International Conference on Magnetism (ICM 2009)
Journal of Physics: Conference Series 200 (2010) 012137

IOP Publishingdoi:10.1088/1742-6596/200/1/012137

Elastic and Thermal Transport Properties of(Pr1−xSmx)0.7Ca0.3CoO3 at metal-insulator Transition
Tomoyuki Naito, Hiroko Sasaki, Hiroyuki Fujishiro andMasahito Yoshizawa
Faculty of Engineering, Iwate University Morioka 020-8551, Japan
E-mail: tnaito@iwate-u.ac.jp
Abstract. In a Pr-Ca-Co-O system, a metal-insulator transition (MIT) accompanied with aspin-state transition (SST) of Co3+ takes place, at which the large volume contraction is also
observed because of the tilting of the CoO6 octahedra. To study lattice dynamics at the MI-SST,
we have measured the thermal conductivity κ(T ), thermal diffusivity α(T ), thermal dilatationdL(T )/L and sound velocity v(T ) for (Pr1−x Smx )0.7 Ca0.3 CoO3 . κ(T ) shows a drop or a kinkand α(T ) shows a dip around the MI-SST temperature, TMI . v(T ) gradually decreases fromthe higher temperature than TMI and abruptly drops at TMI . About 50 % of the dip of α(T )originates from the decrease of the sound velocity at TMI . The gradual decrease in v(T ) above
TMI suggests that the anomaly of the lattice linked to the MI-SST starts from fairly highertemperature.

1. Introduction
Thermal conductivity, κ(T ), of RECoO3 (RE=rare earth element) strongly depends on thespecies of RE. For example, in LaCoO3 , the κ(T ) shows a large peak near 35 K and rapidly
decreases with increasing temperature. The κ(T ) peak originates from the spin-state transition(SST) of Co3+ from the low spin state (LS; t62g e0g , S=0) to the intermediate spin (IS; t52g e1g ,S=1) or the high spin state (HS; t42g e2g , S=2) which starts from 35 K [1, 2]. The κ(T ) forPrCoO3 shows a small and broad peak around 180 K, which was also attributed to the SSTof Co3+ [3, 4]. For both cases, no anomaly appears in the electrical resistivity at the SST. On
the other hand, a metal-insulator transition (MIT) accompanied with the SST appears in aPr-Ca-Co-O system [5, 6, 7, 8]. At the MI-SST, the large volume contraction due to the tilting
the CoO6 octahedra occurs [5, 6]. Such a lattice deformation would strongly affect the heattransport. In this paper, we measure the temperature dependences of the thermal conductivity
κ(T ), thermal diffusivity α(T ), thermal dilatation dL(T )/L and sound velocity v(T ) for the(Pr1−x Smx )0.7 Ca0.3 CoO3 samples and discuss the origin of anomalous lattice dynamics aroundthe MI-SST.
2. Experimental procedure(Pr1−x Smx )0.7 Ca0.3 CoO3 samples were prepared by a solid-state reaction method [8]. Powder
X-ray diffraction patterns were taken at room temperature and the single-phase crystal structure
was confirmed for all samples. The measured densities of each sample were greater than 90 % ofthe ideal density. Thermal conductivity, κ(T ), was measured by a steady-state heat-flow method
c 2010 IOP Publishing Ltd1

International Conference on Magnetism (ICM 2009)
Journal of Physics: Conference Series 200 (2010) 012137

1010065(Pr xSmx) Ca CoO1-10)mcΩ 10m(ρ 10101010IOP Publishingdoi:10.1088/1742-6596/200/1/012137

0.70.3(Pr1-xSmx)0.7Ca0.3CoO3330.220.4

10.50.6

dx=00-100.6) -0.2%()
x=0K00 -0.4(3L0.2/L4

0.5-0.6

0.4
50

100

150T (K)200

250-0.8300

Figure 1.
The temperature dependence of the resistivity,
ρ(T ),
for(Pr1−x Smx )0.7 Ca0.3 CoO3samples
withvarious x.

