1. Thermoelectric and magnetic properties of Ca3Co4–xCuxO9 + d with
x = 0.00, 0.05, 0.07, 0.10 and 0.15
Ankam Bhaskar, Z.R. Lin, Chia-Jyi Liu *
Department of Physics, National Changhua University of Education, Changhua 500, Taiwan
1. Introduction
Thermoelectric (TE) generators are considered as energy
conversion systems, which can convert heat energy into electric
energy directly from vast amounts of waste heat emitted by
automobiles, factories, and similar sources without using moving
parts such as turbines and without producing CO2 gas, radioactive
substances, or other emissions [1]. Wide attention has been
focused on the exploration of thermoelectric materials recently.
Good thermoelectric materials require a large thermopower (S) for
generating a large thermal voltage, a low electrical resistivity (r)
for minimizing the Joule heating, and a low thermal conductivity
(k) for retaining the heat at the junctions in order to obtain a high
figure of merit ZT = S2
T/rk [2]. Besides, thermoelectric materials
are required to be stable at high temperatures. In recent years,
layered cobalt oxides have gained great attention since NaCo2O4
single crystal is found to exhibit good thermoelectric properties
[3]. The misfit cobalt oxides (Ca3Co4O9 + d) have been investigated
extensively as potential thermoelectric material because it has
large S, low r, and low k [4–9]. The crystal structure of Ca3Co4O9 + d
system consists of two subsystems, viz., the distorted NaCl-type
(Ca2CoO3) sublattice and the CdI2-type (CoO2) sublattice, alterna-
tively stacking along the c-axis [10]. The polycrystalline bulk
Ca3Co4O9 + d samples are still at a relatively low level for industrial
applications. Many attempts have been made to optimize the
thermoelectric performance of Ca3Co4O9 + d by either partially
substituting cations or using appropriate fabrication methods such
as hot pressing (HP) or spark plasma sintering (SPS) techniques.
Partial replacement of cations in the Ca3Co4O9 + d has been carried
out on either the Ca site [11–22] or the Co sites [5,8,23–26]. Many
groups have attempted to prepare Ca3Co4–xCuxO9 + d system with
lower concentration (x = 0.00, 0.05, 0.10) of dopants [21,24], and
with higher concentration (x ! 0.2) of dopants [27–29], reporting
remarkable changes in the thermoelectric properties. In this paper,
we report the low-temperature (<300 K) thermoelectric and
magnetic properties of Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07,
0.10 and 0.15) samples.
2. Experimental
Polycrystalline samples of Ca3Co4–xCuxO9 + d with x = 0.00, 0.05,
0.07, 0.10 and 0.15 were synthesized by conventional solid state
reaction from CaCO3, Co3O4, and CuO powders. The powders were
heated at 900 8C for 24 h with intermediate grinding. The resulting
powders were then pressed into parallelepiped and sintered in air
at 900 8C for 24 h. The phase purity of resulting powders was
examined by a Shimadzu XRD-6000 powder X-ray diffractometer
equipped with Fe Ka radiation. The electrical resistance measure-
ments were carried out using standard four-probe techniques. The
thermopower measurements were performed between 80 K and
300 K using steady-state techniques with a temperature gradient
of 0.5–1 K across the sample. A type E differential thermocouple
Materials Research Bulletin 48 (2013) 4884–4888
A R T I C L E I N F O
Article history:
Received 28 March 2013
Received in revised form 13 May 2013
Accepted 5 July 2013
Available online 13 July 2013
Keywords:
Oxides
X- ray diffraction
Electrical properties
Magnetic properties
Thermal conductivity.
A B S T R A C T
Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10 and 0.15) samples were prepared by conventional solid-state
synthesis and their thermoelectric properties were systematically investigated. The thermopower of all
the samples was positive, indicating that the predominant carriers are holes over the entire temperature
range. Ca3Co3.85Cu0.15O9 + d had the highest power factor of 2.17 mW cmÀ1
KÀ2
at 141 K, representing an
improvement of about 64.4% compared to undoped Ca3Co4O9 + d. Magnetization measurements
indicated that all the samples exhibit a low-spin state of cobalt ions. The observed effective magnetic
moments decreased with increasing copper content.
ß 2013 Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: +886 4 723 2105x3337; fax: +886 4 728 0698.
E-mail address: liucj@cc.ncue.edu.tw (C.-J. Liu).
