Physica B 316–317 (2002) 261–264

Thermal diffusivity of La1XCaX MnO3 up to 1200 K
Hiroyuki Fujishiroa,*, Manabu Ikebea, Takaya Akashib, Takashi GotobaFaculty of Engineering, Iwate University, 4-5-3 Ueda, Morioka 020-8551, Japanb
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

Abstract
The thermal diffusivity aðTÞ and the normalized speciﬁc heat CðTÞ=C(300 K) of the colossal magnetoresistive
manganite, La1XCaX MnO3 (LCMO; 0pX p1:0), were measured in the temperature range from 10 to 1200 K. At hightemperatures above 300 K, the temperature differential of the diffusivity, da=dT; changes from negative to positive withincreasing Ca concentration X : The positive da=dT coefﬁcient is not standard and a possible origin correlated with thelocal distortion around the Jahn–Teller active Mn3+ ions is proposed. The phase diagram of the LCMO system iscompleted up to 1200 K using the anomalies of aðTÞ; CðTÞ and other physical quantities. r 2002 Elsevier Science B.V.
All rights reserved.
Keywords: Thermal diffusivity; La1XCaX MnO3; Speciﬁc heat; Phase diagram; Laser ﬂush method

La1XCaX MnO3 (LCMO) is a typical material
which exhibits colossal magnetoresistance (CMR)
and undergoes a paramagnetic insulator (PM-I) toa ferromagnetic metal (FM-M) transition for theCa concentration regime 0:2oX o0:5 [1]. For thehigh doping range, X X0:5; the ground state is an
antiferromagnetic insulator (AFM-I) accompaniedwith the charge ordering (CO) and a canted
antiferromagnetic (CAF) order also takes place
at 0:82pX p1:0 [2]. We have noticed that thethermal diffusivity a (¼ k=C; k is the thermal
conductivity and C the speciﬁc heat) of the X ¼
0:25 sample, for example, abruptly increases justbelow the FM transition temperature Tc ¼ 232 K
and then continues to increase gradually withdecreasing temperature T: We have also noticed*Corresponding author. Tel.: +81-19-621-6363; fax: +8119-621-6373.
E-mail address: fujisiro@iwate-u.ac.jp (H. Fujishiro).

that, for T > Tc ; aðTÞ slightly increases withincreasing temperature up to room temperature[3]. In ordinary insulators, aðTÞ is mainly contributed by phonon heat conduction and isexpected to decrease monotonously or remains
constant with increasing temperature (da=dTp0).
This is because aph is given by vs lph =3 (vs : soundvelocity, lph : phonon mean free path), and lph ðTÞ
limited by the phonon–phonon scattering should
be a decreasing function of T or approach
constant because of saturation at high temperatures. The positive gradient, da=dT > 0 for T > Tc ;suggests that the phonon scattering is actuallyenhanced on approaching Tc from higher temperatures. It should be interesting to measure thethermal diffusivity aðTÞ above room temperatureand to discuss the origin of the positive da=dT: In
this paper, we investigate the thermal diffusivityaðTÞ of the La1XCaX MnO3 system for a wider Ca
concentrations X ð0pX p1:0Þ; extending thetemperature range up to 1200 K.

0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
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H. Fujishiro et al. / Physica B 316–317 (2002) 261–264The samples used in this study were thepolycrystal of which the densities are higher than
85% of the ideal ones [4]. Below 300 K, aðTÞ wasmeasured by the arbitrary heating method by theuse of the measuring system of our own making[5]. The heat pulse was applied to the slendershaped sample and the thermal diffusivity wascalculated from the heat pulse propagation speed.
Above room temperature, aðTÞ and the normalized speciﬁc heat CðTÞ=C(300 K) were measured
by the laser ﬂush method (SINKU-RICO Inc.,TC-7000) up to 1200 K using the disk-shapedsamples.
Figs. 1(a)–(c) show the temperature dependenceof the thermal diffusivity aðTÞ of the typicalsamples. According to an estimation based on
the Wiedemann–Franz law, the heat conduction is
overwhelmingly due to phonons in this system.
For X ¼ 0 sample, aðTÞ increases with decreasing

