A new experimental set-up controlled by computer has been made to measure the pyroelectric charge of ferroelectric materials with a relatively high conductivity at slow rates of temperature variation. It allowed us to obtain the polarisation vs. temperature behaviour of PZT with various compositions in the whole range of temperatures with a high accuracy. The ferroelectric Zr-rich region of the PZT phase diagram, where the isostructural phase transition FRL-FRH takes place, has been studied

Cereceda, Noé

Duan, Ning

Gonzalo, Julio A.

Noheda, Beatriz

English

University of Groningen Research Database

  
 University of Groningen
Pyroelectric Charge Release in Rhombohedral PZT
Noheda, Beatriz; Duan, Ning; Cereceda, Noé; Gonzalo, Julio A.
Published in:
Journal of the Korean Physical Society
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
1998
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Noheda, B., Duan, N., Cereceda, N., & Gonzalo, J. A. (1998). Pyroelectric Charge Release in
Rhombohedral PZT. Journal of the Korean Physical Society, 32(5), S256-S259.
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Download date: 11-02-2018
Journal of the Korean Physical Society, Vol. 32, February 1998, pp. S256∼S259
Pyroelectric Charge Release in Rhombohedral PZT
Beatriz Noheda, Ning Duan*, Noe´ Cereceda and Julio A. Gonzalo
Dept. F´ısica de Materiales, C-IV. Universidad Auto´noma de Madrid, Cantoblanco, 28049-Madrid, Spain
* On leave from Shanghai Inst. of Ceramics, Shanghai, China
A new experimental set-up controlled by computer has been made to measure the pyroelectric
charge of ferroelectric materials with a relatively high conductivity at slow rates of temperature
variation. It allowed us to obtain the polarisation vs. temperature behaviour of PZT with various
compositions in the whole range of temperatures with a high accuracy. The ferroelectric Zr-rich
region of the PZT phase diagram, where the isostructural phase transition FRL-FRH takes place,
has been studied.
]
I. INTRODUCTION
The isostructural phase transition between two ferro-
electric rhombohedral phases (FRL and FRH) of PZT is
gathering increasing interest due to its large pyroelectric
figure of merit, caused by its high pyroelectric coefficient
combined with a low dielectric constant [1]. From the
structural point of view, the tilt of the oxygen octahedra
in the low temperature phase (FRL) is responsible for
the phase transition [2]. The tilt is coupled with the po-
larization in Zr-rich compositions, giving place to small
anomalies in the polarization and the dielectric constant
behaviour [3]. The disappearance of these discontinu-
ities with increasing Ti content has been recently dis-
cussed by Viehland et al. [4] in thin plates, proposing
different kinds of short-range oxygen rotations also in
the ferroelectric high temperature phase (FRH). Never-
theless there are other factors, such as the compositional
disorder, which makes the transition broader and the
anomalies smaller, and could contribute to the vanish-
ing of the anomalies at the FRL-FRH transition as the Ti
content increases.
The tilt-polarization coupling has been studied by the
authors by means of neutron diffraction and hysteresis
loops data [3]. Nevertheless, the available experimental
data on spontaneous polarization (Ps) vs. temperature
were scarce and not accurate, due to the experimental
difficulties. To obtain better experimental data has been
the main motivation of this work.
It is not easy to characterize dielectrically, as a func-
tion of temperature, ferroelectric ceramics such as PZT
and similar. The low impedance found in these materials
close to the ferroelectric transition makes very difficult
the right compensation of the P-E hysteresis loops. The
difficulties increases at low frequencies. These materi-
als are insulators at room temperature but they become
notably semiconductors at temperatures close to the fer-
roelectric phase transition (200 − 300 ◦C). The high in-
crement of the conductivity produces strong variations
with temperature of the optimum electric compensation
in the measurement circuit, which gives rise unreliable
results. The same problem is found measuring the dielec-
tric permitivity, where the high temperature response is
very often masked by the conductivity. On the other
hand, the high coercive fields existing far bellow from
the ferroelectric transition hinder the observation of the
hysteresis loops.
Polarization measurements in rhombohedral PZT by
P-E loops have both above mentioned problems: high
coercive fields and low impedance. Due to the fact that
the ferroelectric (FRL) - ferroelectric (FRH) phase tran-
sition occurs close to room temperature, where the high
coercive fields avoid the P-E loops, it is one case where
a different procedure to determine spontaneous polariza-
tion is really necessary.
Pyroelectric charge measurements are an excellent al-
ternative to the hysteresis loops. If accurate temperature
determination is required (slow measurements), using an
operational amplifier (OA) as charge integrator, is nec-
essary to keep the sample at zero electric field. In this
way the resulting charge integration is independent of the
sample impedance. This consideration led to the system
used by Bernard et al. [5] to study rhombohedral PZT.
An improved version of such a system has been devel-
oped in this work and it has been used to investigate
Zr-rich PZT.
II. EXPERIMENTAL
-S256-
Pyroelectric Charge Release in Rhombohedral PZT – Beatriz Noheda et al. -S257-
Fig. 1. Scheme of the charge integrator developed to mea-
sure pyroelectric charge in ferroelectric materials with rela-
tively high conductivity at slow rates of temperature varia-
tion.
The experimental set-up is based, as it has been
pointed out, in that used by Bernard et al. [5], which
allows polarization and charge integration, alternatively,
by means of two switches. The system has three main
parts: a high voltage variable source, a charge integrator
and a temperature sensor. The switches are computer
controlled and the data are automatically recorded.
The use of charge integrators with ferroelectric materi-
als is frequently avoided because of the extreme fragility
of the operational amplifier (OA) needed. These devices
must be capable of integrating very low currents, of the
order of nA’s, which implies bias currents of the order of
fA’s. But, at the same time, they are very often exposed
to short-circuits resulting in high voltages, which can de-
stroy the OA. A substantial improvement in the system
has consisted in carefully protecting the device against
high voltage peaks. This, however, is not a trivial ac-
complishment in our case because p-n unions, currently
used to this end, produce offset currents of nA’s, of the
same order of magnitude as that of the currents to be
measured. This problem has been solved here using an
Fig. 2. Normalized spontaneous polarization (ps =
Ps/Pso) versus temperature of PZT(100-X)/X with 1% by
weight of Nb2O5, for nine different compositions in the rhom-
bohedral region.
Fig. 3. Phase diagram of PZT(100-X)/X + 1 wt% Nb2O5
(0<X<25) from pyroelectric charge data shown in Fig. 2.
Previously reported data are also included.
