1. Low-temperature thermoelectric and magnetic characteristics
of Ca2.9Bi0.1Co42xFexO9+d (0 £ x £ 0.10)
Ankam Bhaskar • Z.-R. Lin • Chia-Jyi Liu
Received: 19 October 2013 / Accepted: 25 November 2013 / Published online: 4 December 2013
Ó Springer Science+Business Media New York 2013
Abstract The effect of Fe ion doping on the low-temper-
ature thermoelectric and magnetic properties of Ca2.9Bi0.1
Co4-xFexO9?d (x = 0.00, 0.025, 0.05 and 0.10) have been
investigated. The samples were prepared by conventional
solid-state synthesis. The X-ray diffraction patterns revealed
that all the samples are single phase. The electrical resistivity
results indicated that all the samples obey the variable range
hopping in the low temperature regime. The T*
(transition
temperature from Fermi liquid metal to incoherent metal)
was increased and the slope of A value (Fermi-liquid trans-
port coefficient) was decreased with increasing Fe content.
The thermopower of all the samples was positive, indicating
that the predominant carriers are holes over the entire tem-
perature range. The electrical resistivity, thermopower and
total thermal conductivity were decreased with increasing Fe
content. Among the doped samples, Ca2.95Bi0.10Co3.90
Fe0.10O9?d had the highest dimensionless figure of merit of
0.056 at 300 K. Magnetic measurements indicated that all
the samples exhibit a low-spin state of cobalt ion. The
effective magnetic moments were decreased with increasing
Fe content.
1 Introduction
Thermoelectric (TE) materials have been the focus of
attention for protecting the environment by saving energy
resources and reducing the release of CO2 into the atmo-
sphere. The efficiency of thermoelectric devices is deter-
mined by materials dimensionless thermoelectric figure of
merit, ZT = S2
T/qj, where S, q, T, and j are the Seebeck
coefficient, the electrical resistivity, the absolute tempera-
ture, and the thermal conductivity, respectively [1–3].
Recently, the misfit cobalt oxides (Ca3Co4O9?d) have been
investigated extensively as potential thermoelectric mate-
rial because it has large S, low q, and low j [4–10]. The
crystal structure of Ca3Co4O9?d system consists of two
subsystems, viz., the distorted NaCl-type [Ca2CoO3] sub-
lattice and the CdI2-type [CoO2] sublattice, alternatively
stacking along the c-axis [11, 12]. 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 [13] or spark
plasma sintering techniques [14]. Partial replacement of
cations in Ca3Co4O9?d has been carried out on either the
Ca site [15–21] or the Co sites [6, 9, 22–24]. According to
previous reports, the Ca-site substitution by Bi has been
improved the electrical conductivity in Ca3Co4O9?d system
[17, 25–29]. On the other hand, the highest thermopower
was obtained for the Co-site substituted by Fe in
Ca3Co4-xMxO9?d (M = Fe, Mn, and Cu; x = 0, 0.05, and
0.1) system [22, 30]. Therefore, it is interesting to inves-
tigate the effects of simultaneous substitution of Bi at the
Ca site and Fe at the Co site in Ca3Co4O9?d. In this paper,
we report the low-temperature (300 K) thermoelectric
and magnetic properties of Ca2.9Bi0.1Co4-xFexO9?d
(x = 0.00, 0.025, 0.05 and 0.10) system.
2 Experimental procedures
Polycrystalline samples of Ca2.9Bi0.1Co4-xFexO9?d (x =
0.00, 0.025, 0.05 and 0.10) were synthesized by conventional
A. Bhaskar Á Z.-R. Lin Á C.-J. Liu (&)
Department of Physics, National Changhua University of
Education, Changhua 500, Taiwan
e-mail: liucj@cc.ncue.edu.tw
123
J Mater Sci: Mater Electron (2014) 25:778–784
DOI 10.1007/s10854-013-1645-9

2. solid-state reaction from CaCO3, Co3O4, Fe2O3 and Bi2O3
powders. The resulting powders were then pressed into
parallelepiped and sintered in air at 900 °C for 24 h. The
phase purity of resulting powders was examined by a Shi-
madzu XRD-6000 powder x-ray diffractometer equipped
with Fe Ka radiation. Electrical resistance measurements
were carried out using standard four-probe techniques.
