Dielectric properties of A- and B-site doped BaTiO3(II):La- and Ga-doped
solid solutions
D. D. Gulwade, S. M. Bobade, A. R. Kulkarni, and P. Gopalan
Citation: J. Appl. Phys. 97, 074106 (2005); doi: 10.1063/1.1879075
View online: http://dx.doi.org/10.1063/1.1879075
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JOURNAL OF APPLIED PHYSICS 97, 074106 共2005兲
Dielectric properties of A- and B-site doped BaTiO3„II… : La- and Ga-doped
solid solutions
D. D. Gulwade, S. M. Bobade, 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 codoping La and Ga on the A and B sites of BaTiO3, respectively,
resulting in a solid solution of the type Ba1−3xLa2xGa4xTi1−3xO3, have been investigated. The
compounds have been prepared by conventional solid-state reaction. The x-ray diffraction 共XRD兲
shows the presence of the tetragonal 共P4 / mmm兲 phase only. The XRD data have been analyzed
using the FULLPROF 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 Curie temperature as well as a significant enhancement in
the dielectric constant. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1879075兴
INTRODUCTION
Around 1912, Debye et al. postulated the existence of
permanent electric dipoles. Ferroelectricity was discovered
around 1920,1 in which the behavior of Rochelle salt as an
example was discussed. These materials were found to be
useful for numerous industrial applications. The earlier discovered materials, Rochelle salt and potassium dihydrogen
phosphate, bore complicated crystallographic structures. After the discovery of BaTiO3 in 1945, it was thought that a
simpler perovskite structure would provide an opportunity
for more insights into the origin of ferroelectricity. Thereafter, a number of theoretical models were developed to explain ferroelectric behavior in the last 50 years, and the application potential of these materials continually explored.
In the first paper of a series on A- and B-site-doped
BaTiO3, we have reviewed the existing literature on doped
BaTiO3 systems.2 A large number of these exhibit a relaxortype behavior. The features of a relaxor behavior are diffused
phase transition, deviation from Curie–Weiss 共CW兲 law,
spontaneous polarization over an extended temperature
range, and frequency dispersion. In such a material, the properties expected of a normal ferroelectric are not observed.
The relaxor behavior is usually explained due to the formation of a short-range-ordered phase within the matrix of a
high-temperature long-range-ordered phase.3,4 They also exhibit compositional inhomogeneity, which is different from
the classical compositional inhomogeneity consisting of
multiphase.5 The inhomogeneity leads to the formation of
polar nanodomains. Thereafter, the reduced interaction
among the polar nanodomains gives rise to a broad transition. In relaxors, the enthalpy of the transition from ferroelectric to paraelectric phase is diminished, leading to the
coexistence of both phases over a wide range of temperature.
Furthermore, inter- as well as intra granular strain caused by
doping is known to be the cause of peak broadening.6 In the
case of doped barium titanate systems, two or all transitions
shift towards each other, giving rise to a broad diffuse transition. The various explanations above for the relaxor behavior are not mutually exclusive. However, not all dopants
bring about the same kind of response. Some dopants shift
the transition temperature rather than affect the shape of the
dielectric spectrum. In the present article, second in the series, a preliminary study of codoping of BaTiO3 with small
amounts of La and Ga is reported. The resulting compositions exhibit a diffused transition as well as a deviation from
CW law. In the Ba1−3xLa2xTi1−3xGa4xO3 共x = 0.002, 0.004,
0.006, and 0.008兲 systems investigated in this work, increasing the dopant concentration results in an increased diffuseness as well as an increased deviation from the CW law. In
conventional relaxors, such an increase in diffuseness would
also imply an increase in frequency dispersion. The system
in this work is unique in the sense that while diffuseness
increases with dopant concentration, the frequency dispersion does not follow the known observed behavior.
EXPERIMENTAL WORK
Nominal
compositions
of
the
type
Ba1−3xLa2xTi1−3xGa4xO3 for x = 0.002, 0.004, 0.006, and
0.008 have been synthesized by conventional solid-state reaction. The starting materials BaCO3 共99.9%兲, TiO2 共99.9
a兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
0021-8979/2005/97共7兲/074106/5/$22.50
FIG. 1. 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.
97, 074106-1
© 2005 American Institute of Physics
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074106-2
Gulwade et al.
J. Appl. Phys. 97, 074106 共2005兲
FIG. 2. Plot of lattice parameters and tetragonality as a function of dopant
concentration 共x兲.
