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Journal of the Physical Society of Japan
Vol. 72, No. 2, February, 2003, pp. 1–5#2003 The Physical Society of JapanLETTERS

New Stacking Variations of the Charge and Orbital Orderingin the Metal-Ordered Manganite YBaMn2 O6
H. K AGEYAMA1 , T. N AKAJIMA1 , M. I CHIHARA1 , Y. U EDA1 , H. Y OSHIZAWA2 and K. OHOYAMA3
1

Material Design and Characterization Laboratory, Institute for Solid State Physics, University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581
2
Neutron Scattering Laboratory, Institute for Solid State Physics, The University of Tokyo, 106-1 Shirakata, Tokai, Ibaraki 319-1106
3
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577(Received July 5, 2002)

We performed transmission electron microscopy (TEM) and powder neutron diﬀraction experiments
on an A-site ordered manganese perovskite YBaMn2 O6 which undergoes unusual and multiple phasetransitions. In the paramagnetic insulating phase, the so-called CE type of charge and orbital ordering
was observed in the monoclinic a–b plane, which has been frequently observed for the ordinary solid02þ
solution of A3þ
1x Ax MnO3 around x ¼ 0:5. TEM revealed, however, a fourfold periodicity along the caxis, suggesting a new stacking pattern, in which planes of the CE type are built up according to thesequence [ . . .]. Interestingly, when the system entered the antiferromagnetic state below 195 K, thisstacking pattern changed into [ . . .] or [ . . .], suggesting a close interplay between spins andorbitals. The obtained stacking patterns were strongly correlated to the inherent structural alternation,i.e., the Y/Ba order along the c axis.
KEYWORDS: YBaMn2 O6 , TEM, neutron diffraction, ordered perovskite, charge/orbital order
DOI: 10.1143/JPSJ.72.dummy

Perovskite (ABO3 ) and its related compounds (A2 BO4 ,A3 B2 O7 , etc.) display interesting electric, magnetic andoptical properties. Most well-known examples are the highTc superconductivity in cuprates (B ¼ Cu)1) and the colossal
magnetoresistance in manganites (B ¼ Mn).2) Emergence ofvarious interesting phenomena in the perovskite systems ispartly due to the versatility of the A site, which forms a wide
range of solid solutions between cations with diﬀerent
valences and ionic sizes. This advantage provides us an
opportunity to systematically tune the doping level x as wellas the Goldschmidt tolerance factor f , as demonstrated byLa2x Srx CuO4 and La1x Cax MnO3 .3,4)
It should be stressed, however, that such a partial
substitution can always be a cause of structural disorder.
In A1x A0x MnO3 , the shape, size and tilting angle of each
MnO6 octahedron would depend largely on nearby A=A0
ions. It means that crystallographic parameters reﬁned using
x-ray diﬀraction (XRD) or neutron diﬀraction (ND) data
correspond only to spatially averaged values. A problem tobe addressed is that the randomness of the underlying lattice
should aﬀect the physical properties to a certain degree. The
eﬀect would become more serious when one deals withmicroscopic phenomena. It is possible that the structural
disorder and spatial heterogeneity trigger or accelerate
electronic phase segregation, as recently observed inLa2x Srx CuO4 5) and La0:5 Sr0:5 MnO3 .6)
Fortunately, one can obtain a ‘clean’ system when x in
7,8)
02þ
A3þ
for example, A1x Ax BO3 is 1/2. In Y0:5 Ba0:5 CoO3y ,site metal ordering takes place in a way that the YO and BaO
layers are built up alternatively. Therefore, the chemicalcomposition should be better denoted as YBaCo2 O6y .
Recently, we have succeeded in synthesizing its manganese
analogue, YBaMn2 O6 (A ¼ Y, A0 ¼ Ba, B ¼ Mn),9,10)
whose structural and physical properties are considerably
diﬀerent from those of the conventional disordered
A0:5 A00:5 MnO3 system with randomly distributed A=A0
ions.11,12) YBaMn2 O6 undergoes unusual phase transitions

