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Search for p-type oxide thermoelectrics - cobaltites

H. Fujishiro1*, Y. Fujine1, J. Hejtmanek2, D. Sedmidubsky2, A. Maignan3, and C. Martin3
1
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
3
Laboratoire CRISMAT, CNRS/ISMRA/Universite de Caen, 6 boulevard du Maréchal Juin, 14050 CAEN cedex, France*email: fujishiro@iwate-u.ac.jp, fax: +81-19-621-6373
Abstract The thermal conductivity, thermoelectric power,
electrical resistivity and magnetization of the 3D
perovskites Ln1-XAEXCoO3 (Ln=La,Y and AE=Ca and Ba)
and the low dimensional Ca3Co2O6 and Ca3Co4O9 cobaltites
were measured in a wide temperature range up to 1000 K.
As a novelty we demonstrate the prospective high power
factor of the Y0.9Ca0.1CoO3 at high temperatures and,simultaneously, that in the La1-XAEXCoO3 system the
maximum Z and P values are observed at the phaseboundary between the FM and SG state at X~0.1 andexceed that reported for layered cobaltite ceramics.
1.

Introduction
Two important points are to be solved prior theapplication of the material in the thermoelectric device: - ahigh figure of merit ZT (=S2/κρ; S the thermoelectric power,κ the thermal conductivity, ρ the electrical resistivity) andthe long term chemical stability. Within a large family of
conducting oxides the complex cobaltites are of thefundamental importance as candidates for the hightemperature p-type thermoelectric applications. This is due
the unusual concurrence of a high electrical conductivity
and a high positive thermoelectric power. Consequently,
the layered Co-based oxides such as NaCo2O4 [1],Ca3Co4O9 [2] and Bi2-XPbXCo2Oy [3] have recently
attracted much attention as thermoelectric materials with ahigh figure of merit Z and power factor P (=S2/ρ).
Within the family of conducting cobalt oxides, however,two sub-groups of octahedrally coordinated cobalt
networks (CoO6) can be distinguished: (i) the first one,mentioned above, represented by low dimensional 2D or
1D compounds where the dominating Co-O-Co bond ismediated via edge/face shared octahedra and (ii) withcorner shared (CoO6) octahedra represented by a large
perovskite family.
We exemplify the thermoelectric properties of therepresentative oxides Ca3Co2O6 and Ca3Co4O9 but the mainsignificance is focused on perspectives 3D perovskites
which thermoelectric performance was up to now
marginalized with respect to a very promising bronze-like
NaCo2O4. We present the experimental data on thermal
conductivity, thermoelectric power, electrical resistivity
and magnetism determined in a wide temperature range
from low temperatures up to 1000 K. The possible link
between a high thermoelectric power and various spin (e.g.
Co3+ in a low-LS, intermediate-IS and high spin-HS)
eventually orbital (eg = dz2, dx2-y2) fluctuations is discussed
as the high degeneracy of Co spin state has been pointed
out as a possible origin of the large S values [4].
The studied Ln1-XAEXCoO3 (Ln=La,Y and AE=Ca andBa) systems crystallize a perovskite structure, which, in
dependence of the size of interpolated cation adopts either
Pbnm or R3c symmetry. Although the cobalt perovskites
have been investigated for a long time, there is no
systematic report as for the thermoelectric potential as far
as we know. Let us remember that in the parent material
LaCoO3, the transition from the low spin of Co3+ (LS; S=0)