specific FET transistor which limits the maximum volt-
age to be applied to the AO, giving, at the same time,
offset currents in the range of fA. The integrator scheme
is shown in Fig. 1.
The working procedure begins with the poling of the
sample somewhat above the FRL-FRH phase transition,
opening the “poling switch”, S1, (see Fig. 1). Sample
poling was performed with a field of 20 kV/cm during
15 min. in a silicon-oil bath at 125 ◦C. After poling,
the sample is taken to the lowest temperature through
the FRL-FRH phase transition and then the tempera-
ture is increased again to 150 ◦C, without integrating
the pyroelectric charge during this first temperature cy-
cle. At this point the charge integration starts, opening
the “integration switch”, S2, which has been closed un-
til this moment to allow the discharge of the measure-
ment capacitor, C. During integration, the sample is
first cooled through FRL-FRH transition (at TLH) and,
second, heated to the paraelectric phase. Operating in
such a way the thermal hysteresis of the FRL-FRH tran-
sition can be observed. The measured voltage is the
OA output, which is proportional to the charge released,
V = ∆Q/C = ∆PsS/C, being Ps the spontaneous po-
Fig. 4. Change in spontaneous polarization (∆Ps) and
thermal hysteresis (∆TLH) of the FRL-FRH transition versus
Ti content (X) for PZT with 1 wt% of Nb2O5.
-S258- Journal of the Korean Physical Society, Vol. 32, February 1998
Fig. 5. Spontaneous polarization vs. T by integrating the
pyroelectric charge measured also at low temperatures, to
determine Pso = Ps(0 K). Hysteresis loops data are shown to
illustrate the experimental difficulties of this method.
larization and S the samples electroded surface. The
constant voltage level at the paraelectric phase serves to
determine the reference point to get the absolute value of
the spontaneous polarization. The rate of temperature
change used was 20 ◦C/h in each case.
Samples were ceramic discs of 1mm in thickness and
15 mm in diameter for nine different compositions with
Zr/Ti ratios, (100-X)/X, between 98/2 and 77/23. Sam-
ple preparation is described in Ref. 3. The sample con-
ductivity was controlled in part adding 1 wt% of Nb2O5.
III. RESULTS
Ps vs. temperature curves are shown in Fig. 2 for the
nine compositions studied. The spontaneous polariza-
tion has been normalized dividing by Pso = Ps(T= 0 K),
obtained fitting the data as in Ref. 3. Both the ferro-
electric (FRH-PC) and the isostructural (FRL-FRH) tran-
sitions are shown.
The first order character of the ferroelectric (FRH-
PC) transition at To is clearly observed at the higher
Zr-content. This character is decreasing with increasing
Ti content. It has been found (not shown in Fig. 2 for
clarity) that the conductivity has a large effect above
temperatures going from 270 to 300 ◦C. Ps vs. T curves
are far from being flat above To for compositions with
X<10, which, at these temperatures, are in the para-
electric phase. This problem leads to consider possible
distortions in the polarization curve above 270 ◦C for
compositions (X>10) still ferroelectric at these tempera-
tures, as the case for PZT 86/14 and 77/23. Due to this
fact, the tricritical point, at which the transition char-
acter changes from first to second order, is difficult to
determine experimentally.
Heating and cooling through the FRL-FRH transition
(TLH) allows the determination of the thermal hysteresis
and the first order character of the isostructural transi-
tion for compositions with higher Zr content.
The phase diagram obtained from the data in Fig. 2
is plotted in Fig. 3. Data from previous work [6] are also
included. It can be observed that the FRL-FRH phase
boundary is shifted to the left, by a constant value of
2.6% along the X axis, with respect to the phase diagram
of pure PZT [7]. The value of the shift corresponds,
in this range of compositions, to the molar content of
Nb added, which shows that the Nb addition does not
modify the FRL-FRH transition behaviour. This supports
the idea of Nb5+ being located at Zr4+ site, and not in
interstitial positions [8,9].
Thermal hysteresis, ∆TLH = TLH(heat.)-TLH(cool.),
and increment of the spontaneous polarization (∆Ps)
both measured through the FRL-FRH transition are
shown in Fig. 4. ∆TLH and ∆Ps, decrease with increas-
ing Ti content. The Ps step occurring at TLH becomes
less sharp, it diffuses and broadens over a wider range
of temperature as the Ti content is increased, becoming
undetectable for the compositions examined with Zr/Ti
ratio lower than 86/14. Something happens, however,
for Zr/Ti above 96/4. These compositions show clear
ferroelectric behaviour above room temperature due to
the Nb diminishing the antiferro-electric region. Their
Ti content (X<4 %molar) is smaller or similar to the Nb
added (2.6 %molar). This reversal in trend of TLH(X) is
clearly at variance with that shown by the data in Jaffe’s
phase diagram for pure PZT.
A modification of the set-up to measure in the low tem-
perature range (160 K - 370 K) has been developed later
and it has allowed us to examine the experimental value
of Pso, as it is shown in Fig. 5 for PZT 96.5/3.5 + 1.5 wt%
Nb2O5. In this figure we also show data obtained in the
same sample by hysteresis loops measurements, with an
available applied field of 10 kV/cm at a frequency of
10 Hz, to illustrate the experimental difficulties found
with this method. Apart from the hysteresis loops de-
pendence on frequency, and the lack of field amplitude
to saturate the loop far from the paraelectric phase, the
bad compensation of the loops caused by the conduc-
tivity, leads to a broadened phase transition, which is,
however, well define by pyroelectric measurements.
In conclusion, a specially designed experimental set-
up, based on one used by Bernard et al., has been devel-
oped to measure pyroelectric response of low impedance
ferroelectric materials at temperature rates as slow as
one may wish. It has allowed to measure accurately
the thermal hysteresis and the discontinuous step of the
spontaneous polarization at the FRL-FRH transition of
Nb-doped Zr-rich PZT ceramics. The phase diagram of
PZT with 1 wt% of Nb2O5 for 300 K < T < 600 K and
2% < X < 25% has been carefully established.
ACKNOWLEDGMENTS
Our thanks to Dr. C. Alemany for his helpful com-
ments and suggestions. The financial support of CICyT
(PB96-0037) and NATO is acknowledged (CGR-0037).
Pyroelectric Charge Release in Rhombohedral PZT – Beatriz Noheda et al. -S259-
REFERENCES
[1] X. Dai and Y. L. Wang, Phys. Stat. Sol.(a) 124, 435
(1991).
[2] A. M. Glazer and S. A. Mabud, Acta Cryst. B 34, 1060
(1978).
[3] N. Cereceda, B. Noheda, T. Iglesias, J. R. Fndez. del
Castillo, J. A. Gonzalo, N. Duan, Y. L. Wang, D. E. Cox
and G. Shirane, Phys. Rev. B 55, 6174 (1997).
[4] D. Viehland, J. -F. Li, X. Dai and Z. Xu, J. Phys. Chem.
Solids 57, 1545 (1996).
[5] M. Bernard, R. Briot and G. Grange. J. Sol. St. Chem.
31, 369 (1980).
[6] B. Noheda, N. Cereceda, T. Iglesias, G. Lifante and J. A.
Gonzalo, Phys. Rev. B 51, 1688 (1995).
[7] B. Jaffe, W. Cook Jr and H. Jaffe, Pyroelectric Ceramics
(Academic, London, 1971).
[8] Z. Ujima, D. Dmytro´w and M. Pawelczyk, Ferroelectrics
120, 211 (1991).
[9] L. Benguigui, J. Solid Stat. Chem. 3, 381 (1971).


