Enhanced Thermoelectric Properties at X~0.1 in La1-XSrXCoO3 andLa1-XSrX(Co1-YMY)O3 (M=Cr, Cu)
H. Fujishiro1*, Y. Fujine1, Y. Kashiwada1, M. Ikebe1 and J. 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
1Abstract
The thermoelectric properties of the La1-XSrXCoO3 systemhave been investigated from 10 K to 300 K. The figure of
merit Z and power factor P increase with increasing Srconcentration X and show a maximum for X=0.10~0.15,
where the ferromagnetic (FM) order starts to appear. Themaximum Z value (Z=150 x 10-6 K-1 at 230 K for X=0.10) islarger than those of layered cobaltites. The Z and P values areenhanced by the 5 % Co-site substitution by Cr for the overdoped concentration (X=0.20), but are depressed for thenearly optimum doped one (X=0.15). The results arediscussed in conjunction with the carrier density n of thissystem.
1. Introduction
The layered Co-based oxides such as NaCo2O4 [1],Ca3Co4O9 [2] and Bi2-XPbXCo2Oy [3] with edge-shared CoO6octahedra have recently attracted much attention as
thermoelectric materials with a high figure of merit Z(=S2/κρ; S the thermoelectric power, κ the thermal
conductivity, ρ the electrical resistivity) and power factor P(=S2/ρ). The high degeneracy of Co spin state has been
pointed out as a possible origin of the large S values [4]. La1XAEXCoO3 (AE=Ba, Sr, Ca) system has a simple perovskite
structure, which consists of three-dimensional network withcorner-shared CoO6 octahedra. Although this system has been
investigated for a long time, there is no systematic report asfor the thermoelectric potential as far as we know. In theparent material LaCoO3, the spin state of Co3+ shows the lowspin (LS; S=0) and/or intermediate-spin (IS; S=1)
configuration and the ferromagnetic metallic (FM-M) state
appears for X≥0.2 by the introduction of Co4+ spins (IS,S=3/2). In this paper, we present the thermoelectric properties
and the magnetism of the La1-XSrXCoO3 system from 10 K to300 K. The substitution effect for the Co ion site by the 3dtransition metal ions such as Ti, Cr, Mn, and Cu is alsostudied and the relation between the thermoelectric properties
and the carrier (hole) concentration n is discussed.
2. Experimental
Polycrystalline samples of La1-xSrxCoO3 (0≤X≤0.50) and
La1-XSrX(Co1-YMY)O3 (X=0.15~0.30; Y=0.05, 0.10; M=Cr, Cuetc.) were prepared by a solid-state reaction method. The
mixtures of raw powders were calcined twice 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 oxygenstoichiometry. The oxygen content was measured by theiodometric titration. The measured densities of samples wereabout 85~90 % of the ideal 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 and thermoelectric power S weremeasured by a steady-state heat flow method [5]. The
magnetization M was measured using a SQUID
magnetometer under the magnetic field of 0.5 T (zero fieldcooling). The Hall coefficient was measured at room
temperature using the 5 T cryo-cooled superconducting
magnet to estimate the carrier concentration n.
3. Results and Discussion
3.1 La1-XSrXCoO3 (0≤X≤0.50) system
Figs. 1(a)-1(d) show the temperature dependence of the(a) electrical resistivity ρ(T), (b) thermoelectric power S(T),(c) thermal conductivity κ(T) and (d) magnetization M(T) ofLa1-XSrXCoO3, respectively. These figures also contain theresults of La1-XSrX(Co0.95Cr0.05)O3 (X=0.15, 0.20), which are
to be discussed in the following subsection. ρ(T) and S(T)decrease with increasing Sr concentration X. The positive
S(T) value reduces suddenly for X>0.20, where the systemshows the FM-M order. The decrease of S(T) comes from theincrease of n and/or the decrease of the degeneracy in the Co
spin state due to the FM transition. κ(T) increases withincreasing X and a slight enhancement in κ(T) for X=0.20 canbe observed below the FM transition temperature Tc (=220 K).
The larger κ(T) value and the κ(T) anomaly below Tc are
attributable to the electronic contribution, κe(T), as estimatedby the Wiedemann-Franz law.
Fig. 2 shows the temperature dependence of the (a) figureof merit Z(T) and (b) power factor P(T) of La1-XSrXCoO3,
which was calculated using the data shown in Figs. 1(a)-1(c).
The Z(T) and P(T) show the peak below the room
temperature, decreasing with the further decrease of T. TheZ(T) and P(T) values increase with increasing X, show amaximum at X=0.10 and decrease with the further increase ofX. The characteristic temperatures, T(Zmax) and T(Pmax), at
which the Z and P values reach the maximum, decrease withincreasing X up to X=0.15.
Fig. 3 presents the X dependence of Z for La1-XSrXCoO3 at
300 K. A clear peak can be seen at X=0.10 and then the Zvalue decreases with increasing X. It is to be noticed that themaximum Z value at 300 K is higher than those of layered
misfit cobaltites such as poly-crystalline (p)- Ca3Co4O9 [2],single-crystalline (s)-Bi2.3-XPbXSr2.6Co2Oy (X=0.44) [3], pNaCo2O4 [6], and p-NaCo2-XCuXO4 [7], which have recently

