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Dhaka Univ. J. Sci. 61(1): 125-129, 2013 (January)
* Corresponding Author: E-mail: hirubd@yahoo.com
Effect of Nd Substitution on the Microstructural and Dielectric Properties of
Polycrystalline Ca(Ti0.5Fe0.5)O3
M. R. Shah1*, M. R. Amin2 and A. K. M. Akther Hossain3
1 Department of Physics, Primeasia University, Banani, Dhaka-1213, Bangladesh
2Department of Theoretical Physics, University of Dhaka, Dhaka-1000, Bangladesh
3 Department of Physics, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh
Received on 05.06.2012. Accepted for Publication on 04. 11. 2012
Abstract
Neodymium substituted calcium iron titanate having the general formula Ca1-xNdx(Ti0.5Fe0.5)O3 were prepared by t he standard solid
state reaction technique at relatively higher temperature (1473 K). X-ray diffraction (XRD) and optical microscopy are used to carry
out the struct ural analysis and surface morphology, respectively. The XRD analysis confirms that all compositions are single phase
orthorhombic in structure. The lattice parameters and the average grain size are found to decrease but the density to increase with the
increase in Nd content. The dielectric constant (ε/), dielectric loss (tanδ) and ac conductivity (ac) are studied at room temperature as a
function of frequency and compositions. The room temperature ε/ is found to decrease with the increase in frequency and Nd content.
On the other hand, the tanδ and ac are observed to increase with the increase in frequency and decrease with the increase in Nd
content.
Keywords: Dielectric properties, Microstructure, Solid state reaction, X-ray diffraction.
I. Introduction
The complex perovskite with general formula (ABO3)
exhibiting high dielectric constant are widely used for
detectors, multi-layer ceramic capacitors, sensors, actuators,
power transmission devices, memory devices, high energy
storage devices, pyroelectric detectors and other electronic
devices1-2. Generally, perovskite materials containing Pb
compound such as PMN, PST, PLZT exhibit outstanding
physical properties, such as giant dielectric constants,
exceptionally large electrostrictive coefficients and electro-
optic constants which are considered potential properties for
use in modern electronic devices3-4. However, these Pb
based perovskite compounds are environment-unfriendly
because PbO is toxic. Researchers have now been
researching for Pb free perovskite materials whose physical
properties will be comparable with those of Pb containing
compounds5-6. Among these Pb free materials, CaTiO3
(CTO) based solid solutions are considered as environment-
friendly with similar performance as Pb based perovskite.
These materials are widely used due to their interesting
mechanical, chemical and ferroelectric properties. However
these properties of CTO may be improved by the
substitution of different types of dopants so that it can be
employed in wide range of devices, such as: multilayer
capacitors, thermistors, sensors, wireless communications.
The substitution can be done either at the Ca and/or Ti site
of CTO7-8. It has been reported that oxide materials that
exhibit simultaneous ferromagnetic and ferroelectric
characteristics are known as multiferroics have attracted
considerable attention of researcher9-10. However, it is a very
rare case in nature that both magnetic and electric
polarization coexist in one substance. Therefore,
multiferroics become the subject of intensive investigations
due to their potential applications in the emerging field of
spintronics, data-storage media, multiple-state memories etc.
As an ABO3-type ferroelectric oxide, CTO has become one
of the most promising systems for multiferroics research. Its
structural characteristics determine that 3d transition metal
Ti can be easily substituted to produce ferromagnetism.
Therefore, Fe is substituted for Ti because Fe has a strong
resemblance to the Ti ion in size and valence and in the
present study, 50% doping of Fe is done to study the
multiferroics properties of CTO. Doped with rare earth
oxide is also an effective method to vary the ferroelectric
and magnetic properties of materials. Several kinds of rare
earth oxide such as: La2O3, Y2O3, Sm2O3 have been used to
improve further dielectric properties of CTO ceramics8, 11–12.
However, the research on Nd2O3 doped CTO ceramics have
not been reported in the literature. The radius of Nd3+ is very
close to the radius of the A-sites (Ca2+), it is therefore
possible for Nd3+ to enter into the A-sites of CTO perovskite
and affect the properties of CTO ceramics. In view of this,
Nd2O3 is selected as the dopant of Ca1-xNdx(Ti0.5Fe0.5)O3
(CNTFO) (where x= 0.0, 0.1, 0.3 and 0.5) and the effects of
Nd2O3 substitution on the phase composition, microstructure
and dielectric properties of CNTFO were investigated.