Pyroelectric Charge Release in Rhombohedral PZT

A new experimental set-up controlled by computer has been made to measure the pyroelectric charge of ferroelectric materials with a relatively high conductivity at slow rates of temperature variation. It allowed us to obtain the polarisation vs. temperature behaviour of PZT with various compositions in the whole range of temperatures with a high accuracy. The ferroelectric Zr-rich region of the PZT phase diagram, where the isostructural phase transition FRL-FRH takes place, has been studied.

Cereceda, Noé,

Duan, Ning,

Noheda, Beatriz,

Gonzalo, Julio A.,

University of Groningen Digital Archive

ARTS repository - University of Groningen

   University of GroningenPyroelectric Charge Release in Rhombohedral PZTNoheda, Beatriz; Duan, Ning; Cereceda, Noé; Gonzalo, Julio A.Published in:Journal of the Korean Physical SocietyIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1998Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Noheda, B., Duan, N., Cereceda, N., & Gonzalo, J. A. (1998). Pyroelectric Charge Release inRhombohedral PZT. Journal of the Korean Physical Society, 32(5), S256-S259.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 21-06-2022Journal of the Korean Physical Society, Vol. 32, February 1998, pp. S256∼S259Pyroelectric Charge Release in Rhombohedral PZTBeatriz Noheda, Ning Duan*, Noé Cereceda and Julio A. GonzaloDept. F́ısica de Materiales, C-IV. Universidad Autónoma de Madrid, Cantoblanco, 28049-Madrid, Spain* On leave from Shanghai Inst. of Ceramics, Shanghai, ChinaA new experimental set-up controlled by computer has been made to measure the pyroelectriccharge of ferroelectric materials with a relatively high conductivity at slow rates of temperaturevariation. It allowed us to obtain the polarisation vs. temperature behaviour of PZT with variouscompositions in the whole range of temperatures with a high accuracy. The ferroelectric Zr-richregion of the PZT phase diagram, where the isostructural phase transition FRL-FRH takes place,has been studied.]I. INTRODUCTIONThe isostructural phase transition between two ferro-electric rhombohedral phases (FRL and FRH) of PZT isgathering increasing interest due to its large pyroelectricfigure of merit, caused by its high pyroelectric coefficientcombined with a low dielectric constant [1]. From thestructural point of view, the tilt of the oxygen octahedrain the low temperature phase (FRL) is responsible forthe phase transition [2]. The tilt is coupled with the po-larization in Zr-rich compositions, giving place to smallanomalies in the polarization and the dielectric constantbehaviour [3]. The disappearance of these discontinu-ities with increasing Ti content has been recently dis-cussed by Viehland et al. [4] in thin plates, proposingdifferent kinds of short-range oxygen rotations also inthe ferroelectric high temperature phase (FRH). Never-theless there are other factors, such as the compositionaldisorder, which makes the transition broader and theanomalies smaller, and could contribute to the vanish-ing of the anomalies at the FRL-FRH transition as the Ticontent increases.The tilt-polarization coupling has been studied by theauthors by means of neutron diffraction and hysteresisloops data [3]. Nevertheless, the available experimentaldata on spontaneous polarization (Ps) vs. temperaturewere scarce and not accurate, due to the experimentaldifficulties. To obtain better experimental data has beenthe main motivation of this work.It is not easy to characterize dielectrically, as a func-tion of temperature, ferroelectric ceramics such as PZTand similar. The low impedance found in these materialsclose to the ferroelectric transition makes very difficultthe right compensation of the P-E hysteresis loops. Thedifficulties increases at low frequencies. These materi-als are insulators at room temperature but they becomenotably semiconductors at temperatures close to the fer-roelectric phase transition (200 − 300 ◦C). The high in-crement of the conductivity produces strong variationswith temperature of the optimum electric compensationin the measurement circuit, which gives rise unreliableresults. The same problem is found measuring the dielec-tric permitivity, where the high temperature response isvery often masked by the conductivity. On the otherhand, the high coercive fields existing far bellow fromthe ferroelectric transition hinder the observation of thehysteresis loops.Polarization measurements in rhombohedral PZT byP-E loops have both above mentioned problems: highcoercive fields and low impedance. Due to the fact thatthe ferroelectric (FRL) - ferroelectric (FRH) phase tran-sition occurs close to room temperature, where the highcoercive fields avoid the P-E loops, it is one case wherea different procedure to determine spontaneous polariza-tion is really necessary.Pyroelectric charge measurements are an excellent al-ternative to the hysteresis loops. If accurate temperaturedetermination is required (slow measurements), using anoperational amplifier (OA) as charge integrator, is nec-essary to keep the sample at zero electric field. In thisway the resulting charge integration is independent of thesample impedance. This consideration led to the systemused by Bernard et al. [5] to study rhombohedral PZT.An improved version of such a system has been devel-oped in this work and it has been used to investigateZr-rich PZT.II. EXPERIMENTAL-S256-Pyroelectric Charge Release in Rhombohedral PZT – Beatriz Noheda et al. -S257-Fig. 1. Scheme of the charge integrator developed to mea-sure pyroelectric charge in ferroelectric materials with rela-tively high conductivity at slow rates of temperature varia-tion.The experimental set-up is based, as it has beenpointed out, in that used by Bernard et al. [5], whichallows polarization and charge integration, alternatively,by means of two switches. The system has three mainparts: a high voltage variable source, a charge integratorand a temperature sensor. The switches are computercontrolled and the data are automatically recorded.The use of charge integrators with ferroelectric materi-als is frequently avoided because of the extreme fragilityof the operational amplifier (OA) needed. These devicesmust be capable of integrating very low currents, of theorder of nA’s, which implies bias currents of the order offA’s. But, at the same time, they are very often exposedto short-circuits resulting in high voltages, which can de-stroy the OA. A substantial improvement in the systemhas consisted in carefully protecting the device againsthigh voltage peaks. This, however, is not a trivial ac-complishment in our case because p-n unions, currentlyused to this end, produce offset currents of nA’s, of thesame order of magnitude as that of the currents to bemeasured. This problem has been solved here using anFig. 