Measurement of Ti, Ta and Sc off-center displacements in relaxor
ferroelectric PST-PT
A.I. Frenkel1, D.M. Pease2, J. Giniewicz3*, E.A. Stern4, D.L. Brewe4,5, M. Daniel2, and J. Budnick2
1Yeshiva University, New York, NY;
2University of Connecticut, Storrs, CT;
3Indiana University of Pennsylvania, Indiana, PA;
4University of Washington, Seattle, WA;
5Advanced Photon Source, Argonne National Laboratory, Argonne, IL
(* deceased 2003)
We probed titanium (Ti), tantalum (Ta), and scandium (Sc)
environments in the (1-x)Pb(Sc,Ta)O3 – xPbTiO3 relaxor ferroelectric
(PST-PT), which displays variable order–disorder, relaxor, a mixed
phase region, and normal ferroelectric behaviors as x is increased.
The abrupt structural phase transition from rhombohedral to
tetragonal is observed by x-ray diffraction at x = 0.45. According
to x-ray absorption fine structure (XAFS) studies, the structure
around Ta, Sc, or Ti atoms changes differently with x and there are
no abrupt changes at any concentration. No displacements of Ta or Sc
atoms from their oxygen octahedron centers were observed. But
surprisingly, we find that Ti is displaced along the (111) direction
for x = 0.05. However, it gradually changes direction from (111) to
(001) as x increases. We suggest that PST-PT consists of mixed
regions, some characterized by a (111) Ti displacement and others by
a (001) displacement. The displacement averaged over all regions
becomes more weighted toward (001) as x increases.
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Anatoly Frenkel |
Relaxor ferroelectrics display a diffuse temperature dependence
of their polarization-related macroscopic properties. The origin of
this behavior lies in the interactions of polarized entities at the nanoscale. Relaxors typically belong to a class of “oxygen-octahedra
compounds,” in which six oxygen ions surround a particular
transition metal ion, such as Ti, Ta, or Sc (Figure 1). The unique
characteristics of the relaxor ferroelectric (1-x) Pb(Sc1/2Ta1/2)O3
– x PbTiO3 (PST-PT) are revealed when one traverses a variety of
compositions, x. A variety of thermally “adjustable” states of
structural ordering, Curie temperatures, and material properties are
accessible for these materials. PST-PT is attractive for device
applications, and also as a model to explore the nature of relaxor
ferroelectrics.
The structure determined by x-ray diffraction (XRD) was
rhombohedral for low x values, but abruptly becomes tetragonal for x
= 0.45. The Ta L3-edge XAFS data of the same samples were measured
at beamline X11A. The Sc and Ti K-edges were measured at the Pacific
Northwest Consortium - Collaborative Access Team beamlines at the
Advanced Photon Source.
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Figure 1. Perovskite pseudo-cubic ABO3 unit
cell (A=Pb, B=Ta,Sc,Ti). The Ta and Sc atoms were shown to be approximately in
the cell center (A). Arrow shows different directions of Ti atom displacements
obtained by XAFS for x=0.05 (B) and x=1 (C).
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At low x values, an energy dispersive detector is required to
separate Ti Kα fluorescence photons from the Sc Kα photons that
dominate the fluorescence background. We used a log-spiral of
revolution analyzer designed by Doug Pease and Joe Budnick, recently
improved by the addition of the annular ion chamber, designed by Ed
Stern. We know of no other detector setup that could successfully
separate the dilute Ti fluorescence from the otherwise overwhelming
Sc fluorescence photons without saturating the detector, and with
much more intensity than any other crystal monochromator.
Ti K-edge x-ray absorption near-edge structure (XANES) data of
all the samples (Figure 2, left) have a feature “A” located in the
region corresponding to the dipole-forbidden 1s-3d transition. In
order to contribute to the XANES data, the p-character of the final
state of the photoelectron has to be added in the solid. That may
occur due to, for example, hybridization between Ti 3d and O 2p orbitals that is enhanced if Ti is displaced away from the inversion
symmetry center. The presence of a large peak “A” is, therefore, a
signature of the off-center displacements of Ti atoms. The area
under the peak can be used to quantify the off-center displacement
d of the Ti atom (A d2 ). In the Ti K-edge XANES data of all the samples
(Figure 2, left), the intensity of the signal in this region is much
larger than in the reference, cubic EuTiO3 system. For all
x < 1, we obtained d ~ 0.23(2) Å. We found that this large Ti atom
displacement is in marked contrast with Ta and Sc atom behaviors,
where no measurable displacement from the oxygen octahedron centers
(Figure 1(A)) was found by their EXAFS or XANES (Figure 2, left
(inset)) analyses.
The EXAFS analysis (Figure 2, right) revealed that Ti atoms were
displaced along the (111) cubic direction from x = 0.05 (Figure
1(B)). However, this displacement gradually changes direction from
(111) to (001) as x increases (Figure 1(C)).
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Figure 2. (Left) XANES spectra of Ti K-edge in
(PST)1-x(PT)x samples. Feature A denotes
the energy region of the dipole-forbidden 1s-3d transition. The inset shows the
featureless 1s-3d transition region (A) in Sc K-edge XANES. (Right) The Ti
K-edge EXAFS: Fourier transform magnitudes of kχ(k) in
(PST)1-x(PT)x samples. Shown by arrows
are groups of Ti-O distances that correspond to either (111) or (001)
displacement of the Ti atom from the center of the TiO6 octahedron,
depicted schematically in Fig. 1 (B) or (C), respectively.
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These results allow two interpretations. First, the Ti atom
displacement direction changes gradually from (111) to (001) as x
increases, going through the intermediate orientations. The second
interpretation, supported by Maaskant and Bersuker (MB) theory,
allows no intermediate orientation between the (111) and (001)
directions. In the framework of the MB model, we propose that the
PST-PT system consists of mixed regions, some having (111) Ti
displacement and others having (001) displacement. The displacement
averaged over all regions becomes more weighted toward (001) as x
increases.
Our work resulted in the atomic-level scenario of the structural
transformations in PST-PT. We found that the changes in the local
structure could not be simply interpolated from the average
structure data: The independent, element-specific measurements were
required to elucidate the structure around each atomic species. Our
results give the first experimental observation of the
111-displacement direction of a Ti atom in the ferroelectric
perovskite system.
BEAMLINE
X11A
FUNDING
U.S. Department of Energy
PUBLICATION
A. I. Frenkel et al., “Concentration-Dependent Short Range Order in
Relaxor Ferroelectric
(1-x)Pb(Sc,Ta)O3 - xPbTiO3”, Physical Review B, 70,
014106 (2004).
FOR MORE INFORMATION
Anatoly I. Frenkel
Physics Department
Yeshiva University
New York, NY
Email: Anatoly.Frenkel@yu.edu
Web page: http://www.yu.edu/faculty/afrenkel
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