Synthesis and characterization of nanosized SrBi2Ta2O9 powder by a novel sol-gel process

Article abstract of DOI:10.1016/S0025-5408(01)00786-3

Chemically homogeneous SrBi₂Ta₂O₉ (SBT) sol was synthesized using ethoxy tantalum, strontium acetate, and bismuth subnitrate as starting materials, methoxyethylene as a solvent and acetic acid (HOAc) as a catalyst. Single-phased perovskite phase SBT ferroelectric ultrafine powder was obtained after the dried gel was treated at 350°C for 30min and calcined at 800°C for 1h. FT-IR, XRD, TEM and TG-DTA were employed to investigate the transformation processes of sol to gel and gel to ultrafine SBT powder. Acetic acid not only acts as an acid catalyst, but also changes the alkoxide precursor as a ligand at a molecular level. Bidentate acetates replace OR groups and are directly bounded to the tantalum, leading to the formation of Ta (OR)_x(OAc)_5-x. The perovskite SBT phase formed via intermediate phase Bi₃TaO₇ and a Bi-deficient pyrocholore phase.

Full text of DOI:10.1016/S0025-5408(01)00786-3

Materials Research Bulletin 37 (2002) 2517±2524
Synthesis and characterization of nanosized
SrBi Ta O powder by a novel sol±gel process
2
2 9
*
Wen Wang , Dechang Jia, Yu Zhou, Feng Ye
School of Materials Science and Engineering, P.O. Box 433,
Harbin Institute of Technology, Harbin 150001, PR China
(
Refereed)
Received 9 July 2001; accepted 31 October 2001
Abstract
Chemically homogeneous SrBi Ta O (SBT) sol was synthesized using ethoxy tantalum,
2
2 9
strontium acetate, and bismuth subnitrate as starting materials, methoxyethylene as a solvent
and acetic acid (HOAc) as a catalyst. Single-phased perovskite phase SBT ferroelectric
ultra®ne powder was obtained after the dried gel was treated at 3508C for 30 min and calcined
at 8008C for 1 h. FT-IR, XRD, TEM and TG-DTA were employed to investigate the
transformation processes of sol to gel and gel to ultra®ne SBT powder. Acetic acid not only
acts as an acid catalyst, but also changes the alkoxide precursor as a ligand at a molecular
level. Bidentate acetates replace OR groups and are directly bounded to the tantalum, leading
to the formation of Ta (OR) (OAc) . The perovskite SBT phase formed via intermediate
x
5Àx
phase Bi TaO and a Bi-de®cient pyrocholore phase.
7
3
#
2002 Elsevier Science Ltd. All rights reserved.
Keywords: A. Ceramics; A. Electronic materials; B. Sol±gel chemistry; C. Infrared spectroscopy; C.
X-ray diffraction
1
. Introduction
Recently, alternate ferroelectric oxides belonging to the family of layered per-
ovskites, e.g., SrBi Ta O (SBT), were identi®ed as promising candidates for
nonvolatile memory applications. SBT thin ®lms were found to exhibit no fatigue
2
2 9
*
Corresponding author. Fax: ‡86-451-6413922.
E-mail address: ce921@hope.hit.edu.cn (W. Wang).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 0 7 8 6 - 3