2. Normalized spontaneous polarization (ps =Ps/Pso) versus temperature of PZT(100-X)/X with 1% byweight of Nb2O5, for nine different compositions in the rhom-bohedral region.Fig. 3. Phase diagram of PZT(100-X)/X + 1 wt% Nb2O5(0<X<25) from pyroelectric charge data shown in Fig. 2.Previously reported data are also included.specific FET transistor which limits the maximum volt-age to be applied to the AO, giving, at the same time,offset currents in the range of fA. The integrator schemeis shown in Fig. 1.The working procedure begins with the poling of thesample somewhat above the FRL-FRH phase transition,opening the “poling switch”, S1, (see Fig. 1). Samplepoling was performed with a field of 20 kV/cm during15 min. in a silicon-oil bath at 125 ◦C. After poling,the sample is taken to the lowest temperature throughthe FRL-FRH phase transition and then the tempera-ture is increased again to 150 ◦C, without integratingthe pyroelectric charge during this first temperature cy-cle. At this point the charge integration starts, openingthe “integration switch”, S2, which has been closed un-til this moment to allow the discharge of the measure-ment capacitor, C. During integration, the sample isfirst cooled through FRL-FRH transition (at TLH) and,second, heated to the paraelectric phase. Operating insuch a way the thermal hysteresis of the FRL-FRH tran-sition can be observed. The measured voltage is theOA output, which is proportional to the charge released,V = ∆Q/C = ∆PsS/C, being Ps the spontaneous po-Fig. 4. Change in spontaneous polarization (∆Ps) andthermal hysteresis (∆TLH) of the FRL-FRH transition versusTi content (X) for PZT with 1 wt% of Nb2O5.-S258- Journal of the Korean Physical Society, Vol. 32, February 1998Fig. 5. Spontaneous polarization vs. T by integrating thepyroelectric charge measured also at low temperatures, todetermine Pso = Ps(0 K). Hysteresis loops data are shown toillustrate the experimental difficulties of this method.larization and S the samples electroded surface. Theconstant voltage level at the paraelectric phase serves todetermine the reference point to get the absolute value ofthe spontaneous polarization. The rate of temperaturechange used was 20 ◦C/h in each case.Samples were ceramic discs of 1mm in thickness and15 mm in diameter for nine different compositions withZr/Ti ratios, (100-X)/X, between 98/2 and 77/23. Sam-ple preparation is described in Ref. 3. The sample con-ductivity was controlled in part adding 1 wt% of Nb2O5.III. RESULTSPs vs. temperature curves are shown in Fig. 2 for thenine compositions studied. The spontaneous polariza-tion has been normalized dividing by Pso = Ps(T= 0 K),obtained fitting the data as in Ref. 3. Both the ferro-electric (FRH-PC) and the isostructural (FRL-FRH) tran-sitions are shown.The first order character of the ferroelectric (FRH-PC) transition at To is clearly observed at the higherZr-content. This character is decreasing with increasingTi content. It has been found (not shown in Fig. 2 forclarity) that the conductivity has a large effect abovetemperatures going from 270 to 300 ◦C. Ps vs. T curvesare far from being flat above To for compositions withX<10, which, at these temperatures, are in the para-electric phase. This problem leads to consider possibledistortions in the polarization curve above 270 ◦C forcompositions (X>10) still ferroelectric at these tempera-tures, as the case for PZT 86/14 and 77/23. Due to thisfact, the tricritical point, at which the transition char-acter changes from first to second order, is difficult todetermine experimentally.Heating and cooling through the FRL-FRH transition(TLH) allows the determination of the thermal hysteresisand the first order character of the isostructural transi-tion for compositions with higher Zr content.The phase diagram obtained from the data in Fig. 2is plotted in Fig. 3. Data from previous work [6] are alsoincluded. It can be observed that the FRL-FRH phaseboundary is shifted to the left, by a constant value of2.6% along the X axis, with respect to the phase diagramof pure PZT [7]. The value of the shift corresponds,in this range of compositions, to the molar content ofNb added, which shows that the Nb addition does notmodify the FRL-FRH transition behaviour. This supportsthe idea of Nb5+ being located at Zr4+ site, and not ininterstitial positions [8,9].Thermal hysteresis, ∆TLH = TLH(heat.)-TLH(cool.),and increment of the spontaneous polarization (∆Ps)both measured through the FRL-FRH transition areshown in Fig. 4. ∆TLH and ∆Ps, decrease with increas-ing Ti content. The Ps step occurring at TLH becomesless sharp, it diffuses and broadens over a wider rangeof temperature as the Ti content is increased, becomingundetectable for the compositions examined with Zr/Tiratio lower than 86/14. Something happens, however,for Zr/Ti above 96/4. These compositions show clearferroelectric behaviour above room temperature due tothe Nb diminishing the antiferro-electric region. TheirTi content (X<4 %molar) is smaller or similar to the Nbadded (2.6 %molar). This reversal in trend of TLH(X) isclearly at variance with that shown by the data in Jaffe’sphase diagram for pure PZT.A modification of the set-up to measure in the low tem-perature range (160 K - 370 K) has been developed laterand it has allowed us to examine the experimental valueof Pso, as it is shown in Fig. 5 for PZT 96.5/3.5 + 1.5 wt%Nb2O5. In this figure we also show data obtained in thesame sample by hysteresis loops measurements, with anavailable applied field of 10 kV/cm at a frequency of10 Hz, to illustrate the experimental difficulties foundwith this method. Apart from the hysteresis loops de-pendence on frequency, and the lack of field amplitudeto saturate the loop far from the paraelectric phase, thebad compensation of the loops caused by the conduc-tivity, leads to a broadened phase transition, which is,however, well define by pyroelectric measurements.In conclusion, a specially designed experimental set-up, based on one used by Bernard et al., has been devel-oped to measure pyroelectric response of low impedanceferroelectric materials at temperature rates as slow asone may wish. It has allowed to measure accuratelythe thermal hysteresis and the discontinuous step of thespontaneous polarization at the FRL-FRH transition ofNb-doped Zr-rich PZT ceramics. The phase diagram ofPZT with 1 wt% of Nb2O5 for 300 K < T < 600 K and2% < X < 25% has been carefully established.ACKNOWLEDGMENTSOur thanks to Dr. C. Alemany for his helpful com-ments and suggestions. The financial support of CICyT(PB96-0037) and NATO is acknowledged (CGR-0037).Pyroelectric Charge Release in Rhombohedral PZT – Beatriz Noheda et al. -S259-REFERENCES[1] X. Dai and Y. L. Wang, Phys. Stat. Sol.(a) 124, 435(1991).[2] A. M. Glazer and S. A. Mabud, Acta Cryst. B 34, 1060(1978).[3] N. Cereceda, B. Noheda, T. Iglesias, J. R. Fndez. delCastillo, J. A. Gonzalo, N. Duan, Y. L. Wang, D. E. Coxand G. Shirane, Phys. Rev. B 55, 6174 (1997).[4] D. Viehland, J. -F. Li, X. Dai and Z. Xu, J. Phys. Chem.Solids 57, 1545 (1996).[5] M. Bernard, R. Briot and G. Grange. J. Sol. St. Chem.31, 369 (1980).[6] B. Noheda, N. Cereceda, T. Iglesias, G. Lifante and J. A.Gonzalo, Phys. Rev. B 51, 1688 (1995).[7] B. Jaffe, W. Cook Jr and H. Jaffe, Pyroelectric Ceramics(Academic, London, 1971).[8] Z. Ujima, D. Dmytrów and M. Pawelczyk, Ferroelectrics120, 211 (1991).[9] L. Benguigui, J. Solid Stat. Chem. 3, 381 (1971).