I. Experimental Details
Synthesis of CNTFO ceramic obtained by solid state
reaction
Polycrystalline CNTFO compositions were prepared by the
standard solid state reaction technique. In this technique, the
powder of CaCO3 (99.9%), Nd2O3 (99.9%), TiO2 (99.9%)
M. R. Shah, M. R. Amin and A. K. M. Akther Hossain
126
and Fe2O3 (99.9%) were used as raw materials. These
powders were stoichiometrically mixed thoroughly by ball
milling in distilled water media for 10 h to obtain a
homogeneous powder. Afterward, the slurry was dried and
the mixed powders were calcined at 1223 K for 12 h in air.
The calcined powders were re-milled for 10 h and then
dried. Finally, the dried powders were grounded and then
disk-shaped pellets of diameter of 10 mm and thickness 1-2
mm were prepared from the fine powders under uniaxial
pressure. The pellets were sintered at 1473 K for 5 h. During
sintering, the heating and cooling rates were maintained at
0.16 K/s and 0.08 K/s, respectively.
Characterizations
The phase-purity and crystal structure of the compositions
were investigated by the XRD with CuKα radiation at room
temperature. The measured bulk density ‘ρB’ of the
compositions was determined using the expression: ρB = (W
ρ) / (W-W/), where W and W/ are the weights of the
compositions in air and water, respectively, and is the
density of water at room temperature. The theoretical
density ‘ρth’ was calculated using the expression: ρth =
{1034MA / (NAabc)} kg/m3, where NA is the Avogadro's
number, MA is the molecular weight, a, b and c are the
lattice parameters for the orthorhombic unit cell. The
porosity ‘P’ was calculated from the relation: P(%) =
{100(ρth – ρB) /ρth}.
The surface morphology of sintered polished pellet was
studied by a high resolution optical microscope (Olympus
DP-70) and the average grain size was determined by the
linear intercept technique13. In order to measure the
dielectric properties, gold electrodes were deposited on both
sides of the pellets and then a gold wire was attached on
each electrode with silver paste. The frequency dependence
of the capacitance and the dielectric loss were measured by
LCR meter at room temperature within the frequency range
10 Hz to 32 MHz. The dielectric constant was calculated
from the equation: ε/ = (Cd) / (ε0A), where C is the
capacitance (Farad), ε0 is the permittivity in free space, A is
the area (m2) of the electrode and d is the thickness (m) of
the pellet. The ac conductivity was calculated using the
dielectric data (obtained at room temperature) from the
relation: σac = ε/ ε0 ω tanδ, where ω is the angular frequency
and tanδ is the dielectric loss.
II. Results and Discussion
Crystal structure, lattice parameters and density
The XRD pattern for various CNTFO is shown in Fig.1. The
XRD pattern indicates that all compositions are single phase
perovskite and no second phases are detected. It implies that
Nd3+ cations have entered into crystalline lattice structure to
form a homogeneous solid solution14. The XRD patterns are
in good agreement with orthorhombic structure15. It is also
evident that the diffraction peaks of the compositions are
shifted to the higher angle with the increase in Nd content,
this is due to smaller ionic radius of Nd3+ (1.27 Å) than that
of Ca2+ (1.34 Å) which causes the decrease in interplaner
spacing between the lattice14.
20 30 40 50 60
Intensity (arb. unit)
2 (degree)
(210)
(240) / (042)
(301) / (222)
(040)
(221) / (122)
(220) / (022)
(200)/(121)/(002)
(111)
(101) / (020)
x=0.0
x=0.1
x=0.3
x=0.5
Fig. 1. X-ray diffraction pattern for various CNTFO sintered
at 1473 K.
The lattice parameters a, b and c for each compositions of
CNTFO plotted as a function of Nd content are shown in
Fig.2. It is observed from Fig.2 that the lattice parameters
decrease with the increase in Nd content. This is due to Nd3+
(1.27 Å) has a smaller ionic radius than Ca2+ (1.34 Å) and
the substitution of A site with an ion having smaller ionic
radii than the host ion will lead to the shrinkage of the lattice
parameters16.
