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Journal of the Physical Society of Japan
Vol. 67, No. 8, August, 1998, pp. 2582-2585

Letters

Charge Ordering and Sound Velocity Anomaly in
La1−X SrX MnO3 (X ≥ 0.5)
Hiroyuki Fujishiro, Tetsuo Fukase1 and Manabu Ikebe
Department of Materials Science and Technology, Faculty of Engineering, Iwate University,
4-3-5 Ueda, Morioka 020-8551
1 Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577(Received March 18, 1998)

The electrical resistivity, magnetization, dilatation and sound velocity have been measured for
La1−X SrX MnO3 (0.48 ≤ X ≤ 0.90) polycrystals. Observed anomalies in the sound velocity andthe dilatation strongly suggest theoccurrence of the charge ordering within the Srconcentration
range 0.48 ≤ X ≤ 0.82. A phase diagram of the charge-ordered state in La1−X SrX MnO3 as afunction of temperature T and the Sr concentration X is proposed.
KEYWORDS: La1−X SrX MnO3 , charge ordering, magnetization, sound velocity, dilatation, electrical resistivity,
antiferromagnetic orderAs recent revived studies on perovskite-based manganites have conﬁrmed, R1−X AX MnO3 -type crystals(R=La and trivalent rare earth ions, A=Sr, Ba, Ca)1-5)
undergo a variety of phase transitions which originate
from the interplay and competition of various physical interactions. The most conspicuous transition may
be the one from an antiferromagnetic (AFM) insulator to a ferromagnetic (FM) metal, leading to a colossal magnetoresistance (CMR) of these systems. The
FM metallic state in the doped manganites is causedby the double exchange mechanism and the ferromagnetic alignment of Mn4+ (S = 3/2) spins is stabilized
by fully polarized eg -band electrons. However, as aresult of competition with AFM superexchange or polaron ordering,6-8) the FM metallic state is often unstable against the formation of charge ordering (CO)and a commensurate fraction of the carrier concentration such as X = 1/8, 1/3, 1/2 and 2/3 is expected
to favor a particular type of CO. The charge ordering
near X = 1/2 has been reported for several systems
such as Pr1−X CaX MnO3 ,9-12) Pr1/2 Sr1/2 MnO3 ,13-15)
Nd1/2 Sr1/2 MnO3 15-17) and La1−X CaX MnO3 .3-5) The
type of AFM order below the CO transition has beenreported mostly to be of the CE-type3) for X ≈ 1/2 withsome exceptions.15) Very recently, Akimoto et al.18) observed the A-type AFM structure for La0.46 Sr0.54 MnO3
single crystals associated with the occurrence of CO.
In a previous study,19) we reported CO in La1−X SrX MnO3 centered at X = 1/8 and pointed out that thesound velocity anomaly can be a very sensitive and usefulmeasure to monitor the occurrence of CO in this system.
For La1−X SrX MnO3 with a large eg -electron bandwidth(W), the charge ordering at X ≈ 0.5 was believed to beabsent. However, we have found a clear sign of the occurrence of CO in La1−X SrX MnO3 polycrystals at X ≈ 0.5
from the magnetization and resistivity measurements.20)
In this letter, we present the data of the sound velocity,
thermal expansion, magnetizationand electrical resistiv-