   University of GroningenPyroelectric Charge Release in Rhombohedral PZTNoheda, Beatriz; Duan, Ning; Cereceda, Noé; Gonzalo, Julio A.Published in:Journal of the Korean Physical SocietyIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1998Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Noheda, B., Duan, N., Cereceda, N., & Gonzalo, J. A. (1998). Pyroelectric Charge Release inRhombohedral PZT. Journal of the Korean Physical Society, 32(5), S256-S259.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 31-10-2023Journal of the Korean Physical Society, Vol. 32, February 1998, pp. S256∼S259Pyroelectric Charge Release in Rhombohedral PZTBeatriz Noheda, Ning Duan*, Noé Cereceda and Julio A. GonzaloDept. F́ısica de Materiales, C-IV. Universidad Autónoma de Madrid, Cantoblanco, 28049-Madrid, Spain* On leave from Shanghai Inst. of Ceramics, Shanghai, ChinaA new experimental set-up controlled by computer has been made to measure the pyroelectriccharge of ferroelectric materials with a relatively high conductivity at slow rates of temperaturevariation. It allowed us to obtain the polarisation vs. temperature behaviour of PZT with variouscompositions in the whole range of temperatures with a high accuracy. The ferroelectric Zr-richregion of the PZT phase diagram, where the isostructural phase transition FRL-FRH takes place,has been studied.]I. INTRODUCTIONThe isostructural phase transition between two ferro-electric rhombohedral phases (FRL and FRH) of PZT isgathering increasing interest due to its large pyroelectricfigure of merit, caused by its high pyroelectric coefficientcombined with a low dielectric constant [1]. From thestructural point of view, the tilt of the oxygen octahedrain the low temperature phase (FRL) is responsible forthe phase transition [2]. The tilt is coupled with the po-larization in Zr-rich compositions, giving place to smallanomalies in the polarization and the dielectric constantbehaviour [3]. The disappearance of these discontinu-ities with increasing Ti content has been recently dis-cussed by Viehland et al. [4] in thin plates, proposingdifferent kinds of short-range oxygen rotations also inthe ferroelectric high temperature phase (FRH). Never-theless there are other factors, such as the compositionaldisorder, which makes the transition broader and theanomalies smaller, and could contribute to the vanish-ing of the anomalies at the FRL-FRH transition as the Ticontent increases.The tilt-polarization coupling has been studied by theauthors by means of neutron diffraction and hysteresisloops data [3]. Nevertheless, the available experimentaldata on spontaneous polarization (Ps) vs. temperaturewere scarce and not accurate, due to the experimentaldifficulties. To obtain better experimental data has beenthe main motivation of this work.It is not easy to characterize dielectrically, as a func-tion of temperature, ferroelectric ceramics such as PZTand similar. The low impedance found in these materialsclose to the ferroelectric transition makes very difficultthe right compensation of the P-E hysteresis loops. Thedifficulties increases at low frequencies. These materi-als are insulators at room temperature but they becomenotably semiconductors at temperatures close to the fer-roelectric phase transition (200 − 300 ◦C). The high in-crement of the conductivity produces strong variationswith temperature of the optimum electric compensationin the measurement circuit, which gives rise unreliableresults. The same problem is found measuring the dielec-tric permitivity, where the high temperature response isvery often masked by the conductivity. On the otherhand, the high coercive fields existing far bellow fromthe ferroelectric transition hinder the observation of thehysteresis loops.Polarization measurements in rhombohedral PZT byP-E loops have both above mentioned problems: highcoercive fields and low impedance. Due to the fact thatthe ferroelectric (FRL) - ferroelectric (FRH) phase tran-sition occurs close to room temperature, where the highcoercive fields avoid the P-E loops, it is one case wherea different procedure to determine spontaneous polariza-tion is really necessary.Pyroelectric charge measurements are an excellent al-ternative to the hysteresis loops. If accurate temperaturedetermination is required (slow measurements), using anoperational amplifier (OA) as charge integrator, is nec-essary to keep the sample at zero electric field. In thisway the resulting charge integration is independent of thesample impedance. This consideration led to the systemused by Bernard et al. [5] to study rhombohedral PZT.An improved version of such a system has been devel-oped in this work and it has been used to investigateZr-rich PZT.II. EXPERIMENTAL-S256-Pyroelectric Charge Release in Rhombohedral PZT – Beatriz Noheda et al. -S257-Fig. 1. Scheme of the charge integrator developed to mea-sure pyroelectric charge in ferroelectric materials with rela-tively high conductivity at slow rates of temperature varia-tion.The experimental set-up is based, as it has beenpointed out, in that used by Bernard et al. [5], whichallows polarization and charge integration, alternatively,by means of two switches. The system has three mainparts: a high voltage variable source, a charge integratorand a temperature sensor. The switches are computercontrolled and the data are automatically recorded.The use of charge integrators with ferroelectric materi-als is frequently avoided because of the extreme fragilityof the operational amplifier (OA) needed. These devicesmust be capable of integrating very low currents, of theorder of nA’s, which implies bias currents of the order offA’s. But, at the same time, they are very often exposedto short-circuits resulting in high voltages, which can de-stroy the OA. A substantial improvement in the systemhas consisted in carefully protecting the device againsthigh voltage peaks. This, however, is not a trivial ac-complishment in our case because p-n unions, currentlyused to this end, produce offset currents of nA’s, of thesame order of magnitude as that of the currents to bemeasured. This problem has been solved here using anFig. 