to intermediate-spin (IS; S=1) configuration was evidenced
at T~100 K when the origin of successive transition at
T~500 K, accompanied by huge increase of electricalconductivity, is not unambiguously elucidated. As mostplausible scenario the spin origin (IS=>HS), orbital order(IS Jan-Teller distorted => IS non distorted) or Mott-like
MI transitions were proposed. Due to the introduction of
Co4+ species a ferromagnetic metallic (FM-M) ground-state
appears in Ln1-XAEXCoO3 for X≥0.2. On the other hand,the cobalt ions in YCoO3 are known to preserve the LS
ground-state up to temperatures well above 300 K, while
the impact of Co4+ species, introduced via the Ca for Y
substitution, on the long range magnetic order is not known.
2. Experimental
Polycrystalline samples of Ca3Co2O6 and Ca3Co4O9
were prepared by a standard ceramic route [5]. The post
annealing in oxygen, which were used to ensure the oxygenstoichiometry in Ca3Co2O6 sample, led however to theformation of the “misfit” Ca3Co4O9 phase. Coherently withthe detection of the Ca3Co4O9 phase by means of X-ray thebi-phasic nature of Ca3Co2O6 post-annealed samples was
confirmed by transport measurements as further
demonstrated in Fig. 1. The perovskites La1-xAExCoO3(0≤X≤0.40; AE=Ca, Ba)) and Y1-XCaXCoO3 (0≤X≤0.20)
were prepared by a solid-state or nitrate decomposition
method, respectively. The samples were heat-treated inflowing oxygen at 900°C for 24 h in order to achieve theoxygen stoichiometry and the measured densities ofsamples were about 85~90 % of the ideal ones. The X-ray
diffraction analyses at room temperature confirmed that all
La-based samples were a single rhombohedral phase andY-based samples posses the Pbnm symmetry. The
electrical resistivity ρ was measured by a standard four
terminal method. The thermal conductivity κ andthermoelectric power S were measured in close cycle
cryostat by a steady-state 4-point heat flow method [6].
The high temperature measurement was performed using
home made apparatus in air. The magnetization M wasmeasured using a SQUID magnetometer and the Hall
coefficient was measured at room temperature using the 5
T cryo-cooled superconducting magnet to estimate thecarrier concentration n.
3. Results and Discussion
3.1 Low dimensional cobaltites
The electric and thermal transport data of 1D and 2D
representative cobaltites are presented in the Fig. 1. The
gradual evolution from the essentially insulating 1D system,
which exhibit the huge positive thermopower, to quasimetallic 2D layered cobaltite is clearly evidenced. At very
high temperatures the high potential for thermoelectric
applications of 1D cobaltite is underlined by a highthermopower of ~ +300 µVK-1 compensated, however, by

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still high resistivity of ~ 100 mΩcm. Let us note also thepresence of low temperature enhancement of thermal
conductivity, observed for pure Ca3Co2O6, which is typicalfor crystalline solids. This phononic peak is evidently
suppressed in layered cobaltite exhibiting “glass-like”
behavior of thermal conductivity.
Fig. 1. The temperature dependence of the electrical

600
400
200
200

400

600

800

1000

200

400

600

800

1000

150 200
T (K)

250

6

10

4

10

2

10

50

100

300

9

resistivity ρ(T)(upper panel), thermoelectric power S(T)(middle panel) and (c) thermal conductivity κ(T) (lower
panel) of Ca3Co2O6 ( ) , intergrowth of Ca3Co2O6 andCa4Co3O9 (Ο) and Ca4Co3O9 (Δ).

108
107
106
105
104
103
102
101
10

Y1-xCaxCoO3

ρ (mΩ cm)

-1 -1

κ (W m K )

0

10
0
10
8
6
4
2
0
0

Magnetic, structural and transport properties were
investigated over a broad temperature region up to 1000 K.
The temperature dependence of magnetic susceptibility isin Fig. 2 compared with lines marking the characteristic
spin states. The magnetic susceptibility data shows that the
Ca doping does not induce the significant change ofmagnetic behaviour and both presented samples, x=0.0 and
x=0.1, preserve the low-spin ground-state up to ~400 K.
Contrary to published data on LaCoO3, the hightemperature metal insulator transition is evidently not
associated with a IS=>HS spin state transition but isaccompanied only by the gradual increase to theintermediate spin state with S=1. Nonetheless, indistinction to the magnetic data, where the Ca doping does
not induce a significant modifications, the important
impact of the Ca to Y substitution on the resistivity andthermoelectric power can be mentioned - see upper andmiddle panel of Fig. 2. Thus, as a consequence ofsignificantly reduced electrical resistivity upon the Ca
doping the high temperature power factor, depicted in Fig.
5, reaches the promising values. In that case, contrary to
the low dimensional cobaltites where the important role ofspin and orbital degeneracy was emphasized, the highvalue of thermoelectric power is conditioned by a low
concentration of charge carries.

