Coondoo I. (ed.) Ferroelectrics

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Effects of B-site Donor and Acceptor Doping in Pb-free (Bi

0.5

)TiO

Ceramics

227

solutions, but it has not been clearly explained, partly because T

is a unique transition in

BNT-based ceramics that cannot be explained in terms of the PZT database. Sung et al.

suggested lattice distortion as a common factor that affects the two properties inversely. If

lattice distortion decreases, d

increases through better poling by electric field, but T

decreases through easier depoling by temperature.

For doping effects in general, all the explanations have been given in terms of A-site and

oxygen-site vacancies. In terms of the ionic sizes, Mn

, Ti

, Nb

, Ta

, and W

(but not

) are similar, as shown in the comparison with Ti

in Table 1. In cases of similar ionic

size, there will be minimal change in the shape and size of the lattice. With a small amount

of doping, therefore, it would be difficult to detect any variation in the XRD patterns.

Indeed, as shown in Fig. 9, there was no apparent change in peak positions caused by Nb

donor or Mn acceptor doping. It would be interesting to know if any variation in lattice

parameters occurs in the case of a substantial amount of doping with a relatively large

difference in ionic radii.

Fig. 9. Detailed XRD patterns of (Bi

0.5

)(Ti

1-x

ceramics (D = Nb, Mn), from the

patterns shown in Fig. 1

There are some data regarding donor doping effects in BNT-based ceramics, but little

information is available about acceptor doping effects. This situation could exist because

acceptors with 3+ valence can be on either the B-site Ti or the A-site vacancy, considering

the volatile A-site Bi

and Na

. For dornors with 5+ or 6+, on the other hand, it is unlikely

that they would occupy the A-site vacancy instead of the B-site Ti. Moreover, even if they

did, the resulting outcomes would still be considered as donor doping effects. As shown in

Table 2, among the data on donor effects, consistent trends can be identified. Grain size, Q

, and E

decreased, while d

and P

increased. These trends become reversed with

acceptor doping.

Finally, it cannot be emphasized enough that hygroscopic Na

must be completely dried

before and during weighing. Otherwise, the nominal compositions of the samples will be

39 40 41 42 45 46 47 48 49

2.0 Mn

1.5 Mn

1.0 Mn

0.5 Mn

111

0.2 Nb

0.4 Nb

0.6 Nb

0.8 Nb

1.0 Nb

BNT

200

111

Intensity (arb. unit)

(degree)

Ferroelectrics

228

incorrect. If the ionic radius of an acceptor with 3+ valence is larger than that of Ti

, doping

of an acceptor with 3+ valence on Na

vacancy, instead of the Ti

site, might occur. In

addition, there is the possibility of an acceptor with 3+ valence substituting for A-site Bi

instead of for B-site Ti

. This appears to explain the inconsistent results and donor-like

effects (see Table 1) that were observed in Sc

, which has a larger ionic radius than the

others do as compared with that of Ti

. To confirm the acceptor effects in BNT, the next

experiment should include the doping of acceptors with 2+ valence and small differences in

ionic radii.

5. Summary

The effects of a B-site donor, Nb

, and an acceptor, Mn

, for Ti

in Pb-free (Bi

0.5

)(Ti

(D = Nb or Mn) ceramics were studied in terms of microstructure and important

properties. Regarding the microstructure, all the samples retained a perovskite structure

with rhombohedral symmetry with both Nb donor and Mn acceptor doping. No secondary

phase was seen up to x = 1.0 mol % of Nb-doped BNT ceramics and x = 2.0 mol % of Mn-

doped BNT ceramics. The grain size, however, decreased with Nb doping and slightly

increased with Mn doping. In addition to producing variation in microstructure, Nb and Mn

yielded opposite piezoelectric and dielectric outcomes. Higher d

, lower Q

, and smaller

grain size were obtained from Nb doping, while lower d

, higher Q

, and larger grain size

were obtained from Mn doping. These results indicate a correlation between piezoelectric

properties and grain size. Regarding the dielectric transition, T

decreased with Nb doping,

but it decreased slightly then held steady with Mn doping. Overall, the donor effects

observed in BNT ceramics were similar to those observed in PZT ceramics, but acceptor

effects need to be studied further in order to produce consistent data. At elevated

temperatures, BNT exhibited n-type conduction.

