original paper -...

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


M. R. M. C. Santos


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


J. C. Sczancoski


F. M. Pontes


E. Longo


J. A. Varela


M. T. Escote

Centro de Engenharia, Modelagem e Ciencias Sociais,

Universidade Federal do ABC,

CEP 09090-400 Santo Andre, SP, Brazil


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


123

http://www.ccp14.ac.uk/tutorial/fullprof/index.html

http://www.ccp14.ac.uk/tutorial/fullprof/index.html

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


123

(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


123

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http://www.km.kongsberg.com/sim

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


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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Fig. 7 EDXS analysis of (Pb0.50Sr0.50)TiO3 powders heat treated at

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Table 2 Percentage of elements presents in (Pb0.50Sr0.50)TiO3

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Element (symbol) (%) Relative in (Pb,Sr)TiO3

Titanium (Ti) 25

Lead (Pb) 66

Strontium (Sr) 09


123

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