Structural and microstructural characterization of SrBi2(Ta0.5Nb0.48W0.02)2O9 powders

Article abstract of DOI:10.1016/j.jallcom.2006.11.194

SrBi₂(Ta_0.5Nb_0.48W_0.02)₂O₉ powders (SBTN-W) were prepared by the polymeric precursor method. The influence of annealing temperature on the phase formation and specific surface area was eval

Full text of DOI:10.1016/j.jallcom.2006.11.194

Journal of Alloys and Compounds 454 (2008) 61–65
Structural and microstructural characterization of
SrBi2(Ta0.5Nb0.48W0.02)2O9 powders
a
N.L. Amsei Junior , A.Z. Simoesa,∗, A.A. Cavalheirob, S.M. Zanettic, E. Longoa, J.A. Varelaa
´
˜
a Instituto de Qu´ımica, UNESP-Universiade Estadual Paulista, C.P. 355, 14801-970 Araraquara, SP, Brazil
b Instituto de Biocieˆncia, UNESP-Universidade Estadual Paulista, Distrito de Rubia˜o Junior, s/n, 18618-000 Botucatu, SP, Brazil
c Instituto Nacional de Pesquisas Espaciais, INPE-Pc¸ Marechal Eduardo Gomes, 50, Vila das Aca´cias, 12228–900 Sa˜o Jose´ dos Campos, SP, Brazil
Received 3 October 2006; received in revised form 28 November 2006; accepted 29 November 2006
Available online 2 January 2007
Abstract
SrBi2(Ta0.5Nb0.48W0.02)2O9 powders (SBTN-W) were prepared by the polymeric precursor method. The influence of annealing temperature on
the phase formation and specific surface area was evaluated. TG/DTA associated with X-ray diffraction (XRD) analyses showed the formation of
perovskite phase at around 500–600 ◦C. An orthorhombic structure with A21am space group was identified by Rietveld refinement. BET analysis
revealed that the specific surface area reduces with increasing thermal annealing. SEM micrographies showed grains in an almost-spherical
morphology with the presence of agglomerates.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Ceramics; Chemical synthesis; Crystal structure
1. Introduction
aqueous precursor solution containing metal ions complexed
with agent coordinating, have been widely used to prepare
ceramic materials [9–11]. The powder characteristics can be
controlled by using chemical methods due to the possibility
to obtain a more homogeneous mixing of the reactant species,
decrease of synthesis temperature and smaller particle with a
more homogeneous sizes distribution. These features improve
reactivity of powders producing high quality ceramic materials
[12]. The most frequently used method in preparation of
solution can be grouped into two categories: (1) sol–gel process
and (2) metallorganic decomposition. There are essentially
three different kinds of sol–gel process, such as: colloidal
sol–gel processes, derivation of inorganic polymeric gels from
organometallic compounds, and gel routes involving formation
of an organic polymeric complex (polymeric precursor method).
The polymeric precursor method can be divided into two groups
[13]. The first is “in situ” polymerization of organometallic
monomers and the second involves the preparation of a viscous
solution system containing metal ions, polymers, and a solvent.
This viscous solution can be easily converted into a ther-
moplastic gel at high polymer concentrations. The “in situ”
polymeric precursor method has been extensively used to obtain
ceramic powders with small particle size and homogeneous
monophasic system [14,15]. This method was originally devel-
Recently, many ferroelectric materials have been investigated
for applications as electronics devices [1–3]. Strontium bis-
muth niobate tantalate (SBTN) is of significant commercial and
industrial importance because of their excellent dielectric and
ferroelectric properties [4]. SBTN belongs to the layered bis-
muth perovskite family as described by Aurivillus and Fang,
generally formulated as (Bi2O2)2+(Am−1BmO3m+1
)
where A
2−
is Bi, Sr, Ba, Ca, Pb, K, Na and B is Ti, Nb, Ta, Mo, W, Fe [5].
Tungsten-doped strontium bismuth niobate and strontium bis-
muth tantalate have been investigated as promising materials for
nonvolatile memory applications due to their excellent fatigue
endurance and low switching voltage [6,7].
Although there is limited literature on the preparation
and characterization of SBTN-W powders, much attention
has been devoted to its synthesis by different methods [8].
Usually, for large-scale preparation of nanosized powders,
chemical process can be more versatile and adaptable than
physical methods. Therefore, chemical synthesis based on
∗
Corresponding author.
E-mail address: alezipo@yahoo.com (A.Z. Simo˜es).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.11.194

