ferroelectric relaxor behavior in hafnium doped barium-titanate ceramic
TRANSCRIPT
Ferroelectric relaxor behavior in hafnium doped barium-titanate ceramic
Shahid Anwar, P.R. Sagdeo, N.P. Lalla *
UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, India
Received 22 June 2005; received in revised form 7 March 2006; accepted 18 March 2006 by B.-F. Zhu
Available online 4 April 2006
Abstract
Temperature and frequency dependence of the real (3 0) and imaginary (3 00) parts of the dielectric permitivity of cubic Ba(Ti0.7Hf0.3)O3 ceramic
has been studied in the temperature range of 100 K to 350 K at the frequencies 0.1 kHz, 1 kHz, 10 kHz, 100 kHz for the first time. Diffuse phase
transition and frequency dispersion is observed in the permittivity-vs-temperature plots. This has been attributed to the occurrence of relaxor
ferroelectric behavior. The observed relaxor behavior has been quantitatively characterized based on phenomenological parameters.
A comparison with the Zr doped BaTiO3 has also been presented. For Hf doped samples transmission electron microscopy (TEM)
characterization do show the presence of highly disordered microstructure at length scales of few tens of nano-meters.
q 2006 Elsevier Ltd. All rights reserved.
PACS: 77.; 77,22.Ch
Keywords: A. Ferroelectric; C. Structure; D. Dielectric
1. Introduction
Perovskite-based ferroelectrics materials attract consider-
able interest owing to rich diversity of their physical properties
and possible applications in various technologies like memory
storage devices [1], micro-electromechanical systems [2],
multilayer ceramic capacitors [3], and recently in the area of
opto-electronic devices [4]. These useful properties have most
often been observed in lead based perovskite compounds, such
as PMN, PST, PLZT [5–7]. The enhanced properties of these
compounds are attributed to their relaxor behavior, observed in
doped (mixed) perovskites. However these compositions have
obvious disadvantages of volatility and toxicity of PbO.
Therefore much effort has been carried out towards investi-
gating environmental friendly ‘Pb-free’ ceramic materials.
Specifically, BaTiO3 and its isovalent substituted materials are
the promising candidates for microwave and opto-electronic
applications.
The effect of substitution on dielectric relaxation, ferro-
electric phase transition and electrical properties of BaTiO3 has
been extensively studied [8,9].On partial substitution of dopants
like Ca, Sr, Zr [10–12], the variation of 3 0 around Tc gets
0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2006.03.018
* Corresponding author. Tel.: C91 731 2463913; fax: C91 731 2462294.
E-mail address: [email protected] (N.P. Lalla).
broadened out in ceramics and single crystal samples both.
Broadening increases with increasing concentration of the
dopant, as also does the deviation fromCurie–Weiss behavior at
temperatures above the peak temperature (Tm) of the 3 0KT
variation. The observed broadening in 3 0KT variation has
generally been attributed to the presence of nano-regions
resulting from local composition variation over length scale of
100–1000 A. Different nano-regions in a macroscopic sample
transform at different temperatures giving rise a range of
transformation temperatures, the so-called ‘Curie range’. Thus
the compositional fluctuation [6,11] in an otherwise composi-
tionally homogenous system leads to diffuse phase transition
(DPT). In compositionally homogenous systems quenched
random disorder breaks the long range polar order at unit cell
level, leading to broad 3 0KT response [7]. Such materials
exhibit slow enough relaxation dynamics and hence have been
termed as ferroelectric relaxors [5–7]. A series of impurity
doped BaTiO3 system such as Sn, Ce, Zr etc have shown
ferroelectric relaxor behavior. Among these the Zr-substituted
BaTiO3 ceramics have received renewed attention due to its
enhanced properties both in single crystals and ceramics [12].
In the present investigation, we have studied the ferro-
electric relaxor behavior in high concentration Hf substituted
BaTiO3, i.e. Ba(Ti0.7Hf0.3)O3 ceramics, by monitoring the
variation of its dielectric permittivity with temperature in the
range of 90–350 K and in the frequency range of 0.1–100 KHz.