6. Acknowledgments

This work was supported by the National Research Foundation of Korea Grant funded by

the Korean Government (2009-0088570) and the Basic Science Research Program through the

National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science

and Technology (2010-0025055). This research was also financially supported by the

Ministry of Education, Science and Technology (MEST) and Korea Institute for

Advancement of Technology (KIAT) through the Human Resource Training Project for

Regional Innovation.

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Enhanced Dielectric and Ferroelectric

Properties of Donor (W

, Eu

) Substituted SBT

Ferroelectric Ceramics

Indrani Coondoo

and Neeraj Panwar

National Physical Laboratory, Dr. K. S. Krishnan Marg,New Delhi-110012,

Department of Physics, University of Puerto Rico, San Juan, PR-00931

India

USA

1. Introduction

In recent years, there has been a tremendous interest in ferroelectric materials from

perspective of their potential applications in electronic devices such as non-volatile random

access memories (NvRAMs), pyroelectric infrared detectors and optical switches [1-4]. On

account of their read-write speed, non volatility, low operating power and radiation

hardness, NvFRAMs are promising candidates for substituting silicon based electrically

erasable programmable read-only memories (EEPROMs) and flash EEPROMs. The materials

for memory applications are required to possess the following properties: a large remanent

polarization (P

), a low coercive field (E

) and sufficient fatigue endurance against repetitive

polarization switching etc. [5]. Among ferroelectrics, lead zirconate titanate PbZr

1-x

(PZT) has been investigated extensively and has been found to be the most promising

material for NvFRAM applications. However, apart from Pb toxicity, PZT suffers from

serious degrading problems such as fatigue, ageing and leakage current that hinder the

usability of this material in devices [6-10]. Among them fatigue is an important reliability

issue for NvRAM devices which is defined as a decrease in switchable polarization with

increasing number of polarization reversals [11].

During the search for an alternate ferroelectric material for these applications, it was found

that bismuth oxide layered structures (BLSFs) originally synthesized by Aurivillius [12] are

the most suitable candidates due to their relatively high Curie temperature (T

), low

dielectric dissipation and anisotropic nature originating from their layered structure [13-15].

The Aurivillius family of layered bismuth-oxides encompasses many ferroelectric materials,

a fact which was known since the pioneering work of Smolenskii [16] and Subbarao [17],

more than forty years ago. SrBi

(SBT), which is an n=2 member of the Aurivillius

family of layered compounds, proved to be a versatile material for multifarious

applications. Since Araujo et. al. [18] reported the fatigue-free behavior of SrBi

(SBT),

it has gained importance among Pb-free ferroelectric memory materials [19-28]. Araujo

recognized the inherent advantage of BLSFs over other ferroelectric materials on account of

the fact that the former have intermediate bismuth oxide layers between the ferroelectric

units [29].

Ferroelectrics

232

Fig. 1. The crystal structure of SrBi

Fig. 1 depicts the crystal structure of SrBi

consisting of (Bi

)

layers and perovskite-

type (SrTa

)

units with double TaO

octahedral layers [30]. Besides improvement in the

fatigue characteristics, a longer polarization retention time, lesser tendency to take an

imprint [30,31], high dielectric constant and low switching fields have also been observed in

SBT [32], which are better than those of PZT [18, 33-34]. The high fatigue endurance to

repetitive switching of polarization (≈10

switching cycles) is believed to originate from the

anisotropic nature of SBT. It has been argued that the (Bi

)

layers control the electronic

response [35-36] and perform the primary function in preventing degradation of remanent

charge [35,37]. The ferroelectricity arises mainly in the perovskite blocks [15, 35, 38-40]. The

dielectric constant peak corresponding to a ferro-paraelectric transition has been reported in

the range of 300–320 ˚C [35, 38].Though SBT shows improved ferroelectric properties [41-

42], its piezoelectric properties have not been investigated in detail. Kholkin et. al. [43]

reported on the electromechanical properties of SBT but the maximum piezoelectric

coefficient that was measured was quite small.

