Correlation between the dielectric properties and local electronic structure of copper doped calcium titanate

Article abstract of DOI:10.1016/j.jallcom.2013.03.070

Copper-doped calcium titanate (Cu_xCa_1-xTiO ₃, x = 0.0, 0.01, 0.02 and 0.03) ceramic system was synthesized using solid state reaction method. X-ray diffraction and Raman spectroscopic studies reveal the presence of Pbnm sp

Full text of DOI:10.1016/j.jallcom.2013.03.070

Journal of Alloys and Compounds 572 (2013) 84–89
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Correlation between the dielectric properties and local electronic
structure of copper doped calcium titanate
b
c
a
d
Jitendra Pal Singh a, , Sanjeev Gautam , Parmod Kumar , Ambuj Tripathi , Jin-Ming Chen ,
⇑
Keun Hwa Chae b, K. Asokan a
a Materials Science Division, Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India
b Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
c Department of Physics, Indian Institute of Technology Delhi, New Delhi 110 016, India
d National Synchrotron Radiation Research Centre, Hsinchu 30076, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Copper-doped calcium titanate (CuxCa1ꢀxTiO3, x = 0.0, 0.01, 0.02 and 0.03) ceramic system was synthe-
sized using solid state reaction method. X-ray diffraction and Raman spectroscopic studies reveal the
presence of Pbnm space group in the samples. The particle size estimated from scanning electron micro-
Received 24 November 2012
Received in revised form 3 March 2013
Accepted 5 March 2013
graph changes from 235 nm to 6.5 lm with copper doping, however crystallite size remain almost con-
Available online 21 March 2013
stant within experimental error. The dielectric study performed on these samples indicates an increase in
the value of dielectric constant with Cu doping. The drastic change in the value of dielectric constant of
the samples has been attributed to the change in microstructure. Local electronic structure deduced from
X-ray absorption near edge structure reveals no distortion at TiO6 octahedra and reduction in O(2p) and
Ca(4sp) hybridized states with increase of Cu2+ doping.
Keywords:
Ceramic
Solid state reactions
Dielectric constant
X-ray absorption near edge structure
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
In temperature range between 1380 and 1500 K phase transition
occurs and changes the space group for Cmcm. At 1500 K, the
Ferroelectric crystals of ABO3 type perovskite structure have
important applications in developing electrical and magnetic
components, capacitors, infrared sensors, etc. Therefore, attempts
have been made in recent years to synthesize materials with a
dielectric constant over a broad range of temperature and fre-
quency [1–3]. Titanates as members of the perovskite structure
family are well known ferroelectric materials with superior
dielectric properties. Among all the titantes, calcium titanate
(CaTiO3) was discovered in 1839 and now has become a strong
candidate for the interest of scientific community due to its inter-
esting dielectric properties and recently observed luminescence
properties [4,5]. This material is not only used for dielectric appli-
cations but it is also an essential component of synthetic rock
used for storing nuclear waste [6,7] and for various catalytic
applications like partial oxidation of light hydrocarbons [8]. Gen-
erally, CaTiO3 exists in two modifications; orthorhombic and cu-
bic depending on the method of preparation. It exhibits an
orthorhombic structure with space group Pbnm below 1380 K.
orthorhombic structure transforms to ‘‘tetragonal’’ structure with
space group I4/mcm. Above 1580 K, this material exhibits a cubic
structure with space group Pm3m [9].
Similar to other ceramics, properties of this material largely de-
pend on the method of preparation, thermal treatments and dop-
ing with other additive oxides. The doping of Cu to Ba- or Sr-
titanate can greatly enhance dielectric constant through the forma-
tion of barrier layer depending upon the temperature and time of
sintering [10,11]. Effect of copper doping on the structural and
dielectric properties of calcium titanate was investigated by a pre-
vious worker [8] and interesting results for dielectric behavior
were obtained. In another report, it has been shown that doped
ions strongly affect local electronic structure and lattice dynamics
of this system, thereby changing the properties of materials but
authors did not provide any dielectric studies to correlate the ob-
served results [9–12]. Therefore, the investigation of local structure
along with dielectric study to establish a correlation among these
properties is indispensible. X-ray absorption near-edge structure
(XANES) spectroscopy has been recognized as a nondestructive
tool to investigate the local electronic structure of oxide materials
like ferrites and titnates [13–25]. Hence, present work combines
the dielectric properties and XANES investigation to establish a
correlation among these properties.
⇑
Corresponding author. Present Address: Department of Applied Science, Krishna
Engineering College, Ghaziabad, Uttar Pradesh 201 007, India. Tel.: +91 120
2657731/32; fax: +91 120 2659513.
E-mail address: jitendra_singh2029@rediffmail.com (J.P. Singh).
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.03.070