Thermopower measurements were performed between 75
and 300 K using steady-state techniques with a temperature
gradient of 0.5–1 K across the sample. A type E differential
thermocouple was used to measure the temperature differ-
ence between hot and cold ends of sample [31], which was
measured using a Keithley 2000 multimeter. The thermo-
power of sample was obtained by subtracting the thermo-
power of Cu Seebeck probe. Carrier concentration and
mobility were determined by Hall measurements using the
van der Pauw method under an applied field of 0.55 T
(ECOPIA: HMS-3000) [32]. Thermal conductivity mea-
surements were carried out using transient plane source
techniques with very small temperature perturbations of
sample material using a Hot Disk thermal constants analyzer
[33]. 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 tem-
perature 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 [32]. A commercial superconduc-
ting quantum interference device magnetometer (Quantum
Design) was used to characterize the magnetic properties of
the samples. The excess of oxygen content and valence state
of cobalt were determined using iodometric titration [34].
3 Results and discussion
Figure 1a shows the X-ray diffraction patterns of
Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05, and 0.10)
samples. The XRD patterns show that all the samples are
single phase, and no secondary phase is detected. All the
diffraction peaks for all the samples can be indexed based
on the monoclinic misfit structure with the superspace
group X2/m(0b0)s0 [4, 8–10, 35]. The similarity between
the XRD patterns of undoped and doped samples suggest
that substituting ions of Fe do not change the crystalline
structure. The structure refinement using a Jana2006 Ri-
etveld program is carried out for all the samples [36].
Figure 1b shows the Rietveld fits to power XRD data for
the Ca2.9Bi0.1Co3.95Fe0.05O9?d using the superspace group
X2/m/(0b0)s0. The calculated and the difference profiles
(Rp = 6.42 %, Rwp = 9.48 %, and GOF = 1.47) are in
good agreement with previous reports [8, 37]. Lattice
parameters are given in Table 1. The lattice parameters do
not show monotonic trend, which may be due to the dual
doped (Bi and Fe ion at Ca-site and Co-site) and excess of
oxygen content. The radius of Bi3?
is 1.03 A˚ , which is
higher than that of Ca2?
radius (1.00 A˚ ). The radius of low
spin (LS) Co4?
, LS Co3?
, intermediate spin (IS) Co3?
, and
high spin (HS) Co3?
is 0.53, 0.545, 0.56, and 0.61 A˚ ,
Fig. 1 a XRD patterns of Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025,
0.05 and 0.10) samples, b Observed (dotted type), calculated (solid
line), and difference powder XRD profiles for the Ca2.9Bi0.1Co3.95
Fe0.05O9?d using the superspace group X2/m/(0b0)s0
Table 1 Lattice parameters of the Ca2.9Bi0.1Co4-xFexO9?d
(x = 0.00, 0.025, 0.05 and 0.10) samples (b axis lengths b1:
Ca2CoO3, b2: CoO2 sublattice)
x a (nm) b1 (nm) b2 (nm) c (nm) b (°)
0.00 0.4830(1) 0.4562(9) 0.2817(3) 1.0846(9) 98.09(5)
0.025 0.4827(1) 0.4557(6) 0.2816(8) 1.0844(4) 98.10(1)
0.05 0.4828(4) 0.4551(1) 0.2817(6) 1.0850(3) 98.10(1)
0.10 0.4829(4) 0.4563(6) 0.2818(1) 1.0858(3) 98.10(1)
J Mater Sci: Mater Electron (2014) 25:778–784 779
123

3. respectively, while the radius of LS Fe3?
, HS Fe3?