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W. Wang et al. / Materials Research Bulletin 37 (2002) 2517±2524
1
2
up to 10 switching cycles [1], display very good retention properties and low
leakage current densities on Pt electrodes.
To date, pulse laser decomposition (PLD) [2,3], metal-organic chemical vapor
deposition (MOCVD) [4], metal-organic decomposition (MOD) [5] and sol±gel [6,7]
methods have been reported for the synthesis of SBT. Among the reports on sol±gel
processes to fabricate SBT materials, it can be found that metal alkoxide compounds
were used as Sr, Bi, and Ta sources. When starting materials of multicomponent
systems are all alkoxides precursors, the system is so susceptible to ambient humidity
that it should be handled under an inert atmosphere. Therefore, the alkoxides of some
metals such as Ta and Bi have been modi®ed using pyridine [8], alcohol-amine [9],
acetylacetone [10], and metal complex compounds in an effort to achieve stability.
However, the addition of complexing agent makes it dif®cult to control the process of
gel polymerization.
Inorganic and organic salts have been used for sol±gel processing of multi-
component systems when it is dif®cult or unnecessary to use alkoxides [11,12].
In this article, bismuth subnitrate and strontium acetate were used to prepare stable
SBT precursor solutions, thus, it is easier to control the hydrolysis and polymerization
reaction. Pure SBT powders were fabricated in a sol alkoxide system. The authors
investigate here the effect of acetic acid (HOAc) on condensation polymerization and
crystallization behavior of the SBT. The precursor structure of the SBT gel also has
been characterized.
2
. Experimental
In this experiment, all chemical compounds did not need to be specially treated and
the precursor solution was suf®ciently stable to allow their handling in air, which
made the operation more convenient. Analytically pure Sr(CH COO) , BiONO ,
Ta(OCH CH ) in a mole ratio of 1:2:2 were precisely weighed and dissolved in 2-
3
2
3
2
3 5
methoxyethanol. Temperature was controlled at about 808C until the raw materials
were completely dissolved and a transparent solution was obtained. The mixed
precursor exhibits an obvious Tyndall phenomenon, indicating the formation of a sol.
Then, the sol was cooled down to 208C, and allowed to stand for 6 h to prepare gel.
The pH value of the solution is a main factor affecting the formation of the sol and gel.
A stable sol and homogenous gel can form when the pH value of the solution is 3.5.
Acetic acid prevents precipitation and the gelation time increases up to several days or
weeks when acetic acid is added. Stable transparent gels can thus be reproducibily
obtained. Fig. 1 shows the process ¯ow chart of the preparation of SBT powder.
Fourier transform infrared (FT-IR) spectroscopy can conveniently characterize
such structural changes that occur in the sol±gel process during the conversions from
sol to gel and then to crystalline phase. Our analysis of FT-IR data was based on the
results published in the literature [13,14], especially those reported by Doeuf.
Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were
performed at a heating rate of 108C/min to examine the thermal reactions of the gel.

W. Wang et al. / Materials Research Bulletin 37 (2002) 2517±2524
2519
Fig. 1. Flow chart of the preparation of SBT ®ne powder by sol±gel process.
The phases present in and the morphology of the powder were examined by X-ray
powder diffraction (XRD) and transmission electron microscopy (TEM), respectively.
3
. Results and discussion
Fig. 2a gives the FT-IR spectrum of acetic acid. As shown, the bands, such as
À1
À1
1
1
760 cm C=O stretching (monomer), 1715 cm C=O stretching (dimer) and
À1
410 cm COH bending vibration appeared.
Fig. 2b illustrates the FT-IR spectrum of the methoxyethanol solution containing
ethoxy tantalum, strontium acetate, bismuth subnitrate and acetic acid. Because there
is a lot of solvent in the sol at room temperature, the FT-IR spectrum is similar to that
of pure methoxyethanol solvent. In addition, the HOAc used to regulate the pH value
À1
shows the strong C=O stretching mode at 1760 and 1715 cm .
Fig. 2c illustrates the FT-IR spectrum of the gel. However, some solvent remained
À
in the gel. It should be noted that the band for the asymmetric COO stretching mode
À1
which is shifted to 1560 cm , is absent in Fig. 2a and b. Such a change may indicate
that stable tantalum alkoxo-acylates actually forms when acetic acid is added to an
À1
4
3
alkoxide, and that COO in HOAc has the character of multi-electron p bonding (p )
that cause the nC=O to shift to lower frequency [15]. The chelating acetates are then
less easily hydrolyzed than the OR groups thus increasing the time of gelation.
The FT-IR spectrum of the dried gel in Fig. 2d is obviously different from that in
Fig. 2b. First, since solvent methoxyethanol has evaporated, the nOH peak intensity
apparently becomes weak; Second, the intensity of C=O stretching in HOAc