as a function of T: a 1st order structural phase transition at
Tc1 ¼ 520 K from triclinic P1 to monoclinic P2, a metal–
insulator (MI) transition at Tc2 ¼ 480 K and antiferromagnetic ordering at Tc3 ¼ 195 K. One of the peculiar features is
the separation of the structural and MI transition temperatures (i.e., Tc2 6¼ Tc1 ), which is strikingly in contrast to theA0:5 A00:5 MnO3 system, where these transitions occur simultaneously (i.e., Tc1 ¼ Tc2 ). Also interesting is the fact thatthe value of Tc2 is, to the authors knowledge, the highest in
the manganese perovskite family.
Structural considerations9,10) indicate that these unusual
properties in YBaMn2 O6 are attributable to (1) the absenceof lattice disorder, (2) the two-dimensionality due to thestacking sequence of –YO–MnO2 –BaO–MnO2 –, and (3) the
MnO6 octahedron distorted heavily from the rigid one.
However, to fully understand the correlation between the
structural and physical properties, it is necessary to understand how the charge, orbital and spin degrees of freedom
crystallize on cooling. In this study, we performed experiments of transmission electron microscopy (TEM) andpowder ND for the paramagnetic insulating phase (PI;Tc3 < T < Tc2 ) and the antiferromagnetic insulating phase(AFI; T < Tc3 ). It was found that the obtained charge andorbital ordering (COO) pattern in the PI phase diﬀers fromthat for the disordered A0:5 A00:5 MnO3 systems. Interestingly
enough, the antiferromagnetic long-range ordering at Tc3
redesigns the COO pattern.
A polycrystalline sample of YBaMn2 O6 was prepared bya conventional solid state reaction as reported in ref. 9.
Superlattices produced by COO were probed by TEM (JOEL
EM2010) operated at 200 kV, using a low-T sample holder,which is controllable from room temperature (RT) down to
24 K. The sample was ﬁnely ground in methanol and thenwas placed on Cu microgrid meshes for TEM observation.
Powder ND experiments, which allow us to determine themagnetic ordering, were conducted for 25 K < T < 350 K
using the Kinken powder diﬀractometer for high-eﬃciency
12

J. Phys. Soc. Jpn., Vol. 72, No. 2, February, 2003

LETTERSand high-resolution measurements, HERMES, at the Institute for Materials Research (IMR), Tohoku University,
installed at the JRR-3M reactor in the Japan Atomic Energy
Research Institute (JAERI), Tokai. Neutrons with a wave were obtained by the 331 reﬂection of thelength of 1.8196 A
Ge monochromator. The 120 -blank-sample-220 collimation
was employed.
Shown in Fig. 1 is the crystal structure of YBaMn2 O6 at
350 K (the PI phase), determined from the Rietveld reﬁnement of both powder XRD and ND patterns. The details of
the structural analysis will be reported elsewhere.10) The
structure of the PI phase adopts
space group P2
p a monoclinicpwith a unit cell dimension of 2ap  2bp  2cp , where ap ,bp and cp denote the cell constants for the simple cubic
perovskite. The analysis of the powder ND data for 350 Kgave
latticeparametersofa ¼ 5:5193ð3Þ A, c ¼ 7:6135ð2Þ A, and  ¼ 90:295ð3Þ . For
b ¼ 5:5131ð3Þ A,
simplicity, we will use simple cubic perovskite notations(indicated by subscript ‘‘p’’) to discuss the experimental
results in the remaining text. The structure contains two
crystallographically inequivalent Mn sites, Mn(1) andMn(2). Viewed along the ap , bp and cp axes, the MnO6octahedra are tilted to give trans conﬁgurations. Eachoctahedron is heavily distorted due to the A-site order.
There is a signiﬁcant diﬀerence in the volume between two
 3 for Mn(1) andinequivalent MnO6 octahedra [9.998 A
3

