Journal of the Physical Society of Japan
Vol. 71, No. 12, December, 2002, pp. 2843–2846#2002 The Physical Society of JapanLETTERS

Structures and Electromagnetic Properties of New Metal-Ordered Manganites:
RBaMn2 O6 (R ¼ Y and Rare-Earth Elements)
Tomohiko N AKAJIMA, Hiroshi K AGEYAMA, Hideki Y OSHIZAWA1 and Yutaka U EDA
1Materials Design and Characterization Laboratory,
Neutron Scattering Laboratory, Institute for Solid State Physics, University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581(Received July 11, 2002)

New metal-ordered manganites RBaMn2 O6 can be classiﬁed into three groups based on the structuraland electromagnetic properties measured. The ﬁrst group (R ¼ Tb, Dy and Ho) shows three successive
phase transitions, namely, structural, charge/orbital order (CO) and antiferromagnetic order (AF)transitions on cooling, as observed in YBaMn2 O6 . The second group (R ¼ Sm, Eu and Gd) exhibits a COtransition, followed by an AF transition. The third group (R ¼ La, Pr and Nd) exhibits a metallic
ferromagnetic transition, followed by an A-type AF transition in PrBaMn2 O6 and NdBaMn2 O6 .
2þCompared with metal-disordered (R3þ
0:5 A0:5 )MnO3 , RBaMn2 O6 has two remarkable features: (1) the highCO transition temperature (TCO ) above 340 K and (2) the presence of a structural transition above TCO inthe ﬁrst group. These are closely related to the structural feature that the MnO2 sub-lattice is sandwichedby two types of rock-salt layers, RO and BaO, with diﬀerent lattice sizes.
KEYWORDS: new metal-ordered manganites RBaMn2 O6 , structural and electromagnetic properties, structuraltransition, charge/orbital order, magnetic transition, phase diagram
DOI: 10.1143/JPSJ.71.2843

Averaged Ionic Size of A site <rA> (Å)
1.28

1.32

1.36

1.40

400
(R, A)=(La, Sr)

R0.5A0.5MnO3Temperature (K)The magnetic and electrical properties of perovskite
2þ
manganites with the general formula (R3þ1x Ax )MnO3 (R =rare earth elements and A = alkaline earth elements) havebeen extensively investigated for the last decade.1) Amongthe interesting features are the so-called colossal magnetoresistance (CMR) and metal–insulator (MI) transition
accompanied by the charge and orbital order (CO). It is
now widely accepted that these interesting phenomena arecaused by the strong correlation/competition of multidegrees of freedom, that is, spin, charge, orbital and lattice.
The structure of perovskite RMnO3 consists of a MnO2
square sublattice and a RO rock-salt sublattice. The
mismatch between the larger MnO2 and smaller RO
sublattices is relaxed by tilting MnO6 octahedra, leading to
the lattice distortion from a cubic structure to, mostly, anorthorhombic GdFeO3 -type structure. In this lattice distor= Mn–O–Mn deviates from 180 ,tion, the bond angle resulting in a signiﬁcant change in the eﬀective one-electron
bandwidth (W) or equivalent eg -electron transfer interaction
2þ(t). In the substitution system of (R3þ1x Ax )MnO3 with aﬁxed x and a random distribution of R3þ and A2þ , thestructural and electromagnetic properties have been explained well by the degree
pﬃﬃof
ﬃ mismatch, that is, the tolerance
factor f ¼ ðhrA i þ rO Þ=½ 2ðrMn þ rO Þ , where hrA i, rMn andrO are (averaged) ionic radii for the respective elements,
because W or t is changed by varying f . Figure 1 shows the1)2þ
generalized phase diagram for (R3þ
0:5 A0:5 )MnO3 , where theferromagnetic metal (FM) generated by the double-exchange(DE) interaction is dominant near f ¼ 1 (maximal W or t),
while the CE-type CO insulator (COI(CE)) is most stabilized
in the lower f region ( f < 0:975). In the middle region( f 0:975), the competition between the ferromagnetic DE
and antiferromagnetic CO interactions results in various
phenomena including CMR.
Recently, it has been argued how the A-site randomness

300

PM(Sm, Ca)(Pr, Sr)(Nd, Sr)(Nd, Ca)(Pr, Ca)

200

AFI
(CE)

100

(Sm, Sr)

0
0.95

FM

COI
(CE)

0.96

0.97

AFI
(CE) AFM
(A)
FM

0.98

0.99

1.00

Tolerance Factor f
2þ
Fig. 1. Generalized phase diagram for (R3þ
0:5 A0:5 )MnO3 (ref. 1). FM:
ferromagnetic metal, AFM(A): A-type antiferromagnetic metal, COI(CE):
CE-type charge/orbital-ordered insulator, AFI(CE): antiferromagnetic
CE-type charge/orbital-ordered insulator, PM(or PI): paramagnetic metal(or paramagnetic insulator) phases.

