2Fe5O12

Solid State Communications 142 (2007) 449–452 www.elsevier.com/locate/ssc

Structural, thermal and dielectric properties of La3/2Bi3/2Fe5O12 K. Jawahar, R.N.P. Choudhary ∗ Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721302, India Received 21 September 2006; received in revised form 12 March 2007; accepted 20 March 2007 by T.T.M. Palstra Available online 27 March 2007

Abstract Polycrystalline La3/2 Bi3/2 Fe5 O12 (LBIO) compound was prepared by a conventional high-temperature solid-state reaction technique. Preliminary X-ray diffraction (XRD) analysis confirms the formation of single-phase compound in an orthorhombic crystal system at room temperature. The elemental content of the compound has been analysed by EDS microanalysis. Microstructural analysis by scanning electron microscopy (SEM) shows that the compound has well defined grains, which are distributed uniformly throughout the surface. Studies of temperature variation of dielectric response at various frequencies show multiple dielectric anomalies at 200 and 370 ◦ C. It is interesting to note that the loss tangent (tan δ) seems to be reduced at higher frequencies after reaching the instrumental saturation. Also, a couple of weak endothermic peaks were observed in differential thermal analysis (DTA) corresponding to 200 and 370 ◦ C phase transition temperature. As soon as the phase transition process sets in, the DTA curve deviate from the basal horizontal line. The ferroelectric hysteresis loop was observed by applying the electric field of 6 kV/cm. c 2007 Elsevier Ltd. All rights reserved.

PACS: 77.22.Ch; 77.22.Gm; 77.80.Bh; 77.22Ej Keywords: D. Dielectric; D. Ferroelectric; D. Phase transition; D. Hysteresis

1. Introduction Some ceramic oxides belonging to the particular structural family have attracted much attention of scientists to design and develop new materials of the family for multifunctional devices. Yttrium iron garnet (YIG) is a well-known ferrimagentic material of the family (A3 Fe5 O12 , where A = Y, Bi, Ca), which is important for microwave applications [1]. A lot of work has been carried out on this material in the past, by partially [2,3] or fully [4,5] replacing yttrium by rare earths and other elements/oxides [6,7]. The spontaneous magnetization observed in YIG with the chemical formula Y3 Fe2 (FeO4 )3 , is attributed from its perovskite-like phase, which actually originates from the garnet phase [8]. Detailed structural studies of some other compounds of YIG family such as Ca3 Fe2 Sn3 O12 showed CaSnO3 perovskite like structure with a small amount of pyrochlor/secondary phase due to impurity of α-Fe2 O3 phase [9]. The existence of spontaneous magnetization in the

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E-mail address: [email protected] (R.N.P. Choudhary). c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.03.028

perovskite like phase (layer of perovskite) has encouraged us to explore the possibility of fabrication of a multiferroic material for multifunctional devices using the concept of magnetoelectric (ME) effect. Though the ME effect was observed for a quite long-time, recent discovery of this effect in a distorted (Pb-free) perovskite, BiFeO3 (BFO) at high temperature (Tc = 1103 K and TN = 650 K) has attracted much attention of researchers for suitable modification of the material for devices. Unfortunately, high leakage current and high dissipation factor limit the applications of the material for devices. In recent past, some attempts have been made to solve these inherent problems of BFO by suitable substitution at different atomic sites. La substitution at Bi site of BFO has solved some of the above problems with significant improvement of its ME coefficient. Further, advantage of La+3 modification in structural stability [10] and magnetoelectric properties [11] of simple BiFeO3 has encouraged us to find out the possibility of preparing structurally stable (La+3 modified) Bi3 Fe5 O12 , otherwise the preparation of Bi3 Fe5 O12 , in its bulk form is thermodynamically unstable [12]. Also, the inclusion of La+3 prefers the formation of perovskite-like phase in the

