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ORIGINAL PAPER
Structural and morphological characteristics of (Pb12xSrx)TiO3
powders obtained by polymeric precursor method
S. H. Leal Æ J. C. Sczancoski Æ L. S. Cavalcante ÆM. T. Escote Æ J. M. E. Matos Æ M. R. M. C. Santos ÆF. M. Pontes Æ E. Longo Æ J. A. Varela
Received: 17 April 2009 / Accepted: 16 July 2009 / Published online: 11 August 2009
� Springer Science+Business Media, LLC 2009
Abstract Pb1-xSrx)TiO3 powders with different compo-
sitions (x = 0, 0.10, 0.50, 0.90 and 1) were synthesized by
the polymeric precursor method and heat treated at 800 �C
for 2 h under air atmosphere. The thermogravimetric and
differential scanning calorimetry analyses were performed
in the range from 25 to 800 �C in order to estimate the
stages corresponding to the water evaporation, organic
decomposition and crystallization of these materials. X-ray
diffraction patterns and Rietveld analyses showed that the
(Pb1-xSrx)TiO3 phases with strontium content up to
x = 0.1 crystallize in a tetragonal structure. The micro-
graphs obtained by scanning electron microscopy and
transmission electron microscopy showed that the powders
have agglomerated nature, presenting irregular morpholo-
gies and polydisperse particle size distribution. The energy
dispersive X-ray spectrometry indicated the presence of
pure (Pb0.50Sr0.50)TiO3 phase.
Keywords (Pb1-xSrx)TiO3 � Inorganic materials �Chemical synthesis � Nanosized powder
1 Introduction
Recently, lead-based perovskite-type structures have been
extensively investigated due to its interesting ferroelectric,
dielectric and luminescence properties [1–6]. In particular,
the lead titanate (PbTiO3) is a ceramic oxide characterized
by a tetragonal structure and space group P4mm at room
temperature [7]. This ferroelectric material exhibits a low
dielectric constant, high pyroelectric coefficient and strong
spontaneous polarization [8, 9]. However, the large
tetragonal strain and high Curie temperature (Tc =
490 �C) presented by this material limit its use for indus-
trial applications. In order to overcome these drawbacks,
the PbTiO3 matrix has been doped with different lantha-
nide and alkaline earth metals, such as: ytterbium [10],
samarium [11], barium [12], calcium [13] and strontium
[14].
In solid solution systems, the Sr doped PbTiO3 ceramics
are considered potential candidates for the development of
S. H. Leal � J. M. E. Matos � M. R. M. C. Santos
Universidade Federal do Piauı, Campus Ministro Reis Velloso e
Petronio Portela, CEP 64202-020 and 64049-550 Parnaıba e
Teresina, PI, Brazil
e-mail: [email protected]
J. M. E. Matos
e-mail: [email protected]
M. R. M. C. Santos
e-mail: [email protected]
J. C. Sczancoski � L. S. Cavalcante (&) �F. M. Pontes � E. Longo � J. A. Varela
IQ-UNESP e DQ-UFSCar, CEP 17033-360, 14800-900,
13565-905 Bauru, Araraquara, Sao Carlos, SP, Brazil
e-mail: [email protected]
J. C. Sczancoski
e-mail: [email protected]
F. M. Pontes
e-mail: [email protected]
E. Longo
e-mail: [email protected]
J. A. Varela
e-mail: [email protected]
M. T. Escote
Centro de Engenharia, Modelagem e Ciencias Sociais,
Universidade Federal do ABC,
CEP 09090-400 Santo Andre, SP, Brazil
e-mail: [email protected]
123
J Sol-Gel Sci Technol (2010) 53:21–29
DOI 10.1007/s10971-009-2049-4
tunable microwave devices and ultra-large scale integration
dynamic random access memory (ULSI DRAM) capacitors
[14, 15]. Particularly, the nanosized (Pb1-xSrx)TiO3 pow-
ders have received special attention because of its new
physicochemical properties, differing from those materials
in bulk shape [16]. When Pb atoms are partially replaced
by Sr atoms into the PbTiO3 matrix, the phase transition
temperature from ferroelectric tetragonal to paraelectric
cubic phase decreases linearly [17]. Thus, the temperature
range in which is verified the ferroelectric property can be
easily controlled adjusting the Pb/Sr ratio.