, and
Fe4?
is 0.55, 0.645, and 0.585 A˚ , respectively. Karppinen
et al. [38] reported that lattice parameters (a and c) are
increased with increasing excess of oxygen content for the
Ca3Co3.95O9?d (d = 0.00, 0.24, 0.29) system. Iodometric
titration results show that the excess of oxygen content
increases with increasing Fe content (Table 2). Therefore,
one can conclude that lattice parameters are random due to
the dual-doped and excess of oxygen content. The oxygen
vacancies are located in the rock salt-type [Ca2CoO3]
subsystem [39–41]. The crystal structure of Ca3Co4O9?d
consists of two subsystems; these are triple rock salt-type
[Ca2CoO3] block (subsystem 1) and a CdI2-type hexagonal
[CoO2] block (subsystem 2). Therefore, the Fe ion can
occupy in either [Ca2CoO3] or [CoO2] block. The bulk
density was measured by applying Archimedes principle at
room temperature. The densities of samples are
*3.92–4.02 g/cm3
, which are 79–81 % of theoretical
density (4.94 g/cm3
). This value is in relatively good
agreement with earlier reports [4, 22, 30, 42–44]. Kenfaui
et al. [13, 42] conducted research on the Ca3Co4O9
ceramics prepared by solid-state reaction method and
ceramics were sintered by different sintering techniques
such as conventional sintering (CS), hot pressing (HP),
spark plasma sintering techniques (SPS). They have
obtained the bulk density of 2.82 g/cm3
(CS), 4.45 g/cm3
(HP), 4.59 g/cm3
(SPS), which corresponds to 60, 95 and
98 % of the theoretical density, respectively. Wang et al.
[22, 30], Pinistoontorn et al. [43, 44] also reported that the
bulk density of samples is in the range of &3.2–3.8 g/cm3
for the doped and undoped of Ca3Co4O9?d samples; this
value is 65–80 % of theoretical density.
Table 2 summarizes the characterization and properties
for the Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05,
and 0.10) samples. The parent (Ca2.9Bi0.1Co4O9?d) sample
shows the highest resistivity among the samples. The
resistivity of undoped sample is 0.0120 X-cm at 300 K.
The magnitude of q for all the doped samples is in the
range of 0.0119–0.0107 X-cm at 300 K. The resistivity of
doped samples is higher than that of previous report [35].
Bhaskar et al. [35] have observed that electrical resistivity
is in the range of 0.0100–0.0093 X-cm at 300 K for the
Ca2.95Bi0.05Co4-xFexO9?d (x = 0.00, 0.05, 0.10 and 0.15).
This difference can probably be attributed to the higher
content of Bi at Ca-site in Ca3Co4O9?d system, which will
reduce the hole carrier concentration. The resistivity of
samples increases with decreasing Fe content due to a
increase in hole carrier concentration (Table 3). The
valence of Fe ion (Fe2?
or Fe3?
) is lower than that of the
average valence of Co (Table 2). On the basis of valence
equilibrium, the substitution of Fe ion for Co ion introduces
positive charge carriers into the system, which will enhance
the hole carrier concentration. Hall effect measurements
reveal that the hole concentration increases for the doped
samples as compared to the undoped sample (Table 3).
Similar results were also found in earlier reports [22, 30].
Wang et al. [22, 30] reported that the hole carrier con-
centration increases with increasing Fe doping level for the
Ca3Co4-xFexO9?d with x = 0.00–0.10. Besides, the excess
of oxygen content also creates the hole carriers into the
system. Karppinen et al. [38] reported that excess of oxy-
gen content decreases the resistivity of samples for the
Ca3Co3.95O9?d (d = 0.00, 0.24, 0.29). Iodometric titration
results show that the excess of oxygen content increases
with increasing Fe content (Table 2), indicating that the
hole carrier concentration increases and hence decreases
the resistivity. On the other hand, the average valence state
of cobalt increases with increasing Fe content; this may be
related to an increase in Co4?
/Co3?
ratio, leading to the
increase of hole carrier concentration (Co4?
) [8, 37].
Therefore, these results suggest that the decrease of elec-
trical resistivity is mainly contributed by the Fe dopant and
excess of oxygen content.
