Dielectric properties of A- and B-site-doped BaTiO3(I):La- and Al-doped
solid solutions
S. M. Bobade, D. D. Gulwade, A. R. Kulkarni, and P. Gopalan
Citation: J. Appl. Phys. 97, 074105 (2005); doi: 10.1063/1.1879074
View online: http://dx.doi.org/10.1063/1.1879074
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JOURNAL OF APPLIED PHYSICS 97, 074105 共2005兲
Dielectric properties of A- and B-site-doped BaTiO3„I… : La- and Al-doped
solid solutions
S. M. Bobade, D. D. Gulwade, A. R. Kulkarni, and P. Gopalana兲
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay,
Powai, Mumbai 400 076, India
共Received 6 July 2004; accepted 2 February 2005; published online 24 March 2005兲
Extremely small amounts of La and Al doping on the A and B site of BaTiO3, respectively, resulting
in a solid solution of the type Ba1–3xLa2xTi1–3xAl4xO3 have been investigated. The compositions
have been prepared by the Pechini process. The x-ray diffraction 共XRD兲 reveals the presence of
tetragonal 共P4 / mmm兲 phase. The XRD data has been analyzed using FULLPROF, a Rietveld
refinement package. The compositions have been characterized by dielectric spectroscopy between
room temperature and 200 ° C. The resulting compounds 共0 艋 x 艋 0.008兲 exhibit a remarkable
decrease in the Curie temperature as well as a significant enhancement of the dielectric constant.
© 2005 American Institute of Physics. 关DOI: 10.1063/1.1879074兴
I. INTRODUCTION
The high dielectric constant of BaTiO3 coupled with a
simpler crystal structure has attracted the attention of many
researchers. Exhaustive theoretical work has been carried out
to understand the phenomenon of ferroelectricity.1–6 While it
was thought that the origin of ferroelectricity in BaTiO3 may
be simple to understand, the goal remains elusive even after
50 years.7,8
A majority of substituted BaTiO3 compositions exhibit a
diffused phase transition, departure from Curie–Weiss 共CW兲
law, pronounced frequency dispersion, and the existence of
spontaneous polarization well above and below the transition
temperature, namely, a relaxor-type behavior. The observed
departure from the CW law in such cases is governed by the
modified CW law9 关Eq. 共1兲兴
1
共T − Tmax兲␥
1
−
.
=
␧ ␧max
C
共1兲
In Eq. 共1兲, C and ␥ are constants, and ␧max is the maximum dielectric constant at the transition temperature Tmax. In
doped systems, unlike a well-defined and sharp TC for the
case of pure BaTiO3, the transition is a diffuse one as it
happens over a range of temperature. The temperature Tmax is
therefore defined as the temperature where the dielectric
peak occurs. The value of ␥ varies between 1 and 2. In a
limiting case for a normal ferroelectric material, ␥ = 1 and the
system behaves in accordance with the CW law. The constants ␥ and C are interdependent and change in accordance
with each other in a reasonable manner, so as to keep the
ratio 共T − Tmax兲␥ / C constant. Though the two constants govern the diffuseness of transition, they do not provide any
insight into the phenomenology of transition.
Among the substituted BaTiO3, the most interesting and
one with potential application is the strontium substituted
family of compounds. Barium and strontium titanate exhibit
isomorphism and form a solid solution over the entire coma兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
0021-8979/2005/97共7兲/074105/7/$22.50
position range, and this has eventually led to the development of barium strontium titanate 共BST兲. The transition temperature, diffuseness, and dielectric constant in these solid
solutions vary with doping.10 The tetragonality of the structure as well as the transition temperature reduces with
doping.11 The diffusive nature of the transition is attributed
to inter- and intragranular strain. Further it is known that
compositional inhomogeneity may cause broadening of
transition.12 The doping of Sr decreases both the Curie 共TC兲
transition and the orthorhombic to tetragonal 共To–t兲 transition
temperature, whereas the rhombohedral to orthorhombic
共Tr–o兲 transition is unaffected.13 This is usually explained on
the basis of cation size effect; the smaller Sr stabilizes the
cubic phase at lower temperature. In BST, with increasing
mol % Sr, TC decreases at a faster rate than To–t. In comparison, while TC increases with Pb doping in BaTiO3, both To–t
and Tr–o decrease. A somewhat different scenario emerges
for Ca doping in BaTiO3; TC remains unaffected up to
10 mol %, whereas two other transition temperatures 共To–t
and Tr–o兲 decrease dramatically.13 It appears that there is no
convincing explanation for the effect of a specific ion on the
three transitions in BaTiO3.14 There have been efforts towards the addition of MgO and MnO2 for grain growth inhibition and for trapping electrons, respectively, to improve
and optimize the overall performance of BST.15 In fact, much
of the focus on BST has been towards growing thin films16–20
and their characterization. This is largely due to the scope for
using this material in memory-related applications.