The effect of Nd on the density and porosity of CNTFO is
shown in Fig.3. The ρB increases with the increase in Nd
content. On the other hand, P of the compositions has
opposite behavior. The increase of ρB with the increase in
Nd content may be attributed to the difference in atomic
weight and density of initial and substituted cations, [the
atomic weight and density of Nd (144.24 amu, 7.00103
kg/m3) > Ca (40.00 amu, 1.55103 kg/m3)]17. The ρth
increases with the increase in Nd content because the
molecular weight of each composition increases
significantly with the substitution of Nd for Ca. It is also
noticed from Fig. 3 that the ρth is larger in magnitude
compared to the corresponding ρB. This is due to the
existence of pores which are formed and developed during
the preparation of compositions or the sintering process17.
Effect of Nd substitution on the microstructural and dielectric properties of polycrystalline Ca(Ti0.5Fe0.5)O3
127
Microstructure
The optical micrographs of various CNTFO sintered at 1473
K are shown in Fig. 4. The substitution of Nd has significant
effect on the grain size of the compositions and the sintered
CNTFO compositions become dense with the increase in Nd
content. It is seen from the Fig. 4 that the average grain size
for all the compositions gradually decreases from 4.41 μm
to 0.56 μm with the increase in Nd content. This result
suggests that the incorporation of Nd2O3 can limit grain
growth in the CNTFO. This behavior is believed to be
caused by the grain-boundary pinning force of Nd2O3,
which depressed the migration of CNTFO grain boundaries
during the sintering process18. The decrease of grain size can
also be interpreted as the suppression of oxygen vacancies
due to charge compensation mechanism, which results in
slower oxygen ions motion and, consequently, lower grain
growth rate19.
0.0 0.2 0.4
5.35
5.40
5.45
5.50
7.4
7.5
7.6
7.7
c
a
b (Å)
a,c (Å)
Nd content, x
b
Fig. 2. Variation of lattice parameters with Nd content of
various CNTFO sintered at 1473 K.
Frequency dependence of dielectric properties
The frequency dependence of dielectric properties measured
at room temperature for all compositions is shown in the
Fig. 5. It is clear from the Fig. 5(a) that ε/ has larger value in
the lower frequency region and remains almost constant up
to a certain frequency and thereafter decreases rapidly with
the increase in frequency.
0.0 0.2 0.4
3
4
5
6
6
9
12
15
18
B
P (%)
(103 Kg/m3
Nd content, x
th
Fig. 3. Variation of density and porosity with Nd content of
various CNTFO sintered at 1473K.
Fig. 4. The optical micrographs of various CNTFO sintered at
1473 K.
The large values of ε/ at low frequencies in case of ionic
crystals are due to voids, dislocations and other defects.
However, in the case of ceramic materials (ferrite and
ferroelectric) the large values of ε/ have been attributed to
the effect of heterogeneity of the compositions like pores
and layered structures20. The large values of ε/ at lower
frequencies may also be attributed to the dipoles resulting
from changes in valence states of cations and space charge
polarization.
x=0.0
x=0.1
x=0.5
x=0.3
M. R. Shah, M. R. Amin and A. K. M. Akther Hossain
128
0
3
6
9
101103105107
0.00
0.02
0.04 x=0.0
x=0.1
x=0.3
x=0.5
(a)
Frequency (Hz)
tan
(104)
(b)
Fig. 5. Frequency dependence of (a) dielectric constant (ε/)
and (b) dielectric loss (tanδ) of various CNTFO sintered
at 1473 K.
At higher frequencies, the values of ε/ remain independent
of frequency, since the inability of electric dipoles to
reorient itself to the applied electric field. These frequency
independent values of ε/ are known as static values of the
dielectric constant and these static values in the present
compositions are observed beyond 13 kHz to 2.5 MHz
(depending upon the composition).
The dielectric loss arises when the polarization lags behind
the applied electric field and is caused by the impurities and
imperfections in the crystal lattice21. Fig. 5(b) shows the
variation of tanδ with frequency for various compositions.