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2013.07.004

2. was used to measure the temperature difference between hot and
cold ends of sample, which was measured using a Keithley 2000
multimeter [30]. The thermopower of the sample was obtained by
subtracting the thermopower of Cu Seebeck probe. The carrier
concentration and mobility were determined by Hall measure-
ments using the van der Pauw method under an applied field of
0.55 T (ECOPIA: HMS-3000) [31]. The thermal conductivity
measurements were carried out using transient plane source
techniques with very small temperature perturbations of sample
material using a hot disk thermal constants analyzer. The transient
plane source technique makes use of a thin sensor element in the
shape of a double spiral. The hot disk sensor acts both as a heat
source for generating temperature gradient in the sample and a
resistance thermometer for recording the time dependent
temperature increase. The encapsulated sensor was sandwiched
between two pieces of samples. During a preset time, 200
resistance recordings were taken and from these a relation
between temperature and time was established [31]. A commercial
superconducting quantum interference device magnetometer
(quantum design) was used to characterize the magnetic proper-
ties of samples. The oxygen content and valence state of cobalt
were determined using iodometric titration [32].
3. Results and discussion
Fig. 1a shows the XRD patterns of Ca3Co4–xCuxO9 + d (x = 0.00,
0.05, 0.07, 0.10 and 0.15) samples. The XRD patterns reveal that all
the samples are single phase, and no secondary phase is detected.
The diffraction peaks are matched with earlier reports of Ca3Co4–
xCuxO9 + d system [24,27–29]. The crystal structure of Ca3Co4O9 + d
consists of two subsystems; these are triple-layered NaCl-type
rocksalt Ca2CoO3 block (subsystem 1) and a CdI2-type hexagonal
CoO2 layer (subsystem 2) [10]. Therefore, the Cu ion may occupy
either Ca2CoO3 subsystem or CoO2 subsystem. The Co cation in the
CoO2 layer are the mixture of Co3+
and Co4+
and their ionic radii in
six-coordination are 0.54 A˚ and 0.53 A˚ , respectively, whereas the
Co cation in the rocksalt structure (Ca2CoO3) is Co2+
with the ionic
radius in six-coordination of 0.74 A˚ [29], while the ionic radius of
Cu2+
is 0.73 A˚ and Cu3+
is 0.54 A˚ .
Table 1 summarizes the characterization and properties of
Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10, and 0.15) samples. The
undoped sample shows the highest resistivity (0.0176 V-cm at
300 K) among the samples. The size of r for all the doped samples is
in the range of 0.0150 V-cm to 0.0117 V-cm; decreasing with
increasing Cu content due to an increase in charge carrier
concentration (Table 2). The substitution of Cu ion creates hole
carriers into the system because the valence state of Cu ion is lower
than the average valence state of Co ion (Table 1). The average
valence state of Co ion is between 3+
and 4+
, while the Cu ions: Cu2+
or Cu3+
. The Hall-effect measurements reveal that hole carrier
concentration increases with increasing Cu content, as shown in
Table 2. Similar results were also reported for Ca3Co4–xCuxO9 + d
system [24,29,33]. On the other hand, the excess of oxygen content
also creates the hole carriers into the system. Karppinen et al. [34]
have reported that the electrical resistivity is affected by the excess
of oxygen content for calcium cobalt oxide ((CoCa2O3)qCoO2)
system. Iodometric titration results show that the excess of oxygen
content increases with increasing Cu content, expect the one with
x = 0.15, indicates an increase in hole carrier concentration and
hence reducing the resistivity of samples. Karppinen et al. [34]
have reported that the resistivity of samples decreases with
increasing excess of oxygen content for Ca3Co3.95O9 + d (d = 0.00,
0.24 and 0.29) system. Therefore, these results suggest that the
excess of oxygen content and Cu content contribute to decreasing
the resistivity of samples. According to the previous reports [35–
37] the doping at Co-site in CoO2 layers can bring notably variation
of electronic correlation in the system, because the carrier
transport mainly occurs in the CoO2 layers and these layers play
a crucial role in determining the electronic structure of system as
revealed by the band calculation.
Fig. 2 shows the temperature dependence of resistivity for
Ca3Co4–xCuxO9+d (x = 0.00, 0.05, 0.07, 0.10, and 0.15) samples. For
all the samples, the electrical resistivity decreases with increasing
temperature, a typical characteristic of nonmetallic-like tempera-
ture dependence (dr/dT <0), then increases with increasing
temperature, a typical characteristic of metallic-like temperature
dependence (dr/dT >0). The electrical resistivity exhibits the
nonmetallic to metallic transition (TIM) occurs at below 90 K for all
the samples, which is similar to other elements doped in the
Fig. 1. XRD patterns of Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10 and 0.15)
samples.