Figure 2 shows the temperature dependence of resis-
tivity for the Ca2.9Bi0.10Co4-xFexO9?d (x = 0.00, 0.025,
0.05, and 0.10) samples. All the samples of electrical
resistivity decreases with increasing temperature, a typical
Table 2 Room temperature characterization of the Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05 and 0.10) samples
x Cov?
d q (X-cm) S (lV/K) jtotal (W/mK) jcarr (W/mK) jph (W/mK) ZT
0.00 3.127(4) 0.304(9) 0.0120 144 1.00 0.059 0.941 0.054
0.025 3.208(1) 0.463(5) 0.0119 139 0.90 0.060 0.840 0.054
0.05 3.218(1) 0.480(8) 0.0113 138 0.94 0.062 0.878 0.053
0.10 3.243(1) 0.523(9) 0.0107 130 0.85 0.066 0.784 0.056
Table 3 Carrier concentration and mobility of the Ca2.9Bi0.1
Co4-xFexO9?d (x = 0.00, 0.025, 0.05, and 0.10) samples
x Carrier
concentration
(1020
cm-3
)
Mobility
(cm2
/Vs)
0.00 2.38 2.16
0.025 2.39 2.17
0.05 2.41 2.26
0.10 2.50 2.30
780 J Mater Sci: Mater Electron (2014) 25:778–784
123

4. characteristic of nonmetallic-like (dq/dT 0) temperature
dependence, then increases with increasing temperature, a
typical characteristic of metallic-like (dq/dT [ 0) temper-
ature dependence. The metal to nonmetal transition (TMI) is
determined by dq/dT. The dq/dT is zero at transition
temperature. The TMI is around 88 ± 2 K for all the
samples. The metal to nonmetal transition occurs in the low
temperature region, which is similar to other elements
doped in Ca3Co4O9?d systems [4, 6, 8, 22, 23, 30, 35]. In
the insert of Fig. 2 shows the metal-like behavior in the
high temperature region from 120 to 300 K for the
Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05 and 0.10)
samples.
In general, the resistivity behavior of a Fermi-liquid
system can be expressed by the following equation:
q ¼ q0 þ AT2
ð1Þ
where q0 is the residual resistivity owing to the domain
boundaries and other temperature-independent scattering
mechanisms, and A is the Fermi-liquid transport coefficient
[45–48]. Limelette et al. [45–48] reported that Ca3Co4O9
system exhibits two electrical resistivity characteristic
temperatures (TMI and T*
) between 5 and 300 K, where
TMI is the metal to nonmetal transition temperature and T*
the transition temperature from Fermi liquid metal to
incoherent metal. The curves are fitted by Eq. (1) for
temperatures above TMI, and are shown in Fig. 3. The
transition temperature from Fermi liquid metal to inco-
herent metal (T*
) is obtained at the end temperature of the
linear dependence in the metal-like range. The T*
lies
between 217 ± 1 and 220 ± 1 K for the x = 0.00–0.10,
respectively. The slope of A value is obtained in the range
between 7.6 9 10-5
and 6.8 9 10-5
mX-cm K-2
for the
x = 0.00–0.10, respectively. According to the dynamical
mean field theory, a key role of effective mass m*
of a
Fermi liquid is predicated as T*
& 1/m*
. The decrease of
A value and increase of T*
indicates a decrease of m*
,
which implies an increase in bandwidth and weakened the
electronic correlation in these system [49].
To facilitate the understanding of transport mechanism
of charge carriers in the nonmetallic regime, the natural
logarithm of reciprocal of resistivity is plotted against
T-1/4
in the nonmetallic regime. For variable-range hop-
ping transport in a disordered system, the temperature
dependence of resistivity would obey following relation at
low temperature [50].
r ¼ r0 exp À
T0
T
1= dþ1ð Þ
#
ð2Þ
where r0 is weakly temperature dependent, T0 associated
with localization length, and d the dimensionality. For a
three-dimensional system, the conductivity ln r should
vary as T-1/4
. Figure 4 shows the ln r versus T-1/4
for the
Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05, and 0.10)
samples. This behavior might be associated with the
positional disorder positional disorder involved in the
incommensurate structure of the misfit layered title system
[35, 51].