62
N.L. Amsei Ju´nior et al. / Journal of Alloys and Compounds 454 (2008) 61–65
oped by Pechini [16] and is based on the chelation of a metallic
cation by a carboxylic acid, such as citric acid, and further poly-
merization promoted by the addition of ethylene glycol and
consequent polyesterification.
In this work, structural and microstructural characterization
of tungsten-doped SBTN powders prepared by the polymeric
precursor method was investigated.
2. Experimental procedure
SrBi2(Ta0.5Nb0.48W0.02)2O9 (SBTN-W) powders were synthesized by the
polymeric precursor method. Strontium carbonate, SrCO3 (Aldrich), niobium
ammonium oxalate NH4H2[NbO(C2O4)3]·3H2O (CBMM, Araxa´, Brazil), tan-
talum ethoxide, Ta(OC2H5)5 (Alfa Aesar), bismuth oxide, Bi2O3 (Aldrich) and
tungsten oxide, WO3 (Alfa Aesar) were used as raw materials. Niobium hydrox-
ide was formed by dissolution of the niobium ammonium oxalate in water
and precipitated as Nb(OH)5 by addition of NH4OH. After filtration, niobium
hydroxide was dissolved in aqueous solution of citric acid to form niobium cit-
rate. In the preparation of tantalum citrate, tantalum ethoxide was dissolved in
ethylene glycol under heating and stirring and then citric acid was added. The
pH was adjusted to 7–8 adding ethylenediamine. Tungsten oxide was dissolved
in citric acid solution and ethylene glycol to form a tungsten citrate. SrCO3 and
Bi2O3 (dissolved in nitric acid) were added to the niobium, tantalum and tung-
sten citrate solutions, previously mixed, in stoichiometric amounts. The citric
acid/ethylene glycol ratio was set at 60/40. Ethylenediamine was added drop-
wise into the solution with constant heating and stirring until the pH reached
7–8. After homogenization of the solution, ethylene glycol was added to promote
polymerization ofmixedcitrateby polyesterificationreactionwith constantheat-
ing at 90–110 ◦C. Next, the temperature was increased to 130–140 ◦C, yielding
a high viscous polyester resin. The organic part was subsequently eliminated
at temperatures as low as 300 ◦C, forming reactive oxides with well-controlled
stoichiometry. The calcination step was performed at various temperatures.
The SBTN-W polymeric solution was characterized by thermogravimetry
(TG) and differential thermal analysis (DTA) aiming to determine the decom-
position temperature of the polymer and phase crystallization. Both analyses
were performed on a TA Instrument Thermal Analyzer, model SDT 2690. The
analyses were carried out in synthetic air atmosphere with a heating rate of
10 ◦C/min. ␣-Al2O3 was used as a standard. X-ray diffraction (XRD) data
for milled powders were collected using a Rigaku RINT2000 diffractome-
Fig. 1. TG/DTA analyses for SBTN-W powder obtained from room temperature
up to 1000 ◦C in flow synthetic air at a heating rate of 10 ◦C/min.
can be detected indicating the decomposition of all organic
material. The endothermic signals, in the range of 50–90 ◦C
were attributed to the evaporation of water (DTA curve). A very
intense exotherm signal observed in the range of 300–520 ◦C
was assigned to the organic ligands combustion and crystalliza-
tion of SBTN-W phase. A stable SBTN-W phase was formed
at around 530 ◦C. No clear exothermic peak corresponding to
SBTN-W crystallization was found. These results are in agree-
ment with those reported in literature [22,23].
XRD pattern of the SBTN-W thermal treated powders at
several temperatures is shown in Fig. 2. At low temperature
(400–600 ◦C) there is coexistence between the perovskite and
fluorite phases. The peaks are broaden, indicating low crys-
tallinity. The intermediate phase of fluorite type is similar to
that observed for SBT powder synthesized by sol–gel method
[24] and is characterized by a bismuth deficiency structure with
Fm–3mcubicsymmetry. Asthetemperatureisincreased, theflu-
orite type phase disappears followed by an accented perovskite
phase crystallization, which is enhanced at 800 ◦C. The com-
plete formation of perovskite phase occurs when bismuth suffer
diffusion into the fluorite matrix. At 800 ◦C the powders are well
˚
˚
ter (42 kV × 120 mA) with Cu K␣ radiation (λk␣1 = 1.5405 A, λk␣2 = 1.5443 A,
Ik␣1/Ik␣2 = 0.5), 2θ range between 20 and 80◦, step size of 0.02◦(2θ), diver-
gence slit = 0.5 mm, receiving slit = 0.3 mm. To the Rietveld analysis, the XRD
data were collected using 2θ range between 20 and 120◦, step size of 0.02◦(2θ),
divergence slit = 0.5 mm, receiving slit = 0.3 mm. The Rietveld analysis was per-
formed through the DBWS-9807 program, by using pseudo-Voigt function to
perform the peak profiles [17]. The peak profile function was modeled using a
convolution of the pseudo-Voigt [18] with the asymmetry function described by
Larson and co-workers et al. [19,20], which accounts for the asymmetry due to
axial divergence. Infrared spectra of SBTN-W were recorded on a Nicolet FTIR-
Impact 400 spectrophotometer in the 400–4000 cm−1 spectral range. Scanning
electron microscopy (SEM, Model JOEL-5300) was used to evaluate the particle
size and the powder morphology. Specific surface area was determined based on
the nitrogen adsorption isotherms using the BET method (Sorptomatic 1990).
3. Results and discussion
Fig. 1 shows the TG/DTA curves obtained from room tem-
perature up to 1000 ◦C. It is clearly evident that there are two
stages of weight loss. The first stage (40–150 ◦C), followed
by a slow weight loss up to 300 ◦C, is related to elimina-
tion of absorbed water and carbonaceous residues. The second
stage (300–520 ◦C) is characterized by a continuous weight loss
associated to the decomposition of organic compounds, as in
agreement with literature data [21]. After that, no weight loss
Fig. 2. X-ray diffraction for SBTN-W powders thermal treated at different
temperatures: (a) 400, (b) 500, (c) 600, (d) 700, and (e) 800 ◦C for 2 h.