0

50

100

150 200
T (K)

250

300

Figure 2.
The temperature dependenceof the thermal dilatation, dL(T )/L(300K),
for (Pr1−x Smx )0.7 Ca0.3 CoO3 samples withvarious x.

and thermal diffusivity, α(T ), was measured by an arbitrary heating one under an identicalexperimental setup [9]. Thermal dilatation, dL(T )/L(300 K)[= (L(T ) − L(300 K))/L(300 K)],
based on the sample length at 300 K, was measured using a commercial strain gauge (Kyowa
KFL-1-120-C1-16). Sound velocity, v(T ), was measured by a pulse echo method. Electricalresistivity, ρ(T ), was measured by a four-probe method. All the measurements were performedby cooling runs.
3. Results and DiscussionFigure 1 shows the temperature dependence of the resistivity, ρ(T ), for (Pr1−x Smx )0.7 Ca0.3 CoO3
samples with various x up to 0.6. For x=0, the ρ(T ) increases moderately with decreasing
temperature. For x = 0.2, ρ(T ) shows an abrupt jump with two orders of magnitude at TMI =42
K. The TMI increases and the width of the transition broadens with increasing x. The MIT wasaccompanied with the SST of Co3+ ions [5, 6, 7, 8].
Figure 2 shows the temperature dependence of the thermal dilatation, dL(T )/L(300 K), for(Pr1−x Smx )0.7 Ca0.3 CoO3 with various x. For x=0, the dL(T )/L(300 K) decreases monotonicallywith decreasing temperature and no anomaly appears. For x=0.2–0.6, the dL(T )/L(300 K)
shows a drop at TMI , which originates from the tilting of the CoO6 octahedra [5, 6]. The abrupt
dilatation broadens with increasing x similarly to the transition in ρ(T ). The absolute valueof the dL(20 K)/L(300 K) for x=0.4 is larger than that for x=0.2, but that for x=0.5 and 0.6
decreases.
Figure 3 shows the temperature dependence of the thermal conductivity, κ(T ), of(Pr1−x Smx )0.7 Ca0.3 CoO3 for x=0.2 and 0.4. For x=0.2, κ(T ) decreases monotonically withdecreasing temperature, abruptly drops at 49 K, which is somewhat higher than TMI =42 K, andthen takes a broad peak at lower temperature. For x=0.4, κ(T ) shows a kink at 114 K, which issimilar to TMI , and takes a quite moderate maximum at 40 K. The measured κ is sum of bothcontributions of phonons, κph , and electrons, κel . κel (T ) was also shown in the figure, which wasestimated using Wiedemann-Franz law (κel = L0 T /ρ), where L0 was Lorenz number. In bothsamples, the magnitude of κel is approximately 10–12 % of the measured κ at 150 K. For x=0.2,
the origin of the κ(T ) drop around TMI comes from the phonons. On the other hand, for x=0.4,2International Conference on Magnetism (ICM 2009)
Journal of Physics: Conference Series 200 (2010) 012137

30

11-IOP Publishingdoi:10.1088/1742-6596/200/1/012137

5(Pr1-xSmx)0.7Ca0.3CoO3) 20KmcWm(κ 10x0.20.44κmes κe)ces 32/mm( 2 x=0.2○□TMI050x=0.4

TMI (x=0.4)

α1TMI0(Pr1-xSmx)0.7Ca0.3CoO3100T (K)150

0200Figure 3. The temperature dependence ofthe measured and the electron thermal conductivities, κmes (T ) and κel (T ), respectively,
for (Pr1−x Smx )0.7 Ca0.3 CoO3 with x=0.2 and0.4.