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Bi3 Fe5 O12 garnet stoichiometry. As per ICDD data (Card No. 21-1450), an orthorhombic crystal system with lattice ˚ b = 5.57 A ˚ and c = 7.59 A ˚ was parameters a = 5.27 A, observed in the Y3 Fe5 O12 garnet stoichiometry (instead of ˚ In view garnet structure with cell parameter a = 12.37 A). of the above, we have studied the effect of La-substitution (at Bi site) on structural, electrical and dielectric properties of Bi3 Fe5 O12 (in its perovskite phase). In this paper we mainly report the structural stability and phase transition in Lamodified BiFeO3 with a new composition, La3/2 Bi3/2 Fe5 O12 (LBFO). 2. Experimental 2.1. Material preparation Polycrystalline sample of La3/2 Bi3/2 Fe5 O12 was prepared by a standard high-temperature solid-state reaction technique using high purity precursors. The oxides; La2 O3 (99.9%, M/s Indian Rare Earth Ltd, India), Bi2 O3 (99.9%, M/s Loba Chemi Pvt. Ltd, India) and Fe2 O3 (99.9%, M/s Loba Chemi Pvt. Ltd, India) were thoroughly mixed in agate mortar for 2 h, including the wet mixing in methanol media for 1 h. The mixture was calcined at 920 ◦ C. The process of grinding and calcination was repeated until the single-phase compound was formed (as verified by XRD). The calcined fine powder was mixed with polyvinyl alcohol (PVA), and pressed into a cylindrical pellet of diameter 10 mm and thickness 3 mm, under an isostatic pressure of about 5 × 107 kg/m2 using a hydraulic press. These pellets were then sintered with optimized temperature and time (960 ◦ C, 6 h) in an air atmosphere. The sintered pellets were polished with zero grain emery paper, and coated with high purity silver paste and then dried for 2 h at 150 ◦ C. 2.2. Material characterization Differential thermal analysis (DTA) was carried out using Perkin–Elmer Thermal Analyser (model: Pyris Diamond). The compound formation was confirmed by X-ray diffraction (XRD) using Rigaku X-ray powder diffractometer (model: Miniflex) in a wide range of the Bragg angles 2θ (20◦ ≤ ˚ The 2θ ≤ 80◦ ) being irradiated by Cu Kα1 (λ = 1.5405 A). surface morphology of gold sputtered sample was recorded using scanning electron microscope JEOL (model: JSM5800F). The same instrument was used for energy dispersive X-ray (EDAX) microanalysis of the sintered pellet. The temperature dependence of dielectric data was obtained at some selected frequencies (1 kHz, 10 kHz, 100 kHz and 1MHz) using a computer-controlled LCR impedance meter (Hioki Hitester model: 3532). The hysteresis loop was observed at room temperature using Precision Material Analyser (Radiant Technologies Inc) integrated with 4 kV high voltage amplifier. 3. Results and discussion 3.1. Structural analysis Fig. 1 shows the X-ray diffraction (XRD) pattern of the LBFO pellet. The sharp and single reflection peaks of the XRD pattern, which were different (in position and

Fig. 1. Room temperature XRD pattern of La3/2 Bi3/2 Fe5 O12 .

intensity) from those of ingredient precursors, confirmed the formation of single-phase compound with a minor trace of pyrochlor phase. All the peaks were indexed in different crystal systems. The lattice parameters were obtained using a standard computer program ‘PowdMult’ [13]. The best agreement between observed (obs) and calculated (cal) interplanar spacing P ( (dobs −dcal ) = minimum) was observed in the orthorhombic crystal system. A small peak just adjacent to (110) peak may be due to unreacted Fe2 O3 or unknown impurities. This secondary/pyrochlor (2%) peak is appeared even in BiFeO3 [14–16]. The least-squares refined lattice parameters ˚ b = 3.8651(20) A ˚ and of the LBFO are: a = 3.9162(20) A, ˚ (with estimated standard deviation in the c = 3.9187(20) A parenthesis). Detailed structural analysis using ‘Structure Prediction Diagnostic Software’ [17] (SPuDS) predicts that La3/2 Bi3/2 Fe5 O12 is nothing but the ‘distorted perovskite structure of BiFeO3 ’ in garnet stoichiometry, in which the Bi site of BiFeO3 is occupied by La+3 ion and forms LaFeO3 . The predicted compound can have a general formula AA0 A2 00 M4 O12 , where A, A0 A00 and M are represented by Fe+3 , La+3 , Bi+3 and Fe+3 . The theoretical visualization of predicted structure infers that a unit cell of La3/2 Bi3/2 Fe5 O12 consists of four unit cells of BiFeO3 (distorted perovskite structure) layer. The layering is commonly observed in BiFeO3 and BiMnO2 [18]. Out of four BiFeO3 layers, one is replaced by FeFeO3 and the other by LaFeO3 , (since, Fe, La and Bi are trivalent cations and having almost similar ionic radius). Substitution of La at the Bi site can help to get stable structure. The instability of forming FeFeO3 results on the formation of Fe2 O3 appears in XRD pattern. In fact, it helps to improve the spontaneous magnetization as reported earlier [8]. Using SPuDS, the lattice parameters and ˚ tolerance factor for BiFeO3 were predicted as: a = 4.0309 A (cubic) and t = 0.9146 respectively. The predicted values are in close agreement with our experimental (as determined our XRD analysis) values. The surface morphology and microstructure of the ceramic have been investigated using scanning electron microscopy. The pellet sample was gold coated by sputtering technique. The SEM micrograph of the compound at 5 K magnifications is shown in Fig. 2. It shows the polycrystalline texture of the material. The highly distinctive, more or less uniform and compact grain distributions (with less voids) are observed. The grains have typical dimension in the range of 1–2 µm approximately, and are mostly spherical in nature. Energy

