Journal of the Physical Society of Japan
Vol. 72, No. 12, December, 2003, pp. 3237–3242#2003 The Physical Society of Japan

Ground State Properties of the A-site Ordered Manganites, RBaMn2 O6 (R = La, Pr and Nd)
Tomohiko N AKAJIMA1 , Hiroshi K AGEYAMA1 , Hideki Y OSHIZAWA2 , Kenji O HOYAMA3 and Yutaka U EDA1
1

Materials Design and Characterization Laboratory, Institute for Solid State Physics, University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581
2
Neutron Science Laboratory, Institute for Solid State Physics, 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 August 27, 2003)

The most signiﬁcant structural feature of the A-site ordered manganese perovskites, RBaMn2 O6 , is that
the MnO2 square sublattice is sandwiched by two types of rock-salt layers, RO and BaO, with much
diﬀerent sizes and consequently the MnO6 octahedron itself is distorted in a noncetrosymmetric manner.
The structures and electromagnetic properties of RBaMn2 O6 (R = La, Pr and Nd) have been investigated.
They have tetragonal structures with no tilt of MnO6 octahedra and show ferromagnetic metal transitions.
The ground state is a coexistence of ferromagnetic metal and antiferromagnetic charge-orbital orderedphases for LaBaMn2 O6 and A-type antiferromagnetic metal for PrBaMn2 O6 and NdBaMn2 O6 . Theseground state behaviors are explained by the eﬀect of Mn–O distance on the bandwidth and the layer
structure with the RO–MnO2 –BaO–MnO2 stacking along the c-axis.
KEYWORDS: new metal-ordered manganites RBaMn2 O6 , neutron diffraction, structural and electromagnetic
properties, magnetic ground states, electronic phase diagram
DOI: 10.1143/JPSJ.72.3237

1.

Introduction

The discovery of novel structural and physical properties
in the A-site ordered manganite RBaMn2 O6 (R = Y and rareearth elements) has demanded new comprehension about
perovskite manganese oxides.1–8) The most signiﬁcantstructural feature of RBaMn2 O6 is that the MnO2 squaresublattice is sandwiched by two types of rock-salt layers, RO
and BaO, with much diﬀerent sizes and consequently the
MnO6 octahedron itself is distorted, as schematically shownin Fig. 1. This means that the structural and physicalproperties of RBaMn2 O6 can be no longer explained in termsof the basic structural distortion, the so-called tolerance
factor f , as in the A-site disordered manganites
R0:5 A0:5 MnO3 (A = alkaline earth elements). Furthermore
such a special deformation may give a new perturbation to
the competition among charge, orbital, spin and lattice
degrees of freedom. Previously we ﬁrst reported theelectronic phase diagram of RBaMn2 O6 expressed as afunction of the ratio of ionic radius of the A-site cations, rR3+  1:36 A 9)).3) Among posrR3+ =rBa2+ (rBa2+ ¼ 1:61 A,
sible combinations of R/Ba, the mismatch between RO andBaO is the smallest in La/Ba and the largest in Y/Ba. When
R3þ is smaller than Sm3þ , the CE-type charge and orbital
ordered (CO) state with a new stacking variation along the caxis is stabilized at the relatively high temperatures (TCO ) far

Fig. 1. (a) A schematic drawing of structural concept for the A-siteordered manganite RBaMn2 O6 . (b) An illustration of the distorted MnO6octahedron observed in RBaMn2 O6 .
E-mail: t-nakaji@issp.u-tokyo.ac.jp