N.L. Amsei Ju´nior et al. / Journal of Alloys and Compounds 454 (2008) 61–65
63
Fig. 3. Rietveld refinement performed for the SBTN-W powders thermal treated
at 800 ◦C for 2 h.
Fig. 4. FTIR spectra for the SBTN-W powder thermal treated at 800 ◦C for 2 h.
crystallinebutafewamountofSr(Ta0.5Nb0.5)O3 asconsequence
of bismuth oxide volatilization is evident.
The crystalline structure of SBTN-W powders thermal
treated at 800 ◦C for 2 h was evaluated from Rietveld refine-
ment (Fig. 3). The quantitative phase analyses were calculated
by the Rietveld method and the Hill & Madsen formalism [25].
The obtained crystal structure of SBTN-W is orthorhombic with
space group A21am, similar to the structure for SrBi2Nb2O9
(PDF card no 99–8160 and ICSD card no 88–478). The lattice
parameters (a, b, and c), unit cell volume (V) and refinement
indexes to the SBTN-W powder are illustrated in Table 1. A
secondary phase was found and identified as Sr(Ta0.5Nb0.5)O3
with space group Pmna, which was quantified as 1.8 mol%. This
phase probably is formed after bismuth oxide evaporation during
the heat treatment. A low FWHM value indicates an organized
crystal structure. The A21am space group available on PDF and
ICSD data banks of SrBi2Nb2O9 and SrBi2Ta2O9 compositions
indicates that niobium and tantalum lead to different structural
changes. The SrBi2Nb2O9 present lower a and b parameters
when compared with SrBi2Ta2O9, specifically between 5.514
Table 2
FTIR frequencies for SBTN-W powder thermal treated at 800 ◦C for 2 h
Band frequency
(cm−1) [26,27]
Attribution
Band frequency in the
SBTN-W (cm−1
)
3419–3451
2339
1638
1380–1435
633–640
ν(O H)
ν(C O)
3300–3620
2370
δ(H2O)
1625
ν(C O), ν(C O)
ν(M O)
1390
630
occupies the strontium vacancies. This usually leads to a reduc-
tion in a and b parameters and increase of c parameters, as can
be verified for (Sr0.82Bi0.12)Bi2Ta2O9 (PDF card no. 89–6491)
when compared with stoichiometric structure SrBi2Ta2O9 (PDF
card no. 89–6490). In the refined structure, a and b parame-
ters are lower than for any other SrBi2Ta2O9 or SrBi2Nb2O9
compositions indicating a strontium deficiency in the perovskite
structure.
˚
˚
and5.520 AforSrBi2Nb2O9 compositionand5.552and5.553 A
for SrBi2Ta2O9 composition. On the other hand, c parameter
FTIR results for the SBTN-W powder thermal treated at
800 ◦C for 2 h is displayed in Fig. 4. A band located at 630 cm−1
is attributed to M–O (metal–oxygen) stretching, characteris-
tics of the perovskite structure. On the other hand, bands
corresponding to hydroxide group and carbon bonds, particu-
larly carboxyl groups are visible in 1000–3000 cm−1 interval.
The small absorption at 1460 cm−1 is due to C O vibration
and can be related to small traces of carbonate. In the range
of 3300–3620 cm−1 a band corresponding to O H bond is
˚
is lower for SrBi2Ta2O9 (24.98–25.08 A) when compared to
˚
SrBi2Nb2O9 (25.07–25.13 A). The niobium ion affects the a
and b parameters, while tantalum ion affects c parameter, in
a way that the unit cell volume is accentually reduced. How-
ever, the density of the refined sample is in the average value
betweenSrBi2Nb2O9 andSrBi2Ta2O9 compositions(∼7.26and
∼8.80 g/cm3, respectively). Strontium deficiency is typically
found in SrBi2Ta2O9 compositions, favoring that bismuth ion
Table 1
Lattice parameters (a, b, and c), unit cell volume (V), and refinement indexes for the SBTN-W powder thermal treated at 800 ◦C for 2 h
Structure: orthorhombic
% Phase SrBi2(Ta0.5Nb0.48W0.02)2O9
98.20
Space group: A21am
Density (g/cm3)
8.105
3
˚
˚
Lattice parameters (A)
Volume (A)
Refinement indexes
FWHM
0.1241
a = 5.511
b = 5.509
c = 25.04
760.2
Rwp = 16.05
REXP = 12.21
S = 1.30