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Fig. 2. SEM micrograph of La3/2 Bi3/2 Fe5 O12 sintered pellet.

dispersive X-ray microanalysis (EDAX) was carried out to verify the content of individual elements in the prepared sample on the sintered pellet. Quantitative analysis shows that the percentage (%) for Fe, La, Bi and O is consistent with our sample preparation. 3.2. Thermal analysis Differential thermal analysis (DTA) of the material was carried out on crushed fine powder of the sintered pellet and is shown in Fig. 3. As reported earlier [19], as soon as the phase transition process sets in, the DTA curve deviates from the basal horizontal line. It was suggested that the temperature corresponds to the end of the endothermic peak has to be considered as the phase transition temperature (in case of diffused endothermic peaks). Three endothermic peaks are

Fig. 3. DTA curve of La3/2 Bi3/2 Fe5 O12 calcined powder (with A, B and C represent phase transition points).

observed in DTA curve at 200, 370 and 510 ◦ C. The enthalpy at the phase transitions was found to be 7.4 mJ/mg, 7.2 mJ/mg and 2.22 mJ/mg, respectively. 3.3. Dielectric properties The temperature dependent dielectric constant (ε) and loss tangent (tan δ) is shown in Fig. 4 at selected frequencies (1 kHz, 10 kHz, 100 kHz and 1MHz). Dielectric anomalies at 200 and 370 ◦ C were observed in the sample. These dielectric anomalies may be related to the phase transitions as observed in our thermal analysis (Fig. 3). The phase transition related to 510 ◦ C

Fig. 4. Variation of dielectric constant ε (-- ◦ --) and tan δ (-- • --) with temperature at (a) 1 kHz, (b) 10 kHz, (c) 100 kHz and (d) 1 MHz.

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reaction technique. The microstructure of the material reveals the uniform distribution of grains throughout the surface. However, La3/2 Bi3/2 Fe5 O12 has an orthorhombic structure at room temperature, which is consistent with the distorted perovskite BiFeO3 . The dielectric anomalies and structural phase transitions of LBFO resembled those of BiFeO3 . The phase transitions observed at 200 and 370 ◦ C were consistent with those observed in DTA technique. The ferroelectric hysteresis loop and the spontaneous polarization of our sample were very much comparable to those of reported polycrystalline BiFeO3 . The loss tangent as a function of frequency shows an interesting result. References Fig. 5. Hysteresis loop of La3/2 Bi3/2 Fe5 O12 at room temperature.

endothermic peak can not be verified in our sample because of the limitation of our temperature measurement set up (limit of maximum temperature 500 ◦ C). The above anomalies or endothermic peaks at 200, 370, 510 ◦ C are consistent with those observed in (Bi1−x Lax )FeO3 earlier [19,20]. The frequency dependence of loss tangent exhibits interesting results. At low frequency (i.e. 1 kHz) the loss tangent reaches the instrumental saturation value (tan δ = 10) but at high frequency (i.e. 1 MHz) the value of tan δ drops down from this saturation drastically. It shows the possibility of using the material for high frequency applications with low dissipation factor. In addition, maximum dielectric constant (εmax ) of 20,000 fell down to 600 as a frequency increases from 1 kHz to 1 MHz. It shows that the dielectric relaxation behaviour in the material is complicated [21]. Attempts are being made to analyse these new results. 3.4. Hysteresis The ferroelectric hysteresis loop of LBFO (poled) was recorded at room temperature with the application of 6 kV/cm (Fig. 5). We have observed a hysteresis loop with near saturation. The variation of hysteresis loop suggests that the sample is lossy as observed in our dielecric studies. The saturation polarization Pmax (1.13 × 10−1 µC/cm2 ) was found to be smaller. However, the value is very much consistent with that observed in BiFeO3 . 4. Conclusion A single-phase La3/2 Bi3/2 Fe5 O12 ceramic (with small amount of pyrochlor ∼ = 2%) was prepared by a solid-state

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