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M. Alguer et al.
Nanostructured ceramics
DOI: 10.1002/smll.200700284
Relaxor Behavior, Polarization Buildup, and Switching
in Nanostructured 0.92 PbZn1/3Nb2/3O3–0.08 PbTiO3
Ceramics
Miguel Alguer,* Teresa Hungra, Harvey Amorn, Jesffls Ricote, Jean Galy, and
Alicia Castro
The relaxor-type behavior, electrical polarization buildup, and switching
in 0.92Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 nanostructured ceramics with a
grain size of 20 nm is reported for the first time. This composition
presents the highest-known piezoelectric coefficients, yet phase stability is
an issue. Ceramics can only be obtained by the combination of mechanosynthesis and spark-plasma sintering. The results raise the possibility of
using nanoscale, perovskite-relaxor-based morphotropic-phase-boundary
materials for sensing and actuation in nanoelectromechanical systems.
Keywords:
· ceramics
· ferroelectrics
· nanostructures
· perovskites
· spark-plasma sintering
1. Introduction
Piezoelectric, ferroelectric ceramic materials are a
mature and ubiquitous technology. These ceramics are the
active elements in a range of piezoelectric devices and perform functions such as sensing and actuation.[1] Piezoelectric
devices are no exception to the current miniaturization
trends in ceramic technology for microelectronics. Ceramic
elements are approaching a characteristic thickness of 1 mm,
which requires submicrometer grain sizes close to the nanoscale for device reliability.[2] Similar and even smaller sizes
are involved in ferroelectric thick and thin films on Si-based
substrates for their integration into piezoelectric microelectromechanical systems.[3] Piezoelectric transduction is also
being considered to implement sensing and actuation in
nanoelectromechanical systems,[4, 5] which require a further
[*] Dr. M. Alguer, Dr. T. Hungra, Dr. H. Amorn, Dr. J. Ricote,
Dr. A. Castro
Instituto de Ciencia de Materiales de Madrid (ICMM)
Cantoblanco, 28049 Madrid (Spain)
Fax: (+ 34) 913-720-623
E-mail: [email protected]
Dr. T. Hungra, Dr. J. Galy
Centre d’Elaboration de Mat5riaux et d’Etudes Structurales
(CEMES)
29 rue Jeanne Marvig
BP 94347, 31055 Toulouse (France)
1906
reduction of dimensions; therefore, ultrathin ferroelectric
films have been the focus of recent studies.[6] Nanoscale epitaxial films and islands have been investigated for high-density, nonvolatile, ferroelectric random-access memory deACHTUNGREvices.[7, 8] Also, nanostructured ferroelectrics have been demonstrated for the tuning of the transport properties of nanofibers of a conductive polymer,[9] and for the electrical
switching of the magnetization of epitaxial columnar nanostructures of a ferrimagnetic spinel.[10]
The use of nanoscale ferroelectrics in these technologies
rests on the downscaling behavior of the properties. FerroACHTUNGREelectricity is a cooperative phenomenon, and it was long
thought that a minimum volume was necessary to sustain
the spontaneous polarization; this raises the existence of a
fundamental size limit below which ferroelectricity vanishes.[11, 12] First-principles calculations and experimental studies
on nanostructured films,[13–16] wires,[17, 18] particles,[19, 20] and
ceramics[21] have shown that ferroelectricity persists at the
nanoscale and thus, that the downscaling of ferroelectricbased piezoelectric technologies is feasible. However, most
studies focused on prototype, simple perovskites, such as
BaTiO3, PbTiO3, or tetragonal PbACHTUNGRE(Zr,Ti)O3, whereas results
on high-piezoelectric-coefficient morphotropic-phase-boundary (MPB) materials are scarce.[22, 23]
The highest piezoelectric coefficients have been reported for relaxor-ferroelectric PbZn1/3Nb2/3O3–PbTiO3 single
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Perovskite Nanostructured Ceramics
crystals with values of 2500 pC N 1.[24] Unfortunately, powders of this phase cannot be synthesized by the solid-state
reaction of precursors and a pyrochlore-type nonferroelectric phase always results.[25] The perovskite has only been
obtained by high-pressure synthesis[26] and as a nanocrystalline powder by mechanosynthesis.[27] However, the phase
decomposes into the pyrochlore phase, ZnO, and PbO
under subsequent heating, which prevents ceramics from
being processed. We have succeeded in processing perovskite-phase 0.92PbZn1/3Nb2/3O3–0.08PbTiO3 nanostructured
ceramics by Spark-plasma sintering (SPS) of a nanocrystalline powder obtained by mechanosynthesis. This allowed us
to study the electrical properties and thus the polar states
present at the nanoscale for this relaxor-based MPB material, which are reported here for the first time.
