Dielectric and pyroelectric properties of Sr-modified (Na0.5Bi0.5) Bi4Ti4O15 ceramics

Journal of Alloys and Compounds 456 (2008) 271–276

Dielectric and pyroelectric properties of Sr-modified (Na0.5Bi0.5) Bi4Ti4O15 ceramics E. Venkata Ramana a , V. Vijay Kiran a , T. Bhima Sankaram b,∗ a

Materials Research Laboratory, Department of Physics, University College of Science, Osmania University, Hyderabad 500007, India b Swarna Bharathi College of Engineering, Khammam 507002, India Received 3 December 2006; received in revised form 2 February 2007; accepted 5 February 2007 Available online 9 February 2007

Abstract Polycrystalline (Na0.5 Bi0.5 ) Bi4 Ti4 O15 (NBT) ceramics were prepared by the solid-state reaction method with the Sr substitution in place of divalent pseudo-cation (Na+ , Bi3+ ) with Sr = 0.1, 0.2, 0.3. X-ray diffraction measurements indicate an increase in the orthorhombic distortion and cell volume with the increase of Sr content in the ceramics. Dielectric measurements show the diffuse type transitions with the change in composition. The reduction in Curie temperature can be attributed to the increased tolerance factor due to the substitution of relatively bigger ion in the pseudo cation- (Na+ , Bi3+ ). Pyroelectric data indicate that the samples with x = 0.2 exhibits the better pyroelectric activity than other components of the series. Pyroelectric figures of merit are calculated from the pyroelectric coefficients. © 2007 Elsevier B.V. All rights reserved. Keywords: Ferroelectrics; Sintering; Dielectric response; X-ray diffraction

1. Introduction Aurivillius [1] reported that the family of Bismuth layer structured ferroelectrics (BLSF) with a general formula show interesting physical properties. Compounds of this group such as SrBi2 Ta2 O9 , Bi4−x Lax Ti3 O12 and SrBi4 Ti4 O15 have attracted much attention due to their applications in non-volatile ferroelectric random access memory (NVFRAM). BLSFs are important from the application viewpoints for electronic materials such as dielectrics, piezoelectrics and pyroelectrics under high frequencies, high temperature due to their high Curie temperature, low dielectric constant, and larger anisotropy in electromechanical coefficients (K33 , K31 ) as compared to those of lead-based materials [2–4]. BLSF materials are generally formulated as (Bi2 O2 )2+ (An−1 Bn O3n+1 )2− . The 12-fold coordinated perovskite A sites can be occupied by mono or di or trivalent cations such as Na+ Ba2+ , Ca2+ , Sr2+ , Bi3+ and the six-fold coordinated B sites are usually occupied by smaller cations such as Ti4+ , Ta5+ , Nb5+ ,

∗

Corresponding author. Tel.: +91 40 27150968. E-mail address: [email protected] (T.B. Sankaram).

0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.02.023

and W6+ , leading to BO6 octahedra. In these crystals, perovskite blocks (An−1 Bn O3n+1 ) are sandwiched between bismuth oxide (Bi2 O2 ) layers, where n denotes the number of BO6 octahedral layers in the perovskite blocks. Many of the compounds with this structure are ferroelectric, and the spontaneous polarization arises from different modes of simultaneous rotation of the oxygen octahedra. The displacements of the ions, in the perovskite B-sites contribute a major component of polarization in the a–b plane of the perovskite-like layers. Bi2 O2 layers act as insulating paraelectric layers and control the electronic response such as electrical conductivity, band gap, etc. [5,6], while the ferroelectricity arises mainly in the perovskite blocks [7–9]. The crystal structure, dielectric and ferroelectric anisotropy of BLSFs depend strongly on the value of n [10]. (Na0.5 Bi0.5 ) Bi4 Ti4 O15 (NBT) belongs to this family of compounds with m = 4. At room temperature, NBT is orthorhombic with a = 0.5427 nm, b/a = 1.006 and c = 4.065 nm and undergoes a ferroelectric (A21 am) to paraelectric (I4/mmm) transition at TC = 655 ◦ C [2,11]. NBT is a classical ferroelectric with a sharp maximum of thermal variation in dielectric constant close to TC . Takenaka and Sakata studied Ca-doped NBT ceramics and reported that the system exhibit good piezoelectric and pyroelectric properties to be useful in pyroelectric applications