ity as a function of temperature, extending the range of
X up to 0.90. The results suggest that CO occurs over awide Sr concentration 0.48 ≤ X ≤ 0.82 besides the one
centered at X = 1/8.8, 19, 20)
La1−X SrX MnO3 samples were prepared from stoichiometric mixtures of La2 O3 , SrCO3 and Mn3 O4 powders.19) The mixtures were calcined twice at 1000◦ C for
24 h in air, pressed into pellets and then sintered at
1500◦C for 8 h in air. The sintered crystals were heattreated at 1500◦ C for 24 h in ﬂowing oxygen. All thesamples were conﬁrmed to be in a single phase at room
temperature using X-ray diﬀraction analyses. The measured densities of each sample were higher than 85% ofthat of the ideal one. The magnetization M (T ) was measured using a commercial SQUID magnetometer from10 K to 400 K under an applied magnetic ﬁeld of 5000 G.
The sound velocity vs (T ) was measured using the pulsesuperposition method between 4.2 K and 290 K, by applying 7 MHz longitudinal sound waves.19) The dilatation dL(T )/L was measured between 90 K and 370 K using the strain-gauge method.
Figures 1(a) and 1(b) show the magnetization M (T )of La1−X SrX MnO3 samples as a function of temperature T . For X = 0.48, the FM transition occurs at
the Curie temperature Tc = 330 K and the FM state isstable over the entire temperature range below Tc . For
X = 0.50, 0.52 and 0.55, however, the FM state becomes
unstable at lower temperatures and the AFM order setsin as indicated by a sharp drop in M (T ) at TN .20) For
X ≥ 0.60, the FM state disappears completely and only
the AFM order occurs as indicated by the cusps of the
M (T ) curves. The behavior of M (T ) bears a close resemblance to that of La1−X CaX MnO3 .4)
Figures 2(a) and 2(b) show the temperaturedependence of the electrical resistivity ρ(T ) of
La1−X SrX MnO3 polycrystals for X ≥ 0.48. The ρ(T )curves for X < 0.48 are reported in our previous papers.19, 20) The resistivity ρ(T ) of La0.52 Sr0.48 MnO3 in

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Letters2583

Fig. 1. The magnetization of La1−X SrX MnO3 samples under anapplied ﬁeld of 5000 G for (a) 0.48 ≤ X ≤ 0.60 and (b) 0.67 ≤X ≤ 0.90.

Fig. 2. The temperature dependence of the electrical resistivity
ρ(T ) of La1−X SrX MnO3 samples for (a) 0.48 ≤ X ≤ 0.60 and(b) 0.67 ≤ X ≤ 0.90.

Fig. 2(a) attains a minimum at around 130 K and showsa semiconductive behavior below this temperature. The
semiconductive behavior of ρ(T ) becomes more distinct
with increasing Sr concentration X. For X = 0.50 and
0.52, abrupt increases in ρ(T ) are noticed at around
150 K and 200 K, which correspond to the onset of the
AFM order observed in the M (T ) curves in Fig. 1(a). It
is worthwhile to notice that the absolute values of ρ(T )increase rather abruptly between X = 0.60 and X = 0.67
with increasing X. The present La1−X SrX MnO3 samples may be divided into two groups; the low-resistivity
group for 0.48 ≤ X ≤ 0.60 given in Fig. 2(a) and thehigh-resistivity group for X ≥ 0.67 given in Fig. 2(b).
Because La1−X SrX MnO3 polycrystals are not metallic
for X ≥ 0.48, at least at low temperatures, we classify
all the samples in this work as insulators.
Figure 3 shows the sound velocity vs (T ) as a functionof T for typical samples. For X = 0.50, 0.52 and 0.55,
the vs (T ) curves show clear anomalies which explicitly
correspond to those of the magnetization at the onset of
the AFM order. However, the anomaly in vs (T ) occurs
not only in the vicinity of X = 0.5 but continuously persists into larger X regions where competition between
the FM and AFM orders is not observable. Theforms ofthe vs (T ) anomalies aresomewhat diﬀerent for X ≤ 0.50
and X ≥ 0.52. For X = 0.48 and X = 0.50, the anomaly