Generally, the (Pb,Sr)TiO3 powders prepared by solid-
state reaction (SSR) involves the mixture and reaction
between PbCO3 or PbO, SrCO3 and TiO2 at high heat
treatment temperatures ([1,000 �C) for long processing
times [18, 19]. Moreover, the (Pb,Sr)TiO3 powders
obtained by this method present several problems, such as:
undesired stoichiometry, contamination by impurities and
polydisperse particle size distribution [20]. In addition,
during the sintering process by SSR, the (Pb,Sr)TiO3
powders must be placed within a double-crucible and heat
treated under air atmosphere to prevent possible losses
associate to the volatilization of PbO [21]. These draw-
backs can be minimized using the soft chemical method,
which are able to immobilize metal-complexes in rigid
organic polymeric networks. This chemical route reduces
the segregation of particular metals, ensuring a composi-
tional homogeneity in molecular scale. Moreover, this
method does not require several milling stages. In this case,
the polymeric precursor method (PPM) based on the
Pechini process [22] has received special attention because
of the advantages in the formation of several oxides
[23, 24] with good stoichiometric control and small particle
sizes.
In the last years, our research group has investigated the
dielectric properties as well as the transition phase from
ferroelectric to paraelectric in (Pb,Sr)TiO3 thin films [25,
26]. The diffuse phase-transition of these thin films has
been analyzed by means of dielectric constant measure-
ments as a function of temperature. On the other hand, the
relaxor behavior has been confirmed through Raman
spectra. However, the literature has not reported studies on
Rietveld refinements for this material. Therefore, in this
work, we report on the structural and morphological
characteristics of (Pb1-xSrx)TiO3 powders with different
compositions (x = 0, 0.10, 0.50, 0.90 and 1) synthesized
by the PPM. These powders were characterized by ther-
mogravimetric analysis (TGA), differential scanning calo-
rimetry (DSC) measurements, X-ray diffraction (XRD),
Rietveld refinement, scanning electron microscopy (SEM),
transmission electron microscopy (TEM), high-resolution
transmission electron microscopy (HR-TEM) and energy
dispersive X-ray spectrometry (EDXS).
2 Experimental details
2.1 Synthesis of (Pb1-xSrx)TiO3 powders
The (Pb1-xSrx)TiO3 powders with different compositions
(x = 0, 0.10, 0.50, 0.90 and 1.0) were synthesized by the
PPM [27]. In this synthesis, lead acetate trihydrate
[Pb(CH3COO)2�3H2O] (99% purity, Aldrich), strontium
carbonate [SrCO3] (98% purity, Aldrich), titanium (IV)
isopropoxide [Ti(OCH(CH3)3)4] (97% purity, Alfa Aesar),
ethylene glycol (C2H6O2) (99%, J.T. Baker) and citric acid
(C6H8O7) (99.5% purity, Mallinckrodt) were used as raw
materials. Initially, C6H8O7 was dissolved in deionized
water heated at 75 �C under constant stirring. Afterwards,
[Ti(OCH(CH3)3)4] was quickly added into this citric acid
aqueous solution to avoid hydrolysis reaction between
alkoxide and air environment. After heating at 80 �C under
constant stirring for several hours occurred the formation
of a clear and homogenous titanium citrate solution. The
gravimetric procedure was performed to estimate the stoi-
chiometric value correspondent to the mass (grams) of
titanium oxide contained into the citrate. In the sequence,
stoichiometric quantities of [Pb(CH3COO)2�3H2O] and
[SrCO3] were dissolved into the titanium citrate solution.
After solution homogenization, C2H6O2 was added into the
citrate heated at 120 �C in order to promote the polymer-
ization by means of the polyesterification reaction. The
obtained polymeric resin was then placed into a conven-
tional furnace and heat treated at 300 �C for 4 h, causing
the organic compound decomposition arising from C6H8O7
and C2H6O2. Finally, the obtained precursors were heat
treated at 800 �C for 2 h under air atmosphere.