Figure 2: The temperature dependence of the (a) Z(T) and (b)P(T) of the La1-XSrXCoO3 (0.08≤X≤0.20) and La1XSrX(Co0.95Cr0.05)O3 (X=0.15, 0.20) systems.

Figure 3: The X dependence of Z for La1-XSrXCoO3 at 300 K.
from the larger S and relatively small ρ values of this system
around X~0.1, where the FM moment starts to appear.
Figure 1: The temperature dependence of (a) ρ(T), (b) S(T),(c) κ(T) and (d) M(T) of the La1-XSrXCoO3 (0.08≤X≤0.20) andLa1-XSrX(Co0.95Cr0.05)O3 (X=0.15, 0.20) systems.
been investigated vigorously. Although the P values obtainedby this work are about one order of magnitude smaller thanthat for the NaCo2O4 single crystal (~5 mW/K2m; [1]), thepresent P values are larger than those of the other layered
cobaltites at 300 K. The large P(T) and Z(T) values originate

3.2 La1-XSrX(Co1-YMY)O3 (M=Cr, Cu) system
In a previous paper [8], we investigated the Co site
substitution effect by the 3d-transition metal elements (M=Ti,Cr, Mn, Fe, Ni, Cu and Zn) on the thermoelectric properties
and the magnetism in the La1-XSrX(Co1-YMY)O3 system(X=0.15~0.30, Y=0.05 and 0.10). These properties depend on
the 3d-transition metal element substituted for the Co ions.
We show the typical results for M=Cr, which results in thebest thermoelectric property improvement, and M=Cu, which
was reported to enhance the P and Z values in NaCo2-YCuYO4[9]. The temperature dependences of ρ(T), S(T), κ(T) andM(T) of La1-XSrX(Co0.95Cr0.05)O3 (X=0.15 and 0.20) are shownin Figs. 1(a)-1(d). ρ(T) and S(T) increase and κ(T) decreases
with the substitution of 5 % Co by Cr for both X=0.15 and0.20. The M(T) value and the FM transition temperature Tcdecrease with the substitution by Cr. The ρ(T) and M(T)behaviors of La0.85Sr0.15(Co0.95Cr0.05)O3 are nearly identical tothoseof
La0.90Sr0.10CoO3,
andthoseof
La0.80Sr0.20(Co0.95Cr0.05)O3 are also nearly identical toLa0.85Sr0.15CoO3. These results suggest that the main effect of
the small amount Co-site substitution by Cr is merely to makethe hole concentration n lower. As a result, the ρ(T) and S(T)values increase and κ(T) decreases because the substituting
element acts as scattering centers at the same time. Z(T) andP(T)oftheLa0.85Sr0.15(Co0.95Cr0.05)O3andLa0.80Sr0.20(Co0.95Cr0.05)O3 samples are contained in Figs. 2(a)and 2(b).
Figs. 4(a) and 4(b) show the P value of the Cr- and Cusubstituted samples at (a) 300 K and (b) 150 K as a functionof X. The P value decreases for X=0.15 and increases for
X=0.20 by the Co-site substitution by Cr, compared with thenon-substituting samples. These results can be understood in
the following way. The thermoelectric properties areenhanced by the Cr-substitution for over-doped Srconcentration (X=0.20), but are depressed by the nearly
optimum one (X=0.15) because n is decreased by the Cr
substitution. X(Pmax) , at which P takes the maximum, shifts
to higher X compared with the non-substituted system. On theother hand, for the Co-site substitution by Cu, there is no
distinct improvement for the P value, which makes a clear
contrast to that for the NaCo2-YCuYO4 system [9].