9.547 A for Mn(2)], suggesting a charge ordering, where
the Mn3þ and Mn4þ ions occupy the Mn(1) and Mn(2) sites,
respectively. In contrast, the structure at 500 K(Tc2 < T < Tc1 ) is very close to that of the PI phase but
Mn(1)O6 and Mn(2)O6 are similar in size, which iscompatible with the metallic conductivity.
Figure 2(a) shows the [100]p -zone TEM image and thecorresponding electron diﬀraction pattern at RT (for the PI
phase), which clearly reveal a modulation originating fromthe alternate stack of the YO and BaO layers along the ccp

ap
Fig. 1. Perspective view of the crystal structure of the monoclinic
YBaMn2 O6 at 350 K (the PI phase) along the [010]p -zone axis, where
the Mn(1)O6 and Mn(2)O6 octahedra are shown as light and dark
polyhedra, respectively, while the Ba2þ and Y3þ ions are shown by dark
and light balls.

H. KAGEYAMA et al.

 periodicity of the modulation deduced
axis. The 7.5(4) Afrom the TEM image is indeed in good agreement with theND result [c ¼ 7:6135ð3Þ A].
The doubling of the cparameter can be conﬁrmed from the electron diﬀraction
pattern, which contains, in addition to the main reﬂection
spots, relatively strong superlattice reﬂections with a twofold
periodicity along [001]p . The high quality and homogeneityof the specimen with the A-site order can be conﬁrmed by
the fact that this modulation spreads over a wide range of the
TEM image.
In manganite perovskites, various COO states can bedirectly probed by TEM.13–15) Figure 2(b) shows the [001]p zone lattice image taken also at RT. Together with thecontrast corresponding to the reduced lattice constants, ap
 we found an 11.0 A
 modulation (q1 ) along
and bp ( 3:9 A),
the [110]p (or ½11 0 p ) direction.16) The corresponding
superlattice reﬂections are shown in the inset of Fig. 2(b).
This superlattice, represented as q1 ¼ ð1=4; 1=4; 0Þp [or (1/4,1=4, 0)p ], can be ascribed to the so-called CE-type COO in
the a–b plane, since the same COO pattern in the a–b plane
has been frequently observed for A1x A0x MnO3 when x isaround 0.5.13,14) Interestingly, a careful observation of theinset of Fig. 2(a) further reveals very weak reﬂections, themodulation of which is commensurate and can be denoted asq2 ¼ ð0; 0; 1=4Þp . The interlayer COO can explain the q2
superlattice reﬂections, which will be discussed later.
It is important to note that the (1/2, 1/2, 0)p reﬂection is
not a direct consequence of the checkerboard-type chargeordering in the ab plane. It is well known that octahedral
tilting distortions, as seen in GdFeO3 , allow this reﬂection.
In fact, the Rietveld reﬁnement of the XRD/ND data even in
the metallic state above Tc2 gave the chemical unit cell ofpp2ap  2bp  2cp .10) It should also be pointed out that theq1 and q2 modulations probed by TEM were invisible fromthe XRD/ND experiments. This is because the latter
experiments were performed with the use of powder
samples. The shifts of atomic positions relevant to thesestructural modulations would be too small to be observed by
the powder XRD/ND experiments.
We obtained electron diﬀraction patterns at RT (the PI
state) along various directions and then plotted the observed
superlattice reﬂections on the reciprocal lattice in Fig. 3(a).
The most intriguing feature in the present study is thepresence of the modulation vector q1 ¼ ð0; 0; 1=4Þp . We
consider that the origin of the q1 modulated structure is thestacking sequence of the CE-type layers. There are various
methods of piling up the CE-typelayersalong the c axis sopp
as to satisfy the unit cell 2 2ap  2bp  4cp . Figures 4(a)and 4(b) show two possible models, both of which have thestacking sequence along the c axis described by [ . . .].
In addition, one can conceive more complicated models withstacking sequences such as [ . . .] and [ . . .] (notshown). Whatever the appropriate pattern is, we would liketo emphasize that it forms a striking contrast to the halfdoped solid solution, where a uniformly stacked pattern [Fig.
4(c)], which is in other words, the original 3D CE-COO
pattern given by Wollan et al.,17) has always been
observed.13,14)
Among the candidates, the model shown in Fig. 4(a)consists of the uniform alignment of each charge and isconsistent with the structure determined by the powder

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J. Phys. Soc. Jpn., Vol. 72, No. 2, February, 2003

LETTERS

H. KAGEYAMA et al.

3

Fig. 2. High-resolution TEM images and electron diﬀraction patterns of YBaMn2 O6 at RT obtained along (a) the [100]p -zone and (b)[001]p -zone axes. Electron diﬀraction patterns at 24 K obtained along (c) the [001]p -zone and (d) [100]p -zone axes.