Till date only limited amount of work has been carried out for
Hf doping on Ti site in BaTiO3 [13,14] ceramics like its effect
Solid State Communications 138 (2006) 331–336
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20 40 60 80 100 120
BaTiO3
Ba(Ti0.7Hf0.3)O3
2θ
43.5 44.0 44.5 45.0 45.5 46.073 74 75 76
Inte
nsity
(a.
u)
Fig. 1. Room temperature XRD pattern of BaTiO3 (grey circles) and
Ba(Ti0.7Hf0.3)O3 (black circles), Fig. also shows the fitted pattern, Braggs
peaks and difference between observed and calculated pattern. Inset show the
expansion around the (200) and (310) peaks.
Fig. 2. Secondary electron micrographs corresponding to (a) BaTiO3 and (b)
Ba(Ti0.7Hf0.3)O3 respectively.
S. Anwar et al. / Solid State Communications 138 (2006) 331–336332
on dielectric and structural properties. Detailed structural and
dielectric studies have been carried out. The data has been
quantitatively analyzed in terms of parameters characterizing
the relaxor behavior.
2. Experimental
BaTiO3 and Ba(Ti0.7Hf0.3)O3 ceramics were prepared
following the conventional solid-state reaction technique.
High purity (99.99%) powders of BaCO3, TiO2 and HfO2
were weighed in stoichiometric proportions and wet-mixed,
taking acetone as mixing medium. After mixing and drying, the
powder was calcined at 1100 8C for 6 h. The calcined powder
was remixed, dried, and then palletized at 100 kN/cm2 pressure
into a 15 mm dia. pallet using polyvinyl alcohol as binder. The
pallets were then sintered at 1250 8C for 12 h and cooled
naturally to room temperature while furnace power was off.
The as prepared pallets of BaTiO3 and Ba(Ti0.7Hf0.3)O3were
subjected to structural and phase purity characterization
employing powder X-ray diffraction (XRD), scanning electron
microscopy (SEM-JEOL 5600) and transmission electron
microscopy (TEM). The XRD analysis was carried out using
CuKaX-ray employing a Rigakumake goniometer and rotating
anode X-ray generator working at 10 kW output power. Sample
for TEM studies were prepared using ion-beam polishing at
well-optimized kV and angle settings (3.3 kV and 38). TEM
characterization of these samples was carried out in diffraction
and imaging modes using FEI-TECNAI G2K20 working at
200 kV. Electroding of the pallets for dielectric measurement
was then done by painting its flat surfaces with high temperature
silver-paint and then firing it at 500 8C for 15 min. Dielectric
characterization of pallets, as a function of temperature, was
carried out employing temperature-controlled measurement of
its dielectric permitivity (3 0) and dielectric loss (tan d) in the
temperature range of 90–350 K. For these measurements a
computer interfaced and programmed LCRmeter (Hioki-3538)
and a Lakeshore temperature controller with a Pt-100,
connected in 4-probe mode, was used for temperature
measurement. The peak-to-peak ac signal of 1.0 V in the
frequency range of 102–105 Hz and a temperature step of 0.5 K
with set-point stability better than 20 mK were used. Thus the
measurement process ensures that no anomaly remains
unobserved in the entire studied temperature range.
3. Results and discussions
3.1. Structural studies
The results of X-ray diffraction characterization of the as
prepared Ba(Ti0.7Hf0.3)O3 and BaTiO3 samples are shown in
Fig. 1. The insets of Fig. 1 show two sets of (200) & (002) and
(301) & (310) diffraction peaks which is a characteristics of the
tetragonal BaTiO3 phase at room temperature. In the case of
Ba(Ti0.7Hf0.3)O3 the two peaks merge into single peak, i.e.
(200) and (310), which is a characteristic of tetragonal to cubic
phase transition. The XRD data of Ba(Ti0.7Hf0.3)O3 and
BaTiO3 samples were subjected to Reitveld refinement with
space-group of Pm3m (cubic) and P4 mm (tetragonal)
respectively. The goodness of fit parameter were found to be
w2.44. The continuous line in Fig. 1 shows the Reitveld fit to
the XRD data of Ba(Ti0.7Hf0.3)O3. The quality of fit and the
absence of any unaccounted diffraction peak indicate that the
synthesized Ba(Ti0.7Hf0.3)O3 ceramic is cubic single phase
material. Fig. 2(a) and (b) depicts the secondary electron
Fig. 3. Transmission electron micrographs corresponding to BaTiO3 (a, c) and
Ba(Ti0.7Hf0.3)O3 (b, d) respectively.