2. Results and Discussion
rates up to 600 K min 1. Hence, densification takes place in
a short time, typically about 15 min, and a powder compact
of small grain size is obtained.[31] The combination of mechanosynthesis and SPS has already been successfully used
for the processing of ferroelectric oxides and the study of
size effects.[32] When applied to the PbZn1/3Nb2/3O3–PbTiO3
system, ceramics can only be obtained by the synergy between the two techniques; the thermal decomposition of the
nanocrystalline perovskite at temperatures between 673 and
873 K shows a slow kinetics as compared with the duration
of the SPS process,[27] and pressure helps to stabilize the
PbZn1/3Nb2/3O3 perovskite.[26] Densification of the nanocrystalline powder started above 773 K and density increased
exponentially, so that values above 90 % were obtained at
898 K. However, perovskite decomposition also occurred in
this temperature range and significant amounts of pyrochlore were already present after SPS at the latter temperature. Figure 2 shows the XRD patterns for ceramics process-
Nanocrystalline 0.92PbZn1/3Nb2/3O3–0.08PbTiO3 powders
were obtained by mechanochemical activation of binary
oxides in a high-energy planetary mill. This is a powerful
technique for the preparation of functional nanocrystalline
materials[28] that allows most of the ferroelectric perovskites
to be mechanosynthesized.[27, 29] A transmission electron microscopy (TEM) image of the powder and its particle size
distribution are shown in Figure 1. The Feret=s diameter was
Figure 2. XRD patterns of the nanostructured 0.92PbZn1/3Nb2/3O3–
0.08PbTiO3 materials processed by SPS at several temperatures. Pe:
perovskite; Py: pyrochlore.
Figure 1. TEM image and particle size distribution of the nanocrystalline 0.92PbZn1/3Nb2/3O3–0.08PbTiO3 powder used for the processing
of nanostructured materials.
measured for an ensemble of more than 100 particles, and
an average size of 12 nm resulted. This value is in a good
agreement with the crystal size obtained from X-ray diffraction (XRD) data using the full width at half-maximum
(FWHF) of the perovskite (110) diffraction peak and the
Scherer equation.
Nanostructured ceramics have been processed by SPS.[30]
In this technique, sintering is activated by the use of uniaxial
pressures and pulsed direct current, which results in heating
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ed at 823, 873, and 898 K. Densification levels of 70, 80,
and 90 % of the theoretical value were achieved at these
temperatures, respectively. Studies were focused on the ceramics with a very minor pyrochlore phase, that is, those
spark-plasma sintered at 823 and 873 K. The amount of nonferroelectric phase for the latter material is of the order of
5 % according to XRD, and the phase has a relative permittivity of 100,[33] so its effect on the properties is not significant as compared to that of porosity.
The crystal sizes obtained from XRD data with the
Scherer equation were 16 and 18 nm for the materials proACHTUNGREcessed at 823 and 873 K, respectively. TEM images and the
Feret=s diameter distribution of the two materials are shown
in Figure 3. Single lognormal distributions were found in
both cases, with an average of 14 and 21 nm for the ceramics spark-plasma sintered at 823 and 873 K, respectively.
Note the consistency among the sizes obtained by the two
techniques, which unambiguously confirms a grain size of
20 nm.
Electrical characterization was carried out on ceramic
disks on which Pt electrodes had been deposited by sputter-
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M. Alguer et al.
Figure 4. Dielectric permittivity and losses as a function of temperature at several frequencies for a nanostructured 0.92PbZn1/3Nb2/3O3–
0.08PbTiO3 material with a grain size of 20 nm.