2þ
aﬀects the physical properties of (R3þ1x Ax )MnO3 . The
phenomena such as the coexistence of FM and CO and theelectronic phase separation2) may be induced by the A-site
randomness. Unfortunately, almost all the works devoted toa series of perovskite manganites so far are on disorderedmanganites with R3þ and A2þ ions being randomly
distributed. This means that, whenever x is ﬁnite, there
inevitably exists a disorder in the lattice. Since the physicalproperties of the perovskite manganite are quite sensitive toeven a tiny change in lattice distortion, it is important toemploy a compound without A-site disorder in order toclarify the eﬀect of A-site randomness.
Very recently, we have successfully synthesized a metalordered manganite YBaMn2 O6 with successive stacking of
YO–MnO2 –BaO–MnO2 –YO [see Fig. 2(a)] and observed
three successive phase transitions, namely, structural transi-E-mail: t-nakaji@issp.u-tokyo.ac.jp
2843

2844

J. Phys. Soc. Jpn., Vol. 71, No. 12, December, 2002(a)

LETTERS

(b)
BaO

Ba
MnO6
Y

MnO2
RO
MnO2
BaO
MnO2

c

RO
MnO2
BaO

Fig. 2. Crystal structure of YBaMn2 O6 (a) and a schematic illustration of
structural concept for RBaMn2 O6 (b). The MnO2 square lattice issandwiched by two types of the rock-salt layers, RO and BaO, with
diﬀerent lattice sizes.

T. NAKAJIMA et al.

pﬃﬃﬃ
pﬃﬃﬃ
2ap  2bp  2cp with a tilt of MnO6 octahedra, where
ap , bp and cp denoteprimitive
cell for the simple cubic
pﬃﬃﬃ the p
ﬃﬃﬃ
perovskite. The 2ap  2bp  2cp unit cells for R ¼ Sm,
Eu and Gd are tetragonal (ap ¼ bp ) but those for R ¼ Tb, Dy
and Ho are distorted to monoclinic as observed in
YBaMn2 O6 .4,5) TEM at room
pﬃﬃtemperatureﬃ
pﬃﬃﬃ has revealed thein-plane superstructure of 2 2ap  2bp characteristic of
the CE-type CO in the compounds with R ¼ Sm Ho, as
observed in YBaMn2 O6 ,4) while no superstructure has been
observed in the compounds with R ¼ La, Pr and Nd. In
addition to such superstructure within the a–b plane, TEM
has revealed a 4-fold periodicity along the c-axis, suggestinga new stacking pattern.4)
RBaMn2 O6 can be classiﬁed into three groups based on
the obtained structural and electromagnetic properties.
Figure 3 shows the temperature dependence of  and  for1020(a) DyBaMn2O6
15

10

Tt

10

0.1T

10
10

5

10
10

0

-2

χ ( x10 emu/mol)

1010(b) EuBaMn2O6

5

10
10

4

10

3

0.1T

2

10
10

1

10

0

10
100 200 300 400 500 600 700

0

5
4
3
2
1
0

-1

5
4

ρ (Ω⋅cm)