2.2 Characterizations of (Pb1-xSrx)TiO3 powders
The thermal behavior of (Pb1-xSrx)TiO3 powders was
investigated by the simultaneous TGA and DSC analyses
using a STA 409 equipment (Netzsch, Germany). These
measurements were performed in the temperature range
from 25 to 800 �C under air atmosphere, keeping a con-
stant heating rate of 10 �C/min. In these experiments, the
precursors were heat treated at 300 �C for 2 h to obtain the
disordered powders [28]. The phases were identified by
means of XRD through a DMax/2500PC diffractometer
(Rigaku, Japan). XRD patterns were obtained using Cu-Karadiation in the 2h range from 10� to 75� with a scanning
rate of 0.02�/min. The Rietveld routine was performed in
the 2h range from 15� to 110�, using a scanning rate of
0.02�/min and exposure time of 2 s. The SEM micrographs
were observed with a DSM 940 scanning electron micro-
scope (Carl Zeiss, Germany) operated at 20 keV. In order
to prepare the samples for the TEM and HR-TEM mea-
surements, the powders were dissolved in an isopropanol
22 J Sol-Gel Sci Technol (2010) 53:21–29
123
solution and ultrasonically dispersed. A drop of the sus-
pension was deposited on the carbon-coated copper grids,
which was dried under air atmosphere at room temperature.
The micrographs were obtained with a CM 200 microscope
(Phillips, USA) operated at 200 keV. An EDXS spec-
trometer (Oxford Instruments, UK) coupled to the TEM
microscope allowed to analyze the chemical composition
of (Pb1-xSrx)TiO3 powders. The SAED technique was
employed to identify the crystal structure of the individual
particles.
3 Results and discussion
3.1 Thermal analyses
Figure 1a–c shows the TGA and DSC curves of disordered
(Pb1-xSrx)TiO3 powders with different compositions
(x = 0.1, 0.5 and 0.9).
TGA profiles indicate that all powders exhibit a typical
thermal decomposition behavior, i.e., presenting two dis-
tinct regions of weight losses (Fig. 1a–c). In the first one,
corresponding to the temperature range from 25 to 96 �C, it
was observed a weight loss of approximately 8%, as con-
sequence of the dehydratation process of absorbed water on
the powders. In the second one, a maximum weight loss
between 35 and 56% was verified on the powders when
heated in the range from 350 to 660 �C, which was ascri-
bed to the residual organic matter decomposition arising
from citric acid and ethylene glycol. Finally, it was not
detected weight losses at temperatures above 660 �C,
suggesting to the formation of ordered or crystalline
(Pb1-xSrx)TiO3 structures.
DSC results showed the presence of small, broad and
strong exothermic peaks. In these materials, the two first
peaks located in the range from 448 to 568 �C are related
to the pyrolysis process or residual organic matter
decomposition (Fig. 1a–c). The third exothermic peak
detected in the range from 506 to 635 �C can be due to the
structural organization (Fig. 1a–c). We believe that the
observed differences on the broadening of exothermic
peaks can be caused by the variation in the quantity of Sr
content into the PbTiO3 matrix. Possibly, the replacement
of Pb2? ions into sites normally occupied by Sr2? is able to
change the transition temperature from ordered–disordered
to ordered structure.
3.2 X-ray diffraction and Rietveld refinement analyses
Figure 2 shows the XRD patterns of (Pb1-xSrx)TiO3
powders with different compositions (x = 0, 0.10, 0.50,
0.90 and 1) heat treated at 800 �C for 2 h under air
atmosphere.
According to the XRD patterns, all diffraction peaks
correspond to the pure PbTiO3 phase, while those with Sr
content up to x = 0.1 were indexed to the perovskite-type
tetragonal structure with space group P4mm, in agreement
with the respective Joint Committee on Powder Diffraction
Standards (JCPDS) card No. 06-0452 [29]. However, for
compositions with x C 0.5, the (Pb1-xSrx)TiO3 powders
(a)
(c)
(b)Fig. 1 TG and DSC curves of
disordered (Pb1-xSrx)TiO3
powders with different
compositions: a x = 0.10,
b x = 0.50 and c x = 0.90
J Sol-Gel Sci Technol (2010) 53:21–29 23
123
crystallize in an cubic structure with space group Pm3m
(JCPDS card No. 35-0734) [30]. Based on these results, it
is possible to suppose that occurs a possible phase transi-
tion from tetragonal to cubic in these oxide compounds
when the Sr content is increased up to x & 0.5 . Probably,
this behavior can be associated to the effect of lattice
contraction effect caused by the replacement of Pb sites
(covalent bond with directional orientation) into the
perovskite structure by Sr atoms (ionic bond with radial
orientation), where both present different ionic radii
[31–33]. Thus, Rietveld analyses were employed in order
to estimate the influence of this specific Sr composition on
the lattice parameters of (Pb1-xSrx)TiO3 powders.
