Size Effect of A-site Cation on n-type Thermoelectric Properties in CaMnO3-based System
Hiroyuki Fujishiro*, Yu Suzuki, Yosuke Fujine and Manabu Ikebe
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan*e-mail: fujishiro@iwate-u.ac.jp, fax: +81-19-621-6373
Abstract
The thermoelectric (TE) properties of the electron-doped(REXAE1-X)MnO3 (RE: rare earth ions such as La, Pr etc., AE:
the alkaline earth ions such as Ca, Sr etc., X=0.05, 0.10) havebeen investigated as a function of the average ionic radius rAof the A-site cation in the perovskite structure (AMnO3). The
electrical resistivity ρ, Seebeck coefficient S and the thermal
conductivity κ change depending on rA. Near the room
temperature, the figure of merit Z shows a maximum atrA(opt)=1.182~1.184Å,
which
isrealizedfor
theLa0.10Ca0.90MnO3 and La0.05(Ca0.98Sr0.02)0.95MnO3. The 5%
substitution by elements M with varying valencies for the Mnsite degrades the TE properties in the LaXCa1X(Mn0.95M0.05)O3 (X=0.05, 0.10) due to the disorder in theMnO6 network. The cracks, inevitably induced in the samples
especially for the small X region, seriously reduce the TEproperties.
1. Introduction
Since the discovery of the high thermoelectric (TE)performance in NaCo2O4 [1], the TE properties of the oxides
have been widely investigated. The TE module using only thep- and n-type oxides has been fabricated and operated [2].
Compared with higher p-type TE performance in NaCo2O4,
the TE potential of the n-type oxides is, at present, lower forthe practical use. The electron-doped MXCa1-XMnO3 shows arelatively high TE performance among oxides and is apossible candidate of the n-type TE materials. Thesubstitution for the Ca-site by Bi has presented the best figureof merit Z for X=0.05~0.10 [3]. Ohtaki et al. investigated the10% substitution effect for the Ca-site by several elements M
in the (M0.10Ca0.90)MnO3 system [4]. It was found that theelectrical conductivity σ at 300 K increased with increasing
average ionic radius of the (M0.10Ca0.90)-site resulting in M=Bi
being the best element for the thermoelectricity. The origin ofthe high performance comes from the increase of the carrier
mobility µ caused by the larger intersite distance for hopping.
However, there have been no systematic investigations as forthe size effect of the A-site cation of the perovskite structure(AMnO3) on the TE properties including the Ca-site
substitution by divalent ions.
In this study, we investigate the relation between theaverage ionic radius rA of A-site cation and thethermoelectricity (the figure of merit Z and the power factorP) in (REXAE1-X)MnO3 (RE=rear earth ions, AE=alkaline
earth ions) at the concentration of X=0.05 and 0.10. The rA
value determines the lattice distortion from the cubic throughthe tolerance factor Γ (=(rA+rO)/√2(rMn+rO), rMn and rO being
the ionic radii of Mn and oxygen) and the one-electron
bandwidth is broadened as Γ approaches 1. The variance σ2 of

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the A-site ionic radius (=Σyiri2-rA2, yi being the fractional
occupancy of the A-site species and ri being its ionic radius)
also affects the magnetic transition [5]. We also report thesubstitution effect of the Mn-site by several elements M withvarious ionic valencies on the thermoelectricity in LaXCa1X(Mn0.95M0.05)O3 (X=0.05 and 0.10). The relation between thethermoelectricity and the lattice distortion and the variance σ2is discussed for the electron-doped system.
2. Experimental
The several series with 1.17Å≤rA≤1.20Å ofRE0.10Ca0.90MnO3 (RE=Dy, Gd, Sm, Nd, Pr, La), La0.10(Ca1ySry)0.90MnO3 (0.05≤y≤0.4) and La0.05(Ca1-yAEy)0.95MnO3(AE=Sr, Ba: 0.02≤y≤0.05) were fabricated by a solid statereaction method. The LaXCa1-X(Mn0.95M0.05)O3 (M=Mg2+,Al3+, Sn4+; X=0.05 and 0.10) samples were also fabricated.
The rA value, the tolerance factor Γ and the radius variance σ2were calculated from the tabulated radii with 9-foldcoordination [6]. The mixtures of raw powders were calcinedat 1000 °C for 24 h in air, pressed into pellets and thensintered at 1400 °C for 8 h in air. Some of the samples were
heat-treated in flowing oxygen at 900 °C for 24 h in order toachieve the oxygen stoichiometry. The measured densities ofsamples were about 85~90 % of the ideal ones. The X-ray
diffraction analyses at room temperature confirmed that all thesamples were in a single orthorhombic phase. The electricalresistivity ρ(T) was measured by a standard four-terminal
method from 10 K to 300 K. The thermal conductivity κ(T)and Seebeck coefficient S(T) were measured by a steady-stateheat flow method. The carrier mobility µ and the carrier
concentration n were estimated measuring the Hall coefficientat room temperature.
3. Results and Discussion
3.1 (REXAE1-X)MnO3 system (X=0.05, 0.10)
Fig. 1 shows the temperature dependence of the figure ofmerit Z(T) of the LaXCa1-XMnO3 and RE0.10Ca0.90MnO3(RE=Nd, Sm). The Z values of all the samples decrease withdecreasing temperature T. In LaXCa1-XMnO3, the Z value near
the room temperature takes a maximum at X=0.05 and thendecreases with increasing X. The electrical resistivity ρ andthe Seebeck coefficient S decrease with increasing X at room
temperature, while the thermal conductivity κ remains nearly
constant. Then the Z enhancement at X=0.05 comes from therelatively large S and small ρ values near the optimum
electron doping level. It has been found that the number ofcracks in the samples increases for 0≤X≤0.05 and that theexistence of the cracks reduces the Z value causing the2004 International Conference on Thermoelectrics