64
N.L. Amsei Ju´nior et al. / Journal of Alloys and Compounds 454 (2008) 61–65
observed. This bond can be attributed to adsorbed water due
to the contact of the sample with the environment. These results
agree with those obtained by DTA and XRD analysis and can
be visualized in Table 2.
Specific surface area values were measured from BET analy-
ses. There is a trend to decrease from 32 (400 ◦C), 29.8 (500 ◦C),
21.2 (600 ◦C), 9.1 (700 ◦C) to 5.1 m2/g (800 ◦C). The significant
decrease of specific surface area at higher temperature is most
probably caused by the presence of strong agglomerates and
coarse-grained structure. This behavior is illustrated in the SEM
analysis of SBTN-W powders.
tendency to increase the agglomerated size as the temperature
goes up resulting in a decrease of the specific surface area. These
results are in accordance with BET analysis. Also, it is interest-
ingtonotethattheagglomeratesarerounded, whichistypicalfor
chemically prepared powders when compared to mechanically
activated syntheses [28].
4. Conclusions
SBTN-W powders with perovskite structure were obtained
by the polymeric precursor at 700 ◦C for 2 h. An orthorhombic
structure with A21am space group was identified by Rietveld
method. SEM micrographies showed the increase of particle size
with the raise of temperature of thermal treatment as confirmed
by BET analysis.
SEM images of SBTN-W powders thermal treated at 400
and 800 ◦C for 2 h is illustrated in Fig. 5. The powders thermal
treated at 800 ◦C for 2 h exhibit particles in an almost-spherical
morphology with strong presence of agglomerates. There is a
Acknowledgements
The authors gratefully acknowledge the financial support of
the Brazilian agencies FAPESP, CNPq, and CAPES.
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