Influence of Co-site Substitution on Thermoelectric Properties in La1-xSrxCoO3
Hiroyuki Fujishiro1*, Yosuke Fujine1, Manabu Ikebe1 and Jiri Hejtmanek2
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
2
Institute of Physics of ASCR, Cukrovarnivka 10, 16200 Praha 6, Czech Republic*email: fujishiro@iwate-u.ac.jp, fax: +81-19-621-6373
1

Abstract
The effect of the Co-site substitution by the various
elements M on the thermoelectric properties has been
investigated for the La0.80Sr0.20(Co0.95M0.05)O3 system. The
figure of merit Z and the power factor P depend on the speciesof the substituted elements through the difference of the ionicradius, the ionic valency, the electronic configuration of 3delectrons and their spin states. Among these factors, the ionicvalency of the substituted elements most strongly influences
the thermoelectricity; the 5% substitution by the +2 ions
(Mg2+, Ni2+) makes the average Co valency (AV) increase to
3.26 from pristine 3.20 and that by the +4 ions (Sn4+, Cr4+)
makes the AV value decrease to 3.16. The substitution by the+4 ions makes the Co valency approach to the optimum AV
value (=3.10), enhancing the thermoelectric properties.
1. Introduction
The layered Co-based oxides such as NaCo2O4 [1],Ca3Co4O9 [2] and Bi2-XPbXCo2Oy [3] have recently attracted
much attention as thermoelectric materials with the highfigure of merit Z (=S2/κρ; the Seebeck coefficient S, thermal
conductivity κ, electrical resistivity ρ) and the power factor P(=S2/ρ). High degeneracy of the Co spin state has been
pointed out as a possible origin of the large S values [4]. La1XAEXCoO3 (AE=Ba, Sr, Ca) has a simple perovskite structure,
which consists of three-dimensional network with cornershared CoO6 octahedra. Although these systems have been
investigated for a long time, there is no systematic report asfor the thermoelectric properties. We have investigated the X
dependence of the thermoelectric properties for these systems[5,6]. It has been found that the Z and P values take themaxima at X=0.10 and the maximum values of Z and P
exceeds those for the polycrystals of the layered cobaltites. In
NaCo2O4, the Co-site substitution by Cu enhanced thethermoelectric properties through the enhancement of theeffective mass of the carrier [7]. We have studied the effect of
the Co-site substitution by the 5% Cu ions in La0.80Sr0.20CoO3
system and have concluded that the thermoelectric properties
cannot be enhanced by the Cu substitution [8].
In this paper, we systematically investigate the influenceof the 5% Co-site substitution with the various elements on
the thermoelectric properties of La0.80Sr0.20CoO3. In the La1XSrXCoO3 system, X=0.10 has been confirmed to be theoptimum hole concentration for the thermoelectric properties,
where the average valency of the Co ion, hereafter weabbreviate as AV, is nominally 3.10. Accordingly, X=0.20
samples investigated in this study are in the over-doped
region. The Co-site substitution may simultaneously introduce
the crystal disorder and the changes of AV value and themagnetic entropy. Then the substitution should affect the1-4244-0128-3/05/$20.00 ©2005 IEEE

electrical resistivity ρ(T), the thermal conductivity κ(T) andthe Seebeck coefficient S(T) which decide the Z and P values.
We try to elucidate the substitution effect on these factors.
2. Experimental
Polycrystalline samples of La0.80Sr0.20CoO3 andLa0.80Sr0.20(Co0.95M0.05)O3 were prepared by a solid-state
reaction method. M=Mg2+ (3d0), Al3+ (3d0), Sn4+ (4d10) ionswere adopted as the non-magnetic ions and M= Ni2+ (3d8),
Fe3+ (3d5), Cr4+ (3d2) as the magnetic ions. The non-magnetic
composite 1%Ti4++4%Ga3+, which does not affect the AV
value, was also substituted in order to clarify the disordereffect solely. The AV value for La0.80Sr0.20CoO3 is 3.20 andvaries depending on the species of M element. The mixturesof raw powders were calcined at 1000 °C for 24 h in air,pressed into pellets and then sintered at 1450°C for 8 h in air.
The samples were heat-treated in flowing oxygen at 900°C for
24 h in order to achieve the oxygen stoichiometry. The
measured densities of samples were about 85~90 % of theideal ones. The X-ray diffraction analyses at room
temperature confirmed that all the samples were in a single
rhombohedral phase.
The electrical resistivity ρ was measured by a standard
four terminal method. The thermal conductivity κ andSeebeck coefficient S were measured by a steady-state heatflow method. The magnetization M0 was measured using a
SQUID magnetometer under the magnetic field of 0.5 T after
Table I. The summary of the substituted element M for
La0.80Sr0.20(Co0.95M0.05)O3 investigated in this study.
Substituted
element

