ARTICLE IN PRESS

Physica B 359–361 (2005) 1360–1362
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Anomalous phonon scattering by Jahn–Teller active Co
intermediate spins in LaCoO3 and doped LaCoO3
Yosuke Fujinea, Hiroyuki Fujishiroa, Yohei Kashiwadaa,
Jiri Hejtmanekb, Manabu Ikebea,
abFaculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
Institute of Physics of ASCR, Cukrovarnivka 10, 16200 Praha 6, Czech Republic

Abstract
Thermal conductivity kðTÞ; Seebeck coefﬁcient S(T), and magnetization M(T) have been measured onLaCo0.98M0.02O3 doped with various valence cations M. A sharp and strong kðTÞ peak of pristine LaCoO3 around
30 K is suppressed for 2þ dopants. Jahn–Teller active Co4+(IS)  6Co3+(HS) clusters introduced concomitantly withthe hole doping may act as the powerful phonon scattering centers at low temperatures.
r 2005 Elsevier B.V. All rights reserved.
PACS: 75.30.Hx; 72.15.Eb; 72.10.Di
Keywords: Phonon scattering; Jahn–teller distortion; Co-spin state transition; Cation doping effect

1. Introduction
LaCoO3 has attracted renewed interest recently.
A main controversy is concerned with Co3+ spinstate transitions, i.e., the role of the intermediate
spin (IS) conﬁguration t52ge1g and the high spin (HS)conﬁguration t42ge2g thermally excited from the seaof Co3+ low spins (LS) with t62g conﬁguration. The

Corresponding author. Tel.: +81 19 621 6362;

fax: +81 19 621 6363.
E-mail address: ikebe@iwate-u.ac.jp (M. Ikebe).

hole doped cobaltites such as La1xSrxCoO3 also
exhibit unusual physical properties which have notyet been fully understood. A variety of the physicalproperties of the cobaltites may partly originate
from the enhanced spin–lattice interaction throughthe Jahn–Teller effect of Co3+(IS) and Co4+(IS).
We report the effect of the doped cation valence inthe phonon thermal conductivity kðTÞ; Seebeck
coefﬁcient S(T), and the magnetization M(T) of
LaCoO3, La0.98Sr0.02CoO3 and LaCo0.98M0.02O3(M : Ni2+, Cu2+, Zn2+, Cr3+, Fe3+, Zr4+, Sn4+).
The phonon scattering mechanism in the hole
doped cobaltites is discussed.

0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2005.01.411

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Y. Fujine et al. / Physica B 359– 361 (2005) 1360–1362

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2. Experimental
The polycrystalline cobaltites were prepared bya solid state reaction method. kðTÞ and S(T) weremeasured by a steady heat ﬂow method. M(T) wasrecorded using a SQUID magnetometer under aﬁeld of 0.5 T.

3. Results and discussion
Fig. 1 shows the temperature dependence ofkðTÞ normalized by the value of kð150 KÞ: Theelectrical resistivity rðTÞ of all the present cobaltites is semiconductive and very high and kðTÞ canbe regarded entirely due to phonons. The behaviorof kðTÞ can be classiﬁed into three groups. kðTÞ ofthe ﬁrst group (G1) shows a characteristic peakaround 30 K. Non-doped LaCoO3 and 2%Cr- andFe-doped samples belong to this group. Thecharacteristic peak of kðTÞ is almost completely
suppressed for the second group (G2), to which
La0.98Sr0.02CoO3 and 2%Zn-, Cu- and Ni-doped
samples belong. The third group (G3) consists of
2%Sn4+- and Zr4+-doped specimens, whose kðTÞpeak is lightly reduced.
Fig. 2(a) shows the temperature dependence ofthe Seebeck coefﬁcient S(T) for the respective

Fig. 1. The normalized thermal conductivity kðTÞ=kð150 KÞ for
LaCoO3, La0.98Sr0.02CoO3 and LaCo0.98M0.02O3. The kðTÞ for
LaCoO3 and 2%Cu2+-doped sample is shown in the inset.

Fig. 2. (a) The Seebeck coefﬁcient S(T) and (b) the magnetization M(T) in the ﬁeld of 0.5 T for the respective samples.
Symbols are the same as Fig. 1.

samples. S(T) of the G2 samples is positive,
suggesting the hole doping and the valencies of
the G2 cations should be 2 þ : S(T) of the G1
samples remains negative and the valencies of Cr
and Fe are inferred to be 3þ similarly to Co3+.
S(T) of Sn4+- and Zr4+-doped specimens is also
negative but the S values are somewhat smaller
than those of G2 ions. Fig. 2(b) shows
the magnetization M(T). It can be seen that theM(T) of the G2 samples is enhanced in comparison to pristine LaCoO3.
The sharp kðTÞ peak of LaCoO3 is not
attributable by usual phonon–phonon scattering,but it is attributable to the phonon scattering due
to thermally excited Jahn–Teller active Co3+ ionswith IS conﬁguration [1]. The local Jahn–Teller
distortion (static or dynamic) has been conﬁrmed

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Y. Fujine et al. / Physica B 359– 361 (2005) 1360–1362

to be very efﬁcient in phonon scattering [2]. The
intact k peak of the G1 group demonstrates that
the Co3+ spin-state transition is hardly affected bythe 3þ cation doping.
The disappearance of the kðTÞ peak for the 2þdopants suggests the appearance of a very powerful phonon scattering center effective at lowtemperatures ðTp50 KÞ: At low temperatures,
effective phonon scattering centers must be ofrather large size because the phonon wavelength isalso large. On the basis of the exact diagonalization method, Tsutsui et al. [3] predicted theformation of Co4+(IS)  6Co3+(HS) clusters uponhole doping. The Co4+(IS) ions with t42ge1g conﬁguration are also Jahn–Teller active and should
accompany the local lattice distortion. Moreover,the lattice distortion around Co4+ becomes largescaled because of the cluster nature. Thus
Co4+(IS)  6Co3+(HS) clusters can be the powerfulphonon scattering centers, enhancing M(T) inFig. 2(b) as well. Upon 4þ cation doping, theappearance of Co2+(LS) is expected becauseof the charge neutrality. The Jahn–Teller active

Co2+(LS) ions with t62ge1g conﬁguration may
possibly be responsible for the moderate reductionof the kðTÞ peak observed for the G3 specimens.
In summary, we discussed the phonon scattering
mechanism in LaCoO3 and doped LaCoO3. Thecharacteristic peak around 30 K of the phonon
thermal conductivity of LaCoO3 is rapidly suppressed by the introduction of holes caused by the2þ cation doping. The relatively large-scale lattice
distortion around Jahn–Teller active intermediate
Co4+ forming Co4+(IS)  6Co3+(HS) clusters isthe most probable candidate for the strong
phonon scattering.

References[1] J.Q. Yan, J.S. Zhou, J.B. Goodenough, Phys. Rev. B 69
(2004) 134409.
[2] H. Fujishiro, M. Ikebe, in: H. Oyanagi, A. Bianconi (Eds.),
Physica in Local Lattice Distortions, American InstitutePhysics, 2001, p. 443.
[3] K. Tsutsui, J. Inoue, S. Maekawa, Phys. Rev. B 59 (1999)
4549.