Physica B 316–317 (2002) 265–268

Effect of oxygen content variation on phonon heat transportin La0.75Ca0.25MnO3+dManabu Ikebe*, Hiroyuki Fujishiro, Makoto Numano
Department of Materials Science, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, JapanAbstract
The thermal conductivity kðTÞ; thermal dilatation dLðTÞ=L; magnetization MðTÞ and the electrical resistivity rðTÞ ofLa0.75Ca0.25MnO3+d have been measured for various oxygen off-stoichiometry d: The phonon thermal conduction, theelectrical conduction and the order of the ferromagnetic phase transition were found to be strongly depending on d: The
scattering of phonon by small (T > TC ) and large (ToTC ) Jahn–Teller polarons was found to be of main importance tolimit the phonon thermal conduction of this system. r 2002 Elsevier Science B.V. All rights reserved.
Keywords: Thermal conductivity; Oxygen off-stoichiometry; Thermal dilatation; Manganese oxides

As recent revived studies have demonstrated,hole-doped (RE1XAEX)MnO3-type manganites(RE=trivalent rare-earth ions, AE=divalent alkaline-earth ions) with the perovskite-based structure show a variety of dramatic phenomena suchas the colossal magnetoresistance (CMR) and themetal-insulator (M2I) transition [1]. Besides thedouble exchange mechanism, the electron–phonon
interaction enhanced by the Jahn–Teller (J2T)
effect of the Mn3+ ions has been widely recognized to be essential [2]. Another important key
factor inﬂuential to the lattice effect is the ionicradius matching (or mismatching) between the(RE1XAEX) and Mn ions, which eventually
determines the Mn 3d electron band width (eg).
The mobility and the band ﬁlling of the doped*Corresponding author. Tel.: +81-19-621-6362; fax: +8119-621-6373.
E-mail address: ikebe@iwate-u.ac.jp (M. Ikebe).

holes mainly determine the electronic properties ofthe strongly correlated manganite systems.
The ionic radii of La3+ and Ca2+ are not muchdifferent and the fundamental structure of
La1XCaXMnO3+d (LCMO) remains orthorhombic over the entire Ca concentration X. LCMO
is a ferromagnetic metal (FMM) below theCurie temperature TC for 0.18pXp0.47 [3,4]and it exhibits a characteristic ﬁrst-order-likeFMM transition around X ¼ 0:25 [5]. The ﬁrstorder-like transition was found to be criticallyrelated to the (RE1XAEX) and Mn ionic radius
matching. However, also the oxygen off-stoichiometry is to crucially affect the magnetic andtransport properties of the manganite systems [6]through the change of the effective band-ﬁlling ne(¼ X þ 2d; nominally). In this report, we present
the effect of the oxygen off-stoichiometry d on thephonon thermal conduction and on the ﬁrst-orderlike phase transition in La0.75Ca0.25MnO3+d.

0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
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M. Ikebe et al. / Physica B 316–317 (2002) 265–268

The La0.75Ca0.25MnO3+d samples were prepared by a standard solid-state reaction method.
The sintering temperature was 15001C. The dvalues were determined by iodine titration. The
precision of the iodine titration was estimated tobe Ddp70:005: The structure of all of the presentsamples was conﬁrmed to be a single orthorhombic phase by X-ray diffraction analysis. Thethermal conductivity kðTÞ was measured by asteady-state heat ﬂow method between 10 and
300 K using a Gifford–MacMahon cycle Herefrigerator as a cryostat. The magnetizationMðTÞ was measured by a SQUID magnetometer.
The thermal dilatation dLðTÞ=L was measured bya strain gauge method.
In this study, the oxygen content 3 þ d wascontrolled by heat-treating the as-sintered starting
sample in Ar–H2 (Ar:H2=10:1) atmosphere atvarious temperatures. From the analyses of iodinetitration, the oxygen off-stoichiometry d of thepresent La1XCaXMnO3+d samples was determined to be, 0.040, 0.032, 0.010 and 0.001 for assintered, heat-treated at 4501C, 5001C and 5201C,respectively. Fig. 1 shows the magnetization MðTÞas a function of temperature T for these foursamples. MðTÞ of the as-sintered (d ¼ 0:040)
sample increases almost discontinuously at andjust below TC ; which is indicative of the ﬁrst-order
FM transition. However, hysteretic behavior,
which is another characteristic feature of theFig. 1. The magnetization MðTÞ of La0.75Ca0.25MnO3+d for
d ¼ 0:040; 0.032, 0.010 and 0.001 under the ﬁeld of m0 H ¼
0:5 T: MðTÞ rises very sharply for d ¼ 0:040 and 0.032 justbelow TC :Fig. 2. The electrical resistivity rðTÞ as a function of temperature T: The samples are the same as those in Fig. 1.

