Relaxor behavior of BaBi4Ti3Fe0.5Nb0.5O15 ceramics

Solid State Communications 147 (2008) 457–460

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Relaxor behavior of BaBi4 Ti3 Fe0.5 Nb0.5 O15 ceramics Sunil Kumar, K.B.R. Varma ∗ Materials Research Centre, Indian Institute of Science, Bangalore, 560012, India

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Article history: Received 9 May 2008 Received in revised form 30 June 2008 Accepted 5 July 2008 by E.V. Sampathkumaran Available online 12 July 2008 PACS: 77.84.-s Keywords: A. Ferroelectrics D. Ceramics D. Dielectric response

a b s t r a c t Monophasic BaBi4 Ti3 Fe0.5 Nb0.5 O15 (BBTFN) which exhibited relaxor ferroelectric properties has been synthesized via the solid-state reaction route. X-ray powder diffraction (XRD) studies carried out at room temperature confirmed the phase to be an n = 4 member of the Aurivillius family of oxides. The XRD pattern was indexed to an orthorhombic cell associated with lattice parameters a = 5.463(3) Å, b = 5.443(3) Å and c = 41.77(2) Å. A broad dielectric peak associated with the frequency dependent dielectric maximum temperature was observed. The high temperature slope of the dielectric peak was characterized by the Lorenz-type quadratic law for relaxors. The dielectric relaxation was modeled using the Vogel–Fulcher relationship with activation energy Ea = 0.017 ± 0.002 eV and freezing temperature TVF = 558 ± 1 K. Relaxor behavior and the incidence of ferroelectric hystersis (P vs E) loop at room temperature suggested the coexistence of long-range polar ordering with the polar nanoregions induced by compositional fluctuations. © 2008 Elsevier Ltd. All rights reserved.

1. Introduction Aurivillius family of oxides, generally formulated as (Bi2 O2 )2+ (An−1 Bn O3n+1 )2− , have received considerable attention in recent times owing to their potential applications in non-volatile random access memory [NVRAM] and high temperature piezoelectric devices [1,2]. Structure of these oxides is generally described as the pseudo-perovskite block (An−1 Bn O3n+1 )2− sandwiched between the bismuth oxide layers (Bi2 O2 )2+ along the c-axis, where 12-fold-coordinated A-site is occupied by the mono-, di-, or trivalent ions, oxygen octahedral coordinated B-site is occupied by tetravalent, pentavalent, or hexavalent ions of appropriate sizes, and n indicates the number of corners sharing BO6 octahedra forming the perovskite like slabs [3]. Majority of these oxides are normal ferroelectrics with fairly high Curie temperatures as a consequence of formation of a short bond between the apex oxygen in the perovskite layer and the bismuth in the [Bi2 O2 ]2+ layer associated with octahedral rotations and cationic displacements [4]. Only a very few compounds of this family such as BaBi2 Nb2 O9 , BaBi2 Ta2 O9 , K0.5 La0.5 Bi2 Nb2 O9 , K0.5 La0.5 Bi2 Ta2 O9 , BaBi4 Ti4 O15 etc. show the relaxor behavior [5–10]. Relaxor ferroelectrics, characterized by the frequency dependent diffuse phase transition, are attractive for various applications because of their exceptionally high dielectric and piezoelectric responses over a wide range of temperatures [11,12]. Much of the literature pertaining to relaxors relates to the studies on lead-based

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Corresponding author. Tel.: +91 80 2293 2914; fax: +91 80 2360 0683. E-mail address: [email protected] (K.B.R. Varma).

