thin films deposited through various methods have been widely studied compared to bulk counterparts as synthesis of bulk ceramics leads to mixture of main phase along with other impurity phases [9–14]. The thin film process like pulsed laser deposition is governed by several thermodynamic and kinetic parameters, which provide flexibility to tune a particular structure. Moreover, the substrate induced strain favors growth of close packed perovskite structure instead of non-pervoskite impurity phases. In our studies on BiFeO3 thin films, we have revealed detailed role of kinetic growth parameters [15], strain engineering [16], domain engineering [17] and domain scaling [18] on various physical properties of BiFeO3 phase. The same principles may not be applicable to the bulk material as it is. The polycrystalline bulk material is more suitable for generalized study and separate strategy is needed for the synthesis of phase pure bulk materials. In comparison to other isostructural perovskite oxides like LaMnO3, SrTiO3, BaTiO3, etc. and their variants, the

J Mater Sci: Mater Electron (2013) 24:2950–2955

synthesis of single phase of BiFeO3 compound is immensely difficult. The constituent oxides like Bi2O3 and Fe2O3 have adverse thermodynamic characters as Bi2O3 is volatile and Fe2O3 is less reactive at low temperature. Therefore, the union of these compounds to yield a single phase ternary compound is possible within a narrow set of synthesis parameters. The problem is further compounded by the prominent intermediate phases viz., Bi2Fe4O9 (mullite) and Bi25FeO39 (sillenite) which can coexist with zero degrees of freedom according to Gibb’s phase rule [19]. The technological importance and synthesizing difficulties jointly stimulated emergence of several strategies for the synthesis of single phase BiFeO3 compounds. These can mainly be categorized into wet chemical and solid state routes. The wet chemical routes like sol–gel [20], hydrothermal [21], co-precipitation [22] were primarily used for nanomaterials synthesis. The conventional ‘removal of impurity by concentrated HNO3’ strategy was first developed by Achenbach et al. [23] and followed by other researchers [24–26]. Chen et al. [27] reported an environment friendly variant of this method employing leaching by deionized water. On the other hand, Wang et al. [28] reported rapid liquid phase sintering method which was adopted by some other groups [29]. Although these methods yield reasonably single phase compound, deterioration of sample quality from other sources may not be overruled. For example, use of concentrated acid in the first approach and impurities from crucibles etc. during rapid firing process in the second case can produce undetectable level of impurities. In addition, cationic vacancies, oxygen nonstoichiometry and grain boundary defects are largely beyond process parameter control. Some researchers also used two stage sintering method with counter-active strategies to mitigate Bi loss [30–32]. Incidentally, there exists zeal for a simple reliable approach for the synthesis of pure BiFeO3 and its validation through compatible characterization techniques. We report here one-step solid state synthesis route and detailed structural analysis through X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy techniques. The one step method will be quite useful for the synthesis of high quality BiFeO3 compound with maximum reproducibility.

2 Experimental details We used conventional solid state reaction with a solitary thermal treatment was used for the preparation of single phase BiFeO3 ceramic. Stoichiometric proportions of high purity Bi2O3 and Fe2O3 compounds were taken as starting materials, mixed in agate mortar pestle to form a fine powder by manual grinding for 10 h using acetone as a medium. The grinding was performed in two split durations

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of 5 h each with an interval of 12 h. The powder thus obtained was pressed into pellets of 12 mm diameter applying a weight force of 7 tons. The pellets were sintered at optimized temperature of 870 °C for 30 min and rapidly cooled to room temperature; the heating rate was maintained at 5 °C/min. The phase purity and crystal structure of the samples were analyzed by powder X-ray diffraction technique using Bruker D8 Advance X-ray diffractometer equipped with ˚´ ) source and Bruker Lynx eyed detector with CuKa (1.54 A step size of 0.02°. A Scanning Electron Microscopy (SEM) technique was used to observe the microstructure. Raman spectroscopy measurements were performed on Horiba JY HR 800 micro Raman system equipped with Argon 488 nm laser as an excitation source. The chemical state and stoichiometry were determined by X-ray photoelectron spectroscopy (XPS, VSW, England) with AlKa source (excitation energy 1,486.6 eV). The field dependent magnetization measurement of the sample was carried out using Quantum Design MPMS Superconducting Quantum Interference Device (SQUID) magnetometer.

