Materials Research Bulletin 43 (2008) 297–304 www.elsevier.com/locate/matresbu
1/3
2/3
3
x
(1 x )
3
J.C. Bruno
a , *
, T.C. Boni
b
, A.A. Cavalheiro a
, M.A. Zaghete b
a
Depto de Quı´mica, Instituto de Biocieˆncias, UNESP, Distrito de Rubia˜o Junior, s/n, P.O. Box 510, Botucatu 18.618-000, SP, Brazil b
Liec, Instituto de Quı´mica, UNESP, Rua Prof. Francisco Degni, s/n, P.O. Box 355, Araraquara 14.801-970, SP, Brazil
Received 8 October 2006; received in revised form 5 March 2007; accepted 7 March 2007
Available online 12 March 2007
Abstract
The present work reports the effects caused by barium on phase formation, morphology and sintering of lead magnesium niobate–lead titanate (PMN–50PT). Ab initio study of 0.5Pb(Mg
1/3
Nb
2/3
)O
3
–0.5(Ba x
Pb
(1 x )
TiO
3
) ceramic powders, with x = 0,
0.20, and 0.40 was proposed, considering that the partial substitution of lead by barium can reestablish the equilibrium of monoclinic–tetragonal phases in the system. It was verified that even for 40 mol% of barium, it was possible to obtain pyrochlorefree PMN–PT powders. The increase of the lattice parameters of PMN–PT doped-powders confirmed dopant incorporation into the perovskite phase. The presence of barium improved the reactivity of the powders, with an average particle size of 120 nm for
40 mol% of barium against 167 nm for the pure sample. Although high barium content (40 mol%) was deleterious for a dense ceramic, contents up to 20 mol% allowed 95% density when sintered at 1100 8 C for 4 h.
# 2007 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics; C. X-ray diffraction; D. Phase equilibria
1. Introduction
Among the actuator materials, lead magnesium niobate–lead titanate ((1 x )PMN– x PT) has been a target of interest because of its high dielectric and piezoelectric constants
[1,2]
. In this system, compositions near the morphotropic phase boundary (MPB) exhibit outstanding electromechanical properties, and for this reason, many researches have focused on ceramics with x 35 mol% of PT. The phase diagram of (1 x )PMN– x PT shows that at about 30–38 mol% of PT, a monoclinic phase separates the rhombohedral and the tetragonal ferroelectric phases
[3] ,
characterizing the MPB of the system. Compositions with titanium content much greater than 40 mol% are not widely used for practical applications, since the temperature of maximum dielectric constant is situated around
250 8 C
[4]
.
Although PMN–PT material has good properties, much research has been conducted in order to improve them, mainly involving doping with small quantities of additive
[2,5]
. The isovalent substitution of the lead-site usually leads to a lower temperature of the phase transformation and a broadening of the temperature-dependent properties. The partial substitution of lead by barium at the A-site of perovskite structure in PMN–PZ–PT
[6]
made the normal
* Corresponding author. Tel.: +55 14 3811 6255; fax: +55 14 3811 6255.
E-mail address: julicatarina@yahoo.com.br
(J.C. Bruno).
0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.
doi: 10.1016/j.materresbull.2007.03.010

298 J.C. Bruno et al. / Materials Research Bulletin 43 (2008) 297–304 ferroelectric behavior of electric-field induced polarization turn to relaxor-like behavior, and by analogy with PMN–
PT ceramic, it is possible to infer that barium can partially return the relaxor behavior and advance the properties of
PMN–50PT.
The PMN–PT powders have been prepared using different techniques
[7,8]
. Although most methods involve the preparation of columbite precursor to avoid the formation of pyrochlore phases, some reports indicated that
(1 x )PMN– x PT could be obtained by a reaction-sintering process without a calcination step, producing high density pyrochlore-free PMN–PT ceramics
[9,10]
. The modified columbite method
[11] , recently developed, showed itself to
be very attractive to synthesize different compositions of PMN–PT. For PT contents higher than 10 mol%, a new phase rises in the columbite precursor, named MNT. This phase, with rutile structure, stays in equilibrium with the orthorhombic-structure MN phase up to a concentration of 50 mol% of titanium. An additional amount of titanium engenders a single rutile phase (Mg
0.167
Nb
0.333
Ti
0.500
O
2
)
[12] .