0038-1098/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2008.07.008

complex perovskites such as lead magnesium niobate (PMN) and lead lanthanum zirconate titanate (PLZT). In comparison to these ferroelectrics, there have been a few reports on the relaxor behavior of lead-free Aurivillius oxides which are indispensable because of their superior polarization fatigue resistant characteristics which make them suitable for nonvolatile random access memory applications [13]. In a bid to synthesize ferroic materials belonging to the Aurivillius family of oxides, we synthesized a monophasic BaBi4 Ti3 Fe0.5 Nb0.5 O15 . In this paper, we report the structural, microstructural and relaxor characteristics of BaBi4 Ti3 Fe0.5 Nb0.5 O15 (BBTFN) which is found to be an n = 4 member of the Aurivillius family of oxides. 2. Experimental Polycrystalline BaBi4 Ti3 Fe0.5 Nb0.5 O15 (BBTFN) powders were synthesized by the conventional solid-state reaction route. The starting materials included powders of BaCO3 , Bi2 O3 , TiO2 , Fe2 O3 and Nb2 O5 of purity >99%. Excess Bi2 O3 (3 wt%) was added to compensate for the volatilization of bismuth during sintering. Stoichiometric mixture of the above reactants was calcined at 1273 K for 24 h in air with intermediate grindings. After calcination, the powder was ground again and mixed with a small amount of PVA (polyvinyl alcohol) as a binder. Subsequently the mixture was cold pressed into disks of 10 mm diameter at a pressure of 250 MPa. The binder was burnt out by slowly heating the pellets at 775 K for 6 h. The pressed pellets were sintered at 1300–1350 K for 4 h in air and furnace cooled to room temperature. Densities of the pellets

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Fig. 2. Scanning electron micrograph of BaBi4 Ti3 Fe0.5 Nb0.5 O15 ceramic sintered at 1348 K. Fig. 1. Room temperature XRD pattern for BaBi4 Ti3 Fe0.5 Nb0.5 O15 .

3.3. Dielectric properties were measured by the Archimedes method using xylene (room temperature density ∼0.86 gm/cm3 ) as the liquid medium. The X-ray structural details of the powdered sample were obtained using Philliphs powder diffractometer with Cu Kα radiation (λ = 1.54 Å). Scanning electron microscope model Quanta 200 SEM was employed to obtain the microstructural and surface morphology of the ceramics. For dielectric and ferroelectric property measurements, pellets were ground and polished to a thickness of about 0.8 mm and 0.3 mm, respectively and then gold electrodes were sputtered on either side. The samples were baked at 473 K for 2 h to dry out the moisture prior to any measurement. The capacitance and loss were measured using an LCR meter (Model 4194A, Hewlett–Packard) at signal strength of 0.5Vrms in the temperature range of 300–725 K at various frequencies (1 kHz to 1 MHz). The ferroelectric hysteresis loops were recorded using a modified Sawyer–Tower circuit. 3. Results and discussion 3.1. XRD studies X-ray powder diffraction pattern (XRD) obtained for the nominal composition BaBi4 Ti3 Fe0.5 Nb0.5 O15 is depicted in Fig. 1. All the Bragg reflections in this pattern could be indexed to an orthorhombic cell associated with the lattice parameters a = 5.463(3) Å, b = 5.443(3) Å and c = 41.77(2) Å (space group A21 am). No second phase was detectable. Indeed, this XRD pattern is in close agreement with that obtained for BaBi4 Ti4 O15 (BBT) which is known to be an n = 4 member of the Aurivillius family of oxides [14]. These lattice parameters were obtained by least square refinement of powder XRD data using the program PROSZKI [15]. Representative partial indexation is shown in Fig. 1.

The temperature dependence of the dielectric constant (εr ) and dielectric loss (D) in the 1 kHz–1 MHz frequency range are depicted in Fig. 3((a) and (b)). At high frequency, the dielectric constant exhibits a broad peak (diffusive) and the temperature of the dielectric maximum (Tm ) shifts towards higher temperatures (574–589 K) with the increase in frequency (2 kHz to 1 MHz) which confirms the relaxor behavior of the present ceramics. The dielectric loss also exhibits a similar anomaly in the 570–590 K temperature region associated with frequency dependent maximum temperature, especially at higher frequencies as shown in the inset of Fig. 3(b). At lower frequencies, this anomaly in the loss curve is not much evident as the sample shows higher values of losses and perhaps due to the presence of the other relaxations particularly at higher temperatures. The values of room temperature dielectric constant and loss at 100 kHz are 303 and 0.032 which increased to 746 and 0.076 respectively at Tm . The observed low values of the dielectric constants of the present compound as compared to those of the conventional leadbased relaxors are attributed to the weak interactions between the dipoles in the layered structure. The Tm for BBT is ∼668 K at 100 kHz [9], while it is significantly lower (581 K at 100 kHz) for the present compound. This could be attributed to the decreased ‘‘rattling space’’ in the BO6 octahedra as the effective ionic radii of (0.5Fe+3 + 0.5Nb+5 ) is greater than the ionic radii of Ti+4 ion. Interestingly, the room temperature dielectric constant for BBTFN is nearly twice that of BBT ceramics [9]. The high dielectric loss at low frequencies could be as a consequence of the partial reduction of Fe3+ to Fe2+ ions during sintering, which is highly sensitive to the sintering temperature, leading to thermally activated hole-conduction mechanism. The defect reaction in Kronig–Vink notation can be written as: FexFe → Fe0Fe + h• .