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W. Wang et al. / Materials Research Bulletin 37 (2002) 2517±2524
Fig. 2. Infrared spectra of the materials used and formed in gelation. (a) Acetic acid, (b) SBT sol,
c) SBT gel, (d) SBT gel dried at 1008C.
(
decreases because acetic acid has evaporated with the increase in temperature. The
À1
absorption at 1560 cm demonstrates the formation of bidentate acetate according to
the literature [13,14]. However, the possible shifting of the symmetric COO stretching
À1
mode at 1470 cm was not completely clear due to its overlap with bismuth
À1
subnitrate. Lastly, the absorption peak at 560±660 cm corresponds to the stretching
À1
vibrations of nBi±O,n
corresponding to NO3 suggest that bismuth subnitrate still exists. The sharp band at
and nSr±O. The absorption peaks at 1380 and 840 cm
Ta±O
À
À1
1
strontium acetate also exists.
578 cm corresponds to the nasym(COO) in strontium acetate. This proves that
The sol±gel process normally involves hydrolysis and condensation of metal
alkoxide and metal inorganic salts in an alcoholic media. As condensation poly-
merization proceeds, the viscosity of the reaction mixture increases and ®nally a gel is
À
obtained. FT-IR spectra indicate that CH COO behaves as a bidentate ligand.
3
Acetate reacts chemically with the alkoxide giving rise to a new molecular precursor
Ta(OR) (OAc) . This precursor favors the polycondensation process, and leads to
the formation of monolithic gels. A stoichiometric chemical reaction takes place for a
x
5Àx
1
:1 ratio that can be written as follows:
TaꢀOC H † ‡ HOAc ! TaꢀOC H † OAc ‡ C H OH
2
5
2
5
2
5
5
4
TaꢀOC H † ꢀOAc† ‡ H O ! TaꢀOC H † ꢀOAc†ꢀOH† ‡ HOCH CH
2
5 5Àx
x
2
2
5 3
2
3
2
TaꢀOC H † ꢀOAc†ꢀOH†
2
5 3
!
For the whole reaction system, therefore, the product is mainly Ta±O±Ta monomer.
ꢀAcO†ꢀOC H † TaÀOÀTaꢀOC H † ꢀOAc† ‡ H O
2
5 3
2
5 3
2
Tantalum alkoxide cannot be used directly in alcoholic media because hydrous

W. Wang et al. / Materials Research Bulletin 37 (2002) 2517±2524
2521
Fig. 3. Infrared spectra of gel calcined at different temperatures. (a) 2008C, (b) 4008C, (c) 6008C,
d) 8008C.
(
tantalum oxide precipitates out before gel formation can take place. However,
tantalum alkoxide can be chemically modi®ed with acetic acid to produce tantalum
alkoxo-acylate precursor, which is more stable. The solution of tantalum alkoxo-
acylate, strontium acetate and bismuth subnitride can then be mixed to produce a
homogeneous Sr±Bi±Ta gel. The amorphous Sr±Bi±Ta gel is usually assumed to
consist of Ta±O±Ta framework. Molecules of solvent, BiO(NO ) and Sr(OAc) , are
3
2
situated in random positions in the Ta±O±Ta framework.
Fig. 3 gives the IR spectra of the SBT gel at 200, 400, 600 and 8008C, respectively.
The FT-IR spectrum at 2008C is similar to that of the gel dried at 1008C. Thus, we can
say that the polymer structure of the SBT gel remains stable at least at 2008C. With the
À
À1
increasing temperature, the absorption peak of the COO group at 1560 cm
weakens gradually until it completely disappears, and CO
2
3
À
group characteristic
peaks at 1450 and 860 cm emerge instead. At the same time, in Fig. 4 the XRD
À1
pattern at 5008C also shows the diffraction peaks of SrCO . All the above results show
3
that Sr(OAc) in the above framework is decomposed to SrCO , and it can be inferred
that in the course of the dried sol turning into SBT, SrCO is formed. The
2
3
3
À
À1
characteristic peak of NO3 at 1380 cm also weakens gradually until it completely
disappears because the bismuth subnitrate decomposes to bismuth oxide. At the same
time, in Fig. 4 the XRD pattern at 5008C also shows the diffraction peaks of Bi O In
addition, in the FT-IR spectra the strong absorption peaks at 600 and 800 cm
indicate that the material is a non-organic polycrystalline powder.
2
3.
À1
Fig. 4 shows the XRD patterns of the SBT powders calcined at different
temperatures. As indicated, when SBT gel is calcined at 5008C, SrCO and Bi O
3
2 3
appear; whereas, when SBT gel is calcined at 600 and 7008C, SrCO diffraction peaks
3
are no longer seen. Rather a Bi-de®cient pyrocholore phase and Bi TaO cubic phase
3
7

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W. Wang et al. / Materials Research Bulletin 37 (2002) 2517±2524
Fig. 4. XRD patterns of gel calcined at different temperatures.
are found. Eventually, when SBT gel is calcined at 8008C, a single perovskite SBT
phase is obtained.
Fig. 5 shows the DTA and TGA curves of the gel obtained from solution with
atomic ratio of strontium:bismuth:tantalum ˆ 1:2:2.
The DTA study of the gel reveals one endothermic peak at 48.78C and two
exothermic peaks at 295.7 and 621.58C. The endothermic peak is due to the evolution
of organic materials (such as HOCH CH OCH and CH COOH). A weight loss of
2
2
3
3
2
0.12% is observed. The ®rst exothermic decomposition peak is due to the combus-
tion of volatile organic composition entrapped among framework of the polymers
with a weight loss of 9.98%. The second one is the result of decomposition and
Fig. 5. TG-DTA curves of the gel.