Figure 3. TEM images and grain size distributions of the nanostructured 0.92PbZn1/3Nb2/3O3–0.08PbTiO3 materials processed by SPS at
a) 823 and b) 873 K.
ing. As-sintered materials presented very high dielectric
losses at room temperature that strongly decreased after annealing in air at 623 K for 2 h. This is a common observation
for nanostructured ceramics processed by SPS, which has
been associated with the reducing sintering conditions
within the graphite die and the formation of large numbers
of oxygen vacancies in the perovskite during the process.[21]
The temperature dependence of the dielectric permittivity
and losses of a ceramic spark-plasma sintered at 873 K is
shown in Figure 4 at several frequencies. The permittivity initially increases on heating and reaches a broad maximum at
a temperature that increases with frequency between 210 and
240 K. Below this maximum, permittivity decreases when the
frequency is increased, while dielectric losses increase with
this parameter. This is the typical behavior of a relaxor material, such as the prototype Pb(Mg1/3Nb2/3)O3.[34] At higher
temperatures, a Debye-type relaxation is observed, which indicates the presence of dipolar defects. Also, dielectric losses
linearly increase with the reciprocal frequency in this temper-
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ature range because of electrical conduction. We found analogous dielectric relaxation and conduction in coarse-grained
PbZn1/3Nb2/3O3–PbFe1/2Nb1/2O3–PbTiO3 ceramics processed
from powders by mechanosynthesis, which indicates that the
defects responsible for the high-temperature relaxation and
conduction are generated during the mechanosynthesis and
not during the SPS; their nature is outside the scope of this
paper. Therefore, the main intrinsic feature is the existence
of a relaxor-type behavior with the dispersive maxima in permittivity below room temperature for this material with a
grain size of 20 nm.
This behavior must be compared with that of
0.92Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 single crystals. The roomtemperature phase is ferroelectric rhombohedral with space
group R3m (No. 160) that transforms into ferroelectric tetragonal P4mm (No. 99) at 340 K, and then to a relaxor state
at 440 K.[35] Permittivity presents a sharp increase at the ferroelectric-to-relaxor transition, and typical relaxor behavior
above this temperature.[36] The absence of any dielectric feature associated with the transition between the relaxor and
ferroelectric states for the nanostructured material down to
77 K strongly suggests that this transition has disappeared as
a size effect. This finding is in good agreement with recent
results for 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3, also a perovskite relaxor-based MPB material; structural analysis and
dielectric measurements showed that the ferroelectric phase
vanished below a size of 200 nm, and that a relaxor state existed down to 30 nm.[23] Below this size, a maximum of dielectric permittivity was not observed, and the authors proposed that a conventional paraelectric phase was present.
However, our results indicate that a relaxor state exists for
0.92Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 with a size of 20 nm.
The presence of such a relaxor state is further supported
by the room-temperature nonlinear electric response, which
is shown in Figure 5 for the ceramic spark plasma sintered
at 873 K. Current density at increasing electric fields up to
1 kV mm 1 and 0.1 Hz is presented, which clearly shows polarization-switching maxima. This result demonstrates that
an electrical polarization builds up and then is switched
under the field at room temperature for this nanostructured
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Perovskite Nanostructured Ceramics
Figure 5. Nonlinear electrical response at 0.1 Hz for a nanostructured
0.92PbZn1/3Nb2/3O3–0.08PbTiO3 material with a grain size of
20 nm.
material. Polarization switching is already observed at a
field of 0.2 kV mm 1, and the polarization continuously increases with the field with no signs of saturation at
1 kV mm 1. This is indicated by the height and width of the
maxima, although quantification is not possible because of
nonlinear conduction. This is why we chose to show the current loops rather than the charge ones, which are strongly
deformed by the conduction, yet ferroelectriclike inflection
points can still be observed, as is also illustrated in Figure 5
for the highest field applied. Note that polarization switching is observed at room temperature, which is slightly above
the temperature of the permittivity maximum. This is a typical behavior of relaxors. Polarization switching disappeared
when the frequency was raised to 1 Hz; the buildup of polarization thus has a characteristic time above 1 s.
The study of macroscopic polarization shows the behavior of the aggregate of crystals, and the measurement is very
sensitive to extrinsic factors, such as porosity, second phases,
and the quality of the electrode/ceramic electrical contact,
which is an issue for the nanostructured materials under investigation. Also, ceramic samples suffered dielectric breakdown under high-voltage dc (or very low frequency) electrical loading, which prevented bulk conventional electromechanical characterization. Local measurements with scanning force microscopy (SFM) provide information closer to
the behavior of individual nanocrystals and are not affected
by these factors. The basis of piezoresponse force microscopy (PFM) is the application of an electric field between a
conductive tip and the sample, which induces a piezoelectric
response that is recorded. With this technique, we obtained
in-field piezoelectric hysteresis loops, such as the one shown
in Figure 6 a. The loop is typical of relaxors; high effective
piezoelectric coefficients are attained under field, associated
with the buildup of polarization, which change sign with
switching, but low remanence is found.[37] Also, the loop is
shifted towards negative voltages, which indicates that an internal electric field has built up under the applied inhomogeneous electric field beneath the tip, most probably associated with the separation of free charge carriers.