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[4,12,13]. Dynamic pyroelectric studies on hot forged NBTCBT samples exhibited higher values of pyroelectric parameters. Ramasastry et al. [14] reported low dielectric constant (143), dielectric loss (0.0015) and moderate pyroelectric coefficient (90 pC cm−2 ◦ C−1 ) at room temperature in calcium doped NBT. They have attributed the increase in pyroelectricity of Ca-doped NBT to the formation of locked-up charge complexes, which at higher temperatures contribute to the pyroelectricity. The necessary parameters for good pyroelectric detection are low dielectric constant, low dielectric loss and high pyroelectric coefficients [15]. Since these BLSF compounds have high TC , they exhibit good thermal stability. The impurity doping seems to have improved the pyroelectric coefficient [13]. Replacement of Ca2+ results in increase of Curie temperature and decrease in density. Calcium has the lower polarizability compared to the ions like Sr2+ or Ba2+ . Present study describes the dielectric and pyroelectric properties of Sr-modified sodium bismuth titanate. The dependence of the dielectric and pyroelectric properties on the amount of doping has been investigated. 2. Experimental (Na0.5 Bi0.5 )1−x Srx Bi4 Ti4 O15 (SNBT) ceramic samples with x = 0.1, 0.2, 0.3 (01SNBT, 02SNBT, 03SNBT, respectively), were prepared by conventional solid state double sintering technique. Reagent grades of Na2 CO3 , Bi2 O3 , TiO2 and SrCO3 were used as the starting raw materials. The raw materials were ground to obtain a fine powder. The powder was uniaxially pressed into cylindrical discs of 2.54 cm diameter. These discs were pre-sintered in a temperature range of 800–850 ◦ C for 2 h. The pre-sintered discs were again crushed to fine powder and pressed into cylindrical discs of diameter 1 cm and thickness 1 cm using uniaxial pressing. These discs were sintered at 1000–1100 ◦ C for 2 h in air. Most suitable firing temperature was determined by studying the density obtained under different firing schedules. The sintered cylindrical discs were diamond cut into discs of thickness 0.5–0.8 mm. X-ray diffraction patterns for the final sintered samples were obtained using the Panalytical (X’Pert) diffractometer. Sputtered gold thin film electrodes were used for the dielectric and pyroelectric measurements. The samples were poled under a dc field of 3–4 kV mm−1 at room temperature for 3 h, by keeping the sample in a silicon oil bath. The temperature dependence of dielectric constant and loss tangent was measured at different frequencies ranging from 10 to 1000 kHz using HP4192A Impedance analyzer interfaced to a computer. The dielectric measurements were made in the temperature range from room temperature to 650 ◦ C at a heating rate of 1 ◦ C/min. The pyroelectric coefficient and spontaneous polarization were measured by the static pyroelectric method (Bayer–Roundy) [16] by heating the samples at a rate of 5 ◦ C/min. All the samples after poling were heated to a temperature about 200 ◦ C less than the transition temperature for about 1 h under short circuit condition prior to pyroelectric measurements. This was to ensure that the thermal depolarization effects were minimized during the pyroelectric measurements. The specific heat of the samples was obtained using differential scanning calorimetry.

Fig. 1. XRD pattern of (Na0.5 Bi0.5 )1−x Srx Bi4 Ti4 O15 ceramics.

