Materials Research Bulletin 46 (2011) 297–302

Effect of Bi and Ti substitutions for Na and Nb on ferroelectric properties oforiented (Bi4.5+xNa0.5�x)(Ti2xNb2�2x)WO15 compounds

Hirotaka Ogawa, Akinori Kan *, Yuki Inami, Tohru Moriyama

Department of Transportation Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan

A R T I C L E I N F O

Article history:

Received 2 July 2010

Received in revised form 8 October 2010

Accepted 20 October 2010

Available online 28 October 2010

Keywords:

A. Ceramics

D. Dielectric properties

D. Ferroelectricity

A B S T R A C T

(Bi4.5+xNa0.5�x)(Ti2xNb2�2x)WO15 (BNTNW) compounds were synthesized and their ferroelectric

properties were characterized. The X-ray powder diffraction patterns of the compounds revealed that

they have a single phase over the whole composition range. The linear variations of the lattice

parameters with composition indicate the formation of solid solutions, resulting in a reduction in the

orthorhombicity of the compounds. The remnant polarization of the BNTNW decreased from 8.5 to

5.1 mC/cm2 with increasing x, which may be related to the orthorhombicity of the compounds. By using

hot forging, an oriented BNTNW compound at x = 0 was obtained. Strong reflections from (0 0 l) were

observed for sample // in which the measurement direction is parallel and the orientation factor of such

sample was approximately 0.72. A remarkable increase in the remnant polarization (Pr) of the compound

was observed for the sample ? in which the direction of applied pressure is perpendicular to the

measurement direction; the highest Pr value was 18 mC/cm2.

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1. Introduction

The ferroelectric materials of the Aurivillius family areattracting considerable interest for use in commercial applications.The general formula for the Aurivillius family is Bi2An�1BnO3n+3,where n is the number of BO6 octahedra in the perovskite-likeblock. The crystal structure of the Aurivillius family consists ofarrays of (Bi2O2)2+ layers and perovskite-like (An�1BnO3n+1)2�

blocks (where the A site is occupied by mono-, di-, or trivalentcations and the octahedral B site is typically occupied by Ti4+, Nb5+,Ta5+, or W6+) [1]. Crystallographic data for the Aurivillius familywith various numbers of perovskite blocks have been reported [1–3]. In the Aurivillius family, the Bi5TiNbWO15 and Bi5Ti1.5W1.5O15

compounds are known to be the mixed-layer Aurivillius phase,which corresponds to an intergrowth structure consisting ofBi2WO6 (n = 1) and Bi3TiNbO9 or Bi3Ti1.5W0.5O9 (n = 2). Althoughthe ferroelectric and dielectric properties of various Aurivilliusphases have been reported [4–6], the detailed ferroelectric anddielectric properties of the Na-containing Bi4.5Na0.5Nb2WO15

compound have not been reported. In the present study,(Bi4.5+xNa0.5�x)(Ti2xNb2�2x)WO15 (BNTNW; x = 0�0.5) compoundswere synthesized and the effect of their chemical composition ontheir crystal structure and ferroelectric properties were investi-gated. Since grain orientation can enhance the ferroelectric

* Corresponding author. Tel.: +81 52 838 2072; fax: +81 52 832 1253.

E-mail address: akan@meijo-u.ac.jp (A. Kan).

0025-5408/$ – see front matter � 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2010.10.007

properties, oriented Aurivillius phases have been synthesized byvarious processes, including templated grain growth [7] and hotforging [8]. In the present study, oriented BNTNW compoundswere also prepared by hot forging and the effect of orientation onthe ferroelectric properties, the temperature dependence of thedielectric constant, and the microstructure of the ceramics wereinvestigated.