Fig. 1. Temperature dependence of the thermal diffusivity aðTÞof La1XCaX MnO3 system for (a) X ¼ 020:25; (b) X ¼
0:5020:75 and (c) X ¼ 0:8221:0:

temperature below 300 K, showing a slight kinkaround the Ne! el temperature TN ¼ 120 K. Twoanomalies at TS1 and TS2 are observable above
300 K; the one at TS1 ¼ 530 K is related with thephase transition between the cooperatively Jahn–Teller (JT) distorted orthorhombic phase (Ophase) and the octahedron rotated orthorhombic
phase (O*-phase) and the other at TS2 ¼ 805 K
may probably come from the structure transition
between the O*-phase and the rhombohedralphase (R-phase) [6,7]. The transition temperature
TS1 decreases to B300 K for X ¼ 0:10 anddisappears for higher X samples. The temperature
differential of the diffusivity, da=dT; changes at
TS1 from negative to positive upon heating. Thediffusivity at TS2 shows a distinct and sharp
upturn on heating for X ¼ 0 and 0.10, but theanomaly reduces to a faint kink around TS2 for
X X0:25: TS2 gradually decreases with increasing
X up to 0:25 and, in turn, increases for higher X
samples.
For X ¼ 0:50 (Fig. 1(b)), where the AFM andCO state is stable below TCO ¼ 230 K, an anomalous decrease of aðTÞ is observed just below TCO :The rather gradual reduction of aðTÞ below TCO isa common and characteristic feature in the COstate, as is shown for X ¼ 0:67 and 0.75 samples.
For X ¼ 0:82 and 0.90 samples, where a canted
AFM order takes place below TCAF [8], aðTÞshows a sharp dip around TCAF : At high temperatures, where the crystal structure belongs to theO*- or R-phase, the gradient da=dT changes
systematically from negative to positive withincreasing X : X ¼ 0:67 is the critical concentration
for this da=dT change. The systematic change ofda=dT suggests that the lattice dynamics of thissystem become rather normal with the decrease of
the JT ion (Mn3+) density. The change of da=dT
with X may be understood as follows. As we havepointed out in our previous papers [3,4,9], phonon
scattering by randomly distributed local JT
distortions is vital to limit the phonon thermal
conductivity kph : The anomalous positive da=dT
occurs for X p0:67 where the density n of the JT
active Mn3+ (n ¼ 1  X ; nominally) is pretty
large. In the higher temperature region where thelattice oscillation amplitudes become large owing
to increasing number of phonons, the local JT

H. Fujishiro et al. / Physica B 316–317 (2002) 261–264

distortions may be obscured and may become lesseffective in phonon scattering. Thus the existenceof the JT active ions may be the main origin for theanomalous positive da=dT: For X ¼ 1:0 sample,
where the Mn3+ ions are nominally absent, theabsolute value of aðTÞ and the negative diffusivity
gradient is very large. It is also worthwhile tonotice that in the cooperatively (and coherently)
JT distorted O-phase of LaMnO3 (X ¼ 0),daðTÞ=dT is again negative and normal.
Fig. 2 shows the speciﬁc heat CðTÞ=C(300 K)
normalized to the room temperature value for thetypical samples. For X ¼ 0 sample, two speciﬁcheat anomalies were observed at TS1 ¼ 530 K andTS2 ¼ 803 K. It should be noticed that the anomaly at TS2 becomes sharp at X ¼ 0:0620:25;
where aðTÞ abruptly enhances upon heating, andthen becomes broad for higher X samples. Thestructural phase transition at TS2 for X p0:25 isnot of the simple ﬁrst-order but of the secondorder-like.
Fig. 3 shows the phase diagram of the LCMO
system determined by the anomalies of aðTÞ andCðTÞ=C(300 K) together with those of otherphysical quantities such as magnetization MðTÞand the sound velocity vs ðTÞ [10]. The phaseboundaries between O- and O*-phases and between O*- and R-phases have been clearly
determined in this work. The obtained phase

263Fig. 3. The determined phase diagram of the LCMO system. ’and E represent the phase transition temperature TS1 between
the O- and O*-phases and TS2 between the O*- and R-phases,respectively (see the text).

diagram is consistent with that proposed by
Uehara et al. [11].
In summary, we measured thermal diffusivityaðTÞ of La1XCaX MnO3 (0pX p1:0) in thetemperature range from 10 to 1200 K. At hightemperatures and in the octahedron rotated O*and R-phases, we conﬁrmed the anomalous
positive temperature dependence of the diffusivityaðTÞ for the Ca concentration X p0:67: The originof the possible da=dT has been attributed to thestrong phonon scattering by the incoherent local
JT distortions around Mn3+ ions, which becomesless effective with increasing phonon amplitude at
higher temperatures. The phase diagram of theLCMO system was completed utilizing the anomalies in various physical quantities such as aðTÞ;CðTÞ=C(300 K), MðTÞ and vs ðTÞ:References
Fig. 2. Temperature dependence of the speciﬁc heatCðTÞ=C(300 K) normalized by the room temperature value
for typical LCMO samples.

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