TMI

050

100T (K)150200Figure 4.
The temperature dependence of the thermal diffusivity, α(T ), for(Pr1−x Smx )0.7 Ca0.3 CoO3 with x=0.2 and 0.4.

an observable small κel decrease can be seen but the measured κ(T ) anomaly around TMI isalmost due to phonons. The tilting of the CoO6 octahedra by the MIT gives rise to the bucklingof the CoO2 planes, which would prevent strongly the phonon transport. In the κ(T ) peak inLaCoO3 , we can interpret the peak in the following; at higher temperatures, Jahn-Teller (JT)
active IS-Co3+ ions scatter the phonons and κ(T ) is suppressed. With decreasing temperature,the population of IS-Co3+ decreases and that of non JT active LS-Co3+ increases. As a result,the phonon scattering decreases and then κ(T ) increases and takes a maximum. For the x=0.2
and x=0.4 samples in this study, the reduction of IS-Co3+ ions enhances the κ(T ) below TMI .
Figure 4 shows the temperature dependence of the thermal diffusivity, α(T ), of(Pr1−x Smx )0.7 Ca0.3 CoO3 for x=0.2 and 0.4.
α(T ) slightly increases with decreasing
temperature or is almost temperature-independent at high-temperatures, shows a dip structureat around TMI and increases rapidly with decreasing temperature.
The dip in α(T )indicates that the sound velocity, v(T ), and/or the phonon mean free path, l(T ), decreased
considering the relation, α = 31 vl. Figure 5 shows the temperature dependence of thesound velocity, ∆v(T )/v(300 K) [= (v(T ) − v(300 K))/v(300 K)] for (Pr0.6 Sm0.4 )0.7 Ca0.3 CoO3 .
∆v(T )/v(300 K) increases with decreasing temperature, starts to decrease at 140 K, and drops
discontinuously at 112 K which corresponds to TMI . The decrease in ∆v(T )/v(300 K) from 140
K to 112 K is approximately 25 % of the ∆v(140 K)/v(300 K) value. As found in Fig. 4, the α(T )also started to decrease at around 137 K and took a minimum at 112 K; α(T )=0.66 mm2 /sec
at 112 K was about 48 % lower than α(T )=1.28 mm2 /sec at 137 K. These results suggest thatabout 50 % of the decrease in α(T ) around the MI-SST is caused by the change in the soundvelocity. Since κ is represented as κ = Cα, a part of anomaly in κ(T ) also originates fromthe decrease of the v(T ), where C is the specific heat. However, to carry out the quantitative
discussion, we must know the C(T ) for this compound. It is noteworthy that the softening ofthe lattice started at around 140 K, demonstrating that the anomalous lattice dynamics begins
to appear at higher temperature than TMI .

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International Conference on Magnetism (ICM 2009)
Journal of Physics: Conference Series 200 (2010) 012137

IOP Publishingdoi:10.1088/1742-6596/200/1/012137

0.1TMI0)K003( -0.1v/v∆(Pr0.7Sm0.4)0.7Ca0.3CoO3-0.2-0.3050

100150 200T(K)250Figure 5.
The temperature dependence of the soundvelocity,∆v/v(300 K),for(Pr0.6 Sm0.4 )0.7 Ca0.3 CoO3 .

3004. Summary
We report the thermal conductivity κ(T ), thermal diffusivity α(T ), thermal dilatation dL/L(300 K) and sound velocity ∆v(T )/v(300 K) at around the MI-SST for the(Pr1−x Smx )0.7 Ca0.3 CoO3 samples. κ(T ) showed a drop or a kink and α(T ) showed a dip atthe MI-SST. The quite sharp drop was also found in ∆v(T )/v(300 K). Comparing the magnitude of the anomalies between α(T ) and ∆v(T )/v(300 K), the change of the sound velocity v(T )contributes to about 50% of the α(T ) anomaly and affects a part of κ(T ) anomaly because of
the relation of κ = Cα. The lattice softening occurred at rather higher temperature than TMI ,which might be important to clarify the mechanism of the MI-SST.
Acknowledgements
We acknowledge Ms. M. Sumomozawa for her performing a parts of the experiments and Mr.
K. Nakashima for his measuring the elastic constant.
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[7] Fujita T et al. 2005 J. Phys. Soc. Jpn. 74 2294
[8] Naito T et al. in preparation[9] Ikebe M et al. 1994 J. Phys. Soc. Jpn. 63 3107

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