2. Normalized spontaneous polarization (ps =Ps/Pso) versus temperature of PZT(100-X)/X with 1% byweight of Nb2O5, for nine different compositions in the rhom-bohedral region.Fig. 3. Phase diagram of PZT(100-X)/X + 1 wt% Nb2O5(0<X<25) from pyroelectric charge data shown in Fig. 2.Previously reported data are also included.specific FET transistor which limits the maximum volt-age to be applied to the AO, giving, at the same time,offset currents in the range of fA. The integrator schemeis shown in Fig. 1.The working procedure begins with the poling of thesample somewhat above the FRL-FRH phase transition,opening the “poling switch”, S1, (see Fig. 1). Samplepoling was performed with a field of 20 kV/cm during15 min. in a silicon-oil bath at 125 ◦C. After poling,the sample is taken to the lowest temperature throughthe FRL-FRH phase transition and then the tempera-ture is increased again to 150 ◦C, without integratingthe pyroelectric charge during this first temperature cy-cle. At this point the charge integration starts, openingthe “integration switch”, S2, which has been closed un-til this moment to allow the discharge of the measure-ment capacitor, C. During integration, the sample isfirst cooled through FRL-FRH transition (at TLH) and,second, heated to the paraelectric phase. Operating insuch a way the thermal hysteresis of the FRL-FRH tran-sition can be observed. The measured voltage is theOA output, which is proportional to the charge released,V = ∆Q/C = ∆PsS/C, being Ps the spontaneous po-Fig. 4. Change in spontaneous polarization (∆Ps) andthermal hysteresis (∆TLH) of the FRL-FRH transition versusTi content (X) for PZT with 1 wt% of Nb2O5.-S258- Journal of the Korean Physical Society, Vol. 32, February 1998Fig. 5. Spontaneous polarization vs. T by integrating thepyroelectric charge measured also at low temperatures, todetermine Pso = Ps(0 K). Hysteresis loops data are shown toillustrate the experimental difficulties of this method.larization and S the samples electroded surface. Theconstant voltage level at the paraelectric phase serves todetermine the reference point to get the absolute value ofthe spontaneous polarization. The rate of temperaturechange used was 20 ◦C/h in each case.Samples were ceramic discs of 1mm in thickness and15 mm in diameter for nine different compositions withZr/Ti ratios, (100-X)/X, between 98/2 and 77/23. Sam-ple preparation is described in Ref. 3. The sample con-ductivity was controlled in part adding 1 wt% of Nb2O5.III. RESULTSPs vs. temperature curves are shown in Fig. 2 for thenine compositions studied. The spontaneous polariza-tion has been normalized dividing by Pso = Ps(T= 0 K),obtained fitting the data as in Ref. 3. Both the ferro-electric (FRH-PC) and the isostructural (FRL-FRH) tran-sitions are shown.The first order character of the ferroelectric (FRH-PC) transition at To is clearly observed at the higherZr-content. This character is decreasing with increasingTi content. It has been found (not shown in Fig. 2 forclarity) that the conductivity has a large effect abovetemperatures going from 270 to 300 ◦C. Ps vs. T curvesare far from being flat above To for compositions withX<10, which, at these temperatures, are in the para-electric phase. This problem leads to consider possibledistortions in the polarization curve above 270 ◦C forcompositions (X>10) still ferroelectric at these tempera-tures, as the case for PZT 86/14 and 77/23. Due to thisfact, the tricritical point, at which the transition char-acter changes from first to second order, is difficult todetermine experimentally.Heating and cooling through the FRL-FRH transition(TLH) allows the determination of the thermal hysteresisand the first order character of the isostructural transi-tion for compositions with higher Zr content.The phase diagram obtained from the data in Fig. 2is plotted in Fig. 3. Data from previous work [6] are alsoincluded. It can be observed that the FRL-FRH phaseboundary is shifted to the left, by a constant value of2.6% along the X axis, with respect to the phase diagramof pure PZT [7]. The value of the shift corresponds,in this range of compositions, to the molar content ofNb added, which shows that the Nb addition does notmodify the FRL-FRH transition behaviour. This supportsthe idea of Nb5+ being located at Zr4+ site, and not ininterstitial positions [8,9].Thermal hysteresis, ∆TLH = TLH(heat.)-TLH(cool.),and increment of the spontaneous polarization (∆Ps)both measured through the FRL-FRH transition areshown in Fig. 4. ∆TLH and ∆Ps, decrease with increas-ing Ti content. The Ps step occurring at TLH becomesless sharp, it diffuses and broadens over a wider rangeof temperature as the Ti content is increased, becomingundetectable for the compositions examined with Zr/Tiratio lower than 86/14. Something happens, however,for Zr/Ti above 96/4. These compositions show clearferroelectric behaviour above room temperature due tothe Nb diminishing the antiferro-electric region. TheirTi content (X<4 %molar) is smaller or similar to the Nbadded (2.6 %molar). This reversal in trend of TLH(X) isclearly at variance with that shown by the data in Jaffe’sphase diagram for pure PZT.A modification of the set-up to measure in the low tem-perature range (160 K - 370 K) has been developed laterand it has allowed us to examine the experimental valueof Pso, as it is shown in Fig. 5 for PZT 96.5/3.5 + 1.5 wt%Nb2O5. In this figure we also show data obtained in thesame sample by hysteresis loops measurements, with anavailable applied field of 10 kV/cm at a frequency of10 Hz, to illustrate the experimental difficulties foundwith this method. Apart from the hysteresis loops de-pendence on frequency, and the lack of field amplitudeto saturate the loop far from the paraelectric phase, thebad compensation of the loops caused by the conduc-tivity, leads to a broadened phase transition, which is,however, well define by pyroelectric measurements.In conclusion, a specially designed experimental set-up, based on one used by Bernard et al., has been devel-oped to measure pyroelectric response of low impedanceferroelectric materials at temperature rates as slow asone may wish. It has allowed to measure accuratelythe thermal hysteresis and the discontinuous step of thespontaneous polarization at the FRL-FRH transition ofNb-doped Zr-rich PZT ceramics. The phase diagram ofPZT with 1 wt% of Nb2O5 for 300 K < T < 600 K and2% < X < 25% has been carefully established.ACKNOWLEDGMENTSOur thanks to Dr. C. Alemany for his helpful com-ments and suggestions. The financial support of CICyT(PB96-0037) and NATO is acknowledged (CGR-0037).Pyroelectric Charge Release in Rhombohedral PZT – Beatriz Noheda et al. -S259-REFERENCES[1] X. Dai and Y. L. Wang, Phys. Stat. Sol.(a) 124, 435(1991).[2] A. M. Glazer and S. A. Mabud, Acta Cryst. B 34, 1060(1978).[3] N. Cereceda, B. Noheda, T. Iglesias, J. R. Fndez. delCastillo, J. A. Gonzalo, N. Duan, Y. L. Wang, D. E. Coxand G. Shirane, Phys. Rev. B 55, 6174 (1997).[4] D. Viehland, J. -F. Li, X. Dai and Z. Xu, J. Phys. Chem.Solids 57, 1545 (1996).[5] M. Bernard, R. Briot and G. Grange. J. Sol. St. Chem.31, 369 (1980).[6] B. Noheda, N. Cereceda, T. Iglesias, G. Lifante and J. A.Gonzalo, Phys. Rev. B 51, 1688 (1995).[7] B. Jaffe, W. Cook Jr and H. Jaffe, Pyroelectric Ceramics(Academic, London, 1971).[8] Z. Ujima, D. Dmytrów and M. Pawelczyk, Ferroelectrics120, 211 (1991).[9] L. Benguigui, J. Solid Stat. Chem. 3, 381 (1971).