120
RE=La, X=0.18
RE=La, X=0.15
RE=La, X=0.10
RE=La, X=0.05
X=0
RE=Nd, X=0.10
RE=Sm, X=0.10

80
60
40
20
0100
200
Temperature (K)300Figure 1: The temperature dependence of Z(T) of the LaXCa1XMnO3 and RE0.10Ca0.90MnO3 (RE=Nd, Sm).
Tolerance Factor Γ0.9396
0.94330.94690.93600.9506(a)La0.1(Ca1-ZSrZ)0.9MnO310-50

5-100S (µµ V/K)

290K
0-150(b)La0.1(Ca1-ZSrZ)0.9MnO36060404040

Gd
Dy01.17

Nd
SmLa
Pr

0.10

0.20

Z=0.05

1.18
1.19
1.2
A Site Ionic Radius r (Å)

01.21

A

Figure 2: (a) ρ and S, (b) κ and Z for RE0.10AE0.90MnO3 at
290 K as a function of rA.
increase in ρ. The Z value for RE0.10Ca0.90MnO3 (RE=Nd, Sm)is lower than that for the RE=La sample.
Fig. 2 shows the average A-site ionic radius (rA)dependence of (a) the resistivity ρ and Seebeck coefficient Sand (b) the thermal conductivity κ and figure of merit Z forvarious RE0.10AE0.90MnO3 samples at 290 K. The ρ values
decrease with increasing rA for RE0.10Ca0.90MnO3 and thentakes a minimum at rA=1.184Å (RE=La) and increases withfurther increase of rA for the La0.10(Ca1-ZSrZ)0.90O3 system. The
temperature dependence of ρ is semiconductive, i.e., ρ1-4244-0128-3/05/$20.00 ©2005 IEEE

0.9506(a)3020
1002020

Tolerance Factor Γ
0.9396
0.94330.94690.9360κ (mW/cmK)(RE0.1Ca0.9)MnO3

80
Z (10-6 K-1)

290K
ρ (mΩΩcm)

ρ (mΩΩcm)(RE0.1Ca0.9)MnO3

S (µµ V/K)

0increases with decreasing T for all the samples in this study.
The S value hardly depends on rA and the κ value slightly
decreases with increasing rA. The increase of rA results in thereduction of the static lattice distortion from the cubicstructure with the tolerance factor Γ approaching 1 as shownin the upper axis of abscissas. The ρ reduction inRE0.10Ca0.90MnO3 may result from the decrease in the lattice
distortion from the cubic, originating from the broadening of
the bandwidth. The ρ increase for rA>1.184 Å may come fromthe introduction of the lattice disorder and/or the increase of
the radius variance σ2 caused by the substitution of Sr for theCa-site.
The similar rA dependences can be seen for the 5%
electron doping system. Fig. 3 shows (a) the resistivity ρ, (b)the Seebeck coefficient S and (c) the thermal conductivity κfor (RE0.05AE0.95)MnO3 samples at 290 K as a function of rA.
In these figures, the bold symbols show the values of which
the samples were carefully cut from the nearly crack-free
region of the sintered pellets. For the majority samples
bearing some cracks as shown in the open symbols, the ρvalue takes a minimum and the S value takes a faint maximum
around rA~1.183 Å for La0.05(Ca0.98Sr0.02)0.95MnO3. The κvalue slightly decreases with increasing rA. These behaviors
are similar to those for the RE0.10AE0.90MnO3 shown in Fig. 2.
For the nearly crack-free samples, the low ρ value and thehigh κ value are naturally expected. The extremely strong
enhancement in the S value was observed for theLa0.05(Ca0.98Sr0.02)0.95O3 and La0.05Ca0.95MnO3 samples. One
possible explanation is that the intrinsic higher TE potential
was masked out by the existence of the cracks.