3.3 Carrier concentration
We arrange the experimental results of the thermoelectric
properties using the carrier concentration n determined Halleffect measurement to discuss the effect of the substitution
for all the La1-XSrX(Co1-YMY)O3 samples. Fig. 5(a) shows theP value of the non-substituted La1-xSrxCoO3 at 300 K as afunction of n. With increasing n, the P value increases, showsa maximum at n~3x1021 cm-3, and then decreases. Fig. 5(b)shows the P value at 300 K for the La1-XSrX(Co1-YMY)O3samples (M=Cr and Cu) as a function of n. For the fixed X, ndecreases with increasing content of the substituted element Y.
In the X=0.20 system, P shows a maximum for Y=0.05 andthen decreases for Y=0.10. On the other hand, in the X=0.15
system, P decreases for the 5% Co-site substitution by Cr andCu. In a conventional theory for thermoelectric
semiconductors such as Bi2Te3, the Z or P values show amaximum at n in the order ~1019 cm-3, which is about twoorders of magnitude smaller than that for the present system.
In general, the smaller n (or X) gives the larger ρ value andthe larger n (or X) gives the smaller ρ and S values. As aresult, the maximum Z or P is necessarily realized at theoptimum carrier concentration. The clear effect on thethermoelectric properties due to the spin state of thesubstituted3d-transition metal element has not been
confirmed

Figure 4: The P values of the La1-XSrX(Co1-YMY)O3 samples(Y=0, 0.05 and M=Cr, Cu) at (a) 300 K and (b) 150 K as afunction of X.

Figure 5: The P values at 300 K of the samples investigatedin this study as a function of the carrier concentration n.
at the present stage. A detailed study, for example, thesubstitution effect for the Co site by non-magnetic elements(Al and Ga), is in progress.
4. Conclusions
The thermoelectric properties of the La1-XSrXCoO3(0≤X≤0.50) and La1-XSrX(Co1-YMY)O3 (X=0.15, 0.20, 0.30;
Y=0.05, 0.10; M=Cr, Cu) systems have been investigatedfrom 10 K to 300 K. Important experimental results andconclusions obtained in this study are summarized as follows.
(1) For the La1-XSrXCoO3 system, the figure of merit Z andpower factor P increase with increasing Sr concentration X,
showing a maximum at X=0.10~0.15 (e.g., Z=150 x 10-6K-1 at 230 K for X=0.10), where the carrier concentration nis 3~4 x 1021 cm-3 and the ferromagnetic (FM) moment
starts to appear. The maximum Z and P values are larger
than those of promising layered cobaltites such aspolycrystalline Ca3Co4O9 at 300 K.
(2) The substitution for the Co-site by Cr most strongly
enhances the Z and P values for the over-doped
concentration (X=0.20), but depresses them for the nearly
optimum doped one (X=0.15). However, these values forthe substituted system cannot exceed those of the nonsubstituted system.
(3) These results can be understood on the basis of the carrier
concentration n of this system. The Co-site substitution by
Cr seems to merely make the n value lower.
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 ResearchProject in 2003.
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