ARTICLE IN PRESS

Journal of Solid State Chemistry 177 (2004) 987–999

Anomalous octahedral distortion and multiple phase transitionsin the metal-ordered manganite YBaMn2O6
T. Nakajima,a H. Kageyama,a, M. Ichihara,a K. Ohoyama,c H. Yoshizawa,b and Y. UedaaaMaterials Design and Characterization Laboratory, Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha,
Kashiwa, Chiba 277-8581, Japanb
Neutron Science Laboratory, Institute for Solid State Physics, University of Tokyo, 106-1 Shirakata, Tokai, Ibaraki 319-1106, Japanc
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
Received 9 June 2003; received in revised form 24 September 2003; accepted 6 October 2003

Abstract
By means of powder X-ray diffraction, powder neutron diffraction and transmission electron microscopy (TEM), we determinedthe crystal structures of a metal-ordered manganite YBaMn2O6 which undergoes successive phase transitions. A high-temperature
metallic phase (Tc1 ¼ 520 KoT) crystallizes in a triclinic P1 with the following unit cell: Z ¼ 2; a ¼ 5:4948ð15Þ Å, b ¼ 5:4920ð14Þ Å,c ¼ 7:7174ð4Þ Å, a ¼ 89:804ð20Þ , b ¼ 90:173ð20Þ , g ¼ 91:160ð4Þ . The MnO6 octahedral tilting is approximately written as a0bc,
leading to a signiﬁcant structural anisotropy within the ab plane. The structure for Tc2 oToTc1 is a monoclinic P2 (Z ¼ 2;
a ¼ 5:5181ð4Þ Å, b ¼ 5:5142ð4Þ Å, c ¼ 7:6443ð3Þ Å, b ¼ 90:267ð4Þ ) with an abc tilting. The structural features suggest a dx2 y2orbital ordering (OO). Below Tc2 ¼ 480 K, crystallographically inequivalent two octahedra show distinct volume difference, due tothe Mn3+/Mn4+ charge ordering. The TEM study furthermore revealed a unique d3x2 r2 =d3y2 r2 OO with a modified CE structure.
It was found that the obtained crystal structures are strongly correlated to the unusual physical properties. In particular, theextremely high temperature at which charge degree of freedom freezes, Tc2 ; should be caused by the absence of the structural
disorder and by heavily distorted MnO6 octahedra.
r 2003 Elsevier Inc. All rights reserved.
Keywords: Metal-ordered perovskite; YBaMn2O6; Charge/orbital ordering; Octahedral tilting; CE structure; X-ray and neutron diffraction;
Transmission electron microscopy

1. Introduction
Manganese
perovskites
withaformula
3+
0 2+
0 2+
A3+
A
MnO
(0oxo1),
where
A
and
A
repre3
1x x
sent in general rare-earth ions and alkaline-earth ions,
respectively, have been extensively investigated for the
past 10 years [1]. The potential application to theindustrial technology has apparently accelerated theresearch activity, and as a result deepened our comprehension of this system. Now it is widely accepted thatthe dopant concentration x and the band width w (or thetolerance factor f ) are dominant two parameters whichenable us to systematically tune the magnetic andtransport properties including the colossal magnetore
Corresponding author. Present address: Department of Chemistry,
Graduate School of Science, Kyoto University, Kyoto 606-8502,
Japan. Fax: +81-75-753-4000.
E-mail address: kage@kuchem.kyoto-u.ac.jp (H. Kageyama).

0022-4596/$ - see front matter r 2003 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2003.10.003

sistance, charge ordering (CO) and orbital ordering(OO) [1].
However, probably underestimated to date or evenneglected in many cases, has been the effect ofrandomness or compositional inhomogeneity in the
crystal structure, which inevitably exists when we deal
with solid solutions except the end members, x ¼ 0 and
1. This effect may destroy some hidden phase which
otherwise may be present at given x and wðf Þ: One also
encounters difﬁculties in estimating the physical quantities precisely. In fact, ferromagnetic ordering temperatures observed experimentally so far have been much
lower than that predicted theoretically [2].
This unfavorable situation can be avoided by preparing the case of x ¼ 0:5; i.e., AA0 Mn2O6, where the AO
and A0 O layers are piled up alternately along the c axis,
as illustrated in Fig. 1. In 1999, Millange et al. [3]succeeded in synthesizing an A-site metal-orderedmanganite LaBaMn2O6 through a topotactic route andARTICLE IN PRESS
988