Ionicradius(Å)
0.62

3d electron
Spin
Averageconfiguconfigu
valency ofMration-ration Co ion (AV)
non
3.20
4+
0
1%Ti
3d
0
3.20
+4%Ga3++3d10
5%Mg2+
0.72
3d0
0
3.26
3+
0
5%Al
0.535
3d
0
3.21
5%Sn4+
0.69
4d10
0
3.16
5%Ni2+
0.69
3d8
1
3.26
3+
5%Fe
0.645
3d5
5/2
3.21
4+
2
5%Cr
0.55
3d
1
3.16
zero field cooling (ZFC). All the measurements were
performed from 10 K to 300 K. The valencies of thesubstituted cations were synthetically estimated by the changeof the unit cell volume and κ(T), S(T) and M0(T) behaviors [9].
Table I summarizes the ionic parameters of the studied
elements.
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1

La0.8Sr0.2(Co0.95M0.05)O3

LSCoO(X=0.20)

non-mag
Mg

0.8

2+

3+

Al

0.6

4+

1%Ti4++4%Ga3+

Sn

0.4
0.2
0

Magnetization [µµB/Co-site]

M=non

La0.8Sr0.2(Co0.95M0.05)O3

1

mag

0.8

2+

Ni

3+

Fe

0.6
1%Ti4++4%Ga3+

4+

Cr

0.4
0.2
0

0

100
200
Temperature [K]300

Figure 1: The temperature dependence of the magnetization
M0(T) of La0.80Sr0.20(Co0.95M0.05)O3 for non-magnetic (upper
panel) and magnetic (lower panel) M elements.

Resistivity [mΩ
Ω cm]

102

La0.8Sr0.2(Co0.95M0.05)O3

non-mag
Mg

1%Ti4++4%Ga3+

2+

3+

101

Al

4+

Sn
100
M=non
10-1
La0.8Sr0.2(Co0.95M0.05)O3

Resistivity [mΩΩ cm]

3. Results and Discussion
Fig. 1 shows the temperature dependence of themagnetization M0(T) of La0.80Sr0.20(Co0.95M0.05)O3 for various
non-magnetic (upper panel) and magnetic (lower panel) M
elements. La0.80Sr0.20CoO3 (LSCoO(0.20); AV=3.20) shows aferromagnetic (FM) transition at Tc=210 K. Tc of the
M=1%Ti4++4%Ga3+ sample (AV=3.20), in which only the
crystal disorder should be introduced, decreases down to 150
K. This large depression of Tc may suggest the importance ofthe double exchange interaction in La0.80Sr0.20CoO3. The M0
and Tc values for the Mg2+ and Ni2+ substituted samples(AV=3.26) are larger and those for the Sn4+ and Cr4+
substituted samples (AV=3.16) are smaller, compared withthose for the 1%Ti4++4%Ga3+ substituted sample. The M0 andTc values for the Al3+ and Fe3+ ions (AV=3.21) are slightly
larger than those for the 1%Ti4++4%Ga3+ substituted sample
and show the intermediate behavior between +2 and +4
substituted ions regardless of the magnetic nature of theelements. Thus the M0(T) behavior is more directly correlatedwith the valency of the substituted elements, that is, the AV
value.
Fig. 2 shows the temperature dependence of ρ(T). The
resistivity ρ(T) of the LSCoO(0.20) shows lower values (<1mΩcm) with a metallic behavior even at low temperatures. On
the other hand, ρ(T) of the 1%Ti4++4%Ga3+ sample is larger(>1 mΩcm at 300 K) and increases with decreasing
temperature T. The ρ(T) values progressively increase for the+2, +3, +4 ion doped samples in this order irrespective of themagnetic nature of the substituted ions. Thus ρ(T) is also
closely correlated with the AV value of Co. It is worthwhile tonote that ρ(T) increase in the Mg2+, Ni2+ and Fe3+ substituted
samples is not so large compared with that for other samples.
Particularly, the small ρ(T) values of the Fe3+ substituted
sample in comparison to the Al3+ substituted one demonstrates
the importance of the existence of available 3d orbitals for the
carrier hopping in the system.
Fig. 3 shows the temperature dependence of the Seebeckcoefficient S(T). In the conventional theory, the Seebeckcoefficient increases with increasing ρ value. However, in thepresent case, the S(T) behavior may be more directly
correlated with the magnetization M0 than the resistivity ρ.
Upon cooling, S(T) decreases with increasing magnitude ofM0(T), i.e., the S value decreases at the onset of theferromagnetism as the freedom of spin directions is quenched
and the magnetic entropy is reduced. This result reflects thefact that the Seebeck coefficient represents “entropy per
carrier”. We can see in Fig. 3 that S(T) begins to decrease
near the FM transition temperature Tc, while ρ(T) continues toincrease with decreasing temperatures.