J.P. Singh et al. / Journal of Alloys and Compounds 572 (2013) 84–89
85
2. Experimental details
The ceramic materials CuxCa1ꢀxTiO3 were synthesized through conventional so-
lid state reaction method. The starting materials were CuO, TiO2 and CaCO3. These
materials were manually mixed for 60 min in the stoichiometric proportion fol-
lowed by wet mixing with acetone for 60 min. The mixture was then calcined at
1000 °C for 10 h. The lumps were thoroughly mixed in fine powdered form for
60 min and then wet mixed with ethyl alcohol for 60 min. The process of calcina-
tions and mixing was repeated three times. Thus obtained powder was pressed in
pellet form and then sintered at 1200 °C for 10 h [26]. The sintered pellets were
characterized by X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractom-
eter. Morphological studies on these samples were carried out by scanning electron
microscope (SEM). The dielectric measurements for various compositions as a func-
tion of frequency at different temperature ranging from 120 to 350 K were recorded
by HP4284A LCR meter attached with a lakeshore 340 temperature controller. The
XANES spectra of these materials were obtained using a high-energy spherical grat-
ing monochromator-HSGM BL20A1 at the National Synchrotron Radiation Center in
Hsinchu, Taiwan. All measurements were processed in an ultra high vacuum cham-
ber (10ꢀ10 Torr) at 300 K in total electron yield (TEY) mode. The incoming radiation
flux (I0) was monitored by the total photocurrent produced in a highly transmissive
Au-mesh inserted into the beam. The overall experimental resolution around O K-
edge was ꢁ0.02 eV. After a constant background subtraction, all spectra were nor-
malized to the post-edge step height using Athena 0.0.061 [27].
Fig. 2. Raman spectra of the copper doped calcium titanate ceramic (" indicates the
bands appearing at 537 cmꢀ1 and 671 cmꢀ1). x is the concentration of Cu2+ ions.
3. Results and discussion
3.1. Structural and morphological study
Presence of bands ꢁ155, 180, 226, 247, 286, 337, 471, 495, 537 and
677 cmꢀ1 in Raman spectra has been attributed to the presence of
Pbnm phase of CT and is in correlation with the results obtained
from XRD. According to Hirata et al. [33], the band ꢁ155 cmꢀ1 is re-
lated to the CaTiO3 lattice mode, however presence of bands at
ꢁ180, 226, 247, 286 and 337 cmꢀ1 are ascribed to the O–Ti–O bend-
ing vibration modes or caused by the tilting phenomenon between
the [TiO6]–[TiO6] clusters [34]. The bands around 471 and 495 cmꢀ1
are due to presence of torsional modes while the band ꢁ677 cmꢀ1
has been assigned to the Ti–O symmetric stretching vibration mode
[35]. Besides these, bands in the range ꢁ639–644 cmꢀ1 appear
depending on the method of preparation for CT with Pbnm space
group. Band at ꢁ537 cmꢀ1 in spectrum of CT system has been as-
cribed to the presence torsional mode. A detailed investigation on
the origin of these different bands in CT has been published else-
where and it has been shown that presence and absence of bands
at position ꢁ537, 642 and 677 cmꢀ1 depends on the method of
preparation [36]. It may be contemplated that microstructure
changes induced by method of preparation are responsible for pres-
ence/absence of these bands even having the Pbnm space group in
the sample. Hence, by analogy changes in Raman spectra with
Cu2+ doping has been attributed to the change in microstructure
with Cu2+ doping. This aspect has also been investigated by using
SEM. Recorded SEM micrographs show drastic change in morphol-
ogy and microstructure with copper concentration (Fig. 3). Pure cal-
cium titanate sample exhibit small crystals of size ꢁ235 nm with
XRD patterns of the CuxCa1ꢀxTiO3 samples indicate presence of
orthorhombically distorted perovskite structure with space group,
Pbnm, JCPDS Card No. 86-1393 (Fig. 1). The extra peaks between
25° and 30° in Cu doped CaTiO3 have been identified to originate
from triclinic phase of CaTiO3 [JCPDS No. 39-0145] reported by
Xiong et al. [28]. Double peak features below 60° in the Cu-doped
CT observed in present case is similar to the previous report of
doped CaTiO3 [29,30] and has been attributed to asymmetrical
change in lattice parameter. Raman spectra of these samples have
also been recorded and shown in Fig. 2. Observed Raman spectra of
these samples are very much similar to the spectrum of CaTiO3
(CT) reported by previous authors [31,32]. The observed bands
are more sharpen with Cu concentration. The major changes in
spectra occur in the spectral region of 400–800 cmꢀ1, where bands
495 and 627 cmꢀ1 do not appear in the spectrum of pure CT. The
spectrum of pure CT exhibit bands around 537 and 677 cmꢀ1
According to factor group analyses following modes were pre-
dicted at the Brillouin zone.
.
C
¼ 7Ag þ 5B1g þ 7B2g þ 5B3g
almost spherical shape. The sizes of particles are 6.5 lm for Cu
doped samples with the change in shape of particles after Cu2+ dop-
ing. In SEM micrographs some small particles are also observed
along with the presence of bulk particles, whose concentration in-
creases with copper doping. The behavior of these systems is very
much similar to the behavior of MgxZn1ꢀxO system, where presence
of small crystallite has been attributed to the presence of impure
phase of MgO [37]. Hence, presence of small particles with not well
defined boundary has been attributed to presence of triclinic phase
along with distorted orthorhombic phase of CaTiO3 in Cu2+ doped
samples. Further, the average crystallite size (D) of these materials
was estimated from the most intense peak of XRD pattern by Scher-
rer’s formula [38]. Average crystallite size of the CT is 37 2 nm and
this value remains almost constant within experimental error for all
copper concentration. Crystallite size of these materials is far differ-
ent from the size estimated from SEM. It has been reported that
XRD measures crystallite size, however SEM measures particle size
Fig. 1. X-ray diffraction patterns of CuxCa1ꢀxTiO3 (x = 0.0, 0.01, 0.02 and 0.03)
system.

