Vol. 34 (2003)

ACTA PHYSICA POLONICA B

No 2

THERMAL TRANSPORT IN FERROMAGNETIC
La1 X AEX MnO3 WITH LARGE DIVALENT IONS
H. Fujishiro, M. Ikebe, S. Kanoh and H. Ozawa

Fa ulty of Engineering, Iwate University
4-3-5 Ueda, Morioka 020-8551, Japan(Re eived July 10, 2002)

The thermal ondu tivity (T) of the La1 X AEX MnO3 manganites(AE: divalent ions; Ca2+ , Sr2+ , Pb2+ , Ba2+ ) has been measured. For small
AE ions (Ca2+ and Sr2+ ), (T), whi h is dominated by the phonon ontribution, shows an enhan ement below the ferromagneti metal transition
temperature T . In ontrast, the enhan ement is ompletely wiped out for
the largest Ba2+ ion. The in rease of the ioni radius of AE ions redu es theaverage stati latti e distortion with the toleran e fa tor approa hing to1, while it in reases the randomness of ioni radii in the (La1 X AEX )-siteations dened by the ation radius varian e  2 . In the Ba2+ system, the(T) redu tion aused by the largest  2 masks out the (T) enhan ement
hara teristi of the ferromagneti -metal manganite systems.
PACS numbers: 65.40.-b; 75.30.Kz; 72.15.Eb
1. Introdu tion

As re ently revived studies on perovskite-based manganese have onrmed, (RE1 X AEX )MnO3 -type rystals (RE= trivalent rare-earth ionssu h as La3+ , Pr3+ : AE= divalent ions su h as Ca2+ , Sr2+ ) undergo a variety of dramati phenomena su h as the olossal magnetoresistan e (CMR)
and the insulator-metal (I-M) transition [1℄, et . The phonon thermal ondu tivity ph (T) is a valuable tool to investigate the ee t of the latti edynami s on the phase transitions of the manganite system [2, 3℄. In theLa1 X CaX MnO3 (LCMO) system, for example, the thermal ondu tivity
(T) in reases abruptly below the ferromagneti metal (FM-M) transition
temperature T for 0.19X0.30 [4℄. The in rease of the AE-ion radius(rAE ) in the ABO3 perovskite manganites results in the redu tion of thestati latti e distortion from the ubi stru ture with the toleran e fa torPresented at the International Conferen e on Strongly Correlated Ele tron Systems,(SCES 02), Cra ow, Poland, July 1013, 2002.

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H. Fujishiro et al.

(=(rA +rO )/( 2(rB +rO ))) approa hing 1, where rA , rB and rO are theatomi radii of A-site (La1 X AEX ) ion, B -site (Mn3+ 1 X Mn4+ X ) ion andoxygen, respe tively. The value ae ts the T value through the one ele tron bandwidth W and the widened W enhan es T as the latti e approa hes
to ubi . On the other hand, T also depends on the size dieren es within
the A-site (La3+ and AE2+ ) ations, even if is kept onstant [5℄. The randomness of the A-site ations is dened by the A-site ation radius varian e2 (=yi ri2 -rA2 , yi is the fra tional o upan y of the A-site spe ies and rA
is the average ioni radius of A-site ations) and  2 in reases with in reasing AE -site ioni radius in the present La-based manganites. In this paper,
(T) of La1 X AEX MnO3 is studied for various sizes of the AE ions. We
dis uss the origin of the (T) anomalies below T from the point of view ofthe latti e distortions.
2. Experimental

La1 X AEX MnO3 samples (AE=Ba2+ , Pb2+ , Sr2+ , Ca2+ ) were preparedby a solid-state rea tion method for the hole on entration X = 0:1  0:5.
The sizes of rAE are r(Ca2+ )<r(Sr2+ )<r(Pb2+ )<r(Ba2+ ) and and  2 were
al ulated using the tabulated radii with ninefold oordination [6℄. Thesamples were fabri ated by solid state rea tion method at 1500Æ C for 8 hin air [4, 8℄. The grain size of ea h sample is about 10  20 m and isindependent of the spe ies of AE ions and the hole on entration X . Thethermal ondu tivity (T) was measured by a steady-state heat ow method
using a GiordM Mahon (GM) y le helium refrigerator as a ryostat.
3. Results and dis ussion

Figure 1 shows (T) of La1 X AEX MnO3 samples ((a) X = 0:20 and(b) X = 0:30). All the samples show the metalli ele tri al ondu tion
below the FM-M transition temperature T [4, 7, 8℄. For X = 0:20, (T)of the Ca2+ , Sr2+ and Pb2+ systems, whi h is overwhelmingly due to thephonon ontribution, shows a hara teristi enhan ement below T (=200 Kfor AE = Ca2+ and = 330 K for AE = Sr2+ ) and then rea hes a maximum at around T'40 K. These anomalous temperature dependen es are
onsidered to result from the redu tion in the phonon s attering related tothe relaxation of the spatially random JahnTeller (JT) distortion in themetalli phase. With initial in rease of rAE from AE = Ca2+ to AE = Sr2+ ,the absolute values of (T) remarkably in reases, whi h suggests that thein rease of rAE enhan es the phonon thermal ondu tion possibly by redu ing the latti e distortion from ubi . It is to be noti ed that the rystal
stru ture is orthorhombi for LCMO and hanges to rhombohedral for theThermal Transport in Ferromagneti La1

X AEX MnO3 : : :825

La1 X SrX MnO3 (LSMO) system. With further in rease of rAE , however,
the absolute value of (T) is redu ed signi antly. For the Ba2+ system(LBMO) with the largest rAE , (T) monotoni ally de reases with the derease in temperature. For X = 0:30 shown in Fig. 1(b), the (T) maximum
at low temperatures is enhan ed for LCMO and LSMO, but it is strongly
suppressed for the Pb2+ system (LPMO). A omparison of high temperature (T) around 300 K for X = 0:20 and X = 0:30 shows that (T) valuesof LPMO and LBMO are somewhat enhan ed with in reasing X . This enhan ement is far learer for LBMO, whi h may suggest that the  enhan ingee t related to in reasing is still at work in these two systems at the hightemperatures [8℄.

Fig. 1. Thermal ondu tivity (T) of La1 X AEX MnO3 ((a): X = 0:20, (b):X = 0:30) for various sizes of AE ions. Arrows show T whi h was magneti ally
determined.
Figure 2 presents the al ulated values of (a)and (b)  2 , as a fun tion of the hole on entration X .
in reases with in reasing X and within reasing rAE , whi h may tend to bring about the (T) enhan ement. Onthe other hand,  2 , the randomness of the A-site ations, also in reases within reasing X and rAE . The in rease in  2 orresponds to the in rease of therandomness of the lo al latti e distortion and should redu e (T). In Fig. 1,
(T) of LBMO is smaller than those of other systems at low temperatures.
This result suggests that the redu tion me hanism due to the large  2 value
is very strong in the Ba2+ system espe ially at low temperatures. least at
low temperatures.

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H. Fujishiro et al.

Fig. 2. Cal ulated values of (a) toleran e fa tor
 2 as a fun tion of hole on entration X .

and (b) ation radius varian e

In summary, the in rease of the AE ion radius brings about both enhan ing and redu ing ee ts on the phonon thermal ondu tivity. In the Ba2+system with the largest AE ion radius, the ee t of the (T) redu tion due
to the large  2 values may be more dominant than the (T) enhan ing ee t
due to the large value. In the Pb2+ system, the redu tion ee t seems tobe dominant at low temperatures.
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