Table 1
Room temperature characterization and properties of Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10 and 0.15) samples.
x Cov+
d r (V-cm) TMI (K) T* (K) S (mV/K) ktotal (W/mK) kcar (W/mK) kph (W/mK) PF (mW/cm-K2
) ZT
0.00 3.168 0.336 0.0176 89 235 130 0.73 0.04 0.69 0.96 0.038
0.05 3.210 0.390 0.0150 86 228 140 0.79 0.05 0.74 1.30 0.047
0.07 3.212 0.390 0.0149 82 224 114 1.17 0.05 1.12 0.87 0.021
0.10 3.249 0.436 0.0142 79 222 125 1.09 0.05 1.04 1.10 0.029
0.15 3.261 0.428 0.0117 70 227 139 0.99 0.06 0.93 1.65 0.050
Table 2
Carrier concentration and mobility of Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10.
and 0.15) samples.
x Carrier concentration (1020
cmÀ3
) Mobility (cm2
/Vs)
0.00 1.88 1.88
0.05 2.56 1.56
0.07 2.78 1.49
0.10 2.99 1.46
0.15 3.11 1.70
A. Bhaskar et al. / Materials Research Bulletin 48 (2013) 4884–4888 4885

3. Ca3Co4O9 + d system [5,7]. This may be due to the incommensurate
spin-density-wave (IC-SDW) [35,38,39]. Similar results was also
observed by Huang et al. [29]. Sugiyama et al. [35,38,39] have
reported that Ca3Co4O9 system exhibits the various magnetic
states. These magnetic states: ferromagnetic insulating state
(%19 K), short-range order IC-SDW insulating state (%100 K),
and paramagnetic semiconducting state to a paramagnetic
metallic state (%380 K). The appearance of IC-SDW localizes the
charge carrier and results in the nonmetallic behavior of system
below TIM. The broad minimum is observed around TIM in the r–T
curve with the nonmetallic to metallic transition (%90 K), which is
consistent the IC-SDW transition (%100 K). Sugiyama et al.
[35,38,39] have reported that there is no significant effect on
the IC-SDW transition and an average valence state of Co ion does
not change for Sr2+
doped in the Ca3–xSrxCo4O9 system. In addition,
they have found that the IC-SDW transition is increased for Y3+
or
Bi3+
doped in the Ca3–xMxCo4O9 system (M = Y or Bi), and Y3+
or Bi3+
doping decreases the average valence of Co ion. These results
suggest that the IC-SDW transition associate with the average
valence of Co ion. In our case, the nonmetallic to metallic transition
temperature (TIM) decreases with increasing average valence of Co
ion for all the samples.
In general, the transport behavior of a Fermi-liquid system can
be expressed by the following equation:
r ¼ r0 þ AT2
(1)
where r0 is the residual resistivity owing to the domain boundaries
and other temperature-independent scattering mechanisms
[29,40–43], and A is the Fermi-liquid transport coefficient [40–
43]. Limelette et al. [40–43] have reported that Ca3Co4O9 system
exhibits two resistivity characteristic temperatures (TIM, T*
)
between 5 K and 300 K, where TIM is the nonmetallic to metallic
transition and T*
is the transition temperature from Fermi-liquid
metal to incoherent metal. The curves are fitted using Eq. (1) at
above TIM in Fig. 3, and T*
is presented in Table 1. The transition
temperature from Fermi-liquid metal to incoherent metal (T*
) is
obtained at the end temperature of linear dependence in metallic
range. Table 1 shows the T*
decreases with increasing Cu content
up to x = 0.10, which is similar to the previous report [29].
However, the T*
slight increases for the x = 0.15. According to the
dynamical mean field theory [29], a key role of effective mass m*
of
a Fermi liquid is predicated as T*
% 1/m*
. The decrease of T*
indicates an increase in m*
with increasing Cu content due to a
decrease in bandwidth and enhance the electronic correlation in
these system.
The temperature dependence of resistivity behavior gradually
varies with Cu content. It is well known that nonmetallic behavior
is obtained for Ca3Co4–xCuxO9 + d at low temperature range, which
obey the variable range hopping (VRH) theory [44,45],
r ¼ r0
0expðT0=TÞ1=4
(2)
where r0
0 is weakly temperature dependent, and T = 24/
[pkBN(eF)lv
3
] is the VRH characteristic temperature associated
with the density of localized states at Fermi energy N(eF), kB is
Boltzman constant, and lv is localization length. By the fitting of
experimental data using Eq. (2), the Fig. 2 displays nonmetallic-like
behavior. The plots of ln versus TÀ1/4
for all the samples lie on
straight lines in the nonmetallic-like behavior, as shown in Fig. 4.
This behavior might be associated with the positional disorder
involved in the incommensurate structure of the misfit layered
title system.