For Ba1−xLaxTi1−x/4O3 共BLT兲, the transition temperature
decreases with increasing level of doping, in good agreement
with the decrease in the tetragonal distortion.14 The Ti vacancy model proposed to account for electroneutrality is supported by electron probe microanalysis14 共EPMA兲 and
neutron-diffraction Rietveld analysis.21 The lower La m / o
containing compositions fired in ambient atmosphere exhibit
higher conductivity, which is attributed to oxygen loss.22 At
higher levels of doping, the compositions exhibit an observable deviation from the CW law. The BLT system is interesting in that it exhibits a pinching effect, i.e., both transitions
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074105-2
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Bobade et al.
tetragonal–cubic 共Tt–c兲 and orthorhombic–tetragonal 共To–t兲
move towards each other. The former transition shifts towards lower temperature, while the latter shifts towards
higher temperature. Eventually, a stage is reached when both
transitions overlap and a broad transition is observed for
around 8 m / o La.22 A slightly different composition
Ba0.997La0.003Ti1.0045O3, in the Ti-rich region, has also been
investigated; being Ti excess, the possibility of Ti vacancies
is excluded, and it has been suggested that the formation of
Ba vacancies accounts for the observed higher resistivity.23
The doping of Bi on Ba site results in the Curie temperature shifting towards room temperature, as also an increasing
diffusivity corresponding to level of doping with a concomitant departure from the CW law.24
Interestingly, the doping of Ce in BaTiO3 could potentially result in a substitution on either site; Ce exists in 3+
and 4+ states and can substitute for Ba2+ or Ti4+ site depending upon the processing conditions,25,26 which in turn govern
the defect mechanism. Since cerium is quite likely to be
accommodated on either site, the interpretation becomes
complicated. The incorporation of Ce on a Ba site decreases
TC at the rate of 21 ° C / mol.27 A Ti vacancy model has been
proposed to account for these observations.28 Likewise, it has
also been proposed that Ce incorporated on a Ti site decreases tetragonality with increase in doping, a relaxor behavior results,29–31 due to the pinching effect, noted earlier.
The pinching occurs at 6 m / o Ce, resulting in a rounded
diffused peak.29 The transitions Tr–o, To–t, and TC move at the
rate of 24, 11, and −7 ° C / mol, respectively. Similar behavior
has been observed in the case of Zr-doped compositions; the
three transitions approach each other resulting in a broad
transition,13,32 at around 20 m / o Zr. As before, the chemical
inhomogeneity and internal stresses are thought to result in a
diffused phase transition.
The doping of yttrium on the Ti site decreases the transition temperature, the diffusivity of transition increases with
the level of doping, and the dielectric constant decreases at
TC.33 The transition temperature decreases at the rate of
27 ° C / mol, a bit higher than those observed for Zr or Ce
doping.34 A relaxor-type behavior with a diffuse transition
and a departure from the CW law is observed. The doping of
Y might create oxygen-ion vacancies for charge balance. It is
possible that locally distorted electrical field and strain field
are the cause for the observed behavior.
The 共1 − x兲BaTiO3 – x KNbO3 system has been studied
for 0 ⬍ x ⬍ 0.20, and it is established that a solid solution
exists over this range.35 Low doping levels 共x ⬃ 0.04兲 exhibit
no frequency dispersion, and thereafter the dependence increases with further doping, accompanied with a departure
from the CW law. The behavior is believed to be a result of
a compositional deviation sufficient enough from BaTiO3, so
as to create local disorder leading to nanoscale inhomogeneity in the sample.
In the 共1 − x兲BaTiO3 – x LaAlO3 system, the solubility
limit of LaAlO3 in BaTiO3 is known to be up to 5 m / o.36 It
has been commented that the charge balance is maintained
by Ti vacancies on the basis of wavelength dispersion spectroscopy results.37,38 Beyond 5 m / o LaAlO3, in addition to
the partial solid solution incorporating La and Al on Ba and
Ti sites, respectively, two parasitic phases are observed. It
has also been possible to suppress the formation of the parasitic phases using a synthesis involving excess Ti. The incorporation of La and Al in BaTiO3 decreases tetragonality and
the dielectric constant, the transition temperature shifts at the
rate of 35 ° C / mol, and the material exhibits low permittivity. In the case of the BaTiO3 – LaAlO3 – LaTi3/4O3, i.e., Ti
excess systems, there is an improvement in the dielectric
constant.