+ % 兲, La2O3 共99.99%兲, and Ga2O3 共99.99%兲 are mixed in
stoichiometric proportion and ground for around half an hour
in acetone. The ground powder is then calcined at 1200 ° C
for 12 h, followed by grinding. X-ray diffraction 共XRD兲 of
the calcined powder is then recorded. Thereafter, the powder
is pelletized and sintered at 1300 ° C for 12 h. The lattice
parameters are extracted by indexing the XRD data in the
tetragonal space group P4 / mmm, using the least-square refinement software FULLPROF.7 The dielectric measurements
of samples have been recorded in the temperature range
40– 200 ° C, and conductivity has been measured between
300 and 500 ° C using a HP 4192A analyzer.
RESULTS AND DISCUSSION
The XRD patterns for all the compositions in Figs.
1共a兲–1共d兲 exhibit a single phase. The least-square fit of the
FIG. 4. Dielectric constant as a function of temperature for 0.006 共a兲 and
0.008 共b兲.
pattern assigned using the P4 / mmm symmetry in FULLPROF
yielded the lattice parameters. The extracted c and a values
as a function of dopant concentration have been plotted in
Fig. 2. It can be observed that the c / a ratio decreases with
the increase in dopant concentration. The dielectric constant
as a function of temperature at five different frequencies is
plotted in Fig. 3 for x = 0.002 共a兲 and 0.004 共b兲 and in Fig. 4
for x = 0.006 共a兲 and 0.008 共b兲, respectively. In our limited
experimental range of frequency 共100 Hz– 1 MHz兲, a change
in transition temperature with frequency is not observed. The
dielectric loss at 1 MHz is plotted in Fig. 5. The dielectric
loss shows a maxima at a temperature slightly lower than the
transition temperature.8 The dielectric loss for all the compo-
FIG. 3. Dielectric constant as a function of temperature for 0.002 共a兲 and
0.004 共b兲.
FIG. 5. Dielectric loss at 1 MHz as a function of temperature.
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074106-3
J. Appl. Phys. 97, 074106 共2005兲
Gulwade et al.
FIG. 6. Dielectric constant at 10 kHz for all compositions as a function of
temperature.
FIG. 7. Plot of 1000/ ⑀ as a function of temperature at 1 MHz.
sitions is around 10−2; there is no consistent trend with the
level of doping.
The dielectric constant for all the compositions along
with that for pure BaTiO3 as a function of temperature at
10 kHz is shown in Fig. 6. The transition temperature decreases from 135 to 90 ° C in going from x = 0 共BaTiO3兲 to
x = 0.008 共see Table I兲, which is in good agreement with the
corresponding decrease in tetragonality. The value of the dielectric constant at 10 kHz at room temperature and Tc is
listed in Table I.
The CW law is not obeyed in the case of the doped
compositions; the plot between the 1000/ ␧ and the temperature deviates from linearity. The deviation from linearity for
x = 0.008 at 1-MHz frequency has been exhibited in Fig. 7.
The temperature where it begins to deviate from linearity
共Tdev兲 signals the onset of transition. The difference between
the Tdev and Tmax governs the ease of transition. 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. In Fig. 8, it can be observed that with the increased
level of doping, Tmax decreases, but ⌬T = Tdev − Tmax increases
with dopant concentration. The increase in ⌬T suggests that
the polar domains are formed and exist over an extended
range of temperature.9 The modified CW law provided below
关Eq. 共1兲兴 has been used for further analysis10
constant ␥ varies between 1 共normal ferroelectric兲 and 2 共relaxor兲. The constant ␥ represents the slope of the graph between log共1 / ␧ − 1 / ␧max兲 and log共T − Tmax兲 and is an indicator
of the diffuseness of the transition. A plot of ␥ versus composition at 1 MHz is exhibited in Fig. 9. The value of the
constant ␥ increases with the increase in dopant concentration, which is in good agreement with the change in ⌬T
共Tdev − Tmax兲 with the composition. In Fig. 10, the normalized
dielectric constant is plotted against the normalized temperature, which facilitates an analysis of all compositions simultaneously. This leads us to the same conclusion that the diffusivity increases from x = 0.002 to 0.008. Figure 11 exhibits
the frequency dispersion for all compositions, a plot between
␧ f / ␧1k and frequency 共f兲, at 90% of Tmax, where ␧ f is the
dielectric constant at frequency f and ␧1k is the dielectric
constant at 1 kHz. It is observed that with the increase in
doping, the graph becomes increasingly parallel to the frequency axis, which means that dispersion decreases with the
increase in doping. An exception is the x = 0.004 composition, which shows an enhancement in dispersion when compared to the x = 0.002 composition. The conductivity and activation energy values are tabulated in Table II; all the
samples are insulating. The activation energy is ⬃0.9 eV,
almost the same for all the compositions. This suggests that
the conductivity mechanism is probably the same for all the
compositions. The PE loops in Figs. 12共a兲 and 12共b兲, for x
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. The
TABLE I. Dielectric constant 共10 kHz兲 at room temperature and Tc.
x
Tmax
共°C兲
Dielectric
constant
共10 kHz兲
at Tc
0.002
0.004
0.006
0.008
120
110
100
90
7929
7525
4207
4700
Dielectric
constant
共10 kHz兲
at room temperature
2072
2229
1958
2411
FIG. 8. Plot of ⌬T 共Tdev − Tmax兲 and Tc as a function of x.