Enhanced Dielectric and Ferroelectric Properties of Donor (W

, Eu

)

Substituted SBT Ferroelectric Ceramics

233

In this chapter effect of tungsten and europium substitution on dielectric, conductivity,

piezoelectric and ferroelectric properties of SBT ceramics have been carried out and the

results are presented.

2. Experimental techniques

The polycrystalline SrBi

(SBT) ferroelectric ceramics doped with W (tungsten) and Eu

(europium) having the following compositions were prepared by the solid state reaction

technique:

SrBi

(Ta

1-x

)

; x = 0.0, 0.025, 0.050, 0.075, 0.10, 0.20

1-x

; x = 0.0, 0.025, 0.050, 0.10, 0.20

The starting chemicals used were strontium carbonate (SrCO

), bismuth oxide (Bi

tantalum oxide (Ta

), tungsten oxide (WO

) and europium oxide (Eu

). The chemicals

were weighed in stoichiometric proportions as mentioned above. The weighed powders

were mixed and thoroughly ground and passed through sieve of appropriate mesh size. The

ground powder was calcined at 900 ˚C in air for two hours. Thereafter, the calcined powder

was pressed into disk shaped pellets. The pellets were sintered in air at 1200 ˚C. The sintered

pellets were polished to a thickness of nearly 1mm. High temperature conductive silver

paste was used for electroding the parallel surfaces. After applying the silver paste, the

pellets were cured at 550˚C for half an hour before electrical characterization. X-ray

diffractograms of all the samples were recorded for the structural analysis using Bruker X-

ray diffractometer with CuK

radiation of wavelength 1.54439 Å in the range from 10˚≤ 2θ

≤70˚ at a scanning rate of 0.05˚ / second. The SEM micrographs of the fractured surfaces of

the samples were obtained using the Cambridge Stereo Scan 360 scanning electron

microscope. A Solartron 1260 Gain-phase impedance analyzer was used for measuring the

dielectric constant and dielectric loss in the present work. The d.c conductivity was

measured using Keithley’s 6517A electrometer. Hysteresis measurements were done at

room temperature using an automatic PE loop tracer based on Sawyer-Tower circuit at

switching frequency of 50 Hz. The piezoelectric coefecient, d

was measured using a

Berlincourt d

meter.

3. Results and discussion

3.1 XRD analysis

The observed XRD patterns of the studied samples having different concentrations of

tungsten and europium are shown in Fig. 2(a) and (b), respectively. The XRD patterns of the

SrBi

(Ta

1-x

)

samples show the characteristic peaks of SBT. The peaks have been

indexed with the help of a computer program – POWDIN [44] using the observed

interplanar spacing d. It is observed that the single phase layered perovskite structure is

maintained in the range 0.0 ≤ x ≤ 0.05. An unidentified peak of very low intensity is

observed in the composition with x > 0.05. In all the diffraction patterns, peaks shift slightly

towards higher diffraction angle with increasing W concentration implying a decrease in

lattice parameters. This can be understood from the fact that the ionic radius of W

(0.60Å)

is smaller in comparison to Ta

(0.64Å). On the other hand no extra peaks are observed in

Ferroelectrics

234

the XRD patterns of Sr

1-x

samples. It can therefore be concluded that the single

phase layered perovskite structure is maintained in all the SEBT samples. The calculated

lattice parameters for both the series are tabulated (Table 1).

(b)

(a)

Fig. 2. XRD patterns of the (a) SrBi

(Ta

1-x

)

and (b) Sr

1-x

samples

Conc

(Å)

Volume

(Å

)

Conc.