The positive thermopower confirms that dominant charge
carriers are holes for all the samples. The undoped sample exhibits
a larger absolute S values 130 mV/KÀ1
at 300 K. Lin et al. [47,48],
Fig. 2. The temperature dependence of electrical resistivity for Ca3Co4–xCuxO9 + d
(x = 0.00, 0.05, 0.07, 0.10 and 0.15) samples.
Fig. 4. Plot of In r versus TÀ1/4
(KÀ1/4
) for Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10
and 0.15) samples.
Fig. 3. Variation of r versus T2
for Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10 and
0.15) samples. The solid lines are linear fitting using r = r0 + AT2
. TIM: transition
temperature of nonmetallic to metallic, T*
: strongly correlated Fermi-liquid regime
up to the temperature.
A. Bhaskar et al. / Materials Research Bulletin 48 (2013) 4884–48884886

4. Chen et al. [4], and Nong et al. [8,9] have also reported that the
undoped (Ca3Co4O9 + d) sample exhibits a large room-temperature
thermopower of 132 mV/K at 300 K. The Hall-effect measurements
confirm that the majority carriers are p-type, which is consistent
with the thermopower data. The thermopower is also affect by the
excess of oxygen content. The excess of oxygen content decreases,
expect the one with x = 0.15, with increasing Cu content, indicating
that the hole carrier concentration decreases and thermopower
increases. Karppinen et al. [34] have reported that the thermo-
power slightly decreases with increasing excess of oxygen content
for Ca3Co3.95O9 + d (d = 0.07, 0.24 and 0.29) system. Fig. 5 shows the
temperature dependence of thermopower (S) for Ca3Co4–xCuxO9 + d
(x = 0.00, 0.05, 0.07, 0.10 and 0.15) samples. It can observe that all
the curves of thermopower are similar, but the absolute thermo-
power values are different.
In general the thermoelectric power can be expressed by the
Mott formula [35,37,45].
SðTÞ ¼
1
eT
R1
À1 sðeÞðe À mÞ @ fðeÞ
@e de
R1
À1 sðeÞ @ fðeÞ
@e de
(3)
where s(e) and f(e) represent electrical conductivity and Fermi–
Dirac distribution function at energy e. The product of the
thermopower coefficient and temperature can therefore be
understood as the mean energy flow carried by a conduction
electron. Using the condition of @f(e)/@e = d (eÀEF), and s = nem(e)
[34,36] Eq. (3) can be written as:
SðTÞ ¼
Ce
n
þ
p2
k2
BT
3e
@lnmðeÞ
@e
e¼EF
(4)
where m(e), Ce, and kB are energy correlated carrier mobility,
electronic specific heat, and Boltzmann constant, respectively. If S
is inversely proportional to the n, it is usually interpret as the
predominance of first term in Eq. (4). If the second term in Eq. (4)
dominant then S closely related to the electronic correlation
[28,29]. The thermopower values of doped samples are random;
this may be related to the Cu occupying at different subsystem
(Ca2CoO3 or CoO2). Huang et al. [29] have reported that the
thermopower of doped samples does not show a monotonic trend
due to Cu occupying at different subsystem (Ca2CoO3 or CoO2) for
Ca3Co4–xCuxO9 + d single crystals. Furthermore, they have found
that the electronic correlation plays a key role in determining the
thermopower. Therefore, further studies are required (ex: specific
heat) to know which term is dominant in our case.
Fig. 6 shows the temperature dependence of power factor (S2
s)
for Ca3Co4–xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10 and 0.15) samples. It
can be seen that the power factor initially rises, reaches a
maximum value (141 K) and then continuously falls with
increasing temperature for all the samples. The highest value of
S2
s = 2.17 mW cmÀ1
KÀ2
at 141 K for Ca3Co3.85Cu0.15O9 + d sample
as a result of its low r value combined with its moderate S value.
This represents a 64.4% increase when compared to undoped
sample at 141 K. It should be noted that higher thermoelectric
power factor leads to higher efficiency of the thermoelectric
generator.