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074106-4
J. Appl. Phys. 97, 074106 共2005兲
Gulwade et al.
FIG. 9. Variation in ␥ as a function of x.
= 0.002 and 0.008, respectively, exhibit a ferroelectric-type
hysteresis.
At this stage, it would be appropriate to compare the
dielectric characteristics of the Ba1−3xLa2xTi1−3xAl4xO3 and
the Ba1−3xLa2xTi1−3xGa4xO3 systems. The dielectric characteristics that are observed in the two systems exhibit more or
less the same features. The dielectric behavior of these two
systems can be related by considering the c / a ratios, relative
ionic radii of the dopant cations, and observed diffuseness.
The dielectric characteristics that are observed in the two
systems exhibit more or less the same features. The only
difference that shows up is in the values of the dielectric
constants; the Ba1−3xLa2xTi1−3xGa4xO3 system exhibits higher
dielectric constant, which can be attributed to better size
compatibility between Ga and Ti 共relative to that between Al
and Ti兲, resulting in an easier movement of Ti. Other observations
include
a
greater
diffuseness
in
the
Ba1−3xLa2xTi1−3xAl4xO3
system
relative
to
Ba1−3xLa2xTi1−3xGa4xO3. The tolerance factor that describes
the stability of the structure is almost the same for both systems; the size difference between Al and Ga, however, does
indicate an increased preference for the cubic phase in the
case of the Ba1−3xLa2xTi1−3xGa4xO3 system. However, the
observed c / a ratios for Ba1−3xLa2xTi1−3xGa4xO3 are always
higher, relative to Ba1−3xLa2xTi1−3xAl4xO3. It may be added
that the c / a ratios are a measure of the tetragonality of the
FIG. 10. Plot of normalized dielectric constant at 10 kHz as a function of
normalized temperature.
FIG. 11. Plot of normalized dielectric constant as a function of frequency 共at
0.9Tmax兲.
TABLE II. Conductivity and activation energy data.
x
Conductivity
共S/cm兲
Activation
energy 共eV兲
共±error兲
0.002
0.004
0.006
0.008
1.05⫻ 10−13
4.7⫻ 10−14
9.8⫻ 10−14
2.9⫻ 10−13
0.93共0.03兲
0.95共0.02兲
0.91共0.03兲
0.88共0.06兲
FIG. 12. P vs E loop for x = 0.002 共a兲 and 0.008 共b兲.
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074106-5
J. Appl. Phys. 97, 074106 共2005兲
Gulwade et al.
structure. The lower extent of the tetragonality in the
Ba1−3xLa2xTi1−3xAl4xO3 system can be viewed as amounting
to a lower enthalpy of transition and thereby a higher observed diffuseness for this system.
CONCLUSIONS
In the present investigations, the changes in the dielectric properties as a result of codoping extremely small
amounts of La and Ga in barium titanate are addressed. The
doping results in an increase in the ease of transition; the
transition persists over a wide temperature range, as does the
diffuseness.
For the x = 0.008 composition, a dielectric constant at a
room temperature of about 2400 ° C at 1 kHz with a dissipation ⬃10−2 and a resistivity in excess of 1011 ⍀ / cm has been
observed. The frequency dispersion does not increase with
an increase in diffuseness, which is the contrary to the observed behavior in well-reported systems.11,12 In the present
investigations, the x = 0.008 composition exhibits the least
frequency dispersion. The increase in diffuseness followed
by a decrease in frequency dispersion is rather strange. Further investigations are needed in an extended microwave region to obtain a better idea of the frequency dispersion. The
La and Ga codoped compositions meet the highly demanding
material characteristics, including a high dielectric constant
at room temperature, a small change in the dielectric constant in the vicinity of room temperature, a low-frequency
dispersion as well as a high resistivity. This appears to be the
BaTiO3-based compound exhibiting high diffuseness and a
high dielectric constant at such a low doping level.
ACKNOWLEDGMENT
The authors thank the ER and IPR Division, DRDO,
Government of India for the generous research support that
helped in the execution of this work.
J. Valasek, Phys. Rev. 17, 475 共1921兲.
S. M. Bobade, D. D. Gulwade, A. R. Kulkarni, and P. Gopalan, J. Appl.
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