appears as an abrupt increase of vs (T ) with decreasing
temperature (one should not overlook the anomalous increase in vs (T ) at around 50 K for X = 0.48). In contrast, the anomaly appears as a local minimum in vs (T )for X ≥ 0.52 accompanied by a considerable softening of
the lattice. The magnitudes of the anomalies seem to be
the largest for X = 0.67 or X = 0.75.
The temperature dependence of the dilatation dL/L
between 100 K and 290 K is shown in Fig. 4. Except for
X = 0.48, dL/L shows anomalies at exactly the same
temperatures where vs (T ) shows signiﬁcant anomalies.
What is to be noticed in this ﬁgure is that the anomalies show quite an opposite character for X ≤ 0.60 andX ≥ 0.67; with decreasing temperature, dL/L abruptly
decreases at the anomaly for X ≤ 0.60, while for X ≥0.67 dL/L abruptly increases at the anomaly. Because
our samples are polycrystals, the meaning of dL/L is
not really clear but it is reasonable to conclude that thefeatures of the lattice deformation at the anomaly are
diﬀerent for X ≤ 0.60 and X ≥ 0.67. Detailed featuresof the lattice distortion at the anomalies must be clariﬁed by diﬀraction studies. For X = 0.90, the anomaly of
dL/L is notdiscernible above 100 K. Preliminary resultsof dL/L for the La0.33 Ca0.67 MnO3 polycrystalsshow thesame behavior as that of La0.33 Ca0.67 MnO3 .21)
Based on the various anomalies observed, let us dis-

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2584

Letters

Fig. 3. Sound velocity vs (T ) vs T for typical La1−X SrX MnO3
samples. Large anomalies associated with the charge-orderingtransition are clearly seen. Note that vs (T ) anomalously increases at around 50 K for La0.52 Sr0.48 MnO3 in which the
charge-order transition is not conﬁrmed by other measurements.

cuss the overall phase diagram of La1−X SrX MnO3 asa function of T and X. The charge-ordering phasetransitions similar to those of present La1−X SrX MnO3
have been conﬁrmed for several compounds, and the
M (T ) curves given in Fig. 1 are quite similar to thoseof La1−X CaX MnO3 .4) For La1−X CaX MnO3 with X ≈2/3, Ramirez et al.22) observed the CO transition with amodulation wavelength q = (2π/a)(1/3, 0, 0). For similar samples, they also observed vs (T ) anomalies justlike the ones we have observed for La1−X SrX MnO3 inthis study. Thus it seems almost indubitable that theanomalies in vs (T ) and the dilatation dL/L are also dueto the CO transitions in La1−X SrX MnO3 and the presented data suggest that the CO transitions occur for0.48 ≤ X ≤ 0.82 over a wide region of the Sr concentration.
On the basis of the data obtained in this work andin the previous reports,19, 20) we have constructed an
overall phase diagram with particular emphasis on the
charge-ordering transitions. The proposed phase diagram is shown in Fig. 5. The Sr composition X is plottedin the nominal quantity. Except for in the vicinities of
X = 0 and X = 1, the phase diagram is fundamentallycomposed of three types of charge transitions, centralcompositions of which are located at X = 1/8 (CO-I),
1/2 (CO-II) and (tentatively) 2/3 (CO-III). Since theanomaly of the dilatation changes its feature between
X = 0.60 and X = 0.67 and the absolute values of ρ(T )jump there as well, we postulate that the character of
the CO transitions also changes at around X ≈ 0.62.
Because dL/L of La0.33 Ca0.67 MnO3 shows a similar increase to that of La0.33 Sr0.67 MnO3 with decreasing temperature,21) we infer that the modulation wavelength of
CO-III of La1−X SrX MnO3 is also q = (2π/a)(1/3, 0, 0),
intrinsically the same value as La0.33 Ca0.67 MnO3 .22) In

Fig. 4. The dilatation dL(T )/L of La1−X SrX MnO3 samples asa function of temperature T . Sudden changes of dL(T )/L canbe seen except for X = 0.48 and X = 0.90. The behavior at theanomaly changes its character between X = 0.60 and X = 0.67.