Magnetization [µµB/Co-site]

1.2

mag
2+

Ni

3+

Fe

101

4+

Cr
1%Ti4++4%Ga3+
100
M=non
10

-1

0

100
200
Temperature [K]300

Figure 2: The temperature dependence of the resistivity ρ(T)of La0.80Sr0.20(Co0.95M0.05)O3 for non-magnetic (upper panel)and magnetic (lower panel) M elements.

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2004 International Conference on Thermoelectrics

Seebeck Coefficient [µµ V/K]

100

non-mag

La0.8Sr0.2(Co0.95M0.05)O3

Mg2+
Al3+
Sn4+

1%Ti4++4%Ga3+

80
60
40
20

M=non

Seebeck Coefficient [µ
µV/K]

0

mag

La0.8Sr0.2(Co0.95M0.05)O3

Ni2+
Fe3+

80

Cr4+

60

1%Ti4++4%Ga3+

40
M=non

20
0

0

100
200
Temperature [K]

300

80

60

Al

Mg

-1

2+

M=non

3+

Sn

non-mag
Figure of Merit Z [K ]non-mag
Mg

150

La0.8Sr0.2(Co0.95M0.05)O3

40

20

1%Ti4++4%Ga3+

3+

Sn

4+

50

M=non

1%Ti4++4%Ga3+

00La0.8Sr0.2(Co0.95M0.05)O3

La0.8Sr0.2(Co0.95M0.05)O3
2+

Ni

60

M=non

3+

Fe

4+

40

20

Cr
1%Ti4++4%Ga3+

50
M=non
0

100
200
Temperature [K]300

Figure4:Thethermalconductivity
κ(T) of
La0.80Sr0.20(Co0.95M0.05)O3 for non-magnetic (upper panel) andmagnetic (lower panel) M elements.

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3+

4+

1%Ti4++4%Ga3+
0

2+

Fe

100

Cr

mag
Ni

-1

mag

0

La0.8Sr0.2(Co0.95M0.05)O3

2+

Al

100

4+

Figure of Merit Z [K ]Thermal Conductivity [mW/cmK] Thermal Conductivity [mW/cmK]Figure3:
The
Seebeck
coefficientS(T)of
La0.80Sr0.20(Co0.95M0.05)O3 for non-magnetic (upper panel) andmagnetic (lower panel) M elements. The arrows show the FMtransition temperature Tc determined by M0(T).

Fig. 4 shows the temperature dependence of the thermal
conductivity κ(T). κ(T) of the LSCoO(0.20) shows the largest
value among the samples studied. κ(T) of all the substituted
samples decreases owing to the introduction of the crystal
disorder. It is worthwhile to note that, in Fig. 4(b), the κ(T)enhancement similar to that of the pristine LSCoO(0.20) canbe seen below Tc for the Ni2+ substituted sample. The κ(T)enhancement below Tc is attributable to the electronic
contribution κe(T) as estimated by the Wiedemann-Franz law.
The thermoelectric figure of merit Z(T) can be estimatedusing the results in Figs. 2 to 4. Fig. 5 shows Z(T) for therespective samples. The Z(T) takes the peak below the room
temperature and decreases with the further decrease of T. All
the Z values of the substituted samples increase in comparison
to that of LSCoO(0.20). The Z value of the 4+ ion substituted
sample (AV=3.16) is the largest and decreases with increasing
AV value for both magnetic and non-magnetic cation
substitutions. As mentioned before, the Z value takes amaximum at X=0.10 (AV=3.10) in La1-XSrXCoO3. The
substitution by the +2 ions makes the AV value increase to
3.26 and AV further deviate from the optimum value. The
substitution by the +4 ions makes AV decrease to 3.16,
approaching to the optimum value. Although the substitution
by the +3 ions (Al3+, Fe3+) does hardly influence to the AV
value, the increase of S(T) and ρ(T) and the decrease of κ(T)take place due to the introduced crystal disorder and resultant
change of Tc.