S. Anwar et al. / Solid State Communications 138 (2006) 331–336 333
micrographs of BaTiO3 and Ba(Ti0.7Hf0.3)O3 phases respect-
ively. Well faceted grains of sizes ranging from 2–8 mm can be
seen.
Fig. 3(a)–(b) shows TEM micrographs of BaTiO3 and
Ba(Ti0.7Hf0.3)O3. The occurrence of large grains of sizes
3–5 mm of BaTiO3 and Ba(Ti0.7Hf0.3)O3 phases can be seen in
Fig. 3(a) and (b), respectively. This is in conformity with the
SEM results. A relatively high-magnification micrograph of
BaTiO3 is shown in Fig. 3(c), which depicts the occurrence of
ferroelectric twin-domains of the tetragonal BaTiO3 phase.
Fig. 3(d) shows a high-magnification micrograph of Ba(Ti0.7Hf0.3)O3 phase. One can see that unlike BaTiO3 phase no
contrast showing the twin-domains are present in its
microstructure but it is rich in nano contrast regions of
4–10 nm apparent sizes. The detailed features of these nano
contrasts are typical to the presence of strained regions [16]. To
confirm that these strains are not due to some secondary phase
inclusions, we carried out selected area diffraction (SAD) from
these areas as shown in the inset of Fig. 3(d). It is obvious from
this SAD pattern that no extra spots are present other than those
from the basic perovskite structure. This confirms that these
contrasts are not due to any secondary phase inclusions. Since
these regions were found to change contrast simultaneously
during tilt, these might originate from strain fields due to some
defect, like tiny dislocation loops extending just to few lattice
sites with their burger vectors parallel. The occurrence of these
defect features in Ba(Ti0.7Hf0.3)O3 will lower its structural
correlation length as compared to BaTiO3 in which no such
features were found. The ‘structural correlation length’ is
basically a measure of the effective extent to which the long-
range order of an atomic arrangement gets limited as a result of
the cumulative effect of all types of defect features, which
some how either interrupt the chemical order or produce strain
in the lattice. Beyond this extent the atoms don’t scatter
coherently and contribute to the width of the diffraction
maxima. Thus the structural correlation length may be used as
a measure of comparison of defects presence in two similar
types of structures. Thus to compare the presence of defects in
the bulk samples of Ba(Ti0.7Hf0.3)O3 and BaTiO3, structural
correlation lengths were calculated from the half-widths of the
(111) XRD peak, after applying instrumental broadening and
Ka2 corrections. These were found to be w980 A and
w1120 A respectively for Ba(Ti0.7Hf0.3)O3 and BaTiO3. It
should be noted that the size of even the smallest grain, as seen
through SEM, is at least an order of magnitude larger than the
structural correlation length. Nearly 10% smaller structural
correlation length was found for Ba(Ti0.7Hf0.3)O3 as compared
to that of the BaTiO3, indicating the presence of more defects
in it. This tells that the defect feature observed through TEM is
a characteristic of the whole bulk and is not only limited to few
grains.
3.2. Dielectric studies
The temperature dependence of real (3 0) and imaginary (3 00)
parts of the dielectric permitivity of Ba(Ti0.7Hf0.3)O3 ceramic
are shown in Fig. 4. Unlike BaTiO3 the transition is quite
diffuse. The paraelectric to ferroelectric phase transition
temperature (Tc) as compared to that of the BaTiO3, have
decreased. The three phase transitions which are observed in
BaTiO3 have got pinched and merged into one round peak in
3 0KT variation. The results obtained can be described as in the
following:
100 150 200 250 300 350
0.0003
0.0006
0.0009
0.0012
0.0015
Tdev
1/ε'
Temperature (K)
Fig. 5. The inverse dielectric constant (1/3 0) as a function of temperature at
10 KHz for ceramics Ba(Ti0.7Hf0.3)O3. The Symbol represents experimental
data points and solid line shows fitting to the Curie–Weiss law.
100 150 200 250 300 350
500
1000
1500
2000
2500
3000
3500
100 KHz
100 Hz
100 KHz
100 Hz
0.1KHz1KHz10KHz100KHz
ε'
100 150 200 250 300 350
0
50
100
150
200
250
300
ε''
Temperature(K)
Fig. 4. Temperature dependence of 3 0 and 300 of Ba(Ti0.7Hf0.3)O3 ceramic at 0.1,
1, 10 and 100 kHz.