(a)

(b)

cp*

001*

001*

100*

ap*

c p*

010*

bp*

010*

bp*

100*

ap*

Fig. 3. Reciprocal lattices with fundamental (closed) and superlattice(open) reﬂections, obtained from TEM at (a) RT and (b) 24 K.

XRD/ND study.10) We conﬁrm that the powder XRD/ND
patterns, in which the fourfold periodicity was not observed,
were analyzed based on the smaller unit cell. However, thereﬁned crystal structure would be almost close to the real
one as long as the structural modulations arising from the
COO are considerably small. Actually, the obtained structural parameters, such as bond length, Mn–O–Mn bridging
angle and so on, are quite reasonable and the reliable factors
are as good as those for the higher temperature phases
without COO. Of course it is impossible to completely
exclude other models from the present study; if one of othermodels is a relevant
model,
the Rietveld analysis using theppunit cell of 2ap  2bp  2cp would provide only anaveraged structure with larger displacement parameters andpossibly with larger reliable factors. In order to conﬁrm the
3D CE-COO structure, we may need to perform structuralcharacterization with the use of single crystals, which havenot been grown yet.
As stated above, the model in Fig. 4(a), where the d3x2 r2 and d3y2 r2 -orbitals stack as [ . . .], is possibly realized
in the PI state. It is interesting to compare this novel stacking
sequence [ . . .] in YBaMn2 O6 with that in NaV2 O5 .18)
In this layered vanadate, a number of diﬀerent charge
stacking patterns appear in the pressure-temperature phase
diagram, which is successfully mapped onto the ANNNI
model (devil’s staircase). In the manganese perovskite
systems, however, the most probable key in determining
the 3D CE-type COO structures lies in the A-site order/
disorder. In the case of A0:5 A00:5 MnO3 , the interlayer orbital
interaction that governs the stacking form is uniform.
Depending on the sign of the orbital interaction, we candeduce two possible superstructures, [ . . .] and[ . . .], shown in Figs. 4(c) and 4(d), respectively,
though the real system favors the former as mentioned
above. On the contrary, the interlayer orbital interactions for
YBaMn2 O6 can be alternated since each MnO2 layer issandwiched by diﬀerent layers (i.e., the YO and BaO layers)with considerably diﬀerent ionic radii (rY3þ ¼ 1:18 A,
rBa2þ ¼ 1:61 A).
The [ . . .] proposed pattern [Fig.

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J. Phys. Soc. Jpn., Vol. 72, No. 2, February, 2003

(a)

(b)

cp

LETTERS

H. KAGEYAMA et al.

cp

α

α

α

β

α

10

ap

β

15

25

30

(1/2 1/2 5/2)

(0 2 0)(1 1 2)(1 0 2)(1 1 1)(1 1 0)(0 0 2)

20

35

40

2θ ( °)

α

bp

α

(e)

cp

(-3/2 1/2 1/2)

(d)

cp

(-1 1/2 3/2)

(c)

20 K

(1/2 1 1/2)

ap

(1/2 0 3/2)
(1/2 1/2 3/2)

ap

bp

β

(1 0 0)βbp

β(1/2 1/2 1/2)

αβ

(1/2 0 1/2)ααIntensity (arb. units)α(0 0 1)250 K

bp

Fig. 5. ND proﬁles of YBaMn2 O6 at 250 K and 20 K, where the peaks arepp
indexed according to the 2ap  2bp  2cp unit cell.

ap
cp

bp
ap

Fig. 4. Various COO patterns (a)–(d) and a spin-ordering pattern (e),where for simplicity only Mn3þ and Mn4þ ions are drawn. (a) and (b):possible COO structures for the PI phase with the fourfold periodicity
along the c axis. In each case, we have to determine the proper location of
the BaO (or YO) layers: the BaO (or YO) layers are intervened either
between out-of-phase planes (i.e., ) or in-phase planes (i.e.,  and). (c) and (d): possible COO structures for the AFI phase. It is notedthat the model (c) having uniform stacking is the original 3D CE-type
COO structure and is obtained for the metal-disordered A0:5 A00:5 MnO3
system.