-100(b)

RE=Nd, AE=Ca
RE=(La,Pr), AE=Ca
RE=La, AE=Ca
RE=La, AE=(Ca,Sr)
RE=La, AE=(Ca,Ba)-120-140-160

κ (mW/cmK)Z (10-6 K-1)100

REXCa1-XMnO3(c)4020290K
01.17

1.18
1.19
1.2
A Site Ionic Radius r (Å)

1.21

AFigure 3: The rA dependence of (a) ρ, (b) S and (c) κ for
RE0.05AE0.95MnO3 at 290 K.

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1.06040

nearly crack-freesamples

1.18
1.19
1.2
A Site Ionic Radius r (Å)

1.21

Figure 4: The rA dependence of Z for RE0.05AE0.95MnO3 at
290 K.
100
Figure of Merit Z (10-6K-1)

0.6
0.4

01.18

290K
A

RE=Nd, AE=Ca
RE=(La,Pr), AE=Ca
RE=La, AE=Ca
RE=La, AE=(Ca,Sr)
RE=La, AE=(Ca,Ba)

80
604020290K
0 -5
100.8

0.2

2001.171022

1.2
RE=Nd, AE=Ca
RE=(La,Pr), AE=Ca
RE=La, AE=Ca
RE=La, AE=(Ca,Sr)
RE=La, AE=(Ca,Ba)

10-4
10-3
2A-site radius distribusion σ (Å2)

10-2

Figure 5: The Z value for the RE0.05AE0.95MnO3 samples at
290 K as a function of σ2.
Fig. 4 shows the figure of merit Z as a function of rA for
RE0.05AE0.95MnO3 samples at 290 K. For both the crackbearing and nearly crack-free samples, the Z value takes amaximum around rA=1.812~1.814 Å, depending on the rAvalue. The Z value obtained in this study was lower than thereported values of Bi0.04Ca0.96MnO3 (Z=98 x 10-6 K-1) [7] andBi0.10Ca0.90MnO3 (Z=350 x 10-6 K-1) [4].
Fig. 5 shows the Z value for RE0.05AE0.95MnO3 at 290 K asa function of the A-site ionic radius distribution (variance) σ2.
The Z value takes a maximum and then decreases withincreasing σ2. The increase of σ2 indicates the increase of theA-site lattice randomness in the perovskite structure. In Fig. 4,the Zincrease with increasing rA for rA<1.184 Å may result
from the decrease in the lattice distortion and the Z decrease
for rA>1.184 Å may result from the increase of σ2 and/or theintroduction of the crystal disorder caused by the substitution
by Sr or Ba ions for the Ca-site.
Fig. 6 shows the mobility µ and the carrier concentration nfor the RE0.05AE0.95MnO3 samples at 290 K as a function ofrA, which were estimated by the Hall coefficient. The mobilityµ shows a maximum at rA~1.184 Å, where the ρ1-4244-0128-3/05/$20.00 ©2005 IEEE

1021-380

3n (cm )MnO

RE=La, AE=Ca0.98Ba0.02

0.95

RE=La, AE=Ca0.98Sr0.02

AE

RE=La, AE=Ca

0.05

RE=La0.5Pr0.5, AE=CaREMobility µ (cm2/Vs)

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

1001020

RE0.05AE0.95MnO3
1.182
1.184
1.186
1.188
A Site Ionic Radius r (Å)