W. Wang et al. / Materials Research Bulletin 37 (2002) 2517±2524
2523
Fig. 6. TEM morphology of powder obtained from the gel (a) dried at 1008C and (b) calcined at
008C.
8
combustion of the residual carbon with a weight loss is about 2%. The small peak at
47.5±7338C is relevant to the crystallization of the SBT powder. It should be pointed
5
out that two exothermic peaks before the crystallization of SBT phase revealed in the
DTA curves of the gel was previously reported [16] but in this work, only one
exothermic peak was observed. This difference may be attributed to the different
fabrication routes.
Fig. 6 shows typical TEM images of SBT dried gel at 1008C and SBT powders
calcined 8008C, respectively. As shown, the particle sizes are less than 50 nm.
4
. Conclusions
A modi®ed sol±gel process has been developed to prepare SrBi Ta O powder. A
2
2 9
stable SBT sol and gel was obtained, using bismuth subnitrate, strontium acetate and
ethoxy tantalum as precursors, methoxyethanol as a solvent and acetic acid as a
À
catalyst. The FT-IR spectra indicate that CH COO behaves as a bidentate ligand.
3
Acetate also reacts chemically with the alkoxide giving rise to the formation of a new
molecular precursor Ta(OR) (OAc) . The chelating acetates are then less easily
x
5Àx
hydrolyzed than the OR groups thereby increasing the time of gelation. XRD patterns
con®rm that a single-perovskite phase SrBi Ta O powder was obtained after SBT gel
2
2 9
was calcined at 8008C for 1 h. At 600 and 7008C a Bi-de®cient pyrocholore phase and
Bi TaO cubic phase were formed.
3
7

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W. Wang et al. / Materials Research Bulletin 37 (2002) 2517±2524
Acknowledgments
The authors would like to give thanks to the project supported by the Scienti®c
Research Foundation of Harbin Institute of Technology (HIT.2001.67) and Chinese
National Natural Science Foundation (50172012).
References
[
1] C.A. Paz De Araujo, J.D. Cuchiare, L.D. Mcminan, M.C. Scott, J.F. Scott, Nature 374 (1995)
27±629.
6
[
[
[
[
[
2] R. Dat, J.K. Lee, O. Aucielli, A.Z. Kinon, Appl. Phys. Lett. 67 (1995) 572±574.
3] S.B. Desu, P.P. Vijay, X. Zhang, B.P. He, Appl. Phys. Lett. 69 (1996) 1719±1721.
4] T.K. Li, Y.F. Zhu, S.B. Desu, C.H. Peng, M. Nagata, Appl. Phys. Lett. 68 (1996) 616±618.
5] Z.G. Zhang, J.S. Liu, Y.N. Wang, J.S. Zhu, F. Yan, Appl. Phys. Lett. 73 (1998) 788±790.
6] Y. Ito, M. Ushikubo, S. Yokoyama, H. Matsunaga, T. Atsuki, T. Yonezawa, K. Ogi, Jpn. J. Appl.
Phys. 35 (1996) 4925±4929.
[
[
[
7] D. Ravichandran, K. Yamakawa, A.S. Bhalla, R. Boy, J. Sol±Gel Sci. Tech. 9 (1997) 95±101.
8] K. Kato, C. Zheng, J.M. Finder, S.K. Dey, J. Am. Soc. 81 (1998) 1869±1875.
9] W. Wang, Y. Zhou, S. Chen, F. Ye, D.C. Jia, J. Mater. Sci. Tech. 17 (2001) 25±26.
[
[
10] T. Hayashi, H. Takahashi, T. Hara, Jpn. J. Appl. Phys. 35 (1996) 4952±4955.
11] T.J. Boyle, C.D. Buchheit, M.A. Rodriguez, H.N. Shareef, B.A. Hernandez, B. Scott, J.W. Ziller,
J. Mater. Res. 11 (1996) 2274±2281.
[
[
[
[
[
12] C.H. Lu, S.K. Saha, Mater. Res. Bull. 35 (2000) 2135±2143.
13] S. Doeuff, M. Henry, C. Sanchez, J. Livage, J. Non-Cryst. Solids 89 (1987) 206±216.
14] S. Li, R.A. Condrate, R.M. Spriggs, Spestr. Lett. 21 (1988) 969±980.
15] M. Brow, J. Am. Chem. Soc. 77 (1995) 1341±1346.
16] Q.F. Zhou, H.L. Chan, C.L. Choy, J. Non-Cryst. Solids 254 (1999) 106±111.