3. Results and discussion According to XRD in Fig. 1, the sintering conditions lead to the formation of single phase with no detectable phase within the experimental range for all compositions of Sr. All the peaks could be indexed on the basis of orthorhombic symmetry with a = 0.5427 nm, b = 0.5459 nm, and c = 4.065 nm. Lattice parameters calculated indicate the orthorhombic structure of the samples at room temperature (Table 1). As the content of Sr increases, cell volume as well as the lattice parameters increases. One possible explanation is that the ionic radius of Sr2+ (0.144 nm) is more than that of the mean ionic radii of Na+ and Bi3+ (0.135 nm) [17]. Similar behavior was observed in the case of Ba2+ replacement for (Na+ , Bi3+ ) [18]. Table 1 also gives the information regarding the tetragonal distortions and strain on the axis with the addition of Sr2+ in place of the divalent pseudo-cation (Na+ , Bi3+ ). Lattice parameters are calculated from the obtained values of d-spacings and 2θ using the XLAT software after several iterations. These values are tabulated in Table 1. A slight increase in the values of a and c is observed while no significant change could be detected in the values of b. Orthorhombic distortion (b/a) decreases with the increase of Sr2+ and no systematic change in the value of tetragonal strain (c/a) is observed. Hence, even after the substitution of Sr2+ in the present study, the system retains the orthorhombic symmetry. According to Cohen and other workers [19,20], the substitution of Pb2+ in A-site increases the tetragonal strain due to the covalent nature of Pb–O bonds while Sr2+ forms ionic bonds with the neighboring oxygen ions, which favors the orthorhombicity. As the BLSF compounds are known to coexist in tetragonal and orthorhombic phases [21], from Table 1, addition of Sr2+ releases the tetragonal strain keeping the present ceramics in orthorhombic symmetry.

Table 1 Lattice parameters of SNBT system x

a (nm)

b (nm)

c (nm)

Volume

b/a

c/a

0.0*

0.5427 0.5431 ± 0.007 0.5437 ± .004 0.5494 ± 0.008

0546 0.5442 ± 0.007 0.5445 ± 0.004 0.5491 ± 0.010

4.065 4.064 ± 0.143 4.067 ± 0.203 4.114 ± 0.055

1204.52 1206.33 1209.20 1241.05

1.006 1.002 1.001 0.999

7.490 7.483 7.480 7.489

0.1 0.2 0.3

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Table 2 Dielectric data: εr , room temperature dielectric constant, tan δ, dielectric loss, Z, atomic displacement x

εr at RT

tan δ

εr max

TC (◦ C)

Z (nm)

0.0a 0.1 0.2 0.3

248 246 219

0.038 0.045 0.053

3177 1776 1115

655 590 569 554

0.0210 0.0203 0.0201 0.0199

a

Ref. [2].

Fig. 2 depicts the dielectric nature of the samples with x = 0.1, 0.2 and 0.3 at selected frequencies 10, 100 and 1000 kHz. The dielectric constant of the samples is found to decrease with the increase of Sr concentration. It is also observed that the transition temperature corresponding to the peak of dielectric constant is found to shift towards low temperatures. The transition temperature of pure NBT is around 640 ◦ C. It is also clear from Fig. 2 that with the increase Sr content, the dielectric behavior deviates from its normal Curie–Weiss nature and exhibits the diffuse phase transitions (DPT). A modified empirical expression was proposed by Uchino et al. [22] to describe the diffuseness of the ferroelectric phase transition: 1 1 (T − Tc)γ − = ε εm C where γ and C are assumed to be constant, the γ value is between 1 and 2. The limiting values γ = 1 and γ = 2 reduce the equation to Curie–Wiess law, valid for the case of normal ferroelectrics and to the quadratic, valid for the relaxor ferroelectric, respectively. The logarithm of (1/ε − 1/εm ) was plotted against logarithm of (T − Tm ) and the curves gives out the values of γ as 1.48 and 1.68 for Sr = 0.2 and 0.3, respectively (Fig. 3). The sample with Sr = 0.1 composition does not obey the equation. This indicates that the transition becomes diffused from Sr = 0.2 which is absent in the pure NBT material [18]. It was pointed out by Tu et al. [23] that the compound Na0.5 Bi0.5 TiO3 is closely related to the relaxor ferroelectrics and has the close analogy with the classical relaxor. The compound under study is the product of Na0.5 Bi0.5 TiO3 and Bi4 Ti3 O12. The presence of two or three types of ions in A-sites of the perovskite layers may give rise to diffuse transitions. The dielectric loss peaks occur 10–20 ◦ C lower than the dielectric peaks. The variation of dielectric loss (tan δ) with temperature is shown in Fig. 4. The values of tan δ at room temperature are 0.038, 0.045 and 0.053 for x = 0.1, 0.2 and 0.3, respectively. The tan δ value remains constant up to 300 ◦ C and starts increasing with temperature. Peaks were observed at temperatures, below the TC for all the compositions. These peaks in tan δ become broader and the temperature decreases with the increase of Sr content. The pertinent data on tan δ are given in Table 2. The temperatures corresponding to the peaks in dielectric constant (Fig. 2) and tan δ were found to shift towards lower temperatures with the increase in concentration of strontium. The divalent pseudo-cation (Na+ , Bi3+ ) is slightly smaller than Sr2+ but much more polarisable, mainly because of the presence of the lonepair of electrons in Bi3+ cation. Hence, the substitution of Sr2+