In order to verify the influence of the monophasic MNT precursor in the synthesis of PMN–50PT powders and investigate the partial substitution at the A-site of perovskite structure, compositions of PMN–50PT containing 20 and
40 mol% of barium were synthesized. All structural changes in the crystal structure were considered via a structural refinement; the morphologic features and the influence of barium in the sintering process were also studied.
2. Experimental
The synthesis of PMN–PT powders consisted in a prior synthesis of the MNT precursor, as described elsewhere
[11]
. Nb
2
O
5
(Aldrich), (MgCO
3
)
4
Mg(OH)
2
5H
2
O (Cine´tica Quı´mica) and Ti(OC
3
H
7
)
4
(Fluka) were used as starting materials in the preparation of MNT precursor samples using the polymeric precursor method
[13] . This
method consists in the conversion of the starting materials into metal citrate solutions. The stoichiometric mixture of cation citrate is then esterified by the addition of ethylene glycol and heated at 250 8 C yielding highly viscous polyester resin. The polymeric precursor is burned at 410 8 C for 2 h, in order to decompose the organic matter, resulting in a porous material. The carbonized resin was pre-calcined at two stages: 500 and 700 8 C, being kept for 2 h within each range of temperature. After each treatment, the powder was ground and sieved with a 200-mesh sieve to avoid the formation of agglomerated powders, and after obtaining the required phase at 900 8 C for 2 h, the
MNT precursor was ball milled for 18 h in isopropyl-alcohol medium. Next, the precursor was dried, passed through
200-mesh sieves and separated into aliquots to effectuate the synthesis of the PMN–PT powders. The systems here studied are: 0.5PbMg
respectively.
1/3
Nb
2/3
O
3
–0.5Ba
x
Pb
(1 x )
TiO
3 with x = 0, 0.2 and 0.4, named as Ba0, Ba02 and Ba04,
To prepare the pure sample, Pb(NO
3
)
2
(CAAL) was firstly dissolved under heat and stir, and subsequently the MNT precursor was added to it. This solution was concentrated and heated at 150 8 C for 30 min to guarantee total water elimination. For doped samples, Ba(H
3
CCOO)
2
(Vetec) was simultaneously dissolved with Pb(NO
3
)
2
, according to the quantity necessary to prepare each composition. After drying the powders, they were ground in a mortar and precalcined at 500 8 C for 2 h, re-ground and calcined at 800 8 C for 2 h in order to form the perovskite PMN–PT phase.
The resulting powders were ball milled for 6 h and passed through 200-mesh sieves.
The powder was characterized by means of X-ray diffraction (XRD) to verify the phase formation and the collected data were used to investigate the structural variations on crystal and the phase amount using the
Rietveld method (DBWS-9807). The Thompson–Cox–Hastings (TCH) model was used as the peak profile function
[14–16]
. The powder morphology was characterized by scanning electron microscopy (SEM) (Topcon SM-300 equipment) and the specific surface area by N
2 adsorption technique and the BET method of analysis (ASAP,
Micrometrics 2010). In order to prepare PMN–50PT ceramics, the powders were pressed into disks (10 mm in diameter and 1–2 mm thick) by unixialy pressing at 50 MPa using Isobutyl methacrylate (IBMA) as a binder, which was burnt-out by heating at 600 8 C for 1 h before the sintering process at 1100 8 C for 4 h. The ceramic density was determined using the Archimedes method and the effect of barium content in the weight loss and density was evaluated.
