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XANES study of lanthanum-doped lead titanate ceramics
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2005 Phys. Scr. 2005 378
(http://iopscience.iop.org/1402-4896/2005/T115/106)
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Physica Scripta. Vol. T115, 378–380, 2005
XANES Study of Lanthanum-Doped LeadTitanate Ceramics
P. P. Neve s1, V. R. Mastelaro1,* , A. Michalowicz2, J. A. Eiras3, Luiz P. Lopes1, A. C. Doriguetto1and Y. P. Mascarenhas1
1Instituto de F´ısica de S˜ao Carlos, USP - S˜ao Carlos – SP, Brazil
2GPMD, Universit´e Paris XII-Val de Marne, 94010 Creteil cedex - France
3Departamento de F´ısica, UFSCar, SP, Brazil
Received June 26, 2003; accepted in revised from November 4, 2003
pacs numbers: 61.10.Ht, 61.66.Fn, 64.70.Kb
Abstract
X-ray absorption spectroscopy was used to probe the short-range structure in
lanthanum-doped lead titanate ceramics (Pb1−xLaxTiO3) for xranging from 0.0 to
0.30. A comparison of the XANES spectra and the X-ray diffraction (XRD) results
indicates that the local structure around Ti atoms had a different compositional
dependence. According to the XANES data, even for the sample containing 30% of
lanthanum, for which the XRD analysis indicated the existence of a cubic structure,
a local distortion around Ti atoms persisted. The intensity of the pre-edge feature
of the PLT30 sample decreased by a factor of only two when compared with the
lanthanum-free sample.
1. Introduction
Pb1−xLaxTiO3(PLT) ceramic materials have been studied
extensively due to their interesting physical properties [1]. The
isomorphic replacement of lead atoms by lanthanum atoms
induces some interesting changes in the physical properties.
Upon increasing the lanthanum content (x>0.25), a diffuse
character of the ferroelectric-paraelectric phase transition (DPT)
is observed. This variation in the phase transition character is
followed by a linear decrease of the Curie temperature (Tc)to
room temperature [1, 3]. It has been suggested that the lanthanum-
induced modification of PbTiO3causes structural changes that
can be directly related to the nature of the phase transition [4].
Although the physical properties of the PLT system have been
studied in some detail [1–7], the literature contains few reports
concerning the structural characteristics of the PLT system.
The PbTiO3crystal structure has been exhaustively studied
[8–10]. Its room temperature structure is tetragonal P4mm (a=
3.902(3) and c=4.156(3) Å) [11]. The notation on perovskite
structures labels the Pb and Ti sites Aand B, respectively and
according to the literature, the La3+ion replaces the Pb2+ion
rather than the Ti4+ion in Pb-based perovskites [12].To maintain
the neutrality of the charge when lead atoms are replaced by
lanthanum atoms, Asite vacancies are created [12].
Using the Raman scattering technique, E.C.S. Tavares et al.
[13] probed the short-range structure in lanthanum-doped lead
titanate ceramics for xranging from 0.0 to 0.30. According to their
report, X-ray diffraction measurements of highly doped samples
(x>0.27) indicated a cubic structure. On the other hand, in highly
doped samples (x>0.27), measurements of Raman scattering at
temperatures above and below the tetragonal phase transition
showed a residual short-range structural disorder in the cubic
phase. These authors suggested that the existence of this short-
range structural disorder may be correlated to the relaxor behavior
in highly doped samples.
*e-mail: valmor@if.sc.usp.Br
Despite this system’s scientific and technological importance,
there is still a paucity of conclusive investigations regarding
its crystalline structure. In this paper, we discuss a short-range
structural study of Pb1−xLaxTiO3ceramic samples as a function
of the lanthanum content. X-ray Absorption Spectroscopy (XAS)
was used to probe the local structure around Ti atoms.
2. Sample Preparation and Experimental Details
Polycrystalline Pb1−xLaxTiO3(PLTx) samples with 0.0 ≤x≤
0.30 were prepared by the conventional mixed oxides method.
PbO, TiO2and La2O3were mixed with distilled water in an
alumina ball mill. Monophasic PLT powders were obtained after
calcination in air for 3 h at 1123K. The calcined powders, pressed
into pellets, were sintered in a covered alumina crucible at
1473K for 3 h. Further details of sample preparation are reported
elsewhere [2].
The titanium K-edge X-Ray absorption spectra were collected
at the LNLS (National Laboratory of Synchrotron Light) in
Campinas, Brazil, a facility that uses the D04B-XAS1 beam line
[14]. The LNLS storage ring was operated at 1.36GeV and 100–
160mA. The pelletized samples obtained after sintering were
ground for XAS measurements. XAS data were collected at the Ti
K-edge (4966 eV) in transmission mode and at room temperature,
using a Si(111) channel-cut monochromator. Ionization chambers
were used to detect the incident and transmitted flux. XANES
(X-ray absorption near edge structures) at the Ti K-edge were
recorded for each sample between 4910 and 5200 eV, using energy
steps of 0.5eV. A qualitative interpretation of XANES spectra
obtained at the Ti K-edge was performed using the software
package developed by Michalowicz and Noinville [15]. For
purposes of comparison among different samples, all the spectra
were background removed and normalized, using as unity the first
EXAFS oscillation.