Figure 3 shows the Rietveld refinements of (Pb1-xSrx)
TiO3 powders with different compositions (x = 0, 0.50
and 1) heat treated at 800 �C for 2 h under air atmosphere.
In this work, the Rietveld refinements were performed
through the FULLPROF program (http://www.ccp14.ac.
uk/tutorial/fullprof/index.html), assuming the space groups
P4mm and Pm3m for the tetragonal and cubic (Pb1-xSrx)
TiO3 structures, respectively. In these analyses, the refined
parameters were scale factor, background, shift lattice
constants, profile half-width parameters (u, v, w), isotropic
thermal parameters, strain anisotropy factor, occupancy,
atomic functional positions, bond lengths and bond angles.
The background was corrected using a Chebyschev poly-
nomial of the first kind. The diffraction peak profiles were
better fitted by the Thompson–Cox–Hastings pseudo-Voigt
(pV-TCH) function and by the asymmetry function
described by Finger et al. [34]. The strain anisotropy was
corrected by the phenomenological model described by
Fig. 2 XRD patterns of (Pb1-xSrx)TiO3 powders with different
compositions (x = 0, 0.10, 0.50, 0.90 and 1) heat treated at 800 �C
for 2 h under air atmosphere
(a)
(c)
(b)
Fig. 3 Rietveld refinements of (Pb1-xSrx)TiO3 powders [x = (a) 0, (b) 0.50 and (c) 1] heat treated at 800 �C for 2 h under air atmosphere
24 J Sol-Gel Sci Technol (2010) 53:21–29
123
Stephens [35]. The obtained results from the Rietveld
refinement are displayed in Table 1.
As it can be seen in this table, c lattice parameter value,
unit cell volume and c/a ratio of (Pb0.50Sr0.50)TiO3 are
significantly lower when compared to the pure PbTiO3
phase. Therefore, this result suggests a decrease in the
degree of tetragonality of this perovskite-type structure
when the Sr is added, confirming the hypothesis previously
described in the text. In fact, some works reported in the
literature have showed the influence of other alkaline earth
metal ions (Ba2? and Ca2?) on the structural properties of
PbTiO3 phase. For example, Bretos et al. [36] verified that
the substitution of Pb2? by Ca2? ions decreases the
c/a ratio, resulting in a pseudocubic structure for the
Table 1 Rietveld refinement results of (Pb1-xSrx)TiO3 powders (x = 0, 0.1, 0.50, 0.90 and 1) heat treated at 800 �C for 2 h under air
atmosphere
(Pb1-xSrx)TiO3 content Lattice a, b (A) Parameters c (A) Tetragonality factor c/a Unit cell volume (A3) RBragg
x = 0 3.9014(2) 4.1515(2) 1.0641 63.189(6) 3.9
x = 0.10 3.9097(2) 4.0913(2) 1.0464 62.538(6) 5.2
x = 0.50 3.8228(2) 3.9571(2) 1.0087 60.893(2) 5.0
x = 0.90 3.9127(1) 3.9127(1) 1.0000 59.900(3) 2.6
x = 1 3.9046(4) 3.9046(4) 1.0000 59.531(2) 2.2
Site/Occupancy PbTiO3 (Pb0.90Sr0.10)TiO3 (Pb0.50Sr0.50)TiO3 (Pb0.10Sr0.90)TiO3 SrTiO3
ZTi 0.533(3) 0.524(3) 0.476(2) – –
ZO1 -0.117(6) -0.106(5) 0.072(3) – –
ZO2 0.606(2) 0.600(2) 0.574(2) – –
BPb 0.74(3) 1.52(6) 3.3(3) 5.04(8) –
BSr – 5.33(3) 0.8(3) 1.08(8) 0.65(2)
BTi 1.6(2) 2.5(1) 1.01(7) 1.49(4) 0.74(4)
BO1 4.6(9) 5.0(7) 0.7(4) 8.6(9) 0.83(9)
BO2 1.7(5) 2.2(4) 0.6(3) 0.81(1) –
OcPb 0.115(4) 0.099(2) 0.0686(9) 0.012(1) –
OcSr – 0.011(2) 0.0686(9) 0.112(1) 0.02(2)
OcTi 0.126(4) 0.123(4) 0.135(2) 0.125(4) 0.02(2)
Fig. 4 Schematic
representation of a PbTiO3,
b (Pb0.50Sr0.50)TiO3 and cSrTiO3 supercells
J Sol-Gel Sci Technol (2010) 53:21–29 25
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(Pb0.50Sr0.50)TiO3 thin films. Yang et al. [37] and Zhai
et al. [38] reported that the replacement of Pb2? by Sr2?