above 300 K, which would be not only due to the absence of
A-site randomness but also due to the tilt of MnO6 octahedra
as well as heavy distortions of MnO6 octahedron.2–5)
Especially RBaMn2 O6 (R = Tb, Dy, Ho and Y) shows the
structural transition above TCO , which is possibly accompanied by the dx2y2 type orbital order.2,3,5) On the other
hand, RBaMn2 O6 (R = La, Pr and Nd) with relatively larger
R3þ has no octahedral tilt and shows a transition from aparamagnetic metal (PM) to a ferromagnetic metal (FM).
Among perovskite manganese oxides studied so far, thestructure without octahedaral tilt is rare. As mentioned
above, the MnO6 octahedra have a noncentrosymmetricdeformation. It is quite interesting how such deformationaﬀect various interactions or competition among them. Since
RBaMn2 O6 (R = La, Pr and Nd) has a simple tetragonal
structure with no tilt of MnO6 octahedra, it is a goodcompound to study an eﬀect of octahedral distortion on suchcompetition. In this paper, we have reﬁned the crystal
structures and investigated the ground state properties of
RBaMn2 O6 (R = La, Pr and Nd). Neutron diﬀraction study
together with magnetic susceptibility revealed interesting
physical properties including electronic phase segregation.
2.

Experimental Details

Powder samples of the A-site ordered RBaMn2 O6 (R =La, Pr and Nd) were prepared by a solid-state reaction of
La2 O3 , Pr6 O11 , Nd2 O3 , BaCO3 and MnO2 with 99.99%
purities (Rare Metallic Inc.). Starting reagents were groundthoroughly, pressed into pellets and calcined in an Ar ﬂow(6N) at 1573 K for 48 h. The use of Ar gas can avoid theformation of the A-site disorder. However, the obtained
specimen contains a large amount of oxygen vacancy in the
RO layer, providing RBaMn2 O6y (y  1). The metalordering in the form of layers and the lack of oxygen in
the RO layers are due to the general preference of the small
R3þ ions to take eight-fold coordination. The second step isannealing the obtained specimen in ﬂowing O2 at 623 K for
48 h, which leads to the fully oxidation of the specimen, i.e.,
to RBaMn2 O6 .

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J. Phys. Soc. Jpn., Vol. 72, No. 12, December, 2003

The X-ray powder diﬀraction experiments were performed using a MXP18 Mac Science diﬀractometer. The
neutron powder diﬀraction experiments were conducted for
T ¼ 20{400 K using the Kinken powder diﬀractometer forhigh eﬃciency and high resolution measurements,
HERMES, at Institute for Materials Research (IMR),
Tohoku University, installed at the JRR-3M reactor in Japan
Atomic Energy Research Institute (JAERI), Tokai, Japan.
 were obtained by
Neutrons with a wavelength of 1.8207 A
the 331 reﬂection of the Ge monochromator. The 120 -blanksample-180 collimation was employed. The magnetic properties were studied using a SQUID magnetometer in atemperature range T ¼ 2{400 K. The magnetic orderedstates at low temperatures were also studied by powder
neutron diﬀraction.
3.

Results

3.1 Crystal structures
The obtained products were checked to be single phase by
X-ray diﬀraction. The order/disorder of R and Ba wascarefully checked by the presence/absence of (0 0 1/2)
reﬂection indexed in the primitive cell. Figure 2 shows theT. NAKAJIMA et al.
Table I. Reﬁned structural parameters for RBaMn2 O6 (R = La, Pr andNd) at 400 K.
Atom

site

x

y

z

 2)
Biso (ALaBaMn2 O6
a ¼ 3:9178ð1Þ A
c ¼ 7:8070ð5Þ A

La

1a

0

0

0

0.6(1)

1b
2h

0

0

1
2

3
V ¼ 119:832ð9Þ A

Ba
Mn
O(1)

1c

1
2
1
2

0.2451(13)Rwp ¼ 10:87%

00.9(2)

Re ¼ 7:37%

O(2)

4i

0

0.2373(5)1.0(1)

RI ¼ 9:79%

O(3)

1d

1
2
1
2
1
2
1
2

0.8(2)
0.2(1)

12

12

1.0(2)

RF ¼ 5:31%
PrBaMn2 O6
a ¼ 3:9088ð1Þ A
c ¼ 7:7649ð4Þ A

Pr

1a

0

0

0

0.5(1)