Following the phase diagram of BaO– TiO2 – Nb2O5,
compositions of Nb close to that of stoichiometric BaTiO3
have been investigated.39 In this case A / B-site ratio varies
from 0.76 to 1.01. It has further been commented that nonstoichiometry plays some role in dielectric behavior but the
exact cause has not been predicted.
The above discussion indicates that the compositional
inhomogeneity, which is quite different from the classical
inhomogeneity of multiphase systems, is responsible for the
diffused phase transition. The heterogeneity is believed to
arise due to a disorder of the A- or B-site cations, or both. It
is also believed to arise on account of the disorder in the tilt
of oxygen octahedra. The compositional inhomogeneity and
formation of the nanosize domains give rise to the diffused
phase transition, and the dynamics of cluster governs the
behavior.
There is very little work that has been carried out on the
codoping on A and B sites that correlates the dielectric properties to the structural attributes. The BaTiO3 – LaAlO3 system, investigated by Skapin et al.,37 appears to result in a
lower dielectric constant, with formation of parasitic phases.
The suppression of the parasitic phases by the addition of Ti
excess enhances the dielectric properties. The roomtemperature dielectric constant is reported to be below 800.
In this work, we have attempted extremely small amounts of
simultaneous doping on the A and B sites, using La and Al,
respectively. However, unlike Skapin et al., we have chosen
the dopant concentrations on both sites to stay within the
solubility limit. The system investigated in this work is
Ba1–3xLa2xTi1–3xAl4xO3. We have attempted to maintain the
charge and site balance so as to avoid the formation of vacancies on either of the Ba or Ti sites. In the present study, a
preliminary study of simultaneous doping of BaTiO3 by La
and Al is reported, which shows very interesting behavior. It
is known that as long as the 关A兴 / 关B兴 ratio stays between 0.98
and 0.99, the perovskite structure remains stable. All the
compositions studied in this work fall within this tolerance
limit. We report here on the synthesis, as well as the structure
and dielectric property correlations.
II. EXPERIMENTAL WORK
Nominal
compositions
corresponding
to
Ba1–3xLa2xTi1–3xAl4xO3, for x = 0.002, 0.004, 0.006, and
0.008, were investigated. The compounds were synthesized
by the Pechini process;40 a flowchart of steps involved is
provided
in
Fig.
1.
The
starting
materials,
Ba共NO3兲2共99.99%兲, TiCl4共99.9+ %兲, La2O3共99.99%兲, and
Al共NO3兲3共99.99%兲, procured from Aldrich Chemicals 共USA兲
were used. Barium nitrate and aluminum nitrate were dis-
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074105-3
Bobade et al.
J. Appl. Phys. 97, 074105 共2005兲
FIG. 2. XRD pattern for x = 0.002 共a兲, 0.004 共b兲, 0.006 共c兲, and 0.008 共d兲.
FIG. 1. Flowchart of synthesis of Ba1–3xLa2xTi1–3xAl4xO3 compositions by
the Pechini process. Abbreviations: CA-citric acid, EG-ethylene glycol, and
DI-de-ionized.
solved in de-ionized water in separate containers. Lanthanum
oxide was dissolved in nitric acid. Titanium hydroxide was
precipitated by addition of NH4OH to TiCl4 solution that had
been formed by dissolution in dilute HCl. Titanium hydroxide thus obtained was dissolved in nitric acid and the resulting solution was mixed with all the other nitrate solutions.
The resulting solution was stirred well using a magnetic stirrer at room temperature. The pH of solution was modified by
adding an appropriate amount of ammonium hydroxide and
nitric acid; the pH of final solution was maintained at 8.
Citric acid 共CA兲 and ethylene glycol 共EG兲 in the 2:4 proportions were added as chelating agents while continuing with
stirring. The resulting solution was left at 100 ° C overnight
for gelation to complete. Thus obtained black resin was fired
at 400 ° C. The resin was then ground in an agate mortar and
thereafter calcined at 1000 ° C. The powder was characterized by x-ray diffraction. The lattice parameters were extracted by indexing the data in tetragonal space group
共P4 / mmm兲, using the FULLPROF least square refinement
software.41
The powder was pelletized using a 10-mm-diameter
stainless-steel die and sintered at 1200 ° C for 6 h. The dielectric measurements of the samples were recorded at different frequencies and in the temperature range between 40
and 200 ° C using a HP impedance analyzer 共4192A兲 and a
Eurotherm temperature controller.
that all the compositions appear to exhibit a single phase.