3. Results and discussion
In
Fig. 1 , the XRD pattern of the MNT50 precursor powder is shown, and according to the structural refinement, all
of the reflections were identified as Mg
0.167
Nb
0.333
Ti
0.500
O
2
(MNT) phase. This result is in agreement with that

J.C. Bruno et al. / Materials Research Bulletin 43 (2008) 297–304 299
Fig. 1. X-ray diffraction pattern of the MNT50 precursor powder and the MNT standard.
expected for the orthorhombic-rutile phase equilibrium of MgNb
2
O
6
–TiO
2 system
[17] . The equilibrium persists
until a pure rutile structure is reached for 50 mol% of Ti, and for x = 1, pure TiO
2 rutile structure is achieved. A single-phase highly crystalline rutile structure could be obtained for the MNT50 precursor synthesized at a temperature lower than that of conventional oxides mixtures
[18]
, demonstrating the efficiency of the modified columbite method.
The calculation of the cell parameters in the MNT50 precursor powder was preformed using the Rietveld method.
Table 1
shows the lattice parameters, fractional atomic coordinates and the refinement indices R wp
, S and R
B
[19]
of the
MNT phase. The cell parameters of the rutile phase in the MNT50 precursor are very close to the ones available in literature (data in the footnote of
Table 1 ). The value of the
R wp index shows the adjust degree between the calculated and observed XRD pattern. The value of goodness of fit ( S ) is low and so the Bragg intensity index ( R
B
), indicating that the refined crystal-structure model is in good agreement with the observed data. The columbite–rutile transformation is related to the reordering of AB
2
O
6 to AO
2 structure, where A and B, specifically Nb and Mg cations are substituted by the Ti cation. The difference between fractional atomic coordinates for Nb and Mg is reduced when titanium substitutes both cations proportionally.
The XRD patterns of PMN–50PT powders obtained from MNT50 precursor are shown in
Fig. 2
. All the observed reflections belong to perovskite phase (JCPDS: 27-1199), indicating the formation of pyrochlore-free PMN–PT powders for all of the samples. In
Fig. 3
the set of peak at 45 8 and 75 8 (2-theta) is exposed, indicating that the peak splitting tends to disappear as the barium content increases, which can be associated with the loss of tetragonality ( c / a ) of the perovskite phase followed by a consequent increase of the rhombohedral phase, tending to reestablish the equilibrium of rhombohedral–tetragonal phases in PMN–50PT, similar to the one found for the PMN–35PT composition
[20] . However, the tentative of refinement of a rhombohedral phase for any of the samples was
unsuccessful. In fact, the monoclinic phase with Pm space group, instead of Cm space group found for PZT, resulted in
Table 1
Final structural data obtained from the Rietveld refinement for the MNT50 precursor
Structure a
Cell parameters Refinement indices Atom Atomic coordinates x y z
Mg
1/6
Nb
(SG: P
1/3
Ti
42/
1/2
O
2
MNM , FU = 2) a c
V
= 4.662(1) A
= 3.016(3) A
= 65.55(9) A
3
R
R
S
WP exp
= 15.47%,
= 13.64%,
= 1.13, R
B
= 2.41
Mg(II)
+
Ti(IV)
+
Nb(V)
+
O
2
0
0
0
0.304 (0)
0
0
0
0.304 (0) a
Structural model used for the refinement taken from ICSD card no. 88627. Lattice parameters of the MNT50 phase (JCPDS: 40-0366): a = 4.6673 A c = 3.0184 A V = 65.75 A
0
0
0
0

300 J.C. Bruno et al. / Materials Research Bulletin 43 (2008) 297–304
Fig. 2. X-ray diffraction patterns of PMN–50PT powders as a function of barium content and the tetragonal and monoclinic standards.
Fig. 3. Set of peak at 45 8 and 75 8 for PMN–50PT powders showing the monoclinic and tetragonal perovskite phases as a function of barium content.
better indices of refinement, mainly for the R
B factor. These results are in accord with those reported by Singh and
Pandey
[18] . They showed that the MPB region (0.30
< x < 0.35) in (1 x )PMN– x PT system is characterized by a
Pm monoclinic phase, separating the tetragonal ( x 0.35) and rhombohedral ( x 0.30) phase fields. However, the width of the MPB region can vary depending on the synthesis procedure adopted. Kelly et al.