(1) 3+

3.2. Microstructural studies Fig. 2 shows the scanning electron micrograph for the title compound that was sintered at 1348 K for 4 h. It is observed that the BBTFN ceramic has the stack of randomly-oriented grains which are in the form of discs. This disc-like morphology is a typical characteristic of Aurivillius family of compounds. It is known that {001} planes posses lower surface energy. Therefore while sintering, the grains grow predominantly in the a–b plane and develop with disc-like morphology [16]. Relative density of the sintered pellets was 96% of the theoretical value.

2+

The coexistence of Fe and Fe cations on equivalent crystallographic sites leads to the occurrence of a finite hopping/jump type of conduction mechanism, which is very effective at low frequencies resulting in thermally activated space-charge polarization. It is well known that dielectric constant of a normal ferroelectric above the Curie temperature follows the Curie–Weiss law, given by

εr = C /(T − Tθ );

(2)

where Tθ is the Curie–Weiss temperature and C is the Curie–Weiss constant. Dielectric constant data obtained at f = 100 kHz fitted with the Curie–Weiss law at temperature much higher than the

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Fig. 4. The inverse dielectric constant (1/εr ) as a function of temperature at 100 kHz. (The open circle represents the experimental data and the solid line is the fit to the Curie–Weiss law).

Fig. 3. Temperature dependence of (a) dielectric constant (εr ) and (b) loss (D) at different frequencies.

temperature of dielectric maximum is shown in the Fig. 4. The parameters obtained from the linear fit above TD (Burn temperature) are C = 1.98 × 105 K and Tθ = 372 K. A deviation from the Curie–Weiss law can be clearly seen below a certain temperature TD . The deviation from the CW law is generally observed in relaxor ferroelectrics, which is explained by the existence of small spontaneously polarized clusters which appear below TD [17,18]. The presence of polar clusters was confirmed in relaxors by the transmission electron microscopy which was corroborated by the persistence of ferroelectric hystersis loops at the temperatures much higher than Tm [19,20]. The broadness in εr versus temperature curve is one of the most important characteristics of the disordered perovskite structure showing relaxor behaviour. An empirical Lorenz-type relation has been found to describe the dependence of the permittivity on temperature at T > Tm in relaxors [21,22].

εA (T − TA )2 =1+ , (3) ε 2δ 2 where TA and εA are the fitting parameters defining the temperature and magnitude of the Lorenz-peak and δ is a measure of the degree of diffuseness of the peak. Fig. 5 shows the plot of εr at 100 kHz as a function of T starting from temperatures a few degrees below Tm . An excellent fit is achieved in the temperature range from 605 K to 690 K. The deviation from the quadratic law near Tm is due to the conventional relaxor dispersion [21,22], which becomes significant below 605 K, while the discrepancy at high temperatures results from the gradual change to Curie–Weiss law. By fitting the experimental data in Eq. (3), the values obtained for the fitting parameters TA , εA and δ are found to be 577.5 ± 0.2 K, 752 ± 1 K and 163.1 ± 0.5, respectively. The broadness or diffusiveness occurs mainly because of the compositional fluctuations and structural disorder in the arrangement of cations in one or more crystallographic sites of the structure. This suggests the existence of microscopic heterogeneity in the compound with different local Curie points [23]. For BBT (n = 4 member of Aurivillius family of oxides, with Tc ∼ 668 K and relaxor-like ferroelectric behavior [9]), these compositional fluctuations are associated with the

Fig. 5. Dielectric constant at 100 kHz vs temperature. (The open circle represents the experimental data and the solid line represents the fitting to the quadratic law Eq. (3)).