–5.5
S. Anwar et al. / Solid State Communications 138 (2006) 331–336334
(1) There is a broad peak around TmZ200 K in the 3 0KT
curve. With increasing frequency Tm increases, while the
magnitude of the peak decreases.
(2) There is a strong dielectric dispersion in radio frequency
region around and below Tm in the 3 0KT.
(3) The value of T 0m (dielectric absorption maxima tempera-
ture), is much less than Tm and around and above T 0m the
dielectric absorption (3 00) exhibits a strong frequency
dependence. With increasing frequency T 0m shifts to
higher temperature with increasing dielectric absorption.
The above described features of (30KT) and (300KT) variations
shown in Fig. 4 are very much similar to the observations by
Cross, Lu, Cheng, Chen and other workers [5,6,10,12,15] for
various lead based and lead free ferroelectric relaxormaterials. In
order to further confirm the relaxor behavior, the quantitative
characterizations as described in the following have been done.
0.25 0.50 0.75 1.00
–7.0
–6.5
–6.0
log(
1/ε'-
1/ε' m
)
log(T-Tε'm)
Fig. 6. ln(1/30K1/3 0m)-vs-ln(TKT3 0m) plot for Ba(Ti0.7Hf0.3)O3 ceramics at
10 kHz Symbol represents experimental data and solid line shows fitting.
3.3. Permittivity variation in the high temperature side
It is known that dielectric permittivity of a normal
ferroelectric above Curie temperature follows the Curie–
Weiss law described by
1=30 Z ðTKTcÞ=C ðTOTcÞ; (1)
Where Tc is the Curie temperature and C is the Curie–Weiss
constant. Fig. 5 shows the inverse of 30 as a function of
temperature at 10 kHz and its fit to the experimental data by
Curie–Weiss law. A deviation from Curie–Weiss law starting at
Tdev can be clearly seen. The parameterDTm, which is often usedto show the degree of deviation from the Curie-Weiss law is
defined as
DTm Z TdevKTm (2)
The Tdev as determined from the Curie–Weiss fit, isTdevZ288 K,
andDTm is thus found to beZ132 K at 10 kHz. For such relaxor
behavior a modified Curie–Weiss law has been proposed by
Uchino and Nomura, [17] to describe the diffuseness of the phase
transition. This is defined as in the following.
1=30K1=30m Z ðTKT30mÞg=C1 (3)
where g and C1 are modified constants, with 1ZgZ2. The
parameter gives the information on the character of the phase
transition. Its limiting values aregZ1 and gZ2 in expression (3)
of the Curie–Weiss law, gZ1 is for the case of a normal
ferroelectric and the quadratic dependence is valid for an ideal
ferroelectric relaxor respectively [12,18]. Thus the value ofg can
also characterize the relaxor behavior. Theplot of log(1/30K1/30m)
as a function of log(TKT3 0m) is shown in the Fig. 6 by fitting
S. Anwar et al. / Solid State Communications 138 (2006) 331–336 335
with Eq. (3), the exponent g, determining the degree of the
diffuseness of the phase transition, is obtained from the slope of
log(1/3 0K1/3 0m)-vs- log(TKT3 0m) plot. We obtained the value
of the parameterg to be 1.88,which is very close to 2, suggestingthat the prepared ceramic is a relaxor ferroelectric [17,18]. Yet
another parameter, which is used to characterize the degree of
relaxation behavior in the frequency range of 100 Hz to
100 kHz, is described [18] as
DTrelax Z T30mð100 KHzÞKT30mð100 HzÞ (4)
The value of DTrelax was determined to be 23 K for the present
sample. The above characterization done on the basis of Curie–
Weiss law and the value of empirical parameters like DTm, g,and DTrelax suggest that the permittivity of Ba(Ti0.7Hf0.3)O3
ceramic follows Curie-weiss law only at temperatures much
higher than Tm. Thus the large deviation from the Curie–Weiss
behavior, large relaxation temperature g Trelax, and gZ1.88,
suggests that Ba(Ti0.7Hf0.3)O3 is a relaxor ferroelectric.
The frequency dependence of Tm is shown in Fig. 7 as
ln f-vsK1000/Tm. The observed frequency dependence of Tmwas empirically evaluated using Vogel–Fulcher’s relationship
given as
f Z f0expfKEa=kBðTmKTfÞg (5)
where Ea is the activation energy, Tf the freezing temperature
of polarization–fluctuation, and f0 is the pre-exponential factor.