The results on macroscopic properties reported here
have only been obtained thanks to the densification levels
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Figure 6. PFM of the nanostructured 0.92PbZn1/3Nb2/3O3–0.08PbTiO3
material with a grain size of 20 nm: a) piezoelectric coefficient hysteresis loop; b) piezoelectric response.
achieved by SPS. The fact that we observed relaxor behavior for a grain size of 20 nm, while paraelectric behavior was
described for MPB Pb(Mg1/3Nb2/3)O3–PbTiO3 below
30 nm,[23] is most probably not due to any intrinsic difference between these relaxor-based systems but to the densification levels of the materials prepared. In this work, a densification of 80 % was obtained in comparison with the 60 %
reported in ref. [23] for the materials with the smallest size.
Electric measurements in such powder compacts are difficult and the real dielectric behavior of the material can be
masked. The effect of densification on the dielectric properties of the Pb(Zn1/3Nb2/3)O3–PbTiO3 nanostructured materials is illustrated in Figure 7, where the temperature dependence of the relative permittivity and dielectric losses at
1 MHz is given for the ceramics spark-plasma sintered at
873 and 823 K, with densification levels of 80 and 70 %,
respectively. Note the increase of permittivity and decrease
of losses associated with electrical conduction with the increase in densification. Nevertheless, the relaxor maximum
is still observed for the material processed at 823 K.
The results clearly demonstrate that a relaxor state
exists in 0.92Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 nanostructured
ceramics with a grain size of 20 nm, and that a macroscopic electrical polarization can be built up and switched with
an electric field at room temperature. This was not the case
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M. Alguer et al.
linear piezoelectric response under bias is found by PFM.
This is the first time that these phenomena have been observed in a nanostructured, ferroelectric, relaxor-based
MPB material.
4. Experimental Section
Figure 7. Dielectric permittivity and losses as a function of temperature at 1 MHz for nanostructured 0.92PbZn1/3Nb2/3O3–0.08PbTiO3
materials with different levels of densification.
for very dense ( 97 %) BaTiO3 nanostructured ceramics
with similar grain size, for which ferroelectric switching
could not be induced in spite of them being in the ferroelectric phase.[21, 38] Lack of switching was proposed to be due to
the clamping of transgranular ferroelectric domains at the
grain boundaries. Porosity might disrupt domain continuity
across the grains and result in smaller domains with lessclamped walls, as is suggested by the report of ferroelectric
switching in BaTiO3 materials with a grain size of 40 nm
and a densification of 90 %.[39] Nevertheless, the mechanism
of polarization in relaxors is not that of conventional ferroelectrics: the nucleation and growth of ferroelectric domains. Polarization for relaxors is built up by the growth
and merging of polar nanoregions within a paraelectric
matrix. This phenomenon seems to scale well in the nanoscale, as a nanostructured relaxor material with a size of a
few tens of nanometers still presents polarization reversal.
It must be pointed out that polarization in relaxors is not
stable after the removal of the electric field above the freezing temperature. However, relaxors present high piezoelectric coefficients under bias field, and a highly linear, nonhysteretic electromechanical response that is currently used in
actuation. This effect is illustrated in Figure 6 b for the nanostructured Pb(Zn1/3Nb2/3)O3–PbTiO3 material, where the
local piezoelectric response under the built-in field measured by PFM is shown.
3. Conclusions
We have succeeded in processing 0.92Pb(Zn1/3Nb2/3)O3–
0.08PbTiO3 nanostructured ceramics with a grain size of
20 nm by the SPS of a nanocrystalline powder obtained
by mechanosynthesis, and in characterizing their electrical
properties. The results indicate that the relaxor-to-ferroelectric phase transition does not occur in the nanoscale material, and that a relaxor state exists instead of the ferroelectric
R3m phase. An electrical polarization can be built up and
switched with an electric field in the 0.92Pb(Zn1/3Nb2/3)O3–
0.08PbTiO3 nanostructured material, and a local, highly
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Mechanosynthesis and powder characterization: A stoichiometric mixture of analytical grade PbO, ZnO, Nb2O5, and TiO2
(3 g) was initially homogenized by hand in an agate mortar, then
placed in a stainless-steel pot with five stainless-steel balls
(2 cm in diameter, weight 35 g) for mechanochemical activation.