ions induces a diffuse phase transition from cubic to
tetragonal into the lattice. Pontes et al. [39] explained that
higher Ba2? concentrations (x C 0.7) into the (Pb1-xBax)
TiO3 matrix are not able to promote a phase transition, but
only cause variations in the unit cell parameters.
3.3 Schematic representation of PbTiO3,
(Pb0.50Sr0.50)TiO3 and SrTiO3 supercells
Figure 4a–c shows the schematic representation of PbTiO3,
(Pb0.50Sr0.50)TiO3 and SrTiO3 supercells (x = 0, 0.50 and
1) modeled through the Java Structure Viewer Program
(Version 1.08lite for Windows) and VRML-View (Version
3.0 for Windows) (http://www.jcrystal.com/steffenweber/
JAVA/JSV/jsv.html, http://www.km.kongsberg.com/sim).
In order to model the structures, it was employed in these
programs the atomic coordinates obtained from the Riet-
veld refinement (Table 1).
In these unit cells, the Ti atoms are coordinated to six
oxygens ([TiO6] clusters), resulting in a polyhedron-type
with octahedral configuration. As previously described, the
PbTiO3 is an oxide with perovskite-type tetragonal struc-
ture, [TiO6] distorted clusters and space group P4mm at
room temperature [7]. Therefore, the addition of Sr2? ions
into the matrix of this compound up to 50 at.%
((Pb0.50Sr0.50)TiO3 phase) promotes a structural modifica-
tion from tetragonal to cubic structure with space group
Pm3m; as observed in Fig. 4c. In a previous work, our
research group reported in details the phase transition for
the (Pb0.50Sr0.50)TiO3 thin films [40]. This phase transition
was investigated by means of dielectric measurements at
different frequencies during the heating cycles.
3.4 Scanning electron microscopy analyses
Figure 5a and b shows the SEM micrographs of (Pb1-xSrx)
TiO3 powders heat treated at 800 �C for 2 h under air
atmosphere.
As it can be seen in this figure, (Pb1-xSrx)TiO3 powders
are composed by several agglomerated particles with
irregular morphologies, resulting in a non-uniform particle
size distribution. In principle, we believe that these mor-
phological characteristics are governed by the chemical
synthesis and heat treatment conditions. According to the
literature [41], the PPM is able to promote the immobili-
zation of metal complexes into the organic polymeric resin,
reducing the phase segregation and ensuring a complete
chemical homogeneity at molecular scale into this system.
However, the preparation of multicomponent oxides com-
prising more than one type of metal ion, such as: (Pb1-xSrx)
TiO3, the situation becomes more complicated. In this case,
the degree of chemical homogeneity of the resulting resin
can be strongly affected not only by the mixing level of
different alkoxides in the precursor solution, but also by the
reactivity of each alkoxide species in water. Moreover, in
this chemical synthesis, the excess C2H6O2 plays an
important role as a solvent to increase the solubilities of
different metal salts into the solution [42]. However, the
high particle reactivity as well as the excess organic
material into the polymeric resin can favor the formation of
partially sintered hard agglomerates [43]. The preheating of
this polymeric resin up to 300 �C promotes the solvent
evaporation as well as organic matter decomposition aris-
ing from C6H8O7 and C2H6O2. As consequence, this
experimental stage leads to the formation of an amorphous
Fig. 5 SEM micrographs of (Pb0.50Sr0.50)TiO3 powders heat treated
at 800 �C for 2 h under air atmosphere
26 J Sol-Gel Sci Technol (2010) 53:21–29
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precursor powder (highly disordered). When the precursor
was submitted to a heat treatment temperature of 800 �C,
the small pores formed during the pyrolysis process were
slowly reduced by the particle growth via grain boundary
motion. In some regions, this matter transport mechanism
resulted in the formation of more dense regions, which are
composed by several aggregated particles.