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University of Groningen

   University of GroningenPyroelectric Charge Release in Rhombohedral PZTNoheda, Beatriz; Duan, Ning; Cereceda, Noé; Gonzalo, Julio A.Published in:Journal of the Korean Physical SocietyIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1998Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Noheda, B., Duan, N., Cereceda, N., & Gonzalo, J. A. (1998). Pyroelectric Charge Release inRhombohedral PZT. Journal of the Korean Physical Society, 32(5), S256-S259.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019Journal of the Korean Physical Society, Vol. 32, February 1998, pp. S256∼S259Pyroelectric Charge Release in Rhombohedral PZTBeatriz Noheda, Ning Duan*, Noe´ Cereceda and Julio A. GonzaloDept. F´ısica de Materiales, C-IV. Universidad Auto´noma de Madrid, Cantoblanco, 28049-Madrid, Spain* On leave from Shanghai Inst. of Ceramics, Shanghai, ChinaA new experimental set-up controlled by computer has been made to measure the pyroelectriccharge of ferroelectric materials with a relatively high conductivity at slow rates of temperaturevariation. It allowed us to obtain the polarisation vs. temperature behaviour of PZT with variouscompositions in the whole range of temperatures with a high accuracy. The ferroelectric Zr-richregion of the PZT phase diagram, where the isostructural phase transition FRL-FRH takes place,has been studied.]I. INTRODUCTIONThe isostructural phase transition between two ferro-electric rhombohedral phases (FRL and FRH) of PZT isgathering increasing interest due to its large pyroelectricfigure of merit, caused by its high pyroelectric coefficientcombined with a low dielectric constant [1]. From thestructural point of view, the tilt of the oxygen octahedrain the low temperature phase (FRL) is responsible forthe phase transition [2]. The tilt is coupled with the po-larization in Zr-rich compositions, giving place to smallanomalies in the polarization and the dielectric constantbehaviour [3]. The disappearance of these discontinu-ities with increasing Ti content has been recently dis-cussed by Viehland et al. [4] in thin plates, proposingdifferent kinds of short-range oxygen rotations also inthe ferroelectric high temperature phase (FRH). Never-theless there are other factors, such as the compositionaldisorder, which makes the transition broader and theanomalies smaller, and could contribute to the vanish-ing of the anomalies at the FRL-FRH transition as the Ticontent increases.The tilt-polarization coupling has been studied by theauthors by means of neutron diffraction and hysteresisloops data [3]. Nevertheless, the available experimentaldata on spontaneous polarization (Ps) vs. temperaturewere scarce and not accurate, due to the experimentaldifficulties. To obtain better experimental data has beenthe main motivation of this work.It is not easy to characterize dielectrically, as a func-tion of temperature, ferroelectric ceramics such as PZTand similar. The low impedance found in these materialsclose to the ferroelectric transition makes very difficultthe right compensation of the P-E hysteresis loops. Thedifficulties increases at low frequencies. These materi-als are insulators at room temperature but they becomenotably semiconductors at temperatures close to the fer-roelectric phase transition (200 − 300 ◦C). The high in-crement of the conductivity produces strong variationswith temperature of the optimum electric compensationin the measurement circuit, which gives rise unreliableresults. The same problem is found measuring the dielec-tric permitivity, where the high temperature response isvery often masked by the conductivity. On the otherhand, the high coercive fields existing far bellow fromthe ferroelectric transition hinder the observation of thehysteresis loops.Polarization measurements in rhombohedral PZT byP-E loops have both above mentioned problems: highcoercive fields and low impedance. Due to the fact thatthe ferroelectric (FRL) - ferroelectric (FRH) phase tran-sition occurs close to room temperature, where the highcoercive fields avoid the P-E loops, it is one case wherea different procedure to determine spontaneous polariza-tion is really necessary.Pyroelectric charge measurements are an excellent al-ternative to the hysteresis loops. If accurate temperaturedetermination is required (slow measurements), using anoperational amplifier (OA) as charge integrator, is nec-essary to keep the sample at zero electric field. In thisway the resulting charge integration is independent of thesample impedance. This consideration led to the systemused by Bernard et al. [5] to study rhombohedral PZT.An improved version of such a system has been devel-oped in this work and it has been used to investigateZr-rich PZT.II. EXPERIMENTAL-S256-Pyroelectric Charge Release in Rhombohedral PZT – Beatriz Noheda et al. -S257-Fig. 1. Scheme of the charge integrator developed to mea-sure pyroelectric charge in ferroelectric materials with rela-tively high conductivity at slow rates of temperature varia-tion.The experimental set-up is based, as it has beenpointed out, in that used by Bernard et al. [5], whichallows polarization and charge integration, alternatively,by means of two switches. The system has three mainparts: a high voltage variable source, a charge integratorand a temperature sensor. The switches are computercontrolled and the data are automatically recorded.The use of charge integrators with ferroelectric materi-als is frequently avoided because of the extreme fragilityof the operational amplifier (OA) needed. These devicesmust be capable of integrating very low currents, of theorder of nA’s, which implies bias currents of the order offA’s. But, at the same time, they are very often exposedto short-circuits resulting in high voltages, which can de-stroy the OA. A substantial improvement in the systemhas consisted in carefully protecting the device againsthigh voltage peaks. This, however, is not a trivial ac-complishment in our case because p-n unions, currentlyused to this end, produce offset currents of nA’s, of thesame order of magnitude as that of the currents to bemeasured. This problem has been solved here using anFig. 