ARTICLE IN PRESS

Physica B 378–380 (2006) 529–531
www.elsevier.com/locate/physb

Metal–insulator transition and phonon scattering mechanisms
in La1xSrxCoO3
Yohei Kashiwadaa, Hiroyuki Fujishiroa,, Yosuke Fujinea, Manabu Ikebea, Jiri HejtmanekbaFaculty of Engineering, Iwate University, Morioka, 020-8551, Japan
Institute of Physics of ASCR, Cukrovarnivka 10, 16200 Praha 6, Czech RepublicbAbstract
The thermal conductivity k(T) and electrical resistivity r(T) have been measured for La1XSrXCoO3 (0pXp0.50). The phonon
scattering is drastically enhanced with the initial hole doping as a result of the appearance of powerful phonon scattering by localized
Co4+. The strong scattering due to the Jahn–Teller active Co4+ intermediate spin is reduced in the metallic region XX0.20 as the doped
holes become itinerant. However, the phonon scattering due to band carriers becomes dominant for XX0.40. k(T) of metallic
La1XSrXCoO3 (0.20pXp0.50) shows an enhancement at the magnetic ordering temperature Tc, which is attributable to the electronic
contribution ke.
r 2006 Elsevier B.V. All rights reserved.
PACS: 65.40.b; 63.20.Kr; 72.15.Jf
Keywords: La1XSrXCoO3; Thermal conductivity; Phonon scattering; Jahn–Teller ions

1. Introduction
LaCoO3 has attracted renewed interest recently mainly
owing to the peculiar Co3+ spin-state transitions. The
thermally driven transition from the low spin Co3+ (LS)ðS ¼ 0; t62g Þ to the Jahn–Teller active intermediate spinCo3+ (IS) ðS ¼ 1; t52g e1g Þ results in a strong phononscattering enhancement in LaCoO3 [1]. The phononscattering enhancement is even more drastically displayedby rightly doping divalent cations [2]. In this note, we study
and analyze the thermal conductivity k(T) of single phaseand stoichiometric La1XSrXCoO3 (LSCO) over a wide
range of Sr concentration X (X p0:50).
2. Results and discussion
The polycrystalline LSCO samples were prepared by asolid-state reaction method. k(T) was measured by a steadyCorresponding author. Tel.: +81 19 621 6363; fax: +81 19 621 6363.

E-mail address: fujishiro@iwate-u.ac.jp (H. Fujishiro).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2006.01.241

heat ﬂow method. X-ray diffraction conﬁrmed a singlephase rhombohedral structure and iodine titration guaranteed the oxygen stoichiometry for X p0:50.
Fig. 1 shows the electrical resistivity r(T). The insulator–metal (I–M) transition takes place between the Srconcentration X ¼ 0:15 and X ¼ 0:20, concomitant withthe appearance of ferromagnetic (FM) order in LSCO
(X ¼ 0:20).
Fig. 2 displays k(T) of very lightly doped LSCO
(X p0:02). For X p0:10, the electrical resistivity r(T) isvery large and k(T) can be regarded entirely as the phonon
contribution kph. The characteristic peak of pristine
LaCoO3, which originates from the spin-state transition
Co3+(LS)-Co3+(IS) [1], completely disappears by only
1% Sr2+-doping. The very strong phonon scattering abilityof the strain ﬁeld around the Jahn–Teller active Co4+ (IS)ðS ¼ 3=2; t42g e1g Þ introduced by Sr2+-doping wipes out thek(T) peak [2].
Fig. 3 shows k(T) of heavily doped LSCO (X X0:10).
The magnetic behavior of insulating LSCO (X ¼ 0:10) isthat of spin-glass, while LSCO is FM metal for X X0:20.

ARTICLE IN PRESS
Y. Kashiwada et al. / Physica B 378–380 (2006) 529–531

80

105

1.0
X=0.10
X=0.15

X=0.20X=0.30X=0.40X=0.50

104



103

0.6
 (T)

T

c

102

0.4101

 (mW/cmK)

60

 (T)

0.8

0.8
Tc

X=0.30

0.6

40

0.4
ph200.2

 (mΩcm)

530

0.2100e00100

10-1300

200

00100

0300200T (K)T (K)Fig. 1. r(T) of the LSCO specimens (0:10pX p0:50).

Fig. 4. Separation of the phonon component kph and the electroncomponent ke for LSCO (X ¼ 0:30). The electrical resistivity r(T) is alsopresented.

100
La1-XSrXCoO380
X=0
X=0.005
X=0.01
X=0.02

60

40

200

X =0.05
X =0.10
X =0.20
X =0.30
X =0.40
X =0.5060
ph (mW/cmK)

 (mW/cmK)80

La1-XSrXCoO340200100

200300T (K)Fig. 2. k(T) of lightly doped LSCO (X p0:02).

00100200300

T (K)Fig. 5. Phonon thermal conductivity kph(T) for various Sr2+ concentration X.

100
X=0.10X=0.20X=0.30X=0.40 (mW/cmK)

80

Tc

60

40

20
La1-XSrXCoO300

100

200300T (K)Fig. 3. k(T) of LSCO (0:10pX p0:40). The arrows indicate the FMtransition temperature Tc.

k(T) is enhanced from X ¼ 0:10 to X ¼ 0:30, taking themaximum for X ¼ 0:30.
Fig. 4 presents the separation of kph and the electronic
component ke on the basis of the Wiedemann-Franz law.
As we can see, the k(T) is enhanced by ke, which is the main
origin of the slight enhancement of k(T) at the FMtransition temperature Tc observed for X X0:20.
Fig. 5 summarizes the phonon contribution kph(T). For
X X0:05, kph(T) is enhanced with increasing X up to X ¼
0:30 because the strain ﬁeld around the hole ( ¼ Co4+(IS))
is reduced with increasing mobility of the holes. Above
X ¼ 0:30, however, the increasing number of doped holes
make the phonon scattering by band carriers dominant andkph decreases.
In summary, with increasing Sr2+ concentration X, kphof La1XSrXCoO3 is at ﬁrst drastically reduced by thestrain ﬁeld around the localized holes (X p0:05). Then kph

ARTICLE IN PRESS
Y. Kashiwada et al. / Physica B 378–380 (2006) 529–531

is gradually enhanced with further increase of X
(0:05pX p0:30) owing to the reduction of the strain ﬁeld
caused by the hole mobility increase and, ﬁnally, kph isagain reduced as the phonon scattering by band carriers
dominates. The k(T) enhancement observed at the FMtransition temperature Tc is of electronic origin.

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References[1] J.-Q. Yan, J.-S. Zhou, J.B. Goodenough, Phys. Rev. B 69 (2004)
134409.
[2] Y. Fujine, H. Fujishiro, Y. Kashiwada, J. Hejtmanek, M. Ikebe,
Physica B 359–361 (2005) 1360.