The XRD could be indexed in the tetragonal space-group
P4 / mmm. The lattice parameters have been extracted from
the refined XRD pattern. The lattice parameters as well as
the degree of tetragonality 共c / a兲 are shown in Fig. 3. It can
be clearly concluded that the extent of tetragonality decreases with increasing dopant concentration. The crystallite
size of the calcined samples determined by x-ray line broadening was observed to be decreasing with an increase in the
dopant concentration till 0.006, and thereafter staying constant. The crystallite size varied between 49 and 60 nm for
the various compositions studied in this work. We also carried out x-ray profile matching and Rietveld analysis, but it
did not result in any additional information, due to the low
dopant concentration encountered in our studies. Further, the
Rietveld analysis clearly ruled out the possibility of a second
phase.
The dielectric behavior as a function of temperature for a
number of different frequencies is plotted in Figs. 4 and 5.
The change in the transition temperature with frequency is
not observed even for higher dopant concentration within the
experimental span of frequency. The steep increase in the
dielectric constant at low frequency is ascribed to space polarization. In particular, the effect of space polarization is
prominently observed for the x = 0.004 composition. The values of the dielectric constant at room temperature and TC for
III. RESULTS AND DISCUSSION
The x-ray diffraction 共XRD兲 pattern for all the compositions between x = 0.002 and 0.008 in Figs. 2共a兲–2共d兲 shows
FIG. 3. Variation in the lattice parameters a and c and tetragonality 共c / a兲 as
a function of dopant concentration.
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074105-4
J. Appl. Phys. 97, 074105 共2005兲
Bobade et al.
FIG. 4. Dielectric constant as a function of temperature for x = 0.002 共a兲 and
0.004 共b兲.
FIG. 5. Dielectric constant as a function of temperature for x = 0.006 共a兲 and
0.008 共b兲.
a fixed frequency of 10 KHz have been listed in Table I. A
plot of the dielectric constant as a function of temperature for
pure BaTiO3 and different dopant concentrations at a fixed
frequency of 10 KHz is shown in Fig. 6. Between pure
BaTiO3 and x = 0.008, the Curie temperature lies in the range
between 135 ° C and 85 ° C, respectively, and the fall in TC
for increasing values of x is in good agreement with a corresponding decrease in tetragonality.
At TC, we believe, there is only a marginal difference
between the dielectric constant between the x = 0.004 and the
x = 0.006 compositions 共2400 and 2700, respectively兲. It is
well known that the grain-size distribution affects the dielectric constant. The slight variation observed in the dielectric
constant could probably be explained by the difference in the
grain-size distribution. We have measured dielectric properties on samples having more than 93% density. Due to our
inability to record good-quality scanning electron microscopy 共SEM兲 micrographs, we have not included the data on
grain size as function of concentration. However, in an independent study made by Roy42 the SEM data, in line with
x-ray broadening observations, indicated a decrease in the
grain size from 0.84 to 0.5 ␮ as the dopant concentration
increased from 0.004 to 0.008. The decrease in the grain size
results in a more diffused transition. Below TC, the dielectric
loss decreases with increasing dopant concentration, as can
be observed in Fig. 7. For the compositions investigated in
this work, the CW law is not obeyed; the plot between
1000/ ␧ and temperature deviates from linearity. The devia-
tion from linearity at 1 MHz for a representative composition, x = 0.004, has been exhibited in Fig. 8. The deviations
from linearity set in at a temperature that is composition
dependent. The difference between the temperature where
deviation sets in 共Tdev兲 and the transition temperature 共TC or
Tmax兲 governs the ease of the Curie transition. With increasing levels of doping the difference ⌬T = Tdev − Tmax increases,
suggesting the occurrence of polar nanodomains, as discussed earlier.43 A plot of ⌬T as a function of the dopant
concentration x is shown in Fig. 9, which suggests an increase in diffuseness. The dielectric data have been fitted to a
modified CW law 关eq. 共1兲兴. A plot of log共1 / ␧ – 1 / ␧max兲 as a
function of log共T − Tmax兲 at 1 MHz at temperatures greater
than TC is shown in Fig. 10 and the constant ␥ extracted from
the slope is plotted in Fig. 11. The constant ␥ that governs
the diffuseness of transition increases with the level of doping and attains a value of 1.8 for the x = 0.008 composition.
The normalized dielectric constant has been plotted against
TABLE I. Dielectric constant 共10 KHz兲 values at room temperature and at
T C.
Composition,
x
0.002
0.004
0.006
0.008
Dielectric constant 共10 KHz兲
at 40 ° C
at TC
1397
1589
2128
938
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3377
2383
2729
1176
074105-5
Bobade et al.