Fig. 5. The proposed phase diagram of La1−X SrX MnO3 system
based on the present work and the data from ref. 19 (X ≤ 0.175).
Tc shows the Curie temperature which is determined from theρ(T ) [ ] and M (T ) [ ] values. Ts shows the structural transition temperature which is determined from the ρ(T ) [] and thedilatation dL(T )/L [] values. Tco shows the charge-ordering
temperature determined from the vs (T ) [] and the dL(T )/L []values. Tp shows the polaron-ordering temperature determinedfrom the vs (T ) minimums19) []. The data point for X = 0.09
which is not contained in ref. 19 is added to this ﬁgure. The
shaded regions correspond to the borders of the three types ofcharge orderings (CO-I, CO-II and CO-III). AFM-I, FM-I and
FM-M imply antiferromagnetic insulator, ferromagnetic insulator and ferromagnetic metal, respectively.

◦•

Figs. 3 and 4, however, the anomalies in vs (T ) and dL/L
are almost the same in magnitude for X = 0.67 and 0.75
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Letters

tered at X = 2/3 or X = 3/4 from only the experimental
observations performed in this study.
As we have already mentioned, Akimoto et al.18)
found the A-type AFM order for La0.46 Sr0.54 MnO3 andconﬁrmed that the A-type order is a very stable andfundamental magnetic structure at around X = 0.54.
The A-type AFM order consists of alternative ferromagnetically ordered layers. On the other hand, Wollman and Koehler3) observed the C-type AFM order forLa0.2 Ca0.8 MnO3 and Jirak et al.23) also observed the Ctype order for Pr0.2 Ca0.8 MnO3 . The C-type order, which
consists of ferromagnetic chains, seems to be stabilized
in a large X region. It may be possible that the AFM
structures of the present La1−X SrX MnO3 crystals alsochange from the A-type in the CO-II region to the C-type
in the CO-III region. As we have noticed in Fig. 2, themagnitude of the electrical resistivity shows an abrupt
increase between X = 0.60and X = 0.67. The relatively
smaller ρ(T ) for 0.5 ≤ X ≤ 0.6 is consistent with the Atype AFM order, because the ferromagnetic layers in the
A-type structure are expected to contribute to the electrical conduction more eﬀectively than the ferromagnetic
linear chains in the C-type structure as a remnant of the
double exchange interaction in this system.18) It would
be interesting and highly desirable to conﬁrm whether
the CE-type AFM structure, which is usually stable in anarrow region around X = 0.50, is realized or not in thissystem. The phase diagram in Fig. 5 is based only on
the anomalies observed in the temperature dependencesof ρ(T ), M (T ), vs (T ) and dL(T )/L. A more detailed andexact diagram is expected to contain minute structuraltransformation lines such as the O∗ → O → O∗ reentrant transition observed by Kawano et al.24) In order tocomplete the phase diagram given in Fig. 5, it is absolutely necessary to perform detailed neutron diﬀraction
and/or electron diﬀraction studies.
In summary, we have measured the electrical resistivity ρ(T ), the magnetization M (T ), the sound velocityvs (T ) and the dilatation dL(T )/L of La1−X SrX MnO3 asa function of temperature T and have observed anomalies which are associated with the charge-ordering transitions. Based on the anomalies, we proposed a phase diagram of the La1−X SrX MnO3 system from the viewpointof the charge ordering. The framework of the phase diagram is based upon three kinds of charge orderings centered at X = 1/8, 1/2 and 2/3 (or 3/4). In the regions of
X where the charge ordering occurs, the system is nonmetallic at low temperatures. The metallic behavior of
ρ(T ) persists to low temperatures only in the X region
where the charge ordering is absent (0.18 ≤ X ≤ 0.45).
The overall phase diagram of La1−X SrX MnO3 appears
to be quite similar to that of La1−X CaX MnO3 in spiteof the diﬀerences in the eg -electron bandwidth.

2585

The authors wish to thank Y. Konno and S. Ohshiden of Iwate University for their assistance in the sample
preparation. The sound velocity measurement was performed under the inter-university cooperative research
program of the Institute for Materials Research, Tohoku University. This work is partially supported bya Research Grant of the “Saneyoshi Scholarship Foundation” and a Grant-in-Aid for Scientiﬁc Research from
the Ministry of Education, Science, Sports and Culture(No. 09640414).
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