2. Normalized spontaneous polarization (ps =Ps/Pso) versus temperature of PZT(100-X)/X with 1% byweight of Nb2O5, for nine different compositions in the rhom-bohedral region.Fig. 3. Phase diagram of PZT(100-X)/X + 1 wt% Nb2O5(0<X<25) from pyroelectric charge data shown in Fig. 2.Previously reported data are also included.specific FET transistor which limits the maximum volt-age to be applied to the AO, giving, at the same time,offset currents in the range of fA. The integrator schemeis shown in Fig. 1.The working procedure begins with the poling of thesample somewhat above the FRL-FRH phase transition,opening the “poling switch”, S1, (see Fig. 1). Samplepoling was performed with a field of 20 kV/cm during15 min. in a silicon-oil bath at 125 ◦C. After poling,the sample is taken to the lowest temperature throughthe FRL-FRH phase transition and then the tempera-ture is increased again to 150 ◦C, without integratingthe pyroelectric charge during this first temperature cy-cle. At this point the charge integration starts, openingthe “integration switch”, S2, which has been closed un-til this moment to allow the discharge of the measure-ment capacitor, C. During integration, the sample isfirst cooled through FRL-FRH transition (at TLH) and,second, heated to the paraelectric phase. Operating insuch a way the thermal hysteresis of the FRL-FRH tran-sition can be observed. The measured voltage is theOA output, which is proportional to the charge released,V = ∆Q/C = ∆PsS/C, being Ps the spontaneous po-Fig. 4. Change in spontaneous polarization (∆Ps) andthermal hysteresis (∆TLH) of the FRL-FRH transition versusTi content (X) for PZT with 1 wt% of Nb2O5.-S258- Journal of the Korean Physical Society, Vol. 32, February 1998Fig. 5. Spontaneous polarization vs. T by integrating thepyroelectric charge measured also at low temperatures, todetermine Pso = Ps(0 K). Hysteresis loops data are shown toillustrate the experimental difficulties of this method.larization and S the samples electroded surface. Theconstant voltage level at the paraelectric phase serves todetermine the reference point to get the absolute value ofthe spontaneous polarization. The rate of temperaturechange used was 20 ◦C/h in each case.Samples were ceramic discs of 1mm in thickness and15 mm in diameter for nine different compositions withZr/Ti ratios, (100-X)/X, between 98/2 and 77/23. Sam-ple preparation is described in Ref. 3. The sample con-ductivity was controlled in part adding 1 wt% of Nb2O5.III. RESULTSPs vs. temperature curves are shown in Fig. 2 for thenine compositions studied. The spontaneous polariza-tion has been normalized dividing by Pso = Ps(T= 0 K),obtained fitting the data as in Ref. 3. Both the ferro-electric (FRH-PC) and the isostructural (FRL-FRH) tran-sitions are shown.The first order character of the ferroelectric (FRH-PC) transition at To is clearly observed at the higherZr-content. This character is decreasing with increasingTi content. It has been found (not shown in Fig. 2 forclarity) that the conductivity has a large effect abovetemperatures going from 270 to 300 ◦C. Ps vs. T curvesare far from being flat above To for compositions withX<10, which, at these temperatures, are in the para-electric phase. This problem leads to consider possibledistortions in the polarization curve above 270 ◦C forcompositions (X>10) still ferroelectric at these tempera-tures, as the case for PZT 86/14 and 77/23. Due to thisfact, the tricritical point, at which the transition char-acter changes from first to second order, is difficult todetermine experimentally.Heating and cooling through the FRL-FRH transition(TLH) allows the determination of the thermal hysteresisand the first order character of the isostructural transi-tion for compositions with higher Zr content.The phase diagram obtained from the data in Fig. 2is plotted in Fig. 3. Data from previous work [6] are alsoincluded. It can be observed that the FRL-FRH phaseboundary is shifted to the left, by a constant value of2.6% along the X axis, with respect to the phase diagramof pure PZT [7]. The value of the shift corresponds,in this range of compositions, to the molar content ofNb added, which shows that the Nb addition does notmodify the FRL-FRH transition behaviour. This supportsthe idea of Nb5+ being located at Zr4+ site, and not ininterstitial positions [8,9].Thermal hysteresis, ∆TLH = TLH(heat.)-TLH(cool.),and increment of the spontaneous polarization (∆Ps)both measured through the FRL-FRH transition areshown in Fig. 4. ∆TLH and ∆Ps, decrease with increas-ing Ti content. The Ps step occurring at TLH becomesless sharp, it diffuses and broadens over a wider rangeof temperature as the Ti content is increased, becomingundetectable for the compositions examined with Zr/Tiratio lower than 86/14. Something happens, however,for Zr/Ti above 96/4. These compositions show clearferroelectric behaviour above room temperature due tothe Nb diminishing the antiferro-electric region. TheirTi content (X<4 %molar) is smaller or similar to the Nbadded (2.6 %molar). This reversal in trend of TLH(X) isclearly at variance with that shown by the data in Jaffe’sphase diagram for pure PZT.A modification of the set-up to measure in the low tem-perature range (160 K - 370 K) has been developed laterand it has allowed us to examine the experimental valueof Pso, as it is shown in Fig. 5 for PZT 96.5/3.5 + 1.5 wt%Nb2O5. In this figure we also show data obtained in thesame sample by hysteresis loops measurements, with anavailable applied field of 10 kV/cm at a frequency of10 Hz, to illustrate the experimental difficulties foundwith this method. Apart from the hysteresis loops de-pendence on frequency, and the lack of field amplitudeto saturate the loop far from the paraelectric phase, thebad compensation of the loops caused by the conduc-tivity, leads to a broadened phase transition, which is,however, well define by pyroelectric measurements.In conclusion, a specially designed experimental set-up, based on one used by Bernard et al., has been devel-oped to measure pyroelectric response of low impedanceferroelectric materials at temperature rates as slow asone may wish. It has allowed to measure accuratelythe thermal hysteresis and the discontinuous step of thespontaneous polarization at the FRL-FRH transition ofNb-doped Zr-rich PZT ceramics. The phase diagram ofPZT with 1 wt% of Nb2O5 for 300 K < T < 600 K and2% < X < 25% has been carefully established.ACKNOWLEDGMENTSOur thanks to Dr. C. Alemany for his helpful com-ments and suggestions. The financial support of CICyT(PB96-0037) and NATO is acknowledged (CGR-0037).Pyroelectric Charge Release in Rhombohedral PZT – Beatriz Noheda et al. -S259-REFERENCES[1] X. Dai and Y. L. Wang, Phys. Stat. Sol.(a) 124, 435(1991).[2] A. M. Glazer and S. A. Mabud, Acta Cryst. B 34, 1060(1978).[3] N. Cereceda, B. Noheda, T. Iglesias, J. R. Fndez. delCastillo, J. A. Gonzalo, N. Duan, Y. L. Wang, D. E. Coxand G. Shirane, Phys. Rev. B 55, 6174 (1997).[4] D. Viehland, J. -F. Li, X. Dai and Z. Xu, J. Phys. Chem.Solids 57, 1545 (1996).[5] M. Bernard, R. Briot and G. Grange. J. Sol. St. Chem.31, 369 (1980).[6] B. Noheda, N. Cereceda, T. Iglesias, G. Lifante and J. A.Gonzalo, Phys. Rev. B 51, 1688 (1995).[7] B. Jaffe, W. Cook Jr and H. Jaffe, Pyroelectric Ceramics(Academic, London, 1971).[8] Z. Ujima, D. Dmytro´w and M. Pawelczyk, Ferroelectrics120, 211 (1991).[9] L. Benguigui, J. Solid Stat. Chem. 3, 381 (1971).

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Pyroelectric Charge Release in Rhombohedral PZT

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