1b
2h

0

0

1
2

3
V ¼ 118:639ð9Þ A

Ba
Mn
O(1)

1c

1
2
1
2

0.2450(11)Rwp ¼ 10:46%

00.9(2)

Re ¼ 6:83%

O(2)

4i

0

0.2333(5)1.0(1)

RI ¼ 9:33%

O(3)

1d

1
2
1
2
1
2
1
2

0.8(2)
0.2(1)

12

12

1.0(2)

RF ¼ 8:56%
NdBaMn2 O6
a ¼ 3:9043ð1Þ A
c ¼ 7:7448ð3Þ A

Nd

1a

0

0

0

0.6(1)

1b
2h

0

0

1
2

3
V ¼ 118:059ð9Þ A

Ba
Mn
O(1)

1c

1
2
1
2

0.2445(12)Rwp ¼ 9:67%

0

1.2(2)

Re ¼ 6:11%

O(2)

4i

0

0.2307(3)1.1(1)

RI ¼ 9:92%

O(3)

1d

1
2
1
2
1
2
1
2

0.6(2)
0.2(1)

12

12

1.0(1)

RF ¼ 7:92%
Note. All RBaMn2 O6 (R = La, Pr and Nd) phases crystallize in spacegroup P4/mmm (No. 123).

Fig. 2. Powder neutron diﬀraction proﬁles for (a) LaBaMn2 O6 , (b)PrBaMn2 O6 and (c) NdBaMn2 O6 at 400 K. The calculated and observed
diﬀraction proﬁles are shown at the top with the solid line and the crossmarkers, respectively. The vertical marks in the middle show positions
calculated for Bragg reﬂections. The lower trace is a plot of the diﬀerence
between calculated and observed intensities.

neutron powder diﬀraction patterns of RBaMn2 O6 (R = La,
Pr and Nd) measured at 400 K. The crystal structures werereﬁned by the Rietveld analysis of powder neutron diﬀraction using RIETAN 2000.10) Independent reﬁnement of thefractional occupancies of the Ba and R sites showed no
antisite disorder to an experimental uncertainty of 3%. The
reﬁned unit cells parameters, positional coordinates, displacement parameters and reﬁnement statistics are summarized in Table I. Table II demonstrates selected bonddistances and angles.
The crystal structure at 400 K (PM state) has a tetragonal
ap  ap  2cp cell with a space group of P4=mmm, where ap
and cp denote the primitive cell for the simple cubic
perovskite, and has no tilt of MnO6 octahedra which is rarein manganese perovskites. On the other hand, the MnO6octahedron itself is distorted in a way as illustrated in Fig.
1(b): Both Mn and oxygen atoms (Opl ) in the MnO2 plane
are displaced toward the RO layer to a larger extent for Opl .
= Opl –Mn–Opl [== , see Fig. 1(b)]As a result, the angle of 
deviates from 180 to 176.4 (La), 174.7 (Pr) and 173.7
= Oap –Mn–Oap (Oap : apical oxygen atom)(Nd), while that of is 180 in all compounds. The distance of Mn–Oap is for La, 1.902 A
 for Pr and 1.894 A
 forshortened (1.913 ANd) in the RO side, while that is elongated (1.989 A for La,
 for Pr and 1.978 A
 for Nd) in the BaO side.
1.979 A
Intermediate values are obtained for the distance of Mn–Opl
 for La, 1.956 A
 for Pr and 1.955 A
 for Nd).
(1.960 A
3.2 Magnetic properties
The magnetic susceptibilities (M=H) of RBaMn2 O6 (R =

J. Phys. Soc. Jpn., Vol. 72, No. 12, December, 2003

T. NAKAJIMA et al.

3239

Table II. Selected interatomic distances and angles of RBaMn2 O6 (R = La, Pr and Nd) at 400 K.
LaBaMn2 O6
distances (A)PrBaMn2 O6
distances (A)

