Journal of Physics and Chemistry of Solids 63 (2002) 913±916

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Successive phase transitions in a metal-ordered manganite
perovskite YBaMn2O6
Tomohiko Nakajima*, Hiroshi Kageyama, Yutaka Ueda
Material Design and Characterization Laboratory, Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha,
Kashiwa, Chiba 277-8581, Japan

AbstractStructural, magnetic and electric properties of a metal-ordered perovskite YBaMn2O6 have been studied by means of powder
X-ray diffraction, differential scanning calorimetric, magnetic susceptibility, and electric resistivity. It is found that this material
undergoes a ®rst-order structural phase transition at Tc1  520 K; from a pseudo orthorhombic (monoclinic) phase (Tc1 , T ) toa pseudo tetragonal (monoclinic) one (T , Tc1). Accompanied by this transition the susceptibility exhibits a sharp drop where
ferromagnetic exchange interaction becomes antiferromagnetic. Furthermore, two more transitions, a metal±insulator transition at Tc2  480 K and an antiferromagnetic ordering at Tc3  200 K; have been observed, respectively. q 2002 Elsevier
Science Ltd. All rights reserved.
Keywords: A. Oxides; D. Crystal structure; D. Electrical properties; D. Magnetic properties; D. Phase transitions

1. Introduction
Recently, magnetic and electric properties of manganite
perovskite with a general formula A12x 31 A 0x 21 MnO3 ;where A 31 and A 0 21 represent rare earth and alkaline earth
ions, respectively, have been extensively investigated.
Among the interesting features are the so-called colossal
magnetoresistance observed in, e.g. La0.7Sr0.3MnO3 [1] andmetal±insulator (M±I) transition accompanied by charge
and orbital ordering observed in, e.g. Pr0.5Sr0.5MnO3 [2]. It
is now widely accepted that these enchanting phenomena
are caused by the strong correlation/competition of multidegrees of freedom that is spin, charge, orbital and latticeitself [3,4].
Unfortunately enough, almost all the works devoted to theseries of manganite perovskite so far are on the disorderedperovskite with the A 31 and A 0 21 ions being randomly
distributed. This means that whenever x is ®nite there
inevitably exists a disorder in the lattice. Since the physicalproperties of the manganite perovskite are quite sensitive toeven a tiny change in lattice distortion [5] it is desirable toemploy `cleaner' compound in order to understand the effectof A-site disorder more systematically.
* Corresponding author. Tel./fax: 181-471-36-3436.
E-mail address: t-nakaji@issp.u-tokyo.ac.jp (T. Nakajima).

When x equals to 0.5, one can overcome this problem by
preparing a metal-ordered double perovskite AA 0 Mn2O6 thestructure of which consists of alternate stack of the A 31 andA 0 21 ions along the c-axis as shown in Fig. 1. As far as theauthors know only the ordered LaBaMn2O6 was reported[6]. In this work, we successfully prepared the ordered
YBaMn2O6 and investigated the relation between structuraland physical properties of this material.
2. Experimental
The synthesis of the ordered perovskite requires several
complex processes as described in Refs. [7,8]. Starting
materials Y2O3, MnO2, and BaCO3 were ground thoroughly,pressed into pellets and calcined in an Ar ¯ow at 1300 8C for
48 h which yields the ordered YBaMn2O62y (y , 0.9) with alarge amount of oxygen vacancy in the YO layer. Then,annealing it in ¯owing O2 at 500 8C for 48 h leads to thefully oxidation of the sample, i.e. to y ! 0.
Thermogravimetric reduction in H2 con®rmed the oxygen
content of the sample to be ideal stoichiometry O6.0. The
crystal structures were determined for 300±573 K bypowder X-ray diffraction using Cu Ka radiation. Differential
scanning calorimetric (DSC) measurements was carried outfor 150±600 K. The heating/cooling rate was typically 5±20 K/min.

0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
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T. Nakajima et al. / Journal of Physics and Chemistry of Solids 63 (2002) 913±916

AO
MnO2
A'O
MnO2
AOaFig. 1. Crystal structure of the ordered AA 0 Mn2O6. The broken lines
represent the unit cell for the simple perovskite (ap £ bp £ cp).