The thermal conductivity is measured at room temperature and
values are presented in Table 1. Total thermal conductivity (ktotal)
can be expressed as ktotal = kcar + kph, where kcar and kph represent
the carrier and the lattice thermal conductivity, respectively. kcar
can be calculated using the Wiedemann–Franz–Lorenz relation-
ship, kcar = LsT, where L = p2
k2
/3e2
= 2.45 Â 10À8
W V KÀ2
is the
Lorenz number and T is the absolute temperature. kph is obtained
by subtracting kcar from ktotal. It can be clearly seen from Table 1
that the total thermal conductivity slightly increases, then
decreases with increasing Cu content; this may be related to the
structure distortion [29]. For materials with r 1 V-cm, kcar is
negligible. But in our case, the resistivity is lower than 1 V-cm, a
fact which leads us to determine the kcar using the Wiedemann–
Franz law. The calculated value of kcar is 0.04 and 0.06 W mÀ1
KÀ1
at 300 K for Ca3Co4O9 + d and Ca3Co3.85Cu0.15O9 + d, respectively. For
Fig. 5. The temperature dependence of thermopower (S) for Ca3Co4–xCuxO9 + d (x =
0.00, 0.05, 0.07, 0.10 and 0.15) samples.
Fig. 6. The temperature dependence of power factor for Ca3Co4–xCuxO9 + d (x = 0.00,
0.05, 0.07, 0.10 and 0.15) samples.
Fig. 7. The temperature dependence of magnetic susceptibility for
Ca3Co3.93Cu0.07O9 + din an applied field of 50,000 Oe. The solid line is a fit to the
Curie–Weiss law.
A. Bhaskar et al. / Materials Research Bulletin 48 (2013) 4884–4888 4887

5. all the samples, the lattice contribution is more important than the
carrier one. Due to the small kcar, ktotal is mainly attributed to the
lattice contribution. The figure of merit (ZT = S2
T/rk) is calculated
for all the samples. The calculated values are tabulated in Table 1.
Among the samples, Ca3Co3.85Cu0.15O9 + d has the highest dimen-
sionless figure of merit (ZT) of 0.050 at 300 K. This value represents
an improvement of about 31% compared to undoped sample. These
results suggest that there is scope for further improvement of
thermoelectric properties.
Fig. 7 shows the temperature dependence of magnetic
properties for Ca3Co3.93Cu0.07O9 + d sample. The observed effective
magnetic moment is derived by fitting the magnetization versus
temperature using the Curie–Weiss law. The observed effective
magnetic moments are 1.37 mB/Co for x = 0.00, 1.36 mB/Co for
x = 0.03, 1.30 mB/Co for x = 0.05, and 1.11 mB/Co for x = 0.07,
respectively. The observed effective magnetic moments decrease
with increasing Cu content. These results suggest that the
ferromagnetic is suppressed by the Cu content. According to the
previous reports [35,38,39], the ferromagnetic of Ca3Co4O9 is
originated by the interlayer coupling between CoO2 and Ca2CoO3
sublayers. The Ca2CoO3 layer consists of two Ca-O planes and one
Co–O plane, where the Co–O plane is sandwiched by the two Ca–O
planes, and the Ca–O planes are located between Co–O plane and
CaO2 sublayers [10]. The Cu ions may disturb the interlayer
coupling between CoO2 and Ca2CoO3 sublayers, which would
decrease the magnetic moments. The observed effective magnetic
moment of undoped is good agreement with the earlier report [48].
Zhao et al. [48] have reported that the undoped sample exhibits the
observed effective magnetic moment is 1.3 mB/Co. The Co3+
and
Co4+
ions with a low-spin configuration have a theoretical effective
magnetic moment of 0 mB/Co and 1.73 mB/Co, respectively. The
Co3+
and Co4+
ions with a high-spin configuration have a
theoretical effective magnetic moment of 4.89 mB/Co and
5.91 mB/Co, respectively. The observed effective magnetic moment
is close to the low-spin configuration of cobalt ion. Chen et al. [4]
and Liu et al. [6] have also observed the low-spin state of cobalt
ions at low temperature range (5 K–300 K) for Ca3Co4–xMxO9 + d
(0.00–0.15) system.
4. Conclusions
The thermoelectric and magnetic properties of Ca3Co4–
xCuxO9 + d (x = 0.00, 0.05, 0.07, 0.10, and 0.15) have been
investigated systematically. The XRD patterns show that all the
samples are single phase. The positive thermopower confirms that
the dominant carriers are holes for all the samples. A maximum
power factor of 2.17 mW cmÀ1
KÀ2
is reached at 141 K for
Ca3Co3.85Cu0.15O9 + d, representing an improvement of about
64.4% compared to undoped sample. The highest figure of merit
(0.050) is obtained for Ca3Co3.85Cu0.15O9 + d as compared to
undoped sample. Magnetization measurements show a low-spin
state of cobalt ion for all the samples. The observed effective
magnetic moments decrease with increasing Cu content.
Acknowledgement
This work was supported by National Science Council of
Republic of China, Taiwan under the Grant No. 101-2112-M-018-
003-MY3. Ankam Bhaskar would like to express thanks to the
postdoctoral fellowship sponsored by NSC of Taiwan.
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