3
2
1
0

-1

T (K)
20

10

χ ( emu/mol)(c) PrBaMn2O6
15

10
10

0.1T

10

10
10
10

5

10

0
0

100

200

300

10
400

6
5
4
3
2
1

ρ (Ω⋅cm)tion without any charge and magnetic order at Tt ¼ 520 K,
CO transition (MI transition) at TCO ¼ 480 K and antiferromagnetic order (AF) transition at TN ¼ 195 K.3–5) The
observed TCO ¼ 480 K is, to the best of our knowledge,
the highest among the perovskite manganites. Across thephase transition at Tt ¼ 520 K, associated with the structuralchange from pseudo-orthorhombic (triclinic) to pseudotetragonal (monoclinic) on cooling, the resistivity () showsslight change and the magnetic susceptibility () exhibits alarge reduction. Furthermore, the magnetic interaction in theparamagnetic metal (PM) phase seems to be changed fromferromagnetic above Tt to antiferromagnetic below Tt . Suchtransition was ﬁrst observed in perovskite manganites. The
expectation that such novel transition could be closely
related to the metal-ordered structure led us to study themetal-ordered perovskite manganites RBaMn2 O6 . In thispaper, we report the structures and physical properties of the
new RBaMn2 O6 . We summarize the results in a phase
diagram and compare the diagram with Fig. 1.
Powder samples were prepared by a solid-state reaction ofR2 O3 , BaCO3 and MnO2 similar to that used for
YBaMn2 O6 .3) No perovskite-type compound was produced
for Ce, Yb and Lu. Since Er and Tm compounds included asigniﬁcant amount of impurities, we excluded them from thepresent study.
The crystal structure was determined at 300–573 K bypowder X-ray diﬀraction using CuK radiation. The superstructure associated with the CO was investigated bytransmission electron microscopy (TEM). The magnetic
properties were studied using a SQUID magnetometer in thetemperature range of T ¼ 5{700 K under a magnetic ﬁeld of
0.1 T. The electric resistivity of a sintered pellet wasmeasured at T ¼ 100{620 K by a conventional four-probe
technique.
The X-ray diﬀraction patterns of all compounds clearlyshow the (0,0,1/2) reﬂection indexed with the simple cubic
perovskite structure, which indicates the same metal-orderedstructure as that of YBaMn2 O6 . The crystal structure at room
temperature is a tetragonal ap  ap  2cp one with no tilt of
MnO6 octahedra in La, Pr and Nd compounds, while in thecompounds with R ¼ Sm Ho, it has a larger cell of

6

0

-1

T (K)
Fig. 3. Temperature dependence of resistivity () and magnetic susceptibility () for (a) DyBaMn2 O6 , (b) EuBaMn2 O6 and (c) PrBaMn2 O6 .
DyBaMn2 O6 shows a structural transition without any charge andmagnetic order at Tt (see text). TCO , TN , and TC represent the charge/
orbital-ordered, antiferromagnetic and ferromagnetic transition temperatures, respectively.

LETTERS(a) DyBaMn2 O6 , (b) EuBaMn2 O6 and (c) PrBaMn2 O6 as
typical examples of each group. The ﬁrst group includes
compounds with R3þ ¼ Tb3þ , Dy3þ and Ho3þ whose ionicradii are close to Y3þ . As can be seen in Fig. 3(a), thesecompounds show distinct three transitions, namely, structural (triclinic to monoclinic) transition at Tt , CO (MI)transition at TCO and AF transition at TN on cooling, as
observed in YBaMn2 O6 .3–5) The change of magnetic
interaction from ferromagnetic above Tt to antiferromagnetic
below Tt is not very evident in these compounds as in
YBaMn2 O6 ,3) because of the signiﬁcant contribution ofmagnetic rare-earth ions to the total . However the
reduction of  at Tt commonly observed in this series canbe ascribed to a change in Weiss temperature, namely, achange of magnetic interaction, between the PM phases
above and below Tt . The second group consists of
RBaMn2 O6 with R3þ ¼ Sm3þ , Eu3þ and Gd3þ . The
compounds exhibit a CO (MI) transition as determined by
p[seeand TEM (the in-plane superstructure of
ﬃﬃﬃ Fig.
pﬃﬃ3(b)]ﬃ
2 2ap  2bp ) measurements. The structural transition as
observed at Tt in the ﬁrst group is absent in the second
group. As shown in Fig. 3(b), the –T curve of EuBaMn2 O6
has two anomalies at around 240 K and 40 K in addition to
the reduction of  at TCO . The reduction of  at TCO ismainly due to the change of magnetic interaction fromferromagnetic to antiferromagnetic, because the ferromagntic spin-spin correlation due to DE interaction is dominant in
the PM phase above TCO , while the paramagnetic CO phase
below TCO has an antiferromagnetic one. Since the CE-type
COI(CE) phase is known to order antiferromagnetically (seeFig. 1), it would be natural to ascribe one of the two
anomalies of  to AF. Actually the neutron magnetic
diﬀraction performed in YBaMn2 O6 ,4) HoBaMn2 O6 andTbBaMn2 O6 has revealed that the anomaly of  at around200 K corresponds to AF. However it is diﬃcult to performthe neutron diﬀraction in the second group, because of thestrong absorption of neutron by rare-earth atoms. Fortunately, all of the compounds with R ¼ Sm Ho and Y have
similar anomalies of  at around 200 K and 50 K below TCO .
Assuming that the spin-spin correlation in the CE-type
COI(CE) phase is not signiﬁcantly changed in RBaMn2 O6 ,we regard the anomaly at around 200 K as TN . No signiﬁcantchange was observed around 50 K in the neutron diﬀraction
pattern. Any slight change in spin structure may occur ataround 50 K. The third group RBaMn2 O6 with R3þ ¼ La3þ ,Pr3þ and Nd3þ exhibits a FM transition at TC , which isevidenced by the rapid increase of , as shown in Fig. 3(c).
The observed TC ¼ 335 K for LaBaMn2 O6 agrees well withthe previous report.6) In PrBaMn2 O6 , the FM transition isfollowed by some transition accompanied by the sharp
reduction of , as shown in Fig. 3(c). The preliminary
neutron magnetic diﬀraction has revealed a transition to Atype antiferromagnetic metal [AFM(A)] from FM in
PrBaMn2 O6 and NdBaMn2 O6 , which is consistent with therelatively low  below TN compared with that in the
COI(CE) state of EuBaMn2 O6 or DyBaMn2 O6 (see Fig. 3).
A similar A-type AFM(A) was previously reported in metaldisordered Pr0:5 Sr0:5 MnO3 with f 0:985 (see Fig. 1).7) A
detailed report of the neutron diﬀraction study is now in
preparation. A semiconductive behavior in the PM state of
PrBaMn2 O6 is due to loosely sintered samples. The