FIG. 6. Dielectric constant at 10 KHz for all compositions as a function of
temperature.
normalized temperature in Fig. 12, which facilitates an
analysis of all compositions simultaneously. The shape of the
maxima in Fig. 12 leads us to the same conclusion that the
diffuseness increases in going from x = 0.002 to 0.008. The
frequency dispersion decreases with an increase in the level
of doping, as depicted in Fig. 13共a兲, a plot between ␧ f / ␧1 k
and frequency 共f兲, at 90% of Tmax. On the other hand, Fig.
13共b兲 exhibits the frequency dispersion over the entire temperature range. It can be observed in Figs. 13共a兲 and 13共b兲
J. Appl. Phys. 97, 074105 共2005兲
FIG. 9. Variation of ⌬T共Tdev − Tmax兲 and TC as a function of dopant
concentration.
that the dispersion decreases as the dopant concentration increases. The P-E loop has been traced for applied voltages
between 100 and 600 V and they are exhibited in Figs. 14共a兲
and 14共b兲 for x = 0.002 and 0.006, respectively. Further attempts to reach saturation or to track for breakdown field
have not been investigated. In the present study, we have
only focused on investigating for existence of the ferroelectric loop. The loss decreases with an increase in the level of
doping, as evident in Fig. 6 as well.
The defect reaction proposed in the BaTiO3 – LaAlO3
systems is the formation of vacancies at B site. As we have
FIG. 7. Variation of dielectric loss with temperature at 1 MHz.
FIG. 10. Plot of log共T − Tmax兲 vs log共1 / ␧-1 / ␧max兲 for all compositions.
FIG. 8. Plot of 1000/ ␧ as a function of temperature at 1 MHz.
FIG. 11. Variation in ␥ as a function of dopant concentration.
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074105-6
J. Appl. Phys. 97, 074105 共2005兲
Bobade et al.
FIG. 12. Plot of normalized dielectric constant vs normalized temperature at
10 KHz.
关Ti4+ + Ti3+兴 excess in our starting compositions, the possibility of the formation of Ti vacancies can be ruled out and the
formation of Ba vacancies is thus favored. It has been suggested that the p-type behavior would not be observed if Ba
vacancies act as deep traps for holes.44 On the contrary,
Lewis and Catlow suggest that the Ba vacancy and holes are
weakly bounded,45 and hence the resistivity will be lower.
There are different views regarding the defect mechanism for
doped and pure barium titanate. The decrease in dielectric
loss, with increasing concentration, and a higher resistivity in
excess of 1012 ⍀ cm at room temperature are suggestive of
Ba vacancies and trapped holes, in agreement with Daniels et
FIG. 14. P-E loop for x = 0.002 共a兲 and 0.008 共b兲.
al.44 The formula accounting the Ba vacancies could be written as Ba1–3xLa2x共VBa
⬙ 兲xTi1–3xAl4xO3. It would be unfair on
our part to comment on the defect mechanism in this system,
as it would demand more investigation.
IV. CONCLUSIONS
FIG. 13. Plot of normalized dielectric constant as a function of frequency 共at
0.9 Tmax兲.
The compositions synthesized by the Pechini process are
single phase. A dielectric constant of about 2500 at 1 KHz,
dissipation factor ⬃10−2, and resistivity⬎ 1011is achieved for
x = 0.006. Furthermore, the change in the dielectric constant
between room temperature and 100 ° C is ⬃20%.
The diffuseness increases with an increase in the level of
doping which is evident from a corresponding increase in a
deviation from the CW law. This is suggestive of the existence of polar nanodomains over a wide range of temperature, which is responsible for breaking long-range ordering
and giving rise to a diffused phase transition. The increase in
diffusivity is not followed by an increase in frequency dispersion, the reason being that there exists enough interaction
among polar nanodomains so as to suppress frequency dispersion. The increase in diffusivity followed by a decrease in
frequency dispersion is contradictory to usually observed
trends,46,47 which does not agree with our observations.
In the present study, we have thrown some light on the
variation in diffusivity and frequency dispersion with dopant
concentration. More vigorous study and theoretical modeling
are needed to understand the system in-depth. These are
presently underway and will be reported at a later date.
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074105-7
The system investigated in this work meets the requisite
material characteristics relating to a high dielectric constant
at room temperature, a low-temperature coefficient of dielectric constant in the vicinity of room temperature, lowfrequency dispersion, and a high resistivity. We have also
embarked on a program to grow thin films of some of these
compositions.
ACKNOWLEDGMENT
The authors thank the ER&IPR Division, DRDO, Government of India for the generous research support that
helped in the execution of this work.
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