The magnetic properties were studied using a SQUID
magnetometer in a temperature range T  5±600 K in amagnetic ®eld of 1000 Oe. The electric resistivity r ofa sintered pellet was measured for T  100±620 K by aconventional four-probe technique.

3. Results and discussion

20012+102

310+Ê-302+032+302-106+106220-204+024+204-112+112

40

60-302+310+302
032+Ê-223+223

024-212+212-114+114-122+122004
200-112+112(b) Ordered YBaMn2O6 at 573K

-204
220+204**

111+003001

100002+010
101011-102

102+110012**(a) Ordered YBaMn2O6 at R.T.

111
003

100+010002
011+101001

Intensity (arb. units)110-102

The X-ray diffraction pro®le scanned at room temperature(Fig. 2(a)) indicates that the obtained sample is a mixture ofa main phase of the ordered YBaMn2O6 and a miner impurity phase of BaMnO32d (about 3%). The (001) re¯ection

210+Ê-202+202-104+104-212-122+122+212-114+114b020
202-104+104+014+022

caround 2u  128 assures the successful alternation of Y 31
and Ba 21 cations along the c-axis, thus doubling the c parameter with respect to the simple perovskite. It appears thatthe crystal structure is tetragonal but the careful analysis of
the data ®nally led to the conclusion that it is monoclinicwith the unit cell ap £ bp £ 2cp, where ap, bp, and cp represent
lattice parameters for the simple perovskite structure. Thestructural parameters are determined as follows: a  b  3:898 1 A; c  7:602 2 A; b  90:2 18;
3:901 9 A;
and V  115:5 9 A 3 :Fig. 3 shows the temperature dependence of the structuralparameters where a clear jump is observed at Tc1  520 K;
suggesting a ®rst-order transition. The nature of the ®rstorder phase transition at Tc1 is also visible in the DSCcurve (Fig. 4) where a huge endothermic peak is observedon heating. We found that the structure of the high-T phaseis also consistent with the monoclinic symmetry with theunit cell of ap £ bp £ 2cp. The structural parameters, forexample, at 573 K are determined as follows: a  b  3:846 3 A; c  7:731 2 A; b  90:2 88;3:930 6 A;
and V  116:9 8 A 3 : It is strange that the high-T phase witha pseudo orthorhombic structure (a . b, b < 908) has
apparently a lower symmetry than the low-T phase with apseudo tetragonal structure (a < b, b < 908). We suppose
the low-T phase may have superstructures originating fromthe charge/orbital ordering.
The careful reading of the DSC curve in Fig. 4 suggeststhat besides the already-mentioned peak at Tc1 there are two

200+020004

914

80

2θ (degree)
Fig. 2. X-ray diffraction data for the ordered YBaMn2O6 at: (a) RT and (b) 573 K. Both pro®les can be indexed by assuming a monoclinicstructure with the ap £ bp £ 2cp unit cell. Peaks marked by asterisks come from an impurity phase BaMnO32d and Y2O3.

3100.008
3.85510χ (emu/molMn)

c/2b4100.006

3.80
300

400

600

92116

91

11590114
300

400

89600

500

Temperature (K)Fig. 3. Temperature variation of the reduced cell parameters a, b, c/2, b , and V for the ordered YBaMn2O6.

weak endothermic peaks at Tc2  480 K and Tc3  200 K:
In Fig. 5, we demonstrate the electric resistivity as a function of T, which can be characterized by the M±I transition
that occurs at Tc2 and by small anomalies at Tc1 and Tc3.
Although r for Tc2 , T decreases with increasing T, it
should be essentially metallic considering the relatively
low and constant resistivity (,10 21 V cm) and the factthat we used polycrystalline sample not a single crystal.
Also shown in Fig. 5 is the temperature dependence of themagnetic susceptibility x measured at 1000 Oe. At hightemperature above Tc1 the x ±T curve shows a paramagnetic
behaviour obeying the Curie±Weiss law with the effective

Heat Flow (mcal/sec)

AFIPIPM1PM22101100

β ( ˚)

117

3100.004

93500

71061000 Oe

3.90118

Volume (Å )0.010

aTc2 Tc1

Tc3

3.95915

20 K/ min

Tc2

Tc3
Tc1200 250 300 350 400 450 500 550

Temperature (K)Fig. 4. DSC curve of the ordered YBaMn2O6 upon heating with aspeed of 20 K/min.