The value of Ea, Tf and f0 for Ba(Ti0.7Hf0.3)O3 are found to be
0.14 eV, 78.5 K, 1.53!1013 Hz respectively. Similar values
have been reported for Ba(Ti0.7Zr0.3)O3 relaxor ferroelectric
system too [12,19].
The occurance of relaxor in Zr substituted barium titanate
[12,20,21] has been attributed to the existence of nano-polar
region due to Zr doping. The replacements of Ti4C by Zr4C
ions is known in the classical ferroelectric [19,20]. The
increasing substitution decreases the Curie temperature, which
is related to the ionic radius of the dopand. In the case of
BaZr0.3Ti0.7O3 (BZT) [12,19,21] where the ionic radius of
Zr4C(0.72 A) is higher than Ti4C(0.605 A) by 0.095 A the
TcZ280 K is lower than that of the pure BaTiO3 (TcZ395 K).
6.0 6.2 6.4 6.6 6.8 7.0 7.24
6
8
10
12
Ln(f
req)
1000/T
Fig. 7. Frequency dependence of Tm for Ba(Ti0.7Hf0.3)O3 ceramics. The
symbols and solid line indicate data points and fit to Vogel–Fulcher
relationship, respectively.
For BaHf0.3Ti0.7O3 (BHT), in the present studies, TcZ156 K,
which is much lower than that of the Zr doped one. Whereas
the ionic radius of Hf4C(0.71 A) is not much different than the
ionic radius of Zr4C. This appears to be due to lower activation
energy of BHT (0.14 eV) as compared to BZT (0.21 eV) [12].
It can be seen from the comparative structural study of
BaHf0.3Ti0.7O3 and BaTiO3 using XRD and TEM, that nano
scale defect features are present in BaHf0.3Ti0.7O3. It appears
that substitution of Hf at Ti site is causing nano-scale
compositional heterogeneity due to presence of defects
[19–21] creating nano polar domains. It has been suggested
that polar nano-domains are responsible for the relaxational
behavior in PMN and PLZT [10] systems. The substitution of
Hf ions tends to make the distance between off center Ti
dipoles larger and thus weakening the correlation between
these dipoles. The mismatch in the size of Ti and Hf ions will
cause substitutional distortion of the oxygen octahedra, giving
rise to the local electric-field and strain-field. Presence of such
strain-field contrast localized in nano-meter range, has been
very clearly observed for BaHf0.3Ti0.7O3 in the present
investigation, see Fig. 3(d). Present TEM investigations reveal
that these local fields and strain fields occur randomly but are
coherent in nature and tend to break the large ferroelectric
domains into polar nano-domain [18]. Also the ferroelectric
behavior of Ba(Ti0.7Hf0.3)O3 depends on the competitions
between long-range ordering owing to strong correlation of the
off center Ti dipoles and the random fields induced by Hf
doping, thus the existence of these fields leads to the
destruction of long-range ferroelectric ordering and nano-
polar coherent domains get formed giving rise to the relaxor
behavior.
4. Conclusions
Based on the X-ray diffraction and dielectric studies of
Ba(Ti0.7Hf0.3)O3 ceramic it can be concluded that Hf
substitution in the place of Ti causes a tetragonal to cubic
transformation at room temperature. The occurrence of diffuse
phase-transition and strong frequency dispersion of maxima in
the permitivity versus temperature, strongly indicate the relaxor
behavior for this Ba(Ti0.7Hf0.3)O3 ceramic. The quantitative
characterization and comparison of the relaxor behavior based
on empirical parameters (DTm, g, gTdif andDTrelax) confirms its
relaxor behavior. The TEM observations suggest that the
observed relaxor behavior is due to the coherent strained nano-
regions, which form on Hf substitution on Ti site.
Acknowledgements
Authors would like to acknowledge Dr P. Chaddah, the
Director UGC-DAE CSR, Prof V.N. Bhoraskar, the Ex-Director
UGC-DAECSR and Prof AjayGupta, the Center Director UGC-
DAE CSR-Indore for their encouragement and interest in the
work. Authors sincerely acknowledge Prof L.E. Cross for
providing some basic references and Dr Archna Jaiswal, for
helping at various stages. Partial support from DST is also
sincerely acknowledged.
S. Anwar et al. / Solid State Communications 138 (2006) 331–336336
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