3.5 Transmission electron microscopy analyses:
morphology and SAED pattern
Figure 6a–d shows the TEM micrographs and SAED pat-
terns of (Pb0.50Sr0.50)TiO3 powders heat treated at 800 �C
for 2 h under air atmosphere.
A close examination of the low magnification TEM
micrographs in Fig. 6a and b revealed that the powders
are formed by the agglomeration of several particles
with irregular morphologies and different particle sizes.
These morphological characteristics can be arising from a
non-controlled particle growth during the heat treatment,
where the main source is the diffusion mechanism between
the grain boundaries due to a reduction in the total grain
boundary surface energies [43]. Figure 6c shows a high
magnification TEM micrograph of an individual particle,
where the crystallographic planes were verified by
HR-TEM. The HR-TEM micrograph taken from the
selected area marked by the rectangle indicated that the
planes present an interplanar spacing of 0.2253 nm, which
was identified as belonging to the (111) plane. In this
context, the respective SAED pattern (Fig. 6d) confirmed
that these particles are well-crystallized, presenting a single
phase with cubic structure.
3.6 Energy dispersive X-ray spectrometry analysis
Figure 7 shows an EDXS analysis of (Pb0.50Sr0.50)TiO3
powders heat treated at 800 �C for 2 h under air atmosphere.
This spectrum revealed that the powders are chemically
composed of titanium, lead, strontium and oxygen atoms.
Therefore, this result confirms that the heat treatment
performed at 800 �C is able to allow the formation of pure
(Pb0.50Sr0.50)TiO3 phase. The presence of Cu atoms in the
spectrum is because of the carbon-coated copper grids. The
quantitative results on the chemical composition analyses
of (Pb0.50Sr0.50)TiO3 powders obtained by EDXS are dis-
played in Table 2.
Fig. 6 a, b Low magnification
TEM micrographs of
(Pb0.50Sr0.50)TiO3 powders heat
treated at 800 �C for 2 h under
air atmosphere; c high
magnification TEM
micrographs of an individual
particle with the respective
crystallographic plane identified
by HR-TEM; d SAED pattern
taken on the individual particle
in c
J Sol-Gel Sci Technol (2010) 53:21–29 27
123
Table 2 presents the average mass values (%), obtained
by EDXS analysis, to the (Pb0.50Sr0.50)TiO3 powder. The
total area corresponding to the each individual peak of Ti,
Sr and Pb were quantitatively estimated by a Gaussian
function, as shown in Fig. 7. In this case, the peaks relate to
the copper and oxygen were excluded of the calculus. The
copper peaks because are arising from the carbon-coated
copper grids employed in the EDSX measurements and the
oxygen peaks because are light elements.
4 Conclusions
In summary, (Pb1-xSrx)TiO3 powders with different com-
positions (x = 0, 0.10, 0.50, 0.90 and 1) were synthesized
by the PPM and heat treated at 800 �C for 2 h under air
atmosphere. XRD patterns and Rietveld refinements
showed that the Sr content up to x & 0.5 into the (Pb1-xSrx)
TiO3 powders is responsible for a phase transition from
tetragonal to cubic in these materials. This behavior was
associated to a lattice contraction effect caused by the dif-
ferent ionic radii between the Pb and Sr. The SEM and TEM
micrographs indicated that the (Pb1-xSrx)TiO3 powders are
composed by several agglomerated particles with irregular
morphologies. This morphological behavior was attributed
to a strong reduction of the small pores formed during the
pyrolysis process, as consequence of the particle growth by
diffusion mechanisms between the grain boundaries.
Finally, EDXS spectrum confirmed that the powders have a
pure (Pb1-xSrx)TiO3 phase due to present only titanium,
lead, strontium and oxygen atoms in its composition.
Acknowledgments The authors thank the financial support of the
Brazilian research financing institutions: CAPES, CNPq and
FAPESP.
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