The tanδ is minimum at lower frequency region and it
increases with the increase in frequency. Such behavior may
be explained on the ground that at lower frequency, the
dipoles which are formed due to impurities or
inhomogeneous structure are able to follow the frequency of
the applied electric field; so small energy is required by the
dipole to orient itself for polarization resulting in low loss of
energy. But with the increase in frequency, the dipoles lag
behind the frequency of the applied electric field and
therefore, more energy is required for dipole orientation.
So, the loss is high at higher frequency.
Composition dependence of dielectric properties
The composition dependence of ε/ and tanδ is shown in Fig.
6. It is noticed from Fig. 6 (a) that the values of ε/ decrease
with the increase in Nd content. This can be explained on
the basis of Maxwell–Wagner theory. According to this
theory the dielectric constant is directly proportional to the
grain size of the samples22. In the present study, it is found
that the grain size of the compositions decrease with the
increase in Nd content (Fig. 4). The decrease in grain size
leads to the decrease in polarizability of atoms in the
structure, which results in a decrease in ε/.
0
3
6
9
0.0 0.2 0.4
0.00
0.02
0.04
0.1 kHz
1 kHz
10 kHz
100 kHz
1000 kHz
(a)
Nd content, x
10 kHz
100 kHz
1000 kHz
10000 kHz
20000 kHz
tan
(104)
(b)
Fig. 6. Composition dependence of (a) dielectric constant
(ε/) and (b) dielectric loss (tanδ) of various CNTFO sintered
at 1473 K.
The tanδ is found to decrease with the increase in Nd
content as shown in Fig. 6 (b). This may be primarily
attributed to the decrease in dielectric constant with the
increase in Nd content as well as increase in frequency. In
general high permittivity materials acquire higher losses.
Careful observation of the Fig. 5 reveals that the loss is
minimum for x=0.5 and maximum for x=0.0 composition. It
is obvious as ε/ is the lowest for x=0.5 and the highest for
x=0.0 composition.
Frequency and Composition dependence of ac conduc-
tivity
Fig. 7 shows the variation of σac with Nd content at different
frequencies. This figure indicates that σac increases with the
increase in frequency of the applied electric field, as σac is
directly proportional to the frequency23. The increase of σac
with frequency may also be explained by the hopping of
charge carriers between Ti4+↔Ti3+ or Fe3+↔Fe2+ ions as
with the increase in frequency of the applied electric field
enhances the hoping of charge carriers resulting in an
increase in the conduction process24. It is also noticed from
Fig. 7 that the σac decrease with the increase in Nd contents.
This can be explained on the basis of average grain size as
the grain boundary areas are highly resistive in oxide
ceramics. The decreased grain size increases the grain
boundary areas or resistance, as in the case of smaller grain,
Effect of Nd substitution on the microstructural and dielectric properties of polycrystalline Ca(Ti0.5Fe0.5)O3
129
the number of grain boundary per unit thickness is higher
than that of large grain size ceramics. Hence the resistivity
of the compositions increases, which in turn decreases the
conductivity of the compositions with Nd content.
0.0 0.2 0.4
-8
-6
-4
-2
logac (-1.m-1)
0.1kHz
1 kHz
10 kHz
100 kHz
10000 kHz
Nd content, x
Fig. 7. Compositions dependence of ac conductivity (ac) of
various CNTFO sintered at 1473 K.
III. Conclusions
The polycrystalline CNTFO are successfully synthesized by
the standard solid state reaction technique. The XRD pattern
confirms that all the CNTFO compositions are crystallized
in a single phase orthorhombic structure. The lattice
parameters and average grain sizes are found to decrease,
but the density increases with the increase in Nd content.
The ε/ is high at lower frequency and remains fairly constant
up to a certain frequency and thereafter, it decreases with
the increase in frequency. In higher frequency, ε/ is
independent of frequency due to the inability of electric
dipoles to reorient itself to the frequency of the applied
electric field. The tan is minimum at lower frequency but
maximum at higher frequency. The tan decreases with
the increase in Nd content since the dielectric constant
decreases with the increase in Nd content. The ac is found
to increase with the increase in frequency.
Acknowledgement
The authors gratefully acknowledge CASR of Bangladesh
University of Engineering and Technology (BUET) for
financial support in this research.
………………..
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