4(a)] is naturally explained if the sign of the interlayer
orbital interactions changes alternatively. So far we do nothave any microscopic picture of the interlayer orbital
interaction. Clarifying the mechanism, qualitatively or
quantitatively, calls for further investigation. It is noteworthy that another model with the sequence of [ . . .] [Fig.
4(b)] can be similarly understood in terms of the Y/Ba
ordering and the interlayer coulomb interaction, while othercomplicated models cannot.
Finally, we would like to discuss the superstructure in the
AFI state below Tc3 . Figure 5 compares the ND patterns
taken at 250 K and 20 K. While all of the peaks for the PI
phase are of nuclear origin and can be indexed by assumingppa monoclinic symmetry with the 2ap  2bp  2cp unit
cell, additional peaks of magnetic origin appear when
T < Tc3 . It is revealed from indexing these magnetic peaks
that the spin-ordering pattern in the a–b plane is the same asthat observed in A0:5 A00:5 MnO3 ,19) but that along the c axis
has a fourfold periodicity. The proposed spin structure isshown in Fig. 4(e). Such a quadruplication along the c axis
has not been observed for the A0:5 A00:5 MnO3 system because
it is uniquely determined that the interlayer magnetic

interaction is ferromagnetic. Again, it is quite natural toconsider that the Y/Ba order along the c axis in YBaMn2 O6
makes the interlayer magnetic interactions alternated between ferro- and antiferromagnetic, resulting in the proposed
spin structure.
The electron diﬀraction pattern at 24 K [Fig. 2(c)] isinterpreted as the superimposition of twin domains both ofwhich have the q1 modulation. This implies that the CE-type
COO is retained within the a–b plane. Figure 2(d) shows,interestingly enough, the disappearance of the modulation
vector q2 in the AFI state, although the system still has thefourfold periodicity due to the spin ordering. The superlattice reﬂections at 24 K obtained by TEM are summarized
in Fig. 3(b) and possible models of the COO are depicted in
Figs. 4(c) and 4(d), where layers of the CE-type COO are
piled up according to [ . . .] and [ . . .], respectively. The change of the 3D superstructures of the COO
strongly suggests the keen competition between the orbital
and magnetic interactions. The observed rearrangement
across Tc3 is partly supported by the nature of a 1st-orderphase transition (relatively large hysteretic behavior in themagnetic susceptibility and heat absorption/release observedin a diﬀerential scanning calorimetry (DSC) measurement).9)
To summarize, we have investigated the superstructures
of YBaMn2 O6 in the charge-, orbital- and spin-ordered states
by TEM and ND experiments. In the PI phase, in addition to
the superlattice reﬂections originating from the A-site metalorder, novel superlattice reﬂections with modulation vectors
q1 ¼ ð1=4; 1=4; 0Þp and q2 ¼ ð0; 0; 1=4Þp appear. These modulations are interpreted as the CE-type COO in the a–b plane
and its stacking arrangement along the c axis. It should be
stressed that the stacking sequence according to [ . . .]was ﬁrst observed among partially doped manganeseperovskite compounds. In the AFI phase, we observed thedisappearance of q2 , indicating the uniform [ . . .] oralternative [ . . .] stacking of the CE-type COO layers
along the c axis. Transformation of the COO would be
mediated by the spin ordering with a quadruplicate periodicity along the same axis.
This work is partly supported by Grants-in-Aid for

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J. Phys. Soc. Jpn., Vol. 72, No. 2, February, 2003

LETTERS

Scientiﬁc Research Nos. 40302640, 407 and 758 and for
Creative Scientiﬁc Research (No. 13NP0201) from the
Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors thank Prof. A. N. Vasiliev for his
critical reading of this manuscript.
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