Thermopower measurements are a very sensitive probe
to the type and characteristic energy of carriers and are a
complementary tool to the resistivity measurements for
transport studies. Since thermopower is a measure of the
heat per carrier over temperature, we can thus view it as a
measure of the entropy per carrier. The positive thermo-
power confirms that the dominant charge carriers are holes
Fig. 2 The temperature dependence of electrical resistivity for the
Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05 and 0.10) samples
Fig. 3 Variation of q versus T2
of Ca2.9Bi0.1Co4-xFexO9?d
(x = 0.00, 0.025, 0.05 and 0.10) samples. The solid lines are linear
fitting using q = q0 ? AT2
. A: Fermi-liquid transport coefficient,
TIM: transition temperature of nonmetallic to metallic, T*
: strongly
correlated Fermi-liquid regime up to the temperature
J Mater Sci: Mater Electron (2014) 25:778–784 781
123

5. for all the samples. Moreover, Hall measurements also
exhibit the majority carriers are p-type. The undoped
sample exhibits a larger absolute S value 147 lV K-1
at
300 K. The thermopower of undoped sample is higher than
that of the doped samples. The thermopower of doped
samples decrease with increasing Fe content due to an
increase in hole carrier concentration (Table 3). Besides,
the excess of oxygen content also affects the thermopower.
Karppinen et al. [38] reported that the thermopower
slightly decreases with increasing excess of oxygen content
for the Ca3Co3.95O9?d (d = 0.07, 0.24, 0.29) system. In our
case, the excess of oxygen content increases with
increasing Fe content (Table 2), indicates an increase in
hole carrier concentration and thus leads to a decrease of
thermopower. Therefore, one can easily conclude that
decreasing the thermopower is mainly contributed by
increasing the hole carrier concentration. In general,
increasing the carrier concentration of materials would
decrease the electrical resistivity and decrease the ther-
mopower. Figure 5 shows the temperature dependence of
thermopower (S) for the Ca2.9Bi0.1Co4-xFexO9?d
(x = 0.00, 0.025, 0.05 and 0.10) samples. The undoped
and doped samples exhibit a similar temperature depen-
dence of thermopower, but the absolute thermopower
values are different. Similar results were observed in other
elements doped in Ca3Co4O9?d system [6, 8].
In general, the thermoelectric power can be expressed
by the Mott formula [22, 52].
SðTÞ ¼
1
eT
R1
À1 rðeÞðe À lÞ ofðeÞ
oe de
R1
À1 rðeÞ ofðeÞ
oe de
ð3Þ
where r(e), f(e), and l represent electrical conductivity,
Fermi–Dirac distribution function at energy e, and
chemical potential. The product of the thermoelectric
coefficient and temperature can therefore be understood as
the mean energy flow carried by a conduction electron.
Using the condition of qf(e)/qe = d (e - EF), and
r = nel(e) [22, 52] Eq. (3) can be written as:
SðTÞ ¼
Ce
n
þ
p2
k2
BT
3e
o ln lðeÞ
oe
!
e¼EF
ð4Þ
where l(e), Ce, and kB are the 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. (2) is dominant then S
closely related to the electronic structure [22, 30, 46].
Therefore, it can be concluded that first term is dominant in
our case.
Total thermal conductivity (jtotal) can be expressed as
jtotal = jcarr ? jph, where jcarr and jph represent the car-
rier and the lattice thermal conductivity, respectively. jcarr
can be calculated using the Wiedemann–Franz–Lorenz
relationship, jcarr = LrT, where L = p2
k2
/3e2
= 2.45 9
10-8
W X K-2
is the Lorenz number and T is the absolute
temperature. The total thermal conductivity slightly
decreases with increasing Fe content. For materials with
q [ 1 X-cm, jcarr is negligible. But in our case, the resis-
tivity is lower than 1 X-cm, a fact which leads us to
determine the jcarr by Wiedemann–Franz law. The calcu-
lated value of jcarr at 300 K is 0.059 and 0.066
W m-1
K-1
for the Ca2.9Bi0.1Co4O9?d and Ca2.9Bi0.1
Co3.90Fe0.10O9?d, respectively. For all the samples, the
lattice contribution is more important than the carrier one.