(Å)

Volume (Å)

5.5212 5.5139 24.9223 758.7182

5.5212 5.5139 25.0122 761.4550

0.025

5.5314 5.5202 25.1079 766.6555

0.025

5.5262 5.5187 25.0282 763.2960

0.05

5.5270 5.5199 25.0585 764.4969

0.05

5.5256 5.5158 24.9765 761.2364

0.075

5.5251 5.5045 25.0567 762.0472

0.1

5.5209 5.5101 24.9555 759.1641

0.1

5.5242 5.5060 25.085 762.9915

0.2

5.5167 5.5042 24.9349 757.1487

0.2

5.5233 5.4939 25.0861 761.2241

Table 1. Lattice parameters and unit cell volume for SrBi

(Ta

1-x

)

and Sr

1-x

samples

3.2 Morphological studies

Figs. 3 and 4 show the surface morphology of the SBTW and SEBT samples respectively. A

systematic study of the micrographs reveals porous and loosely packed grains for the pure

sample. From the figures one can notice an increase in the average grain size, homogeneity

and relatively packed microstructure with W and Eu substitution. Randomly oriented and

anisotropic plate-like grains are observed in all the W substituted samples. The average

Enhanced Dielectric and Ferroelectric Properties of Donor (W

, Eu

)

Substituted SBT Ferroelectric Ceramics

235

grain size in the SEBT sample with x = 0.025 is ~ 5-6 µm while that in the sample with x =

0.20 the size increases to ~ 7-8 µm.

(a)

(b) (c)

(d)

(e)

(f)

Fig. 3. SEM images with (a) x = 0.0, (b) x = 0.025, (c) x = 0.05, (d) x = 0.075, (e) x = 0.10 and

(f) x = 0.20 of SrBi

(Ta

1-x

)

samples

Fig. 4. SEM images with (a) x = 0.0, (b) x = 0.025, (c) x = 0.05, (d) x = 0.10 of Sr

1-x

samples

Ferroelectrics

236

3.3 Dielectric studies

It is well known that the dielectric permittivity and loss of ferroelectric materials in most

cases depend upon the composition, grain size, secondary phases, etc [45]. Since bismuth

layered perovskites are anisotropic in nature, its dielectric behavior is often influenced by

the crystal structure [45]. All ferroelectric materials are characterized by a transition

temperature known as Curie temperature (T

) at which the dielectric constant is maximum.

At temperatures T>T

, the crystal does not exhibit ferroelectricity while for T<T

it is

ferroelectric [46].

0 100 200 300 400 500

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

(b)

tanδ

Temperature (

0.0

0.025

0.050

0.075

0.10

0.20

0 100 200 300 400 500

100

150

200

250

300

350

0.00 0.05 0.10 0.15 0.20

4.520

4.524

4.528

4.532

Concentration of Eu

c / a

Temperature (

0.0

0.025

0.050

0.10

0.20

(c)

0 100 200 300 400 500

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

(d)

tan δ

Temperature (

0.0

0.025

0.050

0.10

0.20

0 100 200 300 400 500

100

200

300

400

500

600

700

Temperature (

0.0

0.025

0.050

0.075

0.10

0.20

(a)

Fig. 5. (a) Temperature dependence of dielectric permittivity and (b) loss tangent for SBTW

samples. (c) Temperature dependence of dielectric permittivity and (d) loss tangent for

SEBT samples. The inset in (c) shows variation of tetragonal strain (c/a) with Eu

concentration

The phase transition temperature of a sample is deduced from its dielectric permittivity (

)

versus temperature curve. Fig. 5(a) shows the temperature dependence of

at selected

frequencies of 100 kHz in SBTW samples. All the doped samples exhibit sharp transition at

their respective Curie temperatures (T

). A shift in T

to higher temperatures and a

corresponding increase in the peak dielectric constant values with increasing concentration

of tungsten are observed.