0

100
200
Temperature [K]300

Figure 5: The temperature dependence of the Figure of merit
Z(T) of La0.80Sr0.20(Co0.95M0.05)O3 for non-magnetic (upper
panel) and magnetic (lower panel) M elements.

2004 International Conference on Thermoelectrics

Figure of Merit Z (10-6K-1)

150

La1-XSrX(Co1-YMY)O3(4) The S(T) behavior is more directly related with themagnetization M0(T) than the resistivity ρ(T). S(T) begins
to decrease near Tc, where the freedom of the magnetic
entropy begins to be quenched. In contrast, ρ(T) continues
to increase with decreasing temperatures.
(5) TheoptimumPandZvaluesforLa0.80Sr0.20(Co0.95Cr0.05)O3 has been realized by themagnetic Cr4+ doping, though these values do not exceed
the corresponding best values of La0.90Sr0.10CoO3.

300K(Ca2Co3.34)0.614(CoO2) (Miyazaki et al.)NaCo2O4 (Terasaki et al.)

100

Y=0
M=Cr, Y=0.05
50

0
0

0.1

0.2
0.3
Sr Concentration X

0.4

Figure 6: The figure of merit Z of La1-XSrXCoO3 and La1-XSrX(Co0.95Cr0.05)O3 at 300 K. The Z values of the polycrystalsof layered cobaltites are also shown.

Fig. 6 shows the Sr concentration dependence of Z for La1-XSrXCoO3 and La1-XSrX(Co0.95Cr0.05)O3. As can be seen, the Z-6-1value of 115 x 10 K at 300 K is the largest forLa0.90Sr0.10CoO3, which is comparable with the reported
values of Ca3Co4O9 and NaCo2O4 polycrystals [10,11], though
the Z values obtained by the present work are about an orderof magnitude smaller than that for the NaCo2O4 single crystal.
The Z value of the La1-XSrX(Co0.95Cr0.05)O3 is larger than thatof the LSCoO(0.20) and shows a peak at X=0.20. The resultsin Fig. 6 suggests that the thermoelectric properties areenhanced by the Cr-substitution for over-doped Srconcentration (X=0.20) and the optimum X value for the 5%
Cr4+ substituted system, X(Zmax), shifts to higher X comparedwith the La1-XSrXCoO3.
4. Conclusions
The influence of the Co-site substitution by 5% with thevarious elements (M=Mg2+, Al3+, Sn4+ for the non-magnetic
ions and M=Ni2+, Fe3+, Cr4+ for the magnetic ions) on thethermoelectric properties has been investigated for theLa0.80Sr0.20(Co0.95M0.05)O3 system. Important experimental
results and conclusions obtained in this study are summarized
as follows.
(1) The thermoelectric properties (the figure of merit Z andthe power factor P) change depending on the species ofthe substituted elements.
(2) The valency of the substituted elements most strongly
influences the thermoelectricity; the substitution by the +2ions (Mg2+, Ni2+) makes the average Co valency (AV)increase to 3.26 and that by the +4 ions makes the AV
value decrease to 3.16 from AV=3.20 for the pristine
La0.80Sr0.20CoO3.
(3) The lattice disorder introduced by the 5% doping results inthe decrease of the thermal conductivity κ and the increaseof the resistivity ρ and Seebeck coefficient S. The
magnetization M0 and the ferromagnetic transition
temperature Tc also decrease by the disorder effect,suggesting the significant contribution of double exchangemechanism to the ferromagnetic order.

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Acknowledgments
This work was partially supported by a Grant-in-Aid for
Scientific Research from the Ministry of the Education,
Culture, Sports, Science and Technology, Japan (No.
14550661). This work was performed under theinteruniversity cooperative research program of the Institutefor Materials Research, Tohoku University (Prof. T. Goto),
and under the Japan-Czech Republic Cooperative Research
Project in 2003.
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2004 International Conference on Thermoelectrics