3 Results and discussion We synthesized several batches of samples under variant conditions and analyzed them through X-ray diffraction techniques [33]. The final optimized conditions for the onestep method were heating rate 5 °C/min, sintering temperature 870 °C and sintering duration 30 min. In addition, grinding of mixed precursors for long duration (10 h) to reduce the particle size is very essential. A representative X-ray diffraction (XRD) pattern of BiFeO3 compound is shown in Fig. 1. The pattern shows BiFeO3 as major phase with 1 % impurity phase identified by small unindexed peaks. The Rietveld refinement of the diffraction pattern was done using software FULLPROF suite for estimation of structural parameters. The refined parameters are given in Table 1. The XRD pattern shows clear single phase behavior with no trace of impurity. The possible secondary phases, which occur on the compound phase diagram, are mainly Bi2O3, Fe2O3, Bi2Fe4O9 and Bi25FeO39. At higher temperature, Bi2O3 decomposes leading to loss of Bi which may result in residual Fe2O3 impurity. On the other hand, at temperature around 820 °C (decomposition temperature of Bi2O3) formation of Bi2Fe4O9 and Bi25FeO39 phase takes place at the expense of BiFeO3 phase. According to Gibb’s phase rule, the three phases can coexist with zero degrees of freedom as the number of components and oxidation state of Fe remains invariant [19]. The impurity phases can react to form perovskite BiFeO3 phase and BiFeO3 may not decompose back to other phase up to the temperature 930 °C [31]. Although temperature is the main

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Fig. 1 Rietveld refined X-ray diffraction pattern of one-step synthesized BiFeO3 compound

thermodynamic driving force, time duration and particle size are very crucial parameters. Longer duration gives ample time for substantial loss of Bi and shorter duration can keep the reaction incomplete due to low ionic mobility. Therefore, reduced particle size is essentially required to complete the reaction in shorter duration. The reported variation in the values of temperature and duration in the literature may be accounted to variation in the particle size [23, 32]. In the rapid liquid phase sintering process, the high heating rate of 100 °C/s is considered necessary so that Bi2O3 and Fe2O3 get reacted directly or through intermediate phases to form BiFeO3 phase before any loss of Bi [28–30]. However, our study indicates that rapid firing is not essential as the samples take very less time to reach 870 °C (optimized value) from 820 °C (decomposition temperature). A deviation from temperature 870 °C and duration of 30 min resulted in the formation of parasitic phases [33]. The conventional sintering was followed by rapid cooling to avoid local temperature deviation and to reduce the possibility of formation of other phases. The Rietveld refinement carried out considering hexagonal unit cell with rhombohedral symmetry gave lattice parameter values a = b = 5.567, c = 13.8403 and a = b = 90°, c = 120°. In the refinements, pseudo-Voigt function and sixth order polynomial were used to define the profile Table 1 Atomic positions, Rietveld refined parameters and lattice parameters for BiFeO3

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Fig. 2 Scanning Electron Microscopy images of BiFeO3 samples sintered at a 860 °C for 30 min. and b 870 °C for 30 min

shape and background respectively. The rhombohedral phase was considered for the refinement as the XRD profile featured characteristics such as doublet of highest intensity peaks and presence of super lattice reflection at 2h = 39° reported previously [34]. Both Bi3? and Fe3? cations are displaced from their centrosymmetric positions which give rise to permanent dipole moment required for ferroelectric order. The asymmetric R3c unit cell of BiFeO3 in the present case has two wychoff sites 6a occupied by Bi and Fe whereas

