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doi:10.1016/j.jcr

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Journal of Crystal Growth 277 (2005) 462–466

www.elsevier.com/locate/jcrysgro

Substitution site of La ions in La-doped Bi4Ti3O12–SrBi4Ti4O15

intergrowth ferroelectrics

Jun Zhua,b,�, Xiang-yu Maoa, Xiao-bing Chena,b

aCollege of Physics Science and Technology, Yangzhou University, No. 180 Siwangting Road, Yangzhou 225002, ChinabThe National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210008, China

Received 12 December 2003; accepted 2 February 2005

Available online 16 March 2005

Communicated by M.S. Goorsky

Abstract

Several lanthanum-doped intergrowth ferroelectric materials of type Bi4�xLaxTi3O12–SrBi4�yLayTi4O15

[BLT–SBLT(x+y)] were prepared with the lanthanum content, (x+y), ranging from 0.00 to 1.50. XRD patterns

imply that the lattice space of (1 1 6) for Bi4Ti3O12–SrBi4Ti4O15 is the average of those of (1 1 5) for Bi4Ti3O12 and (1 1 7)

for SrBi4Ti4O15. By assuming that this relationship is not affected by doping, the La ion substitution site is calculated.

The results indicate that La ions show pronounced site selectivity for the BTO blocks when the La content is lower than

1.25. As (x+y) ¼ 1.50, more La ions substitute the Bi ions in SBTi blocks. These calculated results are consistent with

the estimation from their Curie temperature.

r 2005 Elsevier B.V. All rights reserved.

PACS: 61.10.N; 77.84.L; 77.80.B; 77.22.G

Keywords: A1. Doping; B1. Bismuth compounds; B2. Ferroelectric materials

1. Introduction

Ferroelectric random access memories haveattracted much attention during the last fewdecades [1]. Due to the excellent fatigue-enduranceproperties, bismuth layer-structured ferroelectrics

e front matter r 2005 Elsevier B.V. All rights reserve

ysgro.2005.02.002

ng author. Tel./fax: +86 514 7975489.

sses: jzhu73@hotmail.com,

u.cn (Jun Zhu).

(BLSFs) have been of considerable researchinterest [2]. The BLSF family has a formula of(Bi2O2)

2+(Am�1BmO3m+1)2�, in which the pseudo-

perovskite blocks [(Am�1BmO3m+1)2�] are inter-

leaved with bismuth oxide layers [(Bi2O2)2+] along

the c-axis. In the formula, A is mono-, di-, ortrivalent ions; B is tetra-, penta-, or hexavalentions; and m is the number of BO6 octahedra in thepseudo-perovskite blocks (m ¼ 1, 2, 3, 4, 5,y) [3].Bi4Ti3O12 (BTO, m ¼ 3) and SrBi4Ti4O15 (SBTi,m ¼ 4) are two typical BLSFs [4,5].

d.

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Jun Zhu et al. / Journal of Crystal Growth 277 (2005) 462–466 463

To meet industrial requirements, a great deal ofresearch has been carried out to improve theferroelectric and fatigue-endurance properties.Doping, constructing intergrowth structure pur-posely, and synthesizing solid solution are allhelpful methods [6]. Substituting more stablelanthanum for volatile Bi in BTO or SBTi canimprove their ferroelectric properties. The rem-nant polarization (2Pr) of La-doped BTO max-imizes with a La content of 0.75 [7,8]. As for La-doped SBTi, the 2Pr reaches a maximum value at adoping content of 0.25 [9]. The variation of Lacontent maximizing 2Pr in the two materialsattributes to the difference of the ions at the Asite [9]. In addition to two Bi ions, there is astrontium ion at the A site in SBTi. The oxygenions near the Sr or La ions are more stable thanthose near Bi ions due to the volatility of the Biions [8]. Namely, the Sr ions can serve as La ions.So the 2Pr of La-SBTi maximizes at a lower Laconcentration [9]. Intergrowth ferroelectric mate-rial is composed of two BLSFs with different m

[10,11]. Bi4Ti3O12–SrBi4Ti4O15 (BTO–SBTi) is astandard intergrowth ferroelectric material, madeup of one-half of the unit cells of BTO and SBTi,with the pseudo-perovskite blocks of BTO andSBTi being in turn sandwiched between (Bi2O2)

2+

layers: ? (Bi2O2)2+–(Bi2Ti3O10)

2�–(Bi2O2)2+–

(SrBi2Ti4O13)2�–(Bi2O2)

2+ ? [12,13]. It is re-ported that the 2Pr of BTO–SBTi ceramic andthin film is larger than 2Pr of its two constituents,i.e., BTO and SBTi [13,14]. This enlargementoriginates from a quite distinct Bi3+ displacementin (Bi2O2)