-1ρ (mΩ cm)

0
0
8
10

3.2 Y1-XCaXCoO3 (0≤X≤0.20) system

x=0.0
x=0.1
x=0.2

1.5

S (mVK )-1

S (µV K )800

µVK-1 over the basic value defined by charge carrier
concentration.

1.0

generalised Heikes formula S = −kBxln [β.
]e1− xsupposes that dominating contribution to the thermopoweris the configurational entropy of quasi-degenerate charge
carriers. The detailed analysis of this formula is, however,out of the scope of this paper. Nonetheless it is evident thatthe high thermopower can be achieved either as a result ofa low hole concentration (small x) or a high difference
between the spin and orbital degeneracy of the Co sites
without (Co3+ ) or with (Co4+) a hole. Based on photoemission studies of the misfit cobaltites [7] complemented
by magnetic susceptibility measurements [8], both Co3+
and Co4+ are found to be in the low spin states. This allows
to derive additional contribution to the thermopower with
β=1/6 providing huge thermopower enhancement of ~+154

0.5

S=1
(IS)

10

-1

-1

-3

χ (emu molOe 10 )As concerns the high and weakly temperaturedependent thermoelectric power of these cobaltites at hightemperatures we propose is to link its absolute value withconfigurational entropy of “hopping” hole-like charge
carriers where the decisive contribution to thethermopower originates from an interplay of the number ofcharge carriers (x) and the difference in the degeneracy
β= g 3 between Co3+ and Co4+ species, respectively. The
g4

1

x=0.1

0.1 x=0.0
0200

400 600
T (K)S~0.15
(LS)
800

1000

Fig. 2. The temperature dependence of the electricalresistivity ρ(T) (upper panel), thermoelectric power S(T)(middle panel) and (c) magnetic susceptibility χ(T) (lower
panel) of Y1-xCaxCoO3.
3.3 La1-XCaXCoO3 and La1-XBaXCoO3 system
In Fig. 3 and Fig. 4 we show the temperaturedependence of the electrical resistivity, thermoelectric
power and magnetic moment measured at 0.5 T of
La1-XCaXCoO3
and
La1-XBaXCoO3(0.1≤X≤0.40),
respectively. The incorporation of formally Co4+ species
via AE2+ substitution leads to the appearance of FM
component below 200 K and, simultaneously, to themetallic-like behaviour of thermoelectric power. In thecase of La-Ca system, however, the positive and weakly
temperature dependent S(T) value reduces suddenly for
X>0.20. Simultaneously the resistivity is depressed down
to ~1 mΩcm but at low temperature the localization occurs

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up to X=0.40. The ferromagnetism is more pronounced incase of La-Ba system with the tolerance factor Γ of thesignificantly larger than that of the LaCa-system.
Coherently with a high metallic electrical conductivity offerromagnetic samples the S(T) reaches very small values
typical for metallic thermopower. Nonetheless the direct
association of the decreased thermopower due to thedecrease of the spin degeneracy of Co ions is also not
excluded see. e.g. sample X=0.2 which exhibit activatedelectrical resistivity below TC but thermopower is close to
zero.

Figure 4: The temperature dependence of the electricalresistivity (a), thermoelectric power (b) and magnetic
moment (at 0.5 T) (c) of the La1-XBaXCoO3 (0.1≤X≤0.40).