NdBaMn2 O6
distances (A)Mn–O(1)

1:913ð10Þ  1

Mn–O(1)

1:902ð7Þ  1

Mn–O(1)

1:894ð6Þ  1

Mn–O(2)

1:960ð1Þ  4

Mn–O(2)

1:956ð1Þ  4

Mn–O(2)

1:955ð1Þ  4

Mn–O(3)

1:989ð10Þ  1

Mn–O(3)

1:979ð7Þ  1

Mn–O(3)

1:978ð7Þ  1

Mn–O (average)

1.957

Mn–O (average)

1.952

Mn–O (average)

1.947

angles ( )angles ( )angles ( )Mn–O(1)–Mn
Mn–O(2)–Mn

180
176.4(7)Mn–O(1)–Mn
Mn–O(2)–Mn

180
174.7(6)Mn–O(1)–Mn
Mn–O(2)–Mn

180
173.7(6)Mn–O(3)–Mn

180

Mn–O(3)–Mn

180

Mn–O(3)–Mn

180

O(1)–Mn–O(3)180

O(1)–Mn–O(3)180

O(1)–Mn–O(3)180

O(2)–Mn–O(2)176.4(7)O(2)–Mn–O(2)174.7(6)O(2)–Mn–O(2)173.7(6)Fig. 4. Temperature dependence of lattice parameters for (a) LaBaMn2 O6 , (b) PrBaMn2 O6 and (c) NdBaMn2 O6 .

Fig. 3. Temperature dependence of magnetic susceptibilities for
RBaMn2 O6 with R = La (a), Pr (b) and Nd (c). The magnetization wasmeasured under 0.1 T on zero-ﬁeld cooled (ZFC) and ﬁeld cooled (FC)processes. The inset in Fig. 3(a) shows the magnetization curves of
LaBaMn2 O6 at 2 K.

La, Pr and Nd) are shown in Fig. 3. As can be seen in Fig.
3(a), LaBaMn2 O6 transforms to ferromagnetic metal (F) at
TC ¼ 330 K,1) accompanying with a sharp increase of M=H
but without any structural change. The M=H then starts todecrease at 200 K, where only slight change of lattice
parameters is observed [Fig. 4(a)], and becomes constant
below 100 K, agreeing with the previous report.1) However,the neutron diﬀraction study reveals an unexpected phenomenon that the CE-type antiferromagnetic CO phase[AFI(CE)] coexists with the FM phase as the ground state of
LaBaMn2 O6 , as shown in Fig. 5(a). Figure 6 shows the3240

J. Phys. Soc. Jpn., Vol. 72, No. 12, December, 2003

Fig. 5. (a) Neutron diﬀraction patterns of powdered (a) LaBaMn2 O6 , (b)PrBaMn2 O6 and (c) NdBaMn2 O6 at 400 K and 20 K. The indexes (hkl)F ,(hkl)A and (hkl)CE represent the peaks characteristic of ferromagnetic
metal, A-type antiferromagnetic metal and CE-type charge/orbital
ordered antiferromagnetic states, respectively.

intensity of neutron diﬀraction peaks for RBaMn2 O6 (R =La, Pr and Nd) as a function of temperature. The AFI(CE)phase appears below Tt ¼ 200 K [Fig. 6(a)], in accordancewith the reduction of M=H. A similar coexistence has been
observed in R1x Ax MnO3 so far and attributed to the A-site
randomness or ﬂuctuation of composition.11) Since thepresent system does not have such randomness, the observed
electronic phase segregation is an essential behavior causedby the interplay among spin, charge, orbital and lattice
degrees of freedom and might be explained by the eﬀect ofMn–O distance on the bandwidth, as discussed in the next
section. Interestingly this AFI(CE) phase is converted to the
FM phase under external magnetic ﬁeld, as shown in theinset of Fig. 3(a). The volume fraction of the AFI(CE) phaseis estimated to be about 30% from the diﬀerence ofmagnetization between 0.3 T and 5 T at 2 K. A similar
behavior has been observed in R1x Ax MnO3 .11) On the otherT. NAKAJIMA et al.