2+ layers along the a-axis, which is dueto the lattice mismatch between two perovskiteblocks and their different chemical characters[13,15].By combining the effects of making intergrowth

and lanthanum doping, La-doped BTO–SBTiintergrowth ferroelectric materials were synthe-sized. The remnant of La-doped BTO–SBTi isenlarged by nearly 60% in comparison with the2Pr at zero doping [16], and the fatigue-enduranceproperty is improved as well [6]. With the aim ofunderstanding ferroelectric properties from thestructural point of view, the La ions concentrationin each unit of the intergrowth has been estimatedwith the aid of the Curie temperature dependences

on La content [16]. However, the results have notbeen validated by other methods. In this article, Laion substitution sites in the intergrowth ferro-electrics are calculated with the aid of the latticespace relationship among the intergrowth andtheir constituent units. The results obtained fromthe two methods are in good agreement with eachother.

2. Experiments

The Bi4�xLaxTi3O12–SrBi4�yLayTi4O15 [BLT–SBLT(x+y), x+y ¼ 0.00, 0.25, 0.50, 0.75, 1.00,1.25, 1.5] ceramic samples were prepared by usingthe standard solid-state reaction method. Afterbeing ball-milled for 24 h, the start powders werecalcined at 800 1C for 7 h. Then the calcinedsamples were reground and fired at 1080�1120 1C for 4 h for X-ray diffraction (XRD)measurements. The powders of La-doped Bi4-Ti3O12, SrBi4Ti4O15 (BLT-x and SBLT-y) forXRD measurements were fabricated with a similarmethod.The crystal structure of each species was

characterized by XRD using M03XHF22 diffrac-tometry with Cu Ka radiation (l ¼ 1:54056 (A) at atube voltage of 40 kV and a tube current of 40mA.

3. Results and discussions

Fig. 1 shows the XRD patterns of Bi4Ti3O12,SrBi4Ti4O15 and Bi4Ti3O12–SrBi4Ti4O15. Thepeaks are indexed according to standard powderdiffraction data. It is seen that the (h k l) peaksðh ¼ k ¼ 0) of BTO–SBTi nearly reside at themiddle of each (h k l�1) peak for BTO and each(h k l þ 1) peak for SBTi. For instance, the 2y forpeak (1 1 5) of BTO is 27.071, the 2y for peak(1 1 7) of SBTi is 27.731, and the peak (1 1 6) ofBTO–SBTi sites at 27.381. Based on Bragg’s law,the lattice space corresponding to these peaks canbe calculated. The lattice spaces from the (1 1 5)peak of BTO (d1), the (1 1 7) of SBTi (d2), and the(1 1 6) of BTO–SBTi (di) are 3.3857, 3.3187, and3.3498 A, respectively. The di is about the averageof d1 and d2. This is expected, for in intergrowth

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18 24 30

Inte

nsity

(a.u

.)

( 1

1 1

)(

1 1

1 )

( 1

1 1

)

( 1

1 9

)(

1 1

8 )

( 1

1 7

)

( 0

0 10

)(

0 0

9 )

( 0

0 8

)

( 1

1 7

)(

1 1

6 )

( 1

1 5

)

( 0

0 8

)(

0 0

7 )

( 0

0 6

)

SBTi

BTO-SBTi

BTO

2� (degree)

Fig. 1. X-ray diffraction patterns of BTO, BTO–SBTi and

SBTi.

Table 1

Lattice parameters a, b, and c of Bi4Ti3O12, SrBi4Ti4O15, and

Bi4Ti3O12–SrBi4Ti4O15

Lattice parameters

a (A) b (A) c (A)

Bi4Ti3O12 5.445 5.411 32.84

SrBi4Ti4O15 5.445 5.437 40.95

Bi4Ti3O12–SrBi4Ti4O15 5.429 5.445 36.92

0.00 0.25 0.50 0.75 1.00 1.25 1.50

3.384

3.392

3.400

3.408

3.416

3.302

3.304

3.306

3.308

3.310

3.312

3.350

3.352

3.354

3.356

3.358

3.360

(b)

La content

Lat

tice

spac

e (A

)o

(c)

(a)

Fig. 2. Dependence of lattice space from (a) peak (1 1 6) of

BLT–SBLT(x+y), (b) peak (1 1 5) of BLT-x, (c) peak (1 1 7) of

SBLT-y on La-doping content.