100.002100.000

010100 200 300 400 500 600 700

Resistivity(Ω⋅cm)

Lattice Parameters (Å)

T. Nakajima et al. / Journal of Physics and Chemistry of Solids 63 (2002) 913±916-1-2

Temperature (K)Fig. 5. Magnetic susceptibility and electric resistivity of the ordered
YBaMn2O6 as a function of temperature.

moment P eff  8:06mB and the Weiss constant u1  286 K:
The positive value of u 1 suggests the ferromagnetic spincorrelation and the value of Peff is compatible with a theoretical value of 7.94m B derived from [Mn 31 1 Mn 41] in
YBaMn2O6. Upon decreasing T, x drops sharply at Tc1
while only small anomaly is observed at Tc2 where the M±
I transition occurs. Below Tc2 the x ±T curve once again
shows a paramagnetic behaviour but the Curie±Weiss ®tting
gave a negative value of u2  2379 K; indicating an antiferromagnetic interaction. Finally, x exhibits a steep drop atTc3 then reaching a constant value ,0.006 emu/mol Mn.
Considering the negative value of u 2, it is natural to assume
that the transition at Tc3 would be due to the antiferromagnetic long-range order.
As mentioned earlier, the ordered YBaMn2O6 shows threetransitions namely from the paramagnetic metal with thenearly orthorhombic structure (PM1) to the paramagnetic
metal with the nearly tetragonal structure (PM2) at Tc1
then to the paramagnetic insulator (PI) at Tc2 and ®nally tothe antiferromagnetic insulator (AFI) at Tc3. Now let us
discuss the successive phase transitions from the viewpointof multi-degrees of freedom. The PM1 state with the positiveu implies that the dominant ferromagnetic interactions athigh temperatures originate from the double-exchangemechanism. Accordingly, without the transitions at Tc1 andTc2 this compound would experience the ferromagneticordering. What we would like to emphasize is that inYBaMn2O6 the charge/orbital ordering occurs at muchhigher temperature compared with the disordered perovskite. Take Sm0.5Ca0.5MnO3 [9] and Pr0.5Sr0.5MnO3 [2] forexample the charge/orbital ordering temperature is reported
to be, respectively, 270 and 140 K. This suggests thatthe disorder in the lattice originating from the random

916

T. Nakajima et al. / Journal of Physics and Chemistry of Solids 63 (2002) 913±916
0distribution of A and A cations signi®cantly lowers thecharge/orbital ordering temperature.
It has been known that the charge/orbital ordering is
usually accompanied by the structural phase transitions as
is seen in Ref. [10]. In the present compound however
charge/orbital ordering temperature (Tc2) is lower than thestructural phase transition (Tc1). This is signi®cantly different from the transitions reported in the disordered manganite
perovskite so far.
To fully understand the nature of the successive phasetransition in particular intermediate phase (PM2) it is necessary to perform microscopic measurements such as neutron
diffraction and NMR, which are now in progress.
4. Conclusion
In summary, we studied the structural and physical properties of the ordered manganite YBaMn2O6 to ®nd threephase transitions at Tc1  520 K PM1 ! PM2 ; at Tc2 480 K PM2 ! PI and at Tc3  200 K PI ! AFI: At Tc1
the structure changes from pseudo orthorhombic to pseudo
tetragonal one. The M±I transition which occurs at Tc2
would be accompanied by the charge/orbital ordering.
Re¯ecting the ordering between Y 31 and Ba 21 ions thetransition temperature is much higher than that reportedfor disordered manganite. The antiferromagnetic orderingoccurs at Tc3.

Acknowledgements
This study was partly supported by Grant-in-Aid for
Scienti®c Research and for Creative Scienti®c Researchfrom the Ministry of Education, Science, Sports andCulture.
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