2. Experimental method

(Bi4.5+xNa0.5�x)(Ti2xNb2�2x)WO15 (BNTNW) compounds wereprepared by the conventional solid-state reaction method usingBi2O3, Na2CO3, TiO2, Nb2O5, and WO3 powders. These materialswere weighed according to the stoichiometric compositions ofBNTNW in the composition range of x = 0�0.5 and mixed withacetone for 40 min using alumina mortar and pestle. Thesepowders were calcined at 700 8C for 5 h in air and ground withethanol for 1 h. The PVA solution was prepared by 5 g of PVAaddition into 60 g of distilled water and added into the calcinedpowder as a binder. The obtained powders were uniaxiallypressed into pellets by applying a pressure of 100 MPa andsintered at 1025�1050 8C for 2 h in air. For the preparation oforiented samples, 10-mm diameter sintered pellets that were15 mm thick were uniaxially pressed by hot forging (H.F.) at atemperature of 800 8C for 2 h in air by using alumina die with25 mm in diameter; as a result, the hot forged samples withapproximately 15 mm in diameter and approximately 6 mmthickness were obtained in this study. The hot forged samples

[()TD$FIG]

Fig. 1. Preparation of (a) sample ? and (b) sample // by hot forging.

H. Ogawa et al. / Materials Research Bulletin 46 (2011) 297–302298

were then cut into samples? and // as shown in Fig. 1. In sample?,the measurement direction is perpendicular to the direction ofapplied pressure, whereas the applied pressure for sample // isparallel to the measurement direction. These samples with andwithout hot forging were polished and annealed at 400 8C for 2 hin air prior to the measuring their electrical properties. Pt and Auelectrodes were attached to the surface of each sample for theelectrical measurements. The P�E hysteresis loops at 50 Hz andthe temperature dependence of the dielectric constant of thesamples at 1 MHz were characterized using an aixACCT TFanalyzer 2000 and an precision LCR meter (Agilent, 4284A),respectively. The crystalline phases of the sintered samples wereidentified by X-ray powder diffraction (XRPD) using Cu Karadiation. The lattice parameters of the BNTNW compounds wererefined by the Le Bail method [9] to investigate the effect ofchemical composition on the crystal structure. The XRPD profilesto refine the lattice parameters were obtained by step scanning ina 2u range of 10�1208 with a step of 0.038 and a counting time of3.0 s. The morphologies of the samples were investigated by field-emission scanning electron microscopy (FE-SEM). As for the FE-SEM observation of orientated samples, the surface on theorientated samples were polished and thermally etched at the

[()TD$FIG]

Fig. 2. XRPD profiles of BNTNW compounds sintered at optimum temperatures for

2 h in air. Intensity of profile was normalized for each composition.

temperature of 800 8C for 30 min in air. The apparent density ofthe samples was measured by the Archimedes method.

3. Results and discussion

Fig. 2 shows the XRPD profiles of BNTNW compounds in thecomposition range x = 0�0.5 sintered at the optimum tempera-tures for 2 h in air. The XRPD profiles do not indicate the presenceof a secondary phase over the whole composition range. Thediffraction peaks were shifted slightly depending on the composi-tion x. To confirm the formation of solid solutions, the latticeparameters of BNTNW compounds were refined by the Le Bailmethod [9]. The compositional dependence of the latticeparameters on the BNTNW compounds is shown in Fig. 3. Thesevalues are also summarized in Table 1. Since all the lattice[()TD$FIG]

Fig. 3. Influence of Bi and Ti substitutions for Na and Nb on lattice parameters of

BNTNW compounds.

Table 1Lattice parameters, unit cell volume, and orthorhombicity of BNTNW compounds.

Composition x Lattice parameters Unit cell volume Orthorhombicity

a (A) b (A) c (A) V (A3)

0 5.4610(2) 5.3949(1) 41.674(2) 1277.8 12.1�10–3

0.125 5.4582(1) 5.3959(2) 41.721(1) 1228.7 11.4� I0–3

0.25 5.4379(2) 5.3965(3) 41.774(3) 1225.9 7.6�10–3

0.375 5.4308(2) 5.3961(1) 41.823(2) 1225.6 6.4�10–3

0.5 5.4290(7) 5.3960(4) 41.836(5) 1225.6 6.1�10–3

[()TD$FIG]

Fig. 4. FE-SEM photographs of BNTNW compounds sintered at optimum temperatures for 2 h in air.