ﬁrst-order phase transition could not detected.
The FM transition becomes gradual and of typicalsecond-order with decreasing d as can be seen in
the ﬁgure.
Fig. 2 shows the temperature dependence of theelectrical resistivity rðTÞ: The rðTÞ values of thed ¼ 0:040 sample show a sizable reduction at TC
with decreasing temperature and clear metallic
behavior can be seen below TC : The rðTÞreduction is smaller for the d ¼ 0:032 sample anddisappears for the d ¼ 0:010 and 0.001 samples.
For T > TC ; rðTÞ of all the samples behave
semiconductive and the electrical conduction isdue to the hopping of small polarons [7,8]accompanied with local J2T lattice distortions.
For d ¼ 0:040 and 0.032 and for ToTC ; themetallic conduction is probably due to much
mobile large polarons accompanied with locallyreduced and spatially somewhat extended J2Tdistortions.
Fig. 3 shows the thermal dilatation dLðTÞ=L as
a function of T: For d ¼ 0:040; dLðTÞ=L exhibits
almost discontinuous contraction with decreasing
temperature. This sharp contraction may reﬂect
the relaxation of the local J2T lattice distortion
associated with the increase in the mobility andwith the change of the charge carrier characters(i.e., from the small to large polarons). The
contraction becomes very small for d ¼ 0:010
and disappears for d ¼ 0:001: It is noticed thatM. Ikebe et al. / Physica B 316–317 (2002) 265–268

Fig. 3. The temperature dependence of the thermal dilatationdLðTÞ=L: The arrow indicates the Curie temperature TC :Fig. 4. The thermal conductivity kðTÞ: kðTÞ above TC is small
and almost equal for all the samples. Below TC ; kðTÞ isenhanced in metallic samples (d ¼ 0:040 and 0.032).

rðTÞ of the d ¼ 0:001 sample remains very large
even for ToTC and that the small-polaron-likeelectrical conduction dominates over the entire
temperature range.
Fig. 4 presents the thermal conductivity kðTÞ forthe respective samples. Based on the Wiedemann–Franz law, the electronic contributions to k arevery small even in the metallic state, and kðTÞ isoverwhelming due to phonons for all the present
La0.75Ca0.25MnO3+d samples. For T > TC ; thekðTÞ values of all the samples are almost the sameand very small. At TETC ; kðTÞ of the d ¼ 0:040

267

and d ¼ 0:032 almost jumps up, continues to beenhanced with decreasing temperature and takes amaximum at around TE40 K: The magnitude ofthe kðTÞ jump and the enhance is more conspicuous for d ¼ 0:040 than for d ¼ 0:032: The kðTÞenhance at TETC is barely discernible for d ¼
0:010 and is completely wiped out for d ¼ 0:001:
The X dependence of kðTÞ clearly corresponds tothose of rðTÞ and dLðTÞ=L: For T > TC ; where theelectrical conduction is due to the small J2T
polarons, kðTÞ is small due to strong phonon
scattering by the large and random J2T local
distortions. For ToTC and in the samples of d ¼
0:040 and 0.032, the phonon scattering is progressively reduced with decreasing temperature asthe local J2T distortion is relaxed with increasing mobility of the large J2T polarons. In thed ¼ 0:010 and 0.001 samples, where rðTÞ islarge even below TC and the small polaron
hopping characters of charge carriers survive evenin the ferromagnetic phase, the strong phonon
scattering by the random local J2T distortions
remains over the entire temperature range of thisstudy.
In summary, we studied the effect of the oxygen
off-stoichiometry on the FM phase transition bymeasuring the thermal conductivity kðTÞ; electricalresistivity rðTÞ and thermal dilatation dLðTÞ=L:
The ﬁrst-order-like FM transition in as-sinteredLa0.75Ca0.25MnO3+d (d ¼ 0:040) promptly disappeared with decreasing d and the transition
became typical of second-order in La0.75Ca0.25MnO3+d (d ¼ 0:010). Thus not only the mismatch of
the ionic radius between (La0.75Ca0.25) and Mn
ions but also the band ﬁlling of holes criticallyaffects the FM transition of the La1XCaXMnO3
system. The main phonon scatters in this system
are the local J2T distortions around Mn3+ ions.
The relaxation of the J2T lattice distortions
associated with the change of the charge carrier
characters from the small to large polaron causes
the observed kðTÞ enhancement for the metallic
d ¼ 0:040 and 0.032 samples below TC :References[1] Y. Tokura, et al., J. Phys. Soc. Japan 63 (1994) 3931.

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