86
J.P. Singh et al. / Journal of Alloys and Compounds 572 (2013) 84–89
x=0.0
x=0.01
x=0.02
x=0.03
Fig. 3. Scanning electron micrographs (SEM) of the CuxCa1ꢀxTiO3 system (x = 0.0, 0.01, 0.02 and 0.03).
and a particle is combination of several crystallites. Hence, discrep-
ancy between the size arises due to measurement of different enti-
ties by different method of measurements [39].
3.2. XANES study
Interesting results obtained from structural and morphological
studies motivated us to investigate the electronic structure of these
samples to know the valance state and local symmetry. Hence
XANES spectra for all the samples have been recorded at O K-edge
and Ti L-edges for the present investigation.
3.2.1. O K-edge XANES
TiO6 octahedra form the basic structural unit in titanates; hence
the information on unoccupied states in these samples will be
important to understand the hybridization of Ti and O states. The
O K-edge XANES spectrum arises from transitions of O 1 s core
electrons to the unoccupied O 2p derived states, which are hybrid-
ized with the relatively narrow 3d and broader 4sp bands of the 3d
transition metal ion with alkaline earth metal bands [12]. Recorded
O K-edge spectra for samples with different copper concentration
consists of spectral features A, B, C, D, E, F, G and H at ꢁ530, 532,
534, 537, 538, 540, 545 and 555 eV (as shown in Fig. 4). Feature
A is associated with the anti-bonding t2g-type molecular orbitals
(MOs) constructed mainly from the O 2p atomic orbitals (AOs)
and the 3d AO of the two neighboring Ti atoms while feature B cor-
responds to the anti-bonding eg-type MO of the same atoms. The
spectral features C and D arise due to O 2p states hybridized with
Fig. 4. O K-edge absorption spectra of samples CuxCa1ꢀxTiO3 (x = 0.0, 0.01, 0.02 and
0.03) at room temperature. Inset shows the de-convolution of XANES spectrum for
sample x = 0.01.