H. Ogawa et al. / Materials Research Bulletin 46 (2011) 297–302 299

parameters of the BNTNW compounds varied almost linearly withthe composition x, the variations in the lattice parameters satisfyVegard’s rule. From these results, it is considered that the BNTNWcompound forms the solid solutions over the whole compositionrange. The lattice parameter a decreased with increasing x,whereas the lattice parameter b was almost constant (seeFig. 3). These variations in the lattice parameters a and b suggestthat the chemical composition of BNTNW compounds affects theirorthorhombicities, defined as 2(a � b)/(a + b) [10]. The variations inthe orthorhombicity are closely related to the ferroelectric proper-ties of Aurivillius phases [10,11] because orthorhombicity impliesthat the octahedra in the ab plane are distorted. The orthorhombi-cities of the compounds were calculated (see Table 1) based on therefined lattice parameters for each compound. The orthorhombi-cities of the compounds decreased linearly from 1.2 � 10�2 to6.4� 10�3 as x was increased from 0 to 0.5. This reduction in theorthorhombicity may degrade the ferroelectric properties.

Table 2Sintering conditions and ferroelectric properties of BNTNW compounds.

Composition x Sintering temperature (8C) Apparent density (g/c

0 1025 7.46

0.25 1050 8.18

0.375 1050 8.11

0.5 1050 8.32

Fig. 4 shows the surface microstructures of the BNTNWcompounds. In all cases, a randomly oriented plate-like morphol-ogy was obtained. The Bi and Ti substitutions for Na and Nb inBNTNW compounds seem to enhance the grain growth of thecompounds. Such a plate-like morphology is typical of Aurivilliusphases; it is considered to be due to the anisotropic nature of thecrystal structure [12]. Although the BNTNW compounds sinteredat the optimum temperatures have dense microstructures, therelative density of the compound at x = 0 is slightly lower thanthose of the compounds in the composition range x = 0.25�0.5 (seeTable 2), though the relative densities of the compounds in thecomposition range x = 0.25�0.5 are almost constant.

Fig. 5 shows the P�E hysteresis loops of BNTNW compounds inan electric field of 280 kV/cm. Table 2 lists the remnantpolarizations (Pr) and the coercive fields (Ec) of the compounds.Pr of BNTNW compounds decreased from 8.5 to 5.1 mC/cm2 withincreasing x, whereas Ec of the compounds did not change greatly.

m3) Relative density (%) Pr (mC/cm2) Ec (kV/cm)

91.9 8.4 113

95.3 7.3 100

96.9 7.1 99

95.3 4.2 99

[()TD$FIG]

Fig. 5. P�E hysteresis loops at an electric field strength of 280 kV/cm of BNTNW

compounds with and without hot forging.

H. Ogawa et al. / Materials Research Bulletin 46 (2011) 297–302300

The highest Pr was obtained at x = 0, despite this compound havinga lower relative density than the compounds with x = 0.25�0.5.Thus, the variations in the orthorhombicities of BNTNW com-pounds caused by substituting Bi and Ti for Na and Nb may have adominant effect on the Pr of the compounds and additionalimprovement in Pr is expected by orienting the grains of thecompound with x = 0.

To improve the ferroelectric properties of BNTNW compounds,an oriented BNTNW compound at x = 0 (i.e., Bi4.5Na0.5Nb2WO15

(BNNW)) was synthesized by hot forging and the relationshipbetween grain orientation and the ferroelectric properties of the

[()TD$FIG]

Fig. 6. XRPD profiles of BNNW compounds that had and had not been hot forged: (a)

BNNW without hot forging, (b) sample //, and (c) sample ?.

compound was investigated. The XRD patterns of oriented BNNWcompound with and without hot forging are shown in Fig. 6.Stronger reflections from the (0 0 l) were observed for sample //than for BNNW without hot forging. In contrast, the reflectionsfrom the (0 0 l) were weak for sample ?. These results imply thathot forging caused the BNNW compound to have highly c-axisorientation. Thus, the orientation factor was calculated for sample// by comparing the relative intensities of the (0 0 l) reflectionswith those of the (h k l) reflections by the Lotgering method [13] toclarify the effect of hot forging on the orientation of the compound.The orientation factor is given by the following equation:

f ¼ ðP � P0Þð1� P0Þ

; (1)

where

P ¼P

Ið0 0 lÞP

Iðh k lÞ ; (2)

whereP

I(0 0 l) andP

I(h k l) are the sums of the relativeintensities of the (0 0 l) and (h k l) reflections for the bulk material,respectively. P0 was calculated from the powders of the BNNWcompound. The orientation factor of sample // was estimated to beapproximately 0.72, which indicates a BNNW compound withrelatively high orientation was obtained by hot forging.