Figure 3: The temperature dependence of the electricalresistivity (a), thermoelectric power (b) and magnetic
moment (at 0.5 T) (c) of the La1-XCaXCoO3 (0.1≤X≤0.40).
We demonstrate in the Fig. 5 the temperaturedependence of the power factor P(T) for Y1-XCaXCoO3,
La1CaXCoO3 and La1BaXCoO3. The P(T) value of Y1XCaXCoO3 is low at 300 K, but is enhanced at the phaseboundary of metal-insulator (M-I) transition (T=700 K) incase of X=0.1 sample which makes this system promisingwith respect to a high temperature applications. The
maximum P(T) value of La1CaXCoO3 is obtained for
X=0.15, but that of La1BaXCoO3 for X=0.10. Thisdifferences can be linked with different tolerance factor
values of both systems.
Figure 5: The temperature dependence of the power factor
P(T) of (a) Y1-XCaXCoO3, (b) La1CaXCoO3 and (c)La1BaXCoO3.
In the Fig. 6 we summaries the X dependence of Z for
La1-XAEXCoO3 ( AE=Ca,Sr,Ba) at room temperature. A
clear sharp maximum can be seen at around X=0.10 for all
systems. However, Z for the La1-XCaXCoO3 system exhibitsa broadened X dependence and the corresponding calcium
concentration X(Zmax) is larger. It is to be noticed that theionic radius of the Ca2+ ion is smaller than those of Sr2+ and-4-

Ba2+ enhancing then the lattice distortion and hindering thedegeneracy of charge carriers.
The maximum Z and P values of the La1-XAEXCoO3
system are higher than those of layered cobaltites, whichhave recently been investigated vigorously. In the Fig. 7
we show the comparison of P(T) based on our data and thereported values of typical layered cobaltites. The P(T)values of La1-XBaXCoO3 (X=0.10) and La1-XCaXCoO3(X=0.15) are confronted with ceramic Ca3Co4O9 [2],single-crystalline Bi2.3-XPbXSr2.6Co2Oy (X=0.44) [3],ceramic NaCo2O4 [9], and ceramic NaCo2-XCuXO4 [10].
Although our P values are about one order of magnitude
smaller than that for the NaCo2O4 single crystal (~5mW/K2m; [1]), the present P values are larger than those of
the other layered cobaltites prepared by ceramic
technology. Let us underline that the largest P(T) values
are observed for X~0.1, where the magnetic properties
changes from the spin-glass state to the FM state.

(1) In the 1D and layered cobaltites the important part ofthermoelectric power up to 1000 K is associated withconfigurational entropy of low spin Co3+ and Co4+
configurations(2) The Y0.9Ca0.1CoO3 exhibit promising termoelectric
properties up to 700 K(3) For the La1-XAEXCoO3 system (AE=Ba, Sr, Ca), thefigure of merit Z and power factor P increase withincreasing AE concentration X and peaks at X=0.10 anddecreases for higher X. The maximum Z is observed at
the phase boundary between the FM and SG state. At
300 K, the best Z and P values are larger than those oflayered cobaltites such as Ca3Co4O9 , NaCo2-XCuXO4.
Acknowledgments
This work was supported by a Grant-in-Aid for
Scientific Research from the Ministry of the Education,
Culture, Sports, Science and Technology, Japan (No.
14550661) and by program KONTAKT ME 572 of the
Ministry of Education, Youth and Sports of the Czech
Republic.
References

Figure 6: The X dependence of Z for La1-XCaXCoO3, La1XSrXCoO3 and La1-XBaXCoO3 systems at 300 K.

Figure 7: P(T) of the studied perovskites (La0.90Ba0.10CoO3
and La0.85Ca0.15CoO3) confronted with the reported valuesof the typical layered cobaltites (Ca3Co4O9 [2], Bi2.3XPbXSr2.6Co2Oy (X=0.44) [3], NaCo2O4 [9] and NaCo2XCuXO4 [10]).
4. Conclusions
The thermoelectric properties of three cobalt perovskite
systems (Y1-XCaXCoO3 (0≤X≤0.20) and La1-XAEXCoO3(0≤X≤0.40, AE=Ca, Ba)) as well as the low dimensional
cobaltites (Ca3Co2O6 and Ca4Co3O9) have been measured
and analyzed. Important experimental results andconclusions obtained in this study are summarized asfollows.

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