T. NAKAJIMA et al.

Ho, Y Dy Tb Gd

Eu Sm

Nd

2845

Pr

La

600

PM'
500

Tt

PM

400

T (K)

J. Phys. Soc. Jpn., Vol. 71, No. 12, December, 2002

TCO
300

TC

COI (CE)
TN

200

FM
AFM (A)

AFI (CE)

100

0
0.74

0.76

0.78

0.80

0.82

0.84

rR 3+/rBa2+
Fig. 4. Phase diagram for RBaMn2 O6 . The notations for each phase andtransition temperature are the same as those deﬁned in Figs. 1 and 3 (see2þtext). Compared with the (R3þ
0:5 A0:5 )MnO3 , RBaMn2 O6 has two remarkable features: (1) relatively high TCO and (2) the presence of structuraltransition above TCO in RBaMn2 O6 with R3þ of small ionic radii (theexistence of the second paramagnetic metal PM’ phase).

measurements of  and  in the last group were not carriedout above 400 K because no transition above this temperature was detected by diﬀerential scanning calorimetry.
The results are summarized in Fig. 4 in a phase diagram.
Here we express the phase diagram as a function of the ratioof ionic radius, rI ¼ rR3þ =rBa2þ ,8) instead of f . In RBaMn2 O6 ,the MnO2 sublattice is sandwiched by two types of rock-saltlayers, RO and BaO, with diﬀerent lattice sizes, as shown in
Fig. 2(b), and therefore the tolerance factor cannot be
deﬁned. rI is a measure of mismatch between RO and BaO
lattices.
It is very interesting to compare Fig. 4 with Fig. 1. Figure
4 is similar to Fig. 1, generally. There exist characteristicphases such as the FM phase and the CE-type COI(CE)phase in both phase diagrams. In Fig. 4, the FM phase isstable around R ¼ La and the COI(CE) phase becomesdominant for Sm Ho and Y. The ionic size of Ba2þ 8) is much larger than those of Sr2þ (1.44 A)
 and all(1.61 A)3þ
of R (1:36 A). In RBaMn2 O6 , the MnO2 layer feels
opposite strain forces from the adjacent RO and BaO layers;
as a result, the MnO6 octahedron itself is distorted in apeculiar manner that the oxygen atoms of the MnO2 squarelattice are strongly bound by R3þ resulting in buckling of Mn
and oxygen atoms in the MnO2 square plane,5) in contrast to2þ
the rigid MnO6 octahedron in (R3þ
0:5 A0:5 )MnO3 . Among the3þ
2þ
combination of R /Ba , the mismatch between RO andBaO lattices is the smallest in La3þ /Ba2 , the second smallest
in Pr3þ /Ba2þ and the third smallest in Nd3þ /Ba2þ . It is
expected that the FM appears in these compounds with arelatively small lattice mismatch and no tilt of MnO6octahedra. In these compounds, TC is about 300 K, and isslightly dependent on rI . This suggests that the ferromagnetic interaction or DE interaction is not highly inﬂuenced
by the lattice distortion in RBaMn2 O6 , which is supported by
the ﬁnding that the positive Weiss temperature  ¼ 286 K is
observed even in the PM region (T > Tt ) of YBaMn2 O6 withthe largest lattice distortion.3) On the other hand, the