[21]
demonstrated that the rhombohedral and tetragonal phases can coexist over a wide range of concentration, since the tetragonal single phase is found only for x 0.65, while the rhombohedral is found as a single phase only for x 0.26. We determined a different MPB region for the PMN–50PT composition, where coexistence between the tetragonal and monoclinic
Table 2
Quantitative phase analysis and refinement indices for Ba-doped PMN–PT powders
Sample QPA (mol%)
Tetragonal Monoclinic
Refinement indices
R wp
(%)
Ba0
Ba02
Ba04
69.2
31.6
12.3
30.8
68.4
87.7
15.63
17.69
13.69
R exp
(%)
12.51
14.69
11.63
S ( R wp
/ R exp
)
1.24
1.19
1.17

J.C. Bruno et al. / Materials Research Bulletin 43 (2008) 297–304
Table 3
Refined structural parameters for P 4 mm tetragonal phase
Sample R
B
(%) a (A c (A
Ba0
Ba02
Ba04
4.35
4.55
5.30
3.9761
3.9804
3.9921
4.0391
4.0389
4.0435
Structural model used for the refinement taken from the ICSD card no. 93553.
c / a (A
1.016
1.015
1.013
V (A
3
)
63.855
63.990
64.443
301
D (g cm
3
)
8.173
7.973
7.738
phases was found for non-doped and barium doped PMN–50PT. Our results are concord with the ones reported by
Kelly et al.
[21]
and contrary to those reported by Singh and Pandey
[18]
, because the modifications introduced into the synthesis procedure in Ref.
[18]
caused a high crystallization of the material.
In the present work, the coexistence of monoclinic and tetragonal phases verified for all of the samples changes with barium concentration (
Table 2 ). Barium insertion increases the amount of the monoclinic phase continuously up to
87.7 mol% for x = 0.4 (Ba04 sample). The refinement indices are also shown in
Table 2 , including the goodness of fit,
where the low value shows a higher quality of refinement. The tendency of the formation of monoclinic phase for barium-doped PMN–50PT compositions is an important fact meaning that barium can change the perovskite structure, turning tetragonal to monoclinic phase directly, without the appearing of the rhombohedral phase. The predominance of the monoclinic phase in this sample will possibly increase the piezoelectric properties due to the high piezoelectric coefficients associated with the MPB, where the monoclinic phase predominates, as discussed by Noheda
[3] .
The reduction of the tetragonal phase amount is followed by the loss of tetragonality with the reduction of c / a ratio
( Table 3
). The increase in the molar volume V as barium quantity gets higher shows that the incorporation of barium in the structure is effective and occurs homogenously for both phase, as can be verified in
Tables 3 and 4
. The increase in the molar volume is followed by an accented decrease in the theoretical density due to the low molar mass (137.33 g) and large ionic radius of Ba
2+
Pb
2+
(1.49 A
[22]
.
The monoclinic phase found in Ba0 sample presents a b angle > 90 8 , different from the b value of the structural model for the Pm monoclinic phase reported by Singh and Pandey
[18]
and from that found in ICSD data bank (card no. 94012). However, for Ba02 and Ba04 samples, the b angle is < 90 8 , which denotes a defined PMN–PT Pm monoclinic phase. The R
B index for the monoclinic phase is lower than that of the tetragonal phase for all of the samples, reaching its lowest value in the Ba04 sample, which in turn is lower than that reported in Ref.
[18]
for highly crystallized powders. The variation in R
B values, which is associated with differences in the stability of the structure, demonstrates that the phase amount depends on the stability of the structure, because a decrease in this index is followed by an increase in the amount for both phases.
The influence of barium in PMN–50PT powders can be verified by the data of specific surface area ( Fig. 4 ), where
the powder reactivity is enhanced as a function of barium. The average particle diameter was also determined for the samples, according to Eq.