Fig. 2. Temperature dependence of dielectric constant for (a) x = 0.1, (b) x = 0.2 and (c) x = 0.3 at fixed frequencies.

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Fig. 3. log (1/ε − 1/εm ) as a function of log(T − TC ) at 100 kHz for (a) x = 0.2 and (b) x = 0.3.

in place of (Na+ , Bi3+ ) reduces the dielectric constant. In layer structured perovskites, the crystal structure may not change as freely as in isotropic perovskites with doping due to structural constraint imposed by [Bi2 O2 ] interlayer. When Sr2+ is doped in place of (Na+ ,Bi3+ ), the Curie temperature decreases. With the bismuth layer structural constraint, the introduction of bigger ions into A-site will consume more space despite the lattice increase resulting in the reduction of “rattling space” for the ions inside the oxygen octahedra resulting in the Curie temperature decrease [24]. The tolerance √ factor t, for the perovskite structures is given by t = (RA + RO )/ 2 (RB + RO ) where RA , RB and RO are the respective ionic radii of A-site, B-site and oxygen ions. The tolerance factor effectively gives the ratio of the size of the A-site cation to that of the cubo-octahedral interstice (BO6 octahedra) of the perovskite structure. In the present system, RA = 0.135 nm (Na+ ,

Fig. 4. Temperature dependence of dielectric loss for (a) x = 0.1, (b) x = 0.2 and (c) x = 0.3 at fixed frequencies.

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Bi3+ ) or 0.144 nm (Sr2+ ), RB = 0.0559 nm (Ti4+ ) and RO = 0.14 [17]. Values of t obtained are 0.970, 0.973, 0.976 and 0.979 for x = 0, 0.1, 0.2 and 0.3, respectively. Therefore the substitution of Sr2+ in the A-site resulted a slight increase of t value from 0.97 (x = 0.0) to 0.98 (x = 0.3). Transition temperature also reduces with this increase of t, which is in good agreement with the report by Suarez et al. [25] for Aurivillius compounds, where a linear decrease of TC was observed with the increase of t in the range t = 0.96 − 1.0. According to Glazer, when t ≤ 0.985, a series of phase transitions occurs involving tilting of octahedra in anti-phase followed by in-phase rotations around the principal axis as t drops to 0.96 [26]. Abrahams et al. [27] have correlated the atomic displacements (z) of certain atoms from their prototype position with the Curie temperature and spontaneous polarization by using the relation: TC = 2.00 ± 0.09 × 104 z2 where z refers to the displacement of Ti ion the compounds. From Table 2, the displacement decrease with the increase of Sr in the ceramics. Kikuchi et al. [28] have attributed this displacement to the cation vacancies or Bi–O bonding. The present samples are studied for pyroelectric activity using the static pyroelectric method. The pyroelectric coefficient is calculated from the measured pyro current following the relation p(T) = ip /A(dT/dt) where p(T) is the pyroelectric coefficient, ip is the pyro current in amperes and dT/dt is the rate of heating and A is the sample area normal to the polar axis. Fig. 5 shows the variation of pyroelectric coefficient and polarization with temperature. The pyroelectric coefficient changes insignificantly with the temperature up to about 350 ◦ C and starts increasing with the increase of temperature thereafter. The pyroelectric current as well as coefficient passes through a peak at a temperature slightly lower than the ferroelectric transition temperature. From Fig. 5, it is seen that the pyroelectric peak tends to become broader with the increase of Sr content. The existence of pyroelectric current is also possible due to the permanent dipoles existing randomly in different regions beyond TC . This can be correlated to the observed broadening in the dielectric behavior, which may be due to the microheterogeneties in the compositions. Fig. 5 also shows the behavior of spontaneous polarization (PS ) calculated from the pyroelectric coefficient following the relation PS = p dt. PS presents a constant value at low temperatures and then tend to decrease to attain zero at a temperature near the TC . The values of PS at room temperature for x = 0.1–0.3 are around 3.3 × 10−3 , 5.1 × 10−3 and 3.0 × 10−3 C/m2 , respectively. The spontaneous polarization becomes zero at 585, 575 and 550 ◦ C respectively for x = 0.1, 0.2 and 0.3 confirming that the present set of compounds are ferroelectric below these temperatures. The value of p(T) varies with the strontium concentration and among the three compositions, the samples with x = 0.2 show the maximum p(T). From the pyroelectric data, voltage responsivity (FV ), detectivity (FD ) are calculated using relations