Fig. 6. Temperature dependence of the intensity of peaks, (110)F , (001)Aand (1/2 1/2 1/2)CE for (a) LaBaMn2 O6 , (b) PrBaMn2 O6 and (c)NdBaMn2 O6 .

hand, PrBaMn2 O6 and NdBaMn2 O6 show the PM-to-FMtransition at TC ¼ 310 K (Pr) and 300 K (Nd), followed by
the magnetic transition at 270 K (Pr) and 290 K (Nd)accompanied by the large reduction of M=H [Fig. 3(b) and3(c)]. The neutron diﬀraction study reveals the transition
from FM to A-type antiferromagnetic metal [AFM(A)] at
TN ¼ 270 K (Pr) and 290 K (Nd) (Figs. 5 and 6). Thistransition associated with dx2y2 orbital order is consistent
with the change of lattice parameters observed, namely theelongation of the a-axis and the contraction of the c-axis at
TN , as shown in Fig. 4(b) and 4(c). The neutron diﬀraction
pattern for PrBaMn2 O6 below Tt ¼ 150 K shows the coexistence of a slight amount of AFI(CE), while no trace of
AFI(CE) phase is seen in NdBaMn2 O6 .
4.

Discussion
Figure 7 shows the electronic phase diagram of

J. Phys. Soc. Jpn., Vol. 72, No. 12, December, 2003

Fig. 7. Electronic phase diagram for RBaMn2 O6 . FM: ferromagnetic
metal phase, AFM(A): A-type antiferromagnetic metal phase, AFI(CE):
antiferromagnetic CE-type charger/orbital ordered insulator phase,
COI(CE): CE-type charger/orbital ordered insulator phase, PM: paramagnetic metal phase.

RBaMn2 O6 (R = Y and rare earth elements) renewed byadding the present results to the previous one. In spite ofapparent similarity to the phase diagram of R0:5 A0:5 MnO3 ,12)
there are several signiﬁcant diﬀerences. In the phase
diagram of R0:5 A0:5 MnO3 , there exists a critical region
where the FM and CO interactions compete with each otherand therefore the transition from FM to AFI(CE) as afunction of temperature, which is responsible for colossal
magnetoresistance, is observable. If the ratio rR3þ =rBa2+
were an appropriate measure of the bandwidth as is the caseof f for R0:5 A0:5 MnO3 , such a critical region would belocated between SmBaMn2 O6 and NdBaMn2 O6 , and a FMto-AFI(CE) transition would be expected in NdBaMn2 O6
adjacent to SmBaMn2 O6 . In reality, however, the groundstate of NdBaMn2 O6 is the AFM(A) phase and a fractionaltransition from FM to AFI(CE) is observed in LaBaMn2 O6
far from SmBaMn2 O6 . This implies that LaBaMn2 O6 might
be located closer to the CO state and the bandwidth would
become larger as LaBaMn2 O6 < PrBaMn2 O6 < NdBaMn2 O6 .
Here it should be noticed again that RBaMn2 O6 (R = La,
Pr and Nd) has no tilt of MnO6 octahedra. It is known thatthe tilt of MnO6 octahedra in ordinary manganese perovskites occurs in order to ﬁt the larger MnO2 sublattice to thesmaller AO sublattice (f < 1). No tilt of MnO6 octahedra is
expected to occur either when the MnO2 lattice has thealmost same size as the AO lattice (f  1) or when the
MnO2 lattice is expanded by the AO lattice (f > 1). In
RBaMn2 O6 (R = La, Pr, Nd) with relatively larger R3þ , theexpansion of the MnO2 lattice by the much larger BaO
lattice (f ¼ 1:071) should overwhelm the contraction by thesmaller RO lattice (f ¼ 0:982 for LaO, f ¼ 0:964 for PrO
and f ¼ 0:950 for NdO), in contrast to the case of
RBaMn2 O6 with R = Sm and later rare earth elements.
 is elongated
Actually, the Mn–Opl distance (1:96 A)compared with that (1.91–1.94 A) for the regular Mn3:5þ O6 .
The increase of Mn–Opl distance and the deviation of the= Mn–Opl –Mn and = Opl –Mn–Opl from 180 would
angles 
result in the decrease of bandwidth.
In addition, the alternate stacking of RO and BaO layers