(1)
:
6 10
3 f ¼
D SA
BET
(1) where f is the average particle size (nm), D the pondered theoretical density obtained by the structural refinement, and
SA
BET is the BET surface area. This calculation assumes that particles have spherical shape.
Table 4
Refined structural parameters for Pm monoclinic phase
Sample R
B
(%) a (A b (A c (A
Ba0
Ba02
Ba04
2.90
2.58
2.31
4.0138
4.0138
4.0172
3.9778
3.9785
3.9897
Structural model used for refinement taken from ICSD card no. 94012.
3.9942
3.9955
4.0027
b (degree)
90.058
89.973
89.912
V (A
3
)
63.772
63.804
64.153
D (g cm
3
)
8.184
7.815
7.417

302 J.C. Bruno et al. / Materials Research Bulletin 43 (2008) 297–304
Fig. 4. Effect of barium concentration on specific surface area (SSA), obtained from BET method, and mean particle diameter, calculated from Eq.
(1)
.
The Ba0 sample presents average particle size of 167 nm and as barium content increases this value decreases reaching 137 nm for Ba02 and 120 nm for Ba04. The powder morphology is also changed by the additive,
corroborating the above data. In Ba0 powder ( Fig. 5
a) some agglomerates with a size of 500 nm are more frequently found, while in Ba02 and Ba04 powders agglomerates close to 300 nm are found and the powders present a lesser degree of agglomeration (
Fig. 5
b and c).
After the sintering process, no significant changes were observed in the phase amount for sintered samples. The possible presence of pyrochlore phases formed by the degradation of perovskite phases, mainly for the Ba04 sample, was eliminated as a consequence of disc-face paring before the XRD characterization. The influence of barium in the weight loss ( D m ) and density ( D
A
) of PMN–PT ceramics was verified (
Table 5 ). The values of
D m during the sintering process showed that barium leads to a greater PbO volatilization, probably due to reduction in ionic diffusivity, making the initial pores more difficult to seal. As a consequence, the sintering process tends to be conduced with PbO deficiency, specifically for the Ba04 sample. It causes a decrease in linear shrinkage during the sintering process, leading to a decrease in relative density ( D
R
). The D
R value was gotten from D
A
/ D
T ratio, where the theoretical density
( D
T
) was obtained from pondered density of each phase. In spite of a continuous increase in D m value as a function of barium content, the D
R value is not significantly affected for the Ba02 sample because the decrease in the consequence of a lesser density of the material (decrease in D
T value). On the other hand, the D
A
D
A value is a value decreases more hastily than the D
T value for the Ba04 sample, affecting the D mainly due to a high weight loss exhibited for this sample.
R value. Thus, a decrease in the linear shrinkage occurs,
Table 5
Data of weight loss and relative density of PMN–50PT ceramic
Sample D m (%) D
A
(g cm
3
)
Ba0
Ba02
Ba04
5.2
5.9
8.6
7.652
7.471
6.553
D
T
(g cm
3
)
8.160
7.865
7.457
D
R
(%)
94
95
88

J.C. Bruno et al. / Materials Research Bulletin 43 (2008) 297–304 303
Fig. 5. SEM micrographies of PMN–50PT powders: (a) Ba0, (b) Ba02 and (c) Ba04.
4. Conclusions
The partial substitution of lead by Ba could reestablish the equilibrium of monoclinic–tetragonal phases in the system. Pyrochlore-free lead magnesium niobate–lead titanate powders were obtained, even for 40 mol% of Ba. The insertion of Ba into the perovskite phase was established by the increase in the lattice parameters of the doped powders.
The presence of Ba contributed to the powder reactivity and reduced the mean particle size. Even though the composition with 40 mol% of Ba was deleterious for the obtaining of a dense ceramic, contents of 20 mol% lead to a ceramic with 95% of density.
Acknowledgments
The authors thank Capes and CNPq Brazilian agencies.
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