Fig. 5. Variation of pyroelectric coefficient and spontaneous polarization with temperature for (a) x = 0.1, (b) x = 0.2 and (c) x = 0.3.

FV =

ε

p , × cV

FD = √

(ε

p × tan δ × cV )

where p is the pyro coefficient, ε the dielectric constant, CV the volume specific heat and tan δ is the dielectric loss. These values are tabulated (Table 3). From the table, the specific heat of the samples is found to decrease with strontium addition in sodium bismuth titanate. Values of figures of merit have also been found

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Table 3 Pyroelectric data: CV , volume specific heat; p, pyroelectric coefficient; FV and FD , pyroelectric figure of merit x

CV (×106 J ◦ C−1 m−3 )

p (C m−2 K−1 ) (max)

FV (C m J−1 )

FD (C m J−1 )

0.1 0.2 0.3

2.72 2.39 2.23

0.00313 0.00966 0.0025

4.64 × 10−13 2.48 × 10−12 8.57 × 10−13

1.36 × 10−11 1.34 × 10−10 3.971 × 10−11

to be higher for 02SNBT. Though the pyroelectric coefficient for 01SNBT is more than that of 03SNBT, the FV and FD for 0.3SNBT is found to be more than that of 01SNBT. This may be due to the fact that the decrease in pyroelectric coefficient of 03SNBT is compensated by corresponding decrease in the dielectric constant and volume specific heat. The values of pyroelectric coefficients and figures of merit in these materials is possibly a function of Na–Sr/Bi ratio. This is very useful in device point of view, since instead of varying material temperature to obtain optimum device characteristics it is possible to vary crystal composition and operate the device at lower temperatures. The values observed in the present study are lower when compared to some of the materials used for pyroelectric device applications like PZT, PbTiO3 and SBN [29,30]. Yet, the present materials, due to the high Curie temperatures and low loss, have the advantage of resistance to depoling, which may help for long term operation. The temperature variation of spontaneous polarization is very little over a large range of temperatures. Takanaka et al. [12] suggested that the solid solutions of ferroelectric would give freedom to offer the possibility to alter the Curie temperature, dielectric loss and decreased thermal hysteresis at will. The smaller values of tan δ are preferable for a good pyroelectric device with high signal to noise ratio. 4. Conclusions The system SNBT prepared through the conventional solid state double sintering route crystallized into the orthorhombic structure with slight increase in cell volume when the content of Sr increased from 0.1 to 0.3. Dielectric measurements indicate a decreasing trend of TC with the increase of Sr exhibiting a diffused type transition. The dielectric constant also decreases with the composition. Pyroelectric coefficient exhibits a peak at temperatures below the TC . The pyroelectric figures of merits are maximum for 0.2SNBT in the series. The results indicate that the system with high Curie temperatures and low dielectric loss presents itself as a candidate material for the pyroelectric applications. Acknowledgements This work is supported by DRDO, New Delhi, India through a research project. Authors acknowledge Dr. R. Chandra Prakash, Scientist, SSPL, New Delhi for encouragement.

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