Fig. 7 shows the temperature dependences of the dielectricconstants of BNNW compounds with and without hot forging. Thedielectric constant of the BNNW without hot forging increasedwith increasing temperature and a maximum value of 950 wasobtained at a temperature of approximately 720 8C. Comparing thedielectric constants of samples ? and // indicates that hot forgingconsiderably alters the temperature dependence of the dielectricconstant of the BNNW compound; similar observations have beenreported for other ferroelectric materials [14,15]. Sample ? had ahigher dielectric constant than sample //; this is considered to berelated to the grain orientation, which is also supported by theresults for XRPD and the orientation factor given above. Fig. 8 showsthe P�E hysteresis curves at an electric field of 190 kV/cm of BNNWcompounds with and without hot forging. Although Pr of the BNNWcompound that had not been hot forged was approximately 4 mC/cm2, a remarkable variation is observed in the P�E hysteresis loopsof the oriented compound as well as in the temperature dependenceof the dielectric constant. Sample ? has a higher Pr than BNNWwithout hot forging and sample //. As a result, sample ? had a Pr of18 mC/cm2. In addition, Pr of sample // decreased much more thanthat of the BNNW compound that had not been hot forged (see[()TD$FIG]

Fig. 7. Temperature dependence of dielectric constant measured at 1 MHz on (a)

BNNW without hot forging, (b) sample //, and (c) sample ?.

[()TD$FIG]

Fig. 8. P�E hysteresis loops of (a) BNNW without hot forging, (b) sample //, and (c)

sample ? measured at 50 Hz.

[()TD$FIG]H. Ogawa et al. / Materials Research Bulletin 46 (2011) 297–302 301

Fig. 8). Fig. 9 shows Pr and Ec of sample? and the BNNW compoundthat had not been hot forged as functions of the electric field. Pr ofsample ? increased dramatically at electric field strengths higherthan 100 kV/cm, whereas Pr of the BNNW compound without hotforging increased gradually when the electric field was increased to320 kV/cm. Thus, the grain orientation induced by hot forgingincreases Pr of BNNW compounds.

To evaluate the relationship between the morphological changesin the ceramics induced by hot forging and their ferroelectric[()TD$FIG]

Fig. 9. Variations in remnant polarization and coercive field measured at 1 MHz of

BNNW with and without hot forging.

Fig. 10. Microstructures of oriented BNNW compounds: (a) sample // and (b) sample

?. Surface on orientated samples was thermally etched at 800 8C for 30 min in air.

properties, the microstructures of the oriented BNNW compoundwere observed by FE-SEM. Fig. 10 shows the microstructures ofsamples ? and //. Although the conventional fabrication processgenerally produces randomly oriented plate-like grains, dense,oriented microstructures were obtained for samples ? and //. Thus,the morphological changes in the ceramic induced by hot forging areconsidered to affect the XRPD patterns and the dielectric andferroelectric properties of these compounds.

4. Conclusions

Mixed-layer Aurivillius compounds (Bi4.5+xNa0.5�x)(Ti2xNb2�2x)WO15 (BNTNW) were prepared by the conventionalsolid state reaction method and the effect of Bi and Ti substitutionsfor Na and Nb on the ferroelectric properties and crystal structuresof these compounds were investigated. XRPD patterns of thecompounds reveal that a single phase was obtained over the wholecomposition range. The lattice parameters of the compoundsvaried linearly with the chemical composition, resulting in areduction in the orthorhombicity. As for the ferroelectric proper-ties of the compounds, Pr decreased from 8.5 to 5.1 mC/cm2 whenBi and Ti were substituted for Na and Nb; the variation in theorthorhombicity of the compounds is considered to dominantfactor for this reduction in Pr. An oriented BNTNW compound atx = 0 was obtained by hot forging. Sample // exhibited strongerreflections from the (0 0 l) plane than ceramics that had not beenhot forged. An orientation factor of 0.72 was obtained for sample //.Hot forging improved Pr remarkably; the highest Pr of 18 mC/cm2

was obtained for sample ?.

H. Ogawa et al. / Materials Research Bulletin 46 (2011) 297–302302

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