positional fluctuations of the A-site cations from one cell to the other. On the basis of single crystal X-ray diffraction studies, Teller et al. [24] proposed that in addition to partial substitution of Bi3+ for Ba2+ in [Bi2 O2 ]2+ slabs, the existence of distinct atomic positions for the Ba2+ and Bi3+ cations has to be taken into consideration to account for the relaxor behavior of BBT. These positional static disorder associated with the compositional fluctuations are responsible for the ferroelectric relaxor behavior through the formation of microdomains with different structural distortion levels. Structure of the compound under investigation is similar to that of BBT but for one of Ti4+ ions is replaced by 1/2(Fe3+ + Nb5+ ) at Bsite. So it is reasonable to assume that the above arguments could be extended to explain the relaxor-type ferroelectric transition in the title compound. However, more structural studies need to be carried out to pin down the actual mechanism to explain this behavior. In relaxor ferroelectrics the frequency dependence of the temperature of the dielectric peak is found to obey the Vogel–Fulcher law [18,25],



 −Ea f = fo exp , kB (Tm − TVF )

(4)

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Fig. 7. Polarization–electric field (P–E) hysteresis loop recorded for the Bi4 Ti3 Fe0.5 Nb0.5 O15 ceramics recorded at 373 K.

Fig. 6. ln(f ) as a function of temperature of dielectric maximum Tm . (The open circle represents the experimental data and the solid line is the fit to the Vogel–Fulcher relation).

where Ea and fo are the activation energy and the attempt frequency of the dipole reorientation respectively and TVF is the freezing temperature of the polarization fluctuations. Fig. 6 shows the variation of Tm with ln(f ) for the BBTFN. Temperature of dielectric peak Tm shows a good fit to the Vogel–Fulcher law, showing typical relaxor behavior. The value of the fitting parameters fo , Ea and TVF are found to be 6.5 × 108 Hz, 0.017 ± 0.002 eV and 558 ± 1 K, respectively. The values of Ea and fo are close to those reported for Aurivillius phases SrBi1.65 La0.35 Nb2 O9 , K0.5 La0.5 Bi2 Nb2 O9 [26,7]. Relaxors belonging to the Aurivillius family of oxides are two dimensional relaxors and the low value of fo (∼109 Hz) suggests the existence of much flatter potential well for BBTFN than in three-dimensional relaxor such as PMN where fo ∼1012 Hz. As compared to that of BaBi2 Nb2 O9 and SrBi1.65 La0.35 Nb2 O9 , the freezing temperature of the present compound is high, which indicates a strong interaction between micropolar regions with a short-range ferroelectric ordering. Interestingly, for the present compound the value of Tm –TVF is 23 K (at 100 kHz) which is much less than that of BBN and is comparable to that of a conventional lead based relaxor such as PMN with Ea and Tm – TVF are about 0.04 eV and 29 K respectively at 100 kHz [18] with rapid broadening of relaxation spectrum. To confirm the ferroelectric nature of the present compound polarization versus electric field measurements were performed at 373 K. Fig. 7 shows the ferroelectric hysteresis loop for BBTFN ceramics. A remnant polarization (Pr ) of 2.1 µC/cm2 with a coercive field (Ec ) of 10.9 kV/cm was obtained under a maximum applied electric field of 67 kV/cm. The P vs E loop that is shown in Fig. 7 is reminiscent of the ferroelectric nature of BBTFN ceramics. 4. Conclusions The Aurivillius type bismuth layer-structured compound

(BaBi4 Ti3 Fe0.5 Nb0.5 O15 ) was synthesized using conventional solidstate processing. Dielectric studies showed a diffusive phase transition with frequency dependent dielectric maximum. The di-

electric relaxation was modeled using Vogel–Fulcher relationship and the fitted values of activation energy Ea , freezing temperature and attempt frequency fo were found to be ∼0.017 eV, 558 K and 6.5 × 108 Hz, respectively. The remnant polarization (Pr ) and coercive field (Ec ) were found to be 2.1 µC/cm2 and 10.9 kV/cm for a maximum applied field of 67 kV/cm. Acknowledgments One of the authors (SK) is grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi, for the award of Senior Research Fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

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