3. Results and Discussion
Figure 1 shows the XANES spectra obtained at the Ti K-edge
of PLT samples. The pre-edge region of the K-edge XANES
spectra of some transition metal oxides are characterized by
a pronounced feature several volts before the main rising edge
[16–18]. In transition metal oxides that crystallize in
centrosymmetric structures, this pre-edge feature is very small
or absent; in noncentrosymmetric structures it can be quite large
[16–18]. According to the literature, the physical origin of the
peak labeled A in Figure 1 is the transition of the metallic 1s
electron to an unfilled d state [16]. This forbidden electronic dipole
transition is normally allowed by the mixture of p character from
surrounding oxygen atoms in the unfilled d states. On the other
Physica Scripta T115 C
Physica Scripta 2005
XANES Study of Lanthanum-Doped Lead Titanate Ceramics 379
Fig. 1 . Ti K-edge XANES spectra of PLTxpowder samples with 0.0 ≤x≤0.30.
hand, the peak labeled B in Figure 1 was found to be caused by
Ti 1s electron transition to the unoccupied 3d-originated eg-type
molecular orbitals of TiO6polyhedra neighboring the absorbing Ti
atoms, which are weakly affected by the core hole potential [18].
The peak B area does not depend strongly on small displacements
of the atoms from their sites in a cubic crystal lattice, but it changes
significantly when 4d atoms appear in the vicinity of the absorbing
Ti atom, for instance, Zr atoms in the PZT solid solution [18].
As can be seen in Figure 1, the intensity of the A feature in the
PLT05 sample is similar to that of the undoped sample. A 10%
increase in the amount of lanthanum causes a significant drop in
the intensity of the A feature.A further increase of lanthanum to a
concentration of up to 30% shows a slight variation in the intensity
of the A feature, indicating a slight dependence of theA feature’s
intensity on the concentration of lanthanum. The intensity of the
A feature was compared with that of the undistorted EuTiO3cubic
compound. In EuTiO3, which is centrosymmetric, the intensity of
the pre-edge A feature is very small [16]. This comparison PLT
suggests that the local distortion around Ti atoms persisted as the
amount of lanthanum increased, even in the PLT30 sample. Thus,
the XANES results of all the samples containing lanthanum shows
that Ti atoms were located in distorted sites.
With regard to the analysis of the B pre-edge feature, since
we found no significant variation in its shape and amplitude when
the content of lanthanum was increased, it is reasonable to assume
that La atoms preferably replaced Pb atoms rather than Ti atoms.
This assumption is in good agreement with our XRD results.
The magnitude of Ti atom distortion from the center of the
oxygen octahedron can be evaluated by measuring the intensity
of the A feature [16, 19]. The intensity of the A feature was
extracted from the XANES spectra by least-square fitting in
the interval from 4965 to 4980eV. A single Gaussian function
was used to model the A pre-edge feature. The results have an
uncertainty of 0.1eV in energy and 0.02 in intensity. Figure 2
compares the magnitude of Ti atoms distortion by measuring the
intensity of the A feature present in the XANES spectra with
the Ti atom distortion obtained from XRD data. The values of
Ti atom distortion obtained from XRD data were obtained by
making a difference between Ti-O distances situated along the
c-axis divided by 2.
As illustrated in Figure 2, the amplitude of the distortion of Ti
atoms decreased as the content of lanthanum increased and was
in good agreement with Ti atom distortion obtained from XRD
data for compositions containing up to 10% of La. For the PLT00
Fig. 2 . Variation of the intensity of A pre-edge peak in XANES spectra and Ti
atom distortion along the c-axis obtained from XRD data.
sample, the amplitude of Ti distortion was found to be equal to
0.29Å, a very close value to that found by other authors [16, 19].
For the samples containing more than 10% of La, the amplitude of
Ti atom distortion obtained by measuring the intensity of the pre-
edge A feature indicate that a local distortion around Ti persisted,
whereas the analysis of XRD data suggested that Ti atoms returned
to the center of the TiO6octahedron, presenting a point m3m
symmetry in the sample containing 30% of La. A certain structural
discrepancy exists between XAS and XRD results concerning the
PLT30 sample.
The same kind of discrepancy between XAS and XRD results
involving local and long-range structural data was observed by
Sicron et al. in their study of the nature of the ferroelectric phase
transition in the PbTiO3compound [20]. Whereas the XAS results
showed that the local distortion around lead and titanium atoms
were nonzero above the transition temperature, the XRD data
suggested that Ti and Pb atoms returned to the center of the atom
site [20].
According to the literature, as long as some long-range
order is present, the diffraction pattern is composed essentially
of functions, even if structural disorder is present [16]. In
this case, there is also a weak broad background, which is
usually ignored. The disorder does, however, affect the relative
integrated intensities of the various functions. Thus, in principle,
the diffractogram do contain information on the disordered
structural distortions. To obtain this information, it is necessary
to introduce the disordered distortions to the fitting model and
to have a sufficient range of kspace to solve the disordered
distortions. Since most X-ray diffraction powder models do
not take disordered distortions into account, these experiments
provided information only on the average structure.
4. Conclusions
The qualitative analysis of the XANES spectra indicated that the
local structure around Ti atoms had a different compositional
dependence than that indicated by the results obtained from the
X-ray diffraction analysis. According to the XANES data, for
the sample containing 30% of lanthanum, for which the XRD
analysis clearly indicated the existence of a cubic structure, a
local distortion around Ti atoms persisted. The intensity of the
A feature of this sample decreased by a factor of only two when
compared to the lanthanum-free sample. Our results are in good
C
Physica Scripta 2005 Physica Scripta T115
380 P. P. Neves et al.
agreement with the results obtained by Raman Spectroscopy, in
which the existence of a local disorder in highly doped lanthanum
samples was observed.
Acknowledgments
Research partially performed at LNLS – National Laboratory of Synchrotron
Light, Brazil. We are also grateful to FAPESP, CNPq (Brazil) and USP-COFECUB
for their financial support.
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