This was carried out in air with a Pulverisette 6 model Fritsch
planetary mill (300 rpm), and mechanosynthesis was completed
after 70 h. Phases and morphology were characterized by XRD
with a Seifert 3000 TT apparatus and Cu Ka radiation, and by
TEM with a Philips CM12ST microscope (120 kV).
Spark-plasma sintering: A cylindrical graphite die (8-mm
inner diameter) was filled with the powder for use with a
SPS 2080 Sumitomo apparatus. A pulsed direct current was
then passed through the die while an increasing uniaxial pressure was applied (up to 100 MPa) and the sample was heated
to the final temperature (100 K min 1). The final conditions of
sintering temperature and 100 MPa were maintained for 3 min.
Ceramic characterization: Phases were characterized by XRD
with a Siemens D500 powder diffractometer with Cu Ka radiation
(2q = 0.058 steps and 5 s dwell time). Samples for TEM were prepared and characterized with the Philips CM12ST microscope for
determining the grain size. The dependences of the dielectric
permittivity and losses on temperature were measured with an
HP 4192A impedance analyzer (100 Hz, 1 kHz, 10 kHz, 100 kHz,
and 1 MHz frequencies). Measurements were dynamically accomplished during heating from 77 to 573 K (1.5 K min 1). For
characterizing the nonlinear electric response at room temperature, voltage sine waves were applied (frequencies of 0.1 and
1 Hz and amplitude up to 1000 V) by the combination of a synthesizer/function generator (HP 3325B) and a bipolar operational power supply/amplifier (Kepco BOP 1000M). Charge was measured with a homebuilt charge-to-voltage converter and software
for loop acquisition and analysis. Current was obtained by numerical differentiation of the charge data. Samples for SFM were
prepared by polishing with Al2O3 suspensions with decreasing
sizes (down to 0.05 mm) and etching in acetic acid (20 min) to
reveal the grain boundaries. Measurements were carried out
with a Nanotec Electr?nica microscope and WSxM software. PFM
was carried out with Pt/Ir-coated tips from Nansensors (ac voltage 1 V, frequency 50 kHz).
Acknowledgements
This research was funded by MEC (Spain) through the
MAT2005-01304 and MAT2004-00868 projects. Drs. T. Hungr,a and H. Amor,n are grateful for financial support by the
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Perovskite Nanostructured Ceramics
EC FPVI NoE MIND (NMP3-CT2005-515757) and MEC (JdC
Program), respectively. Collaboration between ICMM and
CEMES is framed within the ESF COST Action 539 ELENA. Technical support by I. Mart,nez (ICMM) and Ph. Salles (CEMES) is
also acknowledged.
[1] Piezoelectric Materials in Devices (Ed.: N. Setter), EPFL, Lausanne, 2002, ISBN 2-9700346-0-3.
[2] C. Pithan, D. Hennings, R. Waser, Int. J. Appl. Ceram. Technol.
2005, 2, 1 – 14.
[3] S. Trolier-McKinstry, P. Muralt, J. Electroceram. 2004, 12, 7 – 17.
[4] H. G. Craighead, Science 2000, 290, 1532 – 1535.
[5] K. L. Ekinci, Small 2005, 1, 786 – 797.
[6] J. Ricote, S. Holgado, P. Ramos, M. L. Calzada, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 2006, 53, 2299 – 2304.
[7] T. Tybell, P. Paruch, T. Giamarchi, J. M. Triscone, Phys. Rev. Lett.
2002, 89, 097 601.
[8] I. Szafraniak, C. Harneaga, R. Scholz, S. Bhattacharyya, D.
Hesse, M. Alexe, Appl. Phys. Lett. 2003, 83, 2211 – 2213.
[9] M. Nikiforov, H. Liu, H. G. Craighead, D. Bonell, Nano Lett. 2006,
6, 896 – 900.
[10] F. Zavaliche, H. Zheng, L. Mohaddes-Ardabili, S. Y. Yang, Q.
Zhan, P. Shafer, E. Reilly, R. Chopdekar, Y. Jia, P. Wright, D. G.
Schlom, Y. Suzuki, R. Ramesh, Nano Lett. 2005, 5, 1793 – 1796.
[11] N. A. Spaldin, Science 2004, 304, 1606 – 1607.
[12] C. H. Ahn, K. M. Rabe, J. M. Triscone, Science 2004, 303, 488 –
491.