J.P. Singh et al. / Journal of Alloys and Compounds 572 (2013) 84–89
87
4sp states of Ca while feature E is formed due to hybridization of
the O 2p AO and Ca 3d AO [13]. For detailed investigation O K-edge
spectra of all the samples have been de-convoluted (inset: Fig. 4)
and normalized intensities of various features have been shown
in Fig. 5. The integral area of these features shows significant
changes with copper doping (Fig. 5). Fig. 5 shows that normalized
area of feature C increases to almost double of its initial value,
hence O 2p states that are mixed with Ca of 4sp states, have strong
impact on the properties of the system. Other major changes are
observed for feature E indicating the change in hybridized states
of the O 2p AO and Ca 3d AO. Distortion of the Ti site from octahe-
dral to lower symmetry results in an asymmetric broadening of the
peaks assigned to eg states i.e. peak B. An inspection of this peak in
Fig. 5a shows no change in the nature of broadening with Cu2+ con-
centration indicating no distortion of Ti site from octahedral sym-
metry. This aspect can be clearly seen from the Fig. 6 where
integral area of features A and B, which are related to the O and
Ti atoms, exhibit non-significant variation with Cu2+ concentration.
3.2.2. Ti L-edge XANES
Ti L-edge spectra of these materials have features (A1, B1, C1, D1)
at positions ꢁ462, 465, 468, and 470 eV respectively (Fig. 6). Be-
sides these pre-edge structures (indicative of local symmetry in
the system) are also observed in spectra of all the samples and gen-
erally originate from the presence of multiplet spd interaction in
the system [14–22]. The splitted peaks ꢁ464 and 467 eV corre-
spond to L3-edge while peaks around 470 and 472 eV are related
to the L2 edge, originating from Ti (2p3/2) and Ti (2p1/2) spin–orbit
coupling [10]. Observed splitting in these peaks, are due to the
presence of crystal field, which separates the 3d band into t2g
and eg bands [23]. Splitting in eg states is sensitive to the deviation
of ligand anions from Ti octahedral symmetry [24]. Splitting is not
observed in feature B1 in the XANES spectra of neither in copper
doped system nor in pure calcium titante (Fig 6: inset). Hence,
no deviation of ligand ions is expected from Ti octahedral symme-
try. Increase in intensity of features A1–D1 has been attributed to
increase in spin–orbit coupling with Cu2+ concentration. This as-
pect was investigated by the EPR spectroscopy on similar systems
and observed that Cu2+ doping leads to broadening in EPR line-
width suggesting increase in spin–orbit coupling and corroborated
the results obtained from Ti L-edge XANES spectra [40].
Fig. 6. Ti L-edge absorption spectra of samples CuxCa1ꢀxTiO3 (x = 0.0, 0.01, 0.02 and
0.03) at room temperature. Inset shows the enlarged view of feature B1.
3.3. Dielectric study
The value of er decreases with the increase in frequency at all
temperature (Fig. 7) and generally observed in titanates ceramics
[10,11]. Decrease in er with frequency is due to the lack of instan-
taneous presence of polarization with the application of the elec-
tric field because of inertia. The delay in response gives rise to
loss and decline in permittivity values. At low frequencies, all types
of polarization like ionic and orientation contributes. As frequency
is increased, those with large relaxation times cease to respond and
hence permittivity of material decreases. At room temperature va-
lue of er is 90, 134, 168 and 171 for Cu concentration of 0.0, 0.01,
0.02 and 0.03 respectively. This indicates that addition of Cu leads
to enhancement in the value of er and the value is almost double
for Cu concentration of 0.03. Similar behavior of increase of er with
copper concentration is observed at all measured temperatures
(Table 1). It is clear from the XANES study that Ti does not exhibit
reduced co-ordination in TiO6 octahedron and also no deviation of
ligand atoms for Ti octahedral symmetry. Since, distortion at TiO6
octahedron is responsible for polarization in titanates, hence huge
enhancement in er is not related to this effect. However behavior of
feature C with copper concentration is providing information that
er in this system may be related to the 2p states that are mixed
with Ca of 4sp states. Microscopically, the rapid increase in er with
copper concentration can be explained by the superionic nature
and spontaneous polarization or non-stoichiometry of samples
due to motion of Ti4+, Ca 2+ and Cu2+ cations in the host lattice site.
Based on Maxwell–Wagner model [41], electrical inhomogenous
structures creates polarization due to presence of mobile charged
species and internal interfaces leading to conducting grains and
hence increase in er value occur with Cu concentration. Since,
dielectric properties of titanates are very much affected by the
microstructure. Hence, drastic change of small spherical shaped
particles into the hexagonal type microcrystal along with the pres-
ence of impurity phase will have impact on the dielectric
properties.
The value of loss tangent is of order of 10ꢀ3 and generally in-
creases with copper concentration (Table 1). This effect may be
attributed to increase in the conductivity and supports the
Fig. 5. Variation of normalized area with copper concentration (x). A, B, C, D and E
are the spectral features observed in O–K edge spectra of the samples.