1019
1.19

A

Figure 6: The mobility µ and the carrier concentration n for
RE0.05AE0.95MnO3 at 290 K as a function of rA.
shows a lower value shown in Fig. 3(a). The n value, at which
Z takes a maximum, is 1 x 1021 cm-3 and these results indicatethat the carrier hopping is the main conduction mechanism inthis system. However, it must be noted that only the averageradius rA is not the main factor to determine µ and σ (=1/ρ) as
clarified the maximum at rA~1.184 Å.
3.2 LaXCa1-X(Mn0.95M0.05)O3 system (X=0.05, 0.10)
We have investigated the effect of the Co-site substitution
by the various elements on the TE properties forLa0.80Sr0.20(Co1-ZMZ)O3 [8,9]. It was found that the ionicvalency of the substituted element M more strongly influenced
the thermoelectricity rather than the ionic radius or spin stateof the element M. The valence effect of the dopants for theMn-site substitution on the TE properties were studied for
LaXCa1-X(Mn0.95M0.05)O3 (X=0.05, 0.10; M=Mg2+, Al3+, Sn4+).
The substitution by the 2+ ions (Mg2+) is expected to makethe average Mn valency increase and the substitution by the4+ ions (Sn4+) is expected to make the average Mn valency
slightly decrease. Fig. 7 shows (a) the resistivity ρ, (b) theSeebeck coefficient S, (c) the thermal conductivity κ and (d)the figure of merit Z for LaXCa1-X(Mn0.95M0.05)O3 (X=0.05,0.10) at 290 K. The ρ values increase and the κ values
decrease for the Mn-site substitution owing to the introductionof the disorder in the MnO6 network. The ρ values for the Al3+
substitution are the largest and are about an order ofmagnitude higher than those of the non-substituted samples.
The absolute values of S slightly increase by the Mn-site
substitution for X=0.10, but decrease for X=0.05. As a result,the Z value decreases by the Mn-site substitution for both
X=0.05 and 0.10 samples relative to the non-substituted
LaXCa1-XMnO3. The Mn-site substitution seriously damages
the TE properties irrespective of the ionic valency of thesubstituted elements in contrast with the La0.80Sr0.20(Co1ZMZ)O3 system.
4. Conclusions
The thermoelectric (TE) properties of the electron-doped(REXAE1-X)MnO3 (X=0.05, 0.10) have been investigated as a2004 International Conference on Thermoelectrics

1030(a)M=non(b)M=Mg 2+

LaXCa1-X(Mn0.95M0.05)O3-50

M=Sn4+-100
101

S (µµ V/K)

Ωcm)
ρ (mΩ

M=Al3+102-150

1000.05

X0.10.05

M=non(d)

X

0.1

80

100(c)2+

M=Mg

LaXCa1-X(Mn0.95M0.05)O33+60

M=Al80

4+

M=Sn

40
40
200-1-660

Z (10 K )

κ (mW/cmK)-200

200.05X

0.10.05

X

0.10Figure 7: The X dependence of (a) ρ, (b) S, (c) κ and (d) Z of
LaXCa1-X(Mn0.95M0.05)O3 at 290 K (M=Mg2+, Al3+, Sn4+).

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 partially 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.
References
1. Terasaki, I. et al, Phys. Rev. B56 (1997), pp. R12685R12687.
2. Matsubara I., Funahashi R., Takeuchi T. and Sodeoka S.,
Appl. Phys. Lett. 78 (2001) 3627.
3. Kobayashi T., Takizawa H., Endo T., Sato T., Taguchi H.
and Nagao M., J. Solid State Chem. 92 (1991) 116.
4. Ohtaki M., Koga H., Tokunaga T., Eguchi K. and Arai H.,
J. Solid State Chem. 120 (1995) 105.
5. Rodrigues-Martinez L.M., Attfield J.P., Phys. Rev. B54(1996) R15622.
6. Shannon R.D., Acta Crystallogr. Sect. A32 (1976) 751.
7. Xu G., Funahashi R., Matsubara I., Shikano M. and Zhou
Y., J. Mater. Res. 17 (2002) 1092.
8. Fujishiro, H. et al., Proc. 22nd Int. Conf. Thermoelectrics (ICT2003), 235.
9. Fujishiro, H. et al., presented in this conference (ICT2004)

function of the average ionic radius rA of the A-site cation in
the perovskite structure. The electrical resistivity ρ, Seebeckcoefficient S and the thermal conductivity κ have beenmeasured and the figure of merit Z and the power factor P
have been estimated. Important experimental results andconclusions obtained in this study are summarized as follows.
(1) The ρ, S and κ values depend on rA. Near the room
temperature, the Z value shows a maximum atrA(opt)=1.182~1.184Å, which is realized for theLa0.10Ca0.90MnO3 and La0.05(Ca0.98Sr0.02)0.95MnO3. The
decrease of Z with decreasing rA from 1.182 Å may result
from the decrease of the tolerance factor Γ and thedecrease of Z with increasing rA from 1.184 Å may result
from the introduction of the crystal disorder and/or theincrease of the radius variance σ2. The P value shows thesimilar rA dependence.
(2) The 5% substitution by elements M for the Mn-site
degrades the TE properties in LaXCa1-X(Mn0.95M0.05)O3(X=0.05, 0.10) compared with the non-substitutedsamples. The TE property degradation of this system ismainly caused by the crystal disorder in the MnO6 network
and is nearly independent of the ionic valency of thesubstituted elements.
(3) The cracks, inevitably induced in the samples during thesintering process, seriously reduce the TE properties
especially for the small X region (0≤X≤0.10).

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2004 International Conference on Thermoelectrics