2846

J. Phys. Soc. Jpn., Vol. 71, No. 12, December, 2002

LETTERS

COI(CE) phase appears in RBaMn2 O6 (R ¼ Sm Ho andY) with a larger lattice mismatch and the tilt of MnO6octahedra. The observed TCO (340 K) is much higher than2þthat (<300 K) for (R3þ
0:5 A0:5 )MnO3 , and it increases withdecreasing rI . This suggests that the lattice distortion in
RBaMn2 O6 eﬃciently enhances the CO interaction incontrast to ferromagnetic interaction; as a result, the COinteraction exceeds the ferromagnetic DE interaction. This
could be one of the reasons for the absence of FM to
antiferromagnetic CE-type COI(AFI(CE)) transition in
RBaMn2 O6 .
There are remarkable features in the phase diagram of
RBaMn2 O6 : (1) the high TCO and (2) the presence of astructural transition at Tt above TCO . It is easy to understandthis relatively high TCO in RBaMn2 O6 , because the absenceof A-site randomness and the layer-type metal-order arefavorable for the CO of Mn3þ /Mn4þ . As mentioned above,
the increase of the mismatch between RO and BaO lattices
also makes the CO stable. The structural transition at Tt is acharacteristic of compounds with R3þ of small ionic radii.
The large mismatch between RO and BaO lattices introduces
a strong frustration to the MnO2 sublattice; as a result, theMnO6 octahedron itself is heavily distorted leading to acomplex structural deformation (triclinic or monoclinic).5)
Such deformation must give a new perturbation to thecompetition of multi-degrees of freedom among charge,
orbital, spin and lattice. The paramagnetic metal phase(PM’) with an antiferromagnetic correlation and a pseudotetragonal structure (isotropic a–b plane) between Tt and TCO
inspire us to study the possibility of orbital ordering,presumably dx2 y2 -type, at Tt . The freezing of the orbital,
charge and spin degrees of freedom at independent
temperatures may become possible in structures with low
symmetry such as asymmetric distortion of the MnO6octahedron.
In summary, the structures and electromagnetic properties

T. NAKAJIMA et al.

of new metal-ordered perovskite manganites RBaMn2 O6(R ¼ Y and rare earth elements) have been investigated.
RBaMn2 O6 can be classiﬁed into three groups based on theobtained structural and electromagnetic properties. The ﬁrst
group (R ¼ Tb, Dy and Ho) shows three successive phasetransitions, namely, structural, CO (MI) and AF transitions.
The second group (R ¼ Sm, Eu and Gd) exhibits the CO(MI) transition, followed by the AF transition. The thirdgroup (R ¼ La, Pr and Nd) exhibits the FM transition,
followed by the A-type AFM transition in PrBaMn2 O6 or
NdBaMn2 O6 .
Comparedwithmetal-disordered
2þ
(R3þ
A
)MnO
,
RBaMn
O
has
two
remarkablefeatures:3
2
6
0:5 0:5(1) the relatively high TCO and (2) the presence of structuraltransition without any charge and magnetic order above TCO
in the ﬁrst group. These are closely related to the metalordered structure with the distorted MnO6 octahedra.
The authors thank T. Yamauchi, M. Isobe, T. Matsushita,
Z. Hiroi and H. Fukuyama for valuable discussion. This
work is partly supported by Grants-in-Aid for ScientiﬁcResearch (Nos. 407 and 758) and for Creative ScientiﬁcResearch (No. 13NP0201) from the Ministry of Education,
Culture, Sports, Science, and Technology.
1) See reviews, C. N. R. Rao and B. Raveau: Colossal Magnetoresistance,
Charge Ordering and Related Properties of Manganese Oxides (World
Scientiﬁc, Singapore, 1998).
2) S. Mori, C. H. Chen and S.-W. Cheong: Phys. Rev. Lett. 81 (1998)
3972.
3) T. Nakajima, H. Kageyama and Y. Ueda: J. Phys. Chem. Solids 63(2002) 913.
4) H. Kageyama et al.: cond-mat/0208518.
5) T. Nakajima et al.: submitted to J. Solid State Chem.
6) F. Millange, V. Caignaert, B. Domengès and B. Raveau: Chem. Mater.
10 (1998) 1974.
7) H. Kawano, R. Kajimoto, H. Yoshizawa, Y. Tomioka, H. Kuwahara andY. Tokura: Phys. Rev. Lett. 78 (1997) 4253.
8) R. D. Shannon: Acta Crystallogr. A 32 (1976) 751.