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J.P. Singh et al. / Journal of Alloys and Compounds 572 (2013) 84–89
Fig. 7. Frequency dependent dielectric behavior of samples CuxCa1ꢀxTiO3 (x = 0.0, 0.02 and 0.03) at different temperature ranging from 120 to 350 K. Value in parenthesis
shows the temperature.
Table 1
Permittivity (er) and loss tanget ‘tand’ of the samples at different temperature at 75 kHz.
Temperature (K)
Copper Concentration (x)
0.00
0.01
0.02
0.03
er
Tand
er
Tand
er
tand
er
tand
120
150
180
210
240
270
300
330
350
122
117
109
103
98
94
90
85
85
ꢀ0.127
ꢀ0.0063
ꢀ0.0064
ꢀ0.0070
ꢀ0.0073
ꢀ0.0075
0.0015
199
182
169
157
148
141
134
129
113
6.22Eꢀ4
4.99Eꢀ4
0.00135
3.2Eꢀ4
5.48Eꢀ4
0.0012
0.0018
0.0029
0.0038
235
214
198
186
176
169
162
157
155
ꢀ0.0023
4.26Eꢀ4
0.00511
0.0064
0.01034
0.01537
0.01977
0.02437
0.03015
243
222
208
195
186
178
171
166
163
0.00498
0.00807
0.01452
0.01685
0.02163
0.02729
0.03277
0.03836
0.04267
0.0030
0.0081
explanation of increase of er with Cu concentration. The low value
of tand is the characteristics of CT [42,43] and is observed in the
present case. It has been reported that this value is 3.3 ꢂ 10ꢀ3 at
230 K and it decreases to 1 ꢂ 10ꢀ3 below 110 K for CT single crystal
[43]. This low value of tand is also expected in present case, how-
ever, due to instrumental limit it appears either negative or very
less in absolute zero (Table 1). It is also clear from Table 1 that er
decreases from 122 to 85, 199 to 113, 235 to 155 and 243 to 163
at 75 kHz for the samples with copper concentration of 0.0, 0.01,
0.02 and 0.03 as the temperature increases from 120 to 350 K. This
behavior of er with temperature is similar to the bulk thin film of
CT and has been attributed to the effect of reduced tendency of
dipoles (or induced dipole) to align itself along external electric
field with the increased kinetic energy due to increase of temper-
ature [42]. Fig. 7 indicates that er for copper concentration of 2%
and 3% is almost same above a particular frequency. This particular
frequency is shifted towards lower values with decrease in temper-
ature. This effect may be explained on the basis of different mech-
anism involved for polarization at low and high frequencies. As it is
well known that grain boundaries contribute to er at low frequen-
cies, however grains contribute to er at higher frequencies. As the
temperature decreases most of the contribution comes from grains
rather than grain boundaries resulting shift in this particular fre-
quency towards lower values.

J.P. Singh et al. / Journal of Alloys and Compounds 572 (2013) 84–89
89
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Orthorhombically distorted perovskites CuxCa1ꢀxTiO3 have been
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some bands in Raman spectra with copper doping has been attrib-
uted to the change in microstructure. Copper doping leads to dras-
tic change in morphology of CaTiO3. Almost double increment in
the dielectric constant with 3% of copper doping was observed in
the present case. XANES study performed for these samples show
no distortion of TiO6 octahedra. Drastic change in value of dielec-
tric constant has been attributed to the change in microstructure
and modified hybridized state of O (2p) and Ca (4sp). Further attri-
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Acknowledgments
Authors are thankful to Mr. Jai Prakash, Mr. Pawan Kularia and
Mrs. Indra Sulania, Materials Science Division, IUAC, New Delhi for
their help in scanning electron microscopy, X-ray diffraction and
Raman spectroscopy. JPS is also thankful to Dr. D.K. Avasthi, Head,
Materials Science Division for acquiring the facilities at the Centre.
S.G. and K.H.C. are also thankful to Jenn-Min Lee (NSRRC) for exper-
imental support. Experiments at NSRRC were supported in part by
Ministry of Education, Science and Technology (MEST), Pohang
Accelerator Laboratory (PAL), and Korea Institute of Science and
Technology (KIST, Project No. V02631).
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