T. NAKAJIMA et al.

3241

would provide another important feature, that is a twodimensional character in the structure, which would be
preferable for the dx2y2 type orbital order [AFM(A)]. These
structural characteristics explain well the present results; Notilt structure stabilizes FM but the two-dimensional characterin the structure leads to AFM(A) associated with dx2y2 typeorbital order as the ground states of NdBaMn2 O6 andPrBaMn2 O6 . The larger elongation of Mn–Opl distance in
LaBaMn2 O6 destabilizes FM to some extent as a result of
the decrease of bandwidth and results in the fractionaltransition from FM to AFI(CE) [coexistence of FM andAFI(CE) phases as the ground state]. The eﬀect of Mn–Opl
distance on the bandwidth has never been seen nor discussed
in the ordinal perovskite manganese oxides, because they
have ﬁnite tilt of MnO6 octahedra, by which the bandwidth
is dominantly controlled.
Lowering of both TC and TCO in the critical region as
observed R0:5 A0:5 MnO3 is not recognized in the phase
diagram of RBaMn2 O6 . The absence of such behavior in
RBaMn2 O6 is partly due to the absence of the A-site
randomness. In the critical region, the FM [AFM(A)] andCO interactions compete with each other and are signiﬁcantly aﬀected by composition, coherent size of crystal,
external ﬁeld etc. The FM [AFM(A)] and CO interactions
are distributed in R0:5 A0:5 MnO3 with the A-site randomness.
Such ﬂuctuation of interactions enhances the criticality. On
the other hand, it could be more deﬁnite which interaction
becomes dominant in RBaMn2 O6 without the A-site randomness.
5.

Summary

To summarize, we have investigated the structures andelectromagnetic properties of the A-site ordered manganite
RBaMn2 O6 (R = La, Pr and Nd). The most signiﬁcantstructural feature of the A-site ordered manganese perovskites, RBaMn2 O6 (R = Y and rare earth elements), is that
the MnO2 square sublattice is sandwiched by two types ofrock-salt layers, RO and BaO, with much diﬀerent sizes andconsequently the MnO6 octahedron itself is distorted in anoncetrosymmetric manner. The crystal structure of the
RBaMn2 O6 (R = La, Pr and Nd) has a tetragonal ap  ap 
2cp cell with a space group of P4=mmm, where ap and cp
denote the primitive cell for the simple cubic perovskite, andhas no tilt of MnO6 octahedra which is rare in manganeseperovskites. Reﬂecting the crystal structures, the ferromagnetic double exchange interaction is dominant in RBaMn2 O6(R = La, Pr and Nd), but the ground state is a mixture of FM
and AFI(CO) for LaBaMn2 O6 and A-type AFM for
PrBaMn2 O6 and NdBaMn2 O6 . These ground state behaviors
are explained by the eﬀect of Mn–O distance on the
bandwidth and the two-dimensional character in the structure.
Acknowledgments
The authors thank T. Yamauchi, M. Isobe, Y. Matsushita,
Z. Hiroi, H. Fukuyama and K. Ueda for valuable discussion.
This work is partly supported by Grants-in-Aid for ScientiﬁcResearch (No. 407 and No. 758) and for Creative ScientiﬁcResearch (No. 13NP0201) from the Ministry of Education,
Culture, Sports, Science, and Technology.

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J. Phys. Soc. Jpn., Vol. 72, No. 12, December, 2003

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