Jun Zhu et al. / Journal of Crystal Growth 277 (2005) 462–466464

ferroelectric material, the lattice parameters a andb are close to those of its constituents. But the c isabout the average of its constituents. As forBTO–SBTi, BTO, and SBTi, their a, b, and c

confirm this relationship [4,13,17], as shown inTable 1.Some XRD peaks of BLT–SBLT(x+y), such

as (0 0 7), (0 0 9), (1 1 6), and (1 1 8), move towardslower angle as La content ranges from 0.00to 1.25. However, the 2y of these peaks of BLT–SBLT(1.50) are higher than those of BLT–SBLT(1.25). It has been described in detail else-where [16]. Fig. 2(a) shows the dependence oflattice space from (1 1 6), di, of BLT–SBLT(x+y)on the total La content.

Since the samples remain the crystal structure ofthe intergrowth ferroelectrics after La doping [16],the relationship of their lattice spaces should notvary with doping. That is to say, the di from (1 1 6)of BLT–SBLT(x þ y) should remain the averagevalue of d1 from (1 1 5) of BLT-x and d2 from(1 1 7) of SBLT-y:

di ¼d1 þ d2

2. (1)

In the XRD patterns of La-doped BTO, the(1 1 5) peak moves towards lower angle withdoping. In contrast, the (1 1 7) peak of La-dopedSBTi shifts towards higher angle with La content.

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Table 2

La content in BLT and SBLT in the BLT–SBLT(x+y) achieved from lattice spaces

Total La content: x+y 0.00 0.25 0.50 0.75 1.00 1.25 1.50

La content in BLT: x 0.00 0.16 0.29 0.43 0.58 0.65 0.70

La content in SBLT: y 0.00 0.09 0.21 0.32 0.42 0.60 0.80

Jun Zhu et al. / Journal of Crystal Growth 277 (2005) 462–466 465

The dependences of d1 and d2 on La content, x andy, can be calculated from their XRD patterns, asdemonstrated in Fig. 2(b) and (c), respectively.Two empirical formulas can be obtained:

d1 ¼ �0:02027x2 þ 0:04833x þ 3:3857 (2)

and

d2 ¼ �0:0072y2 � 0:000384y þ 3:31087. (3)

For BLT–SBLT(x þ y), (x þ y) is the total lantha-num content:

x þ y ¼ 0:00; 0:25; 0:50; 0:75; 1:00; 1:25; 1:50.

(4)

In the intergrowth ferroelectrics, La dopingbrings about microstructural variations of BTOor SBTi units. The variations should be differentfrom the microstructural variation in La-dopedBTO and SBTi ferroelectric ceramics. However,the difference is so very small that we may ignoreit. Therefore, the La contents in each constituent,x and y, may be derived from formulas (1)–(4).The calculation result is shown in Table 2. It canbe concluded that the La content in BLT is higherthan that in SBLT when doping concentration isless than 1.25. With the increase of La dopingcontent to 1.50, La ions prefer substituting Bi ionsin SBTi: x ¼ 0.70 and y ¼ 0.80, respectively. Thedifferent concentration of La ions in BLT andSBLT blocks may relate to the different ions attheir A sites. Strontium ions occupy the A sites inSBTi units, leading to the less acceptability tolanthanum ions.We have reported that the La distribution in La-

doped BTO–SBTi intergrowth ferroelectrics isestimated from the Curie temperature [16].Although each of x and y obtained from theirCurie temperature relationship is different fromthose in Table 2, the trend of La distribution isconsistent with each other. La concentrationcalculation results agree well with the relaxation

characteristics observed in BLT–SBLT(1.50) [16].Relaxation characteristics are also observed in La-doped SBTi with La contents higher than 0.75 [18],while La-doped BTO may display relaxationcharacteristics with La contents above 1.10 [19].We have found the relaxation characteristics inBLT-1.25, which will be reported elsewhere. Thus,the relaxation characteristics of BLT–SBLT(1.50)may be associated with the higher La content inthe SBTi constituent. Hence, the above two simplemethods may be applied to describe the Ladistribution in the doped intergrowth ferroelectricmaterials.

4. Conclusions

Lanthanum doping does not significantly affectthe fundamental crystal structure of BTO–SBTiintergrowth ferroelectric material. The La substi-tution site is calculated based upon the latticespace relationship between the intergrowth ferro-electrics and its constituents. When the total Lacontent is less than 1.25, La ions prefer enteringthe BTO block. But when (x þ y) reaches 1.50,more La ions incorporate into SBTi. The Ladistribution trend achieved from this method is ingood agreement with other results attained fromthe relationship of their Curie temperatures. Theresults also qualitatively coincide with the relaxa-tion characteristics detected in BLT–SBLT(1.50).

Acknowledgments

The authors acknowledge the financial supportfrom the National Natural Science Foundationof China (Grant no. 10274066) and the NaturalScience Foundation of Education Bureau ofJiangsu Province, China (Grant no. 01KJB140011).

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