[13] P. Ghosez, K. M. Rabe, Appl. Phys. Lett. 2000, 76, 2767 – 2769.
[14] J. Junquera, P. Ghosez, Nature 2003, 422, 506 – 509.
[15] D. D. Fong, G. B. Stephenson, S. K. Streiffer, J. A. Eastman, O.
Auciello, P. H. Fuoss, C. Thompson, Science 2004, 304, 1650 –
1653.
[16] V. Nagarajan, S. Prasertchoung, T. Zhao, H. Zheng, J. Ouyang, R.
Ramesh, W. Tian, X. Q. Pan, D. M. Kim, C. B. Eom, H. Kohlstedt,
R. Waser, Appl. Phys. Lett. 2004, 84, 5225 – 5227.
[17] W. S. Yun, J. J. Urban, Q. Gu, H. Park, Nano Lett. 2002, 2, 447 –
450.
[18] J. E. Spanier, A. M. Kolpak, J. J. Urban, I. Grinberg, L. Ouyang,
W. S. Yun, A. M. Rappe, H. Park, Nano Lett. 2006, 6, 735 – 739.
[19] H. Fu, L. Bellaiche, Phys. Rev. Lett. 2003, 91, 257 601.
small 2007, 3, No. 11, 1906 – 1911
[20] S. Ray, Y. V. Kolen’ko, D. Fu, R. Gallage, N. Sakamoto, T. Watanabe, M. Yoshimura, M. Itoh, Small 2006, 2, 1427 – 1431.
[21] M. T. Buscaglia, M. Viviani, V. Buscaglia, L. Mitoseriu, A. Testino,
P. Nanni, Z. Zhao, M. Nygren, C. Harneaga, D. Piazza, C. Galassi,
Phys. Rev. B 2006, 73, 064 114.
[22] C. Liu, B. Zou, A. J. Rondinone, Z. J. Zhang, J. Am. Chem. Soc.
2001, 123, 4344 – 4345.
[23] J. Carreaud, P. Gemeiner, J. M. Kiat, B. Dkhil, C. Bogicevic, T.
Rojac, B. Malic, Phys. Rev. B 2005, 72, 174 115.
[24] S. E. Park, T. R. Shrout, J. Appl. Phys. 1997, 82, 1804 – 1811.
[25] A. Halliyal, U. Kumar, R. E. Newham, L. E. Cross, Am. Ceram.
Soc. Bull. 1987, 66, 671 – 676.
[26] T. Fujiu, A. Tanaka, T. Takenaka, Jpn. J. Appl. Phys. 1991, 30,
L298 – L301.
[27] M. Alguer?, J. Ricote, A. Castro, J. Am. Ceram. Soc. 2004, 87,
772 – 778.
[28] A. Giri, Adv. Mater. 1997, 9, 163 – 166.
[29] J. Wang, W. Dongmei, X. Junmin, N. W. Beng, Adv. Mater. 1999,
11, 210 – 213.
[30] Z. J. Shen, Z. Zhao, H. Peng, M. Nygren, Nature 2002, 417,
266 – 269.
[31] Z. A. Munir, U. Anselmi-Tamburini, M. Ohyanagi, J. Mater. Sci.
2006, 41, 763 – 777.
[32] T. HungrKa, M. Alguer?, A. B. HungrKa, A. Castro, Chem. Mater.
2005, 17, 6205 – 6212.
[33] H. C. Ling, M. F. Yan, W. W. Rhodes, J. Mater. Sci. 1989, 24,
541 – 548.
[34] L. E. Cross, Ferroelectrics 1994, 151, 305 – 320.
[35] G. Y. Xuo, P. M. Gehring, G. Shirane, Phys. Rev. B 2006, 74,
104 110.
[36] M. L. Mulvihill, S. E. Park, G. Risch, Z. Li, K. Uchino, T. R. Shrout,
Jpn. J. Appl. Phys. 1996, 35, 3984 – 3990.
[37] V. V. Shvartsman, A. L. Kholkin, M. Tyunina, J. Levoska, Appl.
Phys. Lett. 2005, 86, 222 907.
[38] X. Y. Deng, X. H. Wang, H. Wen, L. L. Chen, L. Chen, L. T. Li, Appl.
Phys. Lett. 2006, 88, 252 905.
[39] M. H. Frey, Z. Xu, P. Han, D. A. Payne, Ferroelectrics 1998, 206,
337 – 353.
Received: April 23, 2007
Revised: August 21, 2007
Published online on October 12, 2007
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