Atomic positions Atom

Site

x

y

Refined parameters

Lattice parameters

Rp = 7.29, Rwp = 9.72, Rexp = 3.96

a = b = 5.567; c = 13.8403; a = b = 90°; c = 120°

z

Bi

6a

0

0

0.3585

Fe

6a

0

0

0.1422

O

18b

0.2538

0.2906

0.1476


18b occupied by O [34] The refined lattice parameters and positional coordinates of the constituent atoms observed are synchronous to those observed for single phase BiFeO3 samples [34, 35]. Since grain size is a crucial parameter for the synthesis of single phase BiFeO3, we used Scanning Electron Microscopy to study the micro structural behavior and grain size of unsintered and sintered samples as shown in Fig. 2a, b. The Fig. 2a shows the SEM image of ground powder before sintering. The manual grinding leads to nonuniform distribution of grains with fractured grain boundaries. The distant agglomerates of small particles having an average grain size of 3.3 lm were observed in unsintered powder. The SEM image in Fig. 2(b) is for the sample sintered at 870 °C for 30 min. A complete grain growth with interdiffused grain boundaries was observed in this sample. It is noteworthy that small particle size is essential for the single phase BiFeO3 formation as the sintering duration is limited to small period. No additional measures like intermediate grinding were used to enhance the reaction rate in this single step method. The reaction had to proceed through the interface of product phase within the duration of 30 min. In this small duration, the grain growth is quite rapid achieving average grain size around 5 lm. The Raman spectra of BiFeO3 sample is shown in Fig. 3. The peak positions obtained are analogous to the natural frequency of Raman active modes for R3c structured BiFeO3 compound. The selection rule predicts 13 Raman active modes for rhombohedral structure using irreducible representation TRaman = 4A1 ? 9E [36]. In addition, there are 5A2 modes which are silent. The Bi atoms participate in low wave number modes while oxygen dominates the high wave number modes. The Fe atoms are mainly involved in intermediate wave number modes and also contribute to some higher wave number modes. Pandit et al. [24]. have reported 10 Raman active modes whereas Kothari et al. [37] observed all the 13 predicted Raman active modes comprising of four A1 at 135, 167, 218 and 430 cm-1 and nine E modes at 255, 283, 351, 321,467, 526, 598, 71 and 98 cm-1 in polycrystalline BiFeO3 samples. Similarly, we observed 4A1 and 6E modes in the frequency range of 100–700 cm-1. The four A1 modes were observed at 134, 170, 217 and 472 cm-1 whereas E modes were observed at 261, 277, 346, 372, 541 and 640 cm-1. The Raman spectroscopy clearly indicates that the BiFeO3 sample prepared in single step is phase pure with no signature of parasitic phases. The slight shift in the peak positions may be attributed to the sample preparation conditions as the peak positions of Raman vibration modes are dependent on oxygen stoichiometry and internal strain within the sample [24]. Previously, we have reported structural modulations for very large strain induced by substrate in BiFeO3 thin films [16]. A stoichiometric BiFeO3 may show a variety of defect structures. The volatility of Bi and change in the oxidation

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Fig. 3 A typical Raman spectra of one-step synthesized BiFeO3 sample indicating various Raman active modes

state of Fe (Fe3? to Fe2?) could be compensated by formation of oxygen vacancies or formation of Bi ions with oxidation states greater than 3. This fact plays a vital role in determining the magnetic and electrical properties. The confirmation of the phase purity of the BiFeO3 sample was achieved by XPS measurements of electron binding energies of the constituent elements (Bi, Fe and O). The typical XPS core level spectra for the three constituent elements are shown in Fig. 4a–c. The narrow scan spectra of Bi4f, Fe2p and O1 s peaks were fitted using software XPS PEAK 4.1 and the binding energies were rectified after correction of charging effects using C1s peak at 284.5 eV. The bismuth ion is found to have an oxidation state of ?3 as the core level spectra for Bi4f are at 157.8 and 163.1 eV for 7/2 and 5/2 spin orbit doublet components respectively with spin orbit splitting energy of 5.3 eV. The pure Fe2p narrow scan shows two wide doublet peaks with spin–orbit splitting energy of 13.2 eV and positioned at 710.3 eV for Fe2p3/2 and 723.5 eV Fe2p1/2 with no indication of any other oxidation state of Fe which is in agreement with reported data by various authors [38–40]. The O1s peak narrow scan is positioned at 529.8 eV which indicates the presence of O2- bonding state. The binding energy positions are also in good agreement with detailed XPS results on BiFeO3 presented by Mandal et al. [41]. The optimum oxidation values within the experimental accuracy indicate absence of iron based impurity phases, which may occur due to loss of Bi during synthesis. The antiferromagnetic ordering of BiFeO3 is G type, in which Fe magnetic moments are aligned ferromagnetically within (111) and antiferromagnetically between adjacent (111) and exhibits spin cycloid structure in the bulk. We measured the magnetization as a function of magnetic field

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4 Conclusions We have developed a simple method for the synthesis of single phase BiFeO3 ceramic. This method avoids the inherent drawbacks of rapid liquid phase sintering or concentrated acid leaching. The phase purity and structure were confirmed through Rietveld refinement of XRD data and peak positions of active modes in Raman spectroscopy data. The microstructure and oxidation states revealed through SEM and XPS study also indicate that the sample quality is good. The typical antiferromagnetic behavior for BiFeO3 is also observed through magnetization study. Acknowledgments This work is financially supported by M. P. Council of Science and Technology, Bhopal and the University Grants Commission, New Delhi. The work was performed at UGCDAE Consortium for Scientific Research, Indore. The authors are thankful to Dr. Mukul Gupta and Dr. Vasant Sathe, UGC-DAE Consortium for Scientific Research, Indore for providing experimental facilities.

References

Fig. 4 XPS core level spectra for a Bi, b Fe, and c O elements

Fig. 5 Room temperature magnetization behavior of BiFeO3 sample

up to 7 Tesla using SQUID. The magnetization curve obtained in Fig. 5 is linear with the field, which is typical for an antiferromagnetic arrangement of Fe3? magnetic moments with a small remnant magnetization of 3 9 10-3 emu/gm as expected.

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