Due to the small jcarr, jtotal is mainly attributed to the
lattice contribution. The figure of merit (ZT = S2
T/qj) is
calculated for all the samples. The highest ZT (0.056) is
reached for the Ca2.9Bi0.1Co3.90Fe0.10O9?d among the
Fig. 5 The temperature dependence of thermopower (S) for the
Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05 and 0.10) samples
Fig. 4 Plot of In r versus T-1/4
(K-1/4
) for the Ca2.9Bi0.1Co4-x
FexO9?d (x = 0.00, 0.025, 0.05 and 0.10) samples
782 J Mater Sci: Mater Electron (2014) 25:778–784
123

6. samples, which is slightly improved as compared to the
undoped Ca2.9Bi0.1Co4O9?d at 300 K. The ZT value of our
samples is higher than those of previous reports [17, 22, 40,
53–55]. Wang et al. [22] have reported the highest ZT of
0.026 at 300 K for the Ca3Co4-xMxO9?d (M = Fe, Mn,
and Cu; x = 0, 0.05, and 0.1). Liu et al. [17] reported the
ZT value of 0.045 at 300 K for the Ca2.85Bi0.15Co4O9
samples sintered by SPS. Moser et al. [40] reported the ZT
of *0.04 at 300 K for the Ca2.8Bi0.2Co4O9?d samples
prepared by conventional solid-state reaction method. Park
et al. [53] reported a ZT value 0.03 at 300 K for the
Ca2.7Bi0.3Co4O9 sample prepared by conventional solid-
state synthesis. Li et al. [54] prepared Ca2.5Bi0.5Co4O9?d
sample by conventional solid-state reaction method and
reported a ZT value of *0.03 at 300 K. Xu et al. [55]
reported that the ZT value of *0.025 at 300 K for the
Ca2.7Bi0.3Co4O9?d sample prepared by hot-pressing tech-
nique. These results suggest there is scope for further
improvement of thermoelectric properties.
Figure 6 shows the temperature dependence of magnetic
susceptibility for the Ca2.9Bi0.1Co3.90Fe0.10O9?d sample.
The susceptibility value monotonically decreases with
increasing temperature. The observed effective magnetic
moment is derived by fitting the magnetic susceptibility
versus temperature using the Curie–Weiss law, as shown in
Fig. 6. The observed effective magnetic moments are
1.28 lB/Co for x = 0.00, 1.24 lB/Co for x = 0.05 and
1.15 lB/Co for x = 0.10, respectively. The observed
effective magnetic moments decrease with increasing Fe
content; this may be related to the increasing super
exchange mechanism, Fe3?
–O2-
–Fe3?
or Fe2?
–O2-
–Fe2?
.
Wang et al. [22] also reported that super exchange mech-
anism is enhanced for the Ca3Co3.9Fe0.1O9?d as compared
to undoped Ca3Co4O9?d sample. These results suggest that
ferrimagnetic is suppressed by the Fe substitution. The
interlayer coupling between CoO2 and Ca2CoO3 sublayers
could be the origin of ferrimagnetism [56–58]. The
Ca2CoO3 layer has 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 the Co–O
plane and CaO2 sublayers [11]. The ferromagnetic cou-
pling would weaken by the distortion of Ca–O and Co–O
planes due to the bismuth substitution in Ca–O planes and
Fe substitution in Co–O planes. The Co3?
and Co4?
ions
with a low-spin configuration have a theoretical effective
magnetic moment of 0 lB/Co and 1.73 lB/Co, respec-
tively. The Co3?
and Co4?
ions with a high-spin configu-
ration have a theoretical effective magnetic moment of
4.98 and 5.91 lB/Co, respectively. The observed effective
magnetic moment is closely to the low-spin configuration
of cobalt. Liu et al. [7] reported that low-spin state of
cobalt is observed in Ca3Co4-xFexO9?d (x = 0.00, 0.05,
0.10, and 0.15) system.
4 Conclusions
In conclusion, we have prepared polycrystalline samples of
Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05 and 0.10)
samples by conventional solid-state synthesis. The XRD
results show that all the samples are single phase. The
electrical resistivity results indicate that all the samples
obey the variable range hopping mechanism in the low
temperature regime. The TMI is observed at around
88 ± 2 K. The T*
and slope of A value decrease with
increasing Fe content. The positive thermopower confirms
that the dominant carriers are holes for all the samples. The
substitution of Fe ion increases the hole carrier concen-
tration, which results in decrease of resistivity and ther-
mopower. The highest figure of merit (ZT) is observed for
the Ca2.9Bi0.1Co3.90Fe0.10O9?d among the samples. Mag-
netic measurements indicate that all the samples exhibit a
low-spin state of cobalt ion. The effective magnetic
moment decreases with increasing Fe content.
Acknowledgments 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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