T. Nakajima et al. / Journal of Solid State Chemistry 177 (2004) 987–999

Fig. 1. Schematic representation of the ideal crystal structure of the
A-site ordered manganese perovskite, A3+A0 2+Mn2O6, showing thecorner-shared MnO6 octahedra. The open and solid balls are theA3+ and A0 2+ ions, respectively. The broken lines denote the unit cellfor the simple cubic perovskite with the A cation at the origin(ap  bp  cp ).

demonstrated that its ferromagnetic ordering temperature is 335 K which is indeed much higher than that forthe metal-disordered correspondence (La0.5Ba0.5MnO3)
by 65 K.
Following this result, we prepared the next ‘clean’manganite perovskite corresponding to A=Y, A0 =Ba
and x ¼ 0:5 (i.e., YBaMn2O6) [4]. Rather preliminary
structural analysis based on X-ray powder diffraction
and physical properties obtained from magnetic susceptibility (w) and electric resistivity (r) experiments suggestthat YBaMn2O6 undergoes, unlike metal-disordered
0 2+
compound A3+
0.5 A 0.5 MnO3, multiple phase transitions
as a function of temperature (T); upon cooling, a ﬁrstorder-like structural transition occurs at Tc1 ¼ 520 K
between paramagnetic and metallic phases (PM1PM2). The second transition at Tc2 ¼ 480 K is ametal–insulator (MI) transition, and the last one at
Tc3 ¼ 200 K induces the paramagnetic insulator (PI) toan antiferromagnetic insulator (AFI). Here, several
prominent features merit to point out as follows: (a)The MI transition temperature TMI (=Tc2 ) is, to thebest of the authors knowledge, the highest among
known manganese perovskite family. (b) In a striking
contrast to the metal-disordered system [1], the structural transition at Tc1 with drastic changes in lattice
constants is not accompanied by the MI transition(i.e., Tc1 aTc2 ). (c) The structural transition at Tc1 froma pseudo-orthorhombic to a pseudo-tetragonal symmetry on cooling is quite rare.
Moreover, a study of a series of the A-site orderedmanganites RBaMn2O6 with R being Y and rare-earth
elements has revealed that the phase diagram of
RBaMn2O6 is considerably different from that of the

0 2+
disordered analogue A3+
0.5 A 0.5 MnO3 [1,5]. It is thus
evident that the structural and physical properties of
YBaMn2O6 cannot be mapped onto such a simple
framework that has been adequate for the disorderedmanganese perovskites [1]. The main purpose of thepresent study is the determination of the crystal
structures of YBaMn2O6 in each phase on the basis of
the Rietveld reﬁnement using the powder neutron and
X-ray diffraction data. As chemical characterizations,
we performed the thermogravimetric analysis (TGA)
and the differential scanning calorimetry (DSC). In
order to observe possible superstructures due to the CO
and OO, we used the transmission electron microscopy(TEM). The magnetic and transport properties were
investigated by the magnetic susceptibility and electric
resistivity measurements as a function of T: These
intensive investigations have allowed us to unveil an
intimate correlation between physical properties andunderlying structures.

2. Experimental procedures
The synthesis of the A-site ordered perovskite
YBaMn2O6 requires the following two procedures as
employed for the homeotype of LaBaMn2O6 [4].
Starting reagents Y2O3, MnO2, and BaCO3 with
99.99% purities (Rare Metallic Inc.) were groundthoroughly, pressed into pellets and calcined in an Ar
ﬂow (6 N) at 1573 K for 48 h. The use of Ar gas can
avoid the formation of the A-site disorder. However, theobtained specimen contains a large amount of oxygen
vacancy in the YO layer, providing YBaMn2O6y(yB0:9) [6]. The metal ordering in the form of layers
and the lack of oxygen in the YO layers are due to thegeneral preference of the small Y3+ ions (and rare-earthions) to take eight-fold coordination. The second step isannealing the obtained specimen in ﬂowing O2 at 773 K
for 48 h, which leads to the full oxidation of thespecimen, i.e., to YBaMn2O6. This reaction is reversible
and indicates the topotactic nature of the transformation between YBaMn2O5 and YBaMn2O6.
The determination of the oxygen content of the ﬁnal
product was made by TGA using a TG2000S Mac
Science analyzer. This experiment was performed in the
T range between 300 and 1273 K with a heating rate
15–20 K/min in ﬂowing H2. The DSC measurement
was carried out on a DSC3200S Mac Science analyzer
for T ¼ 140–600 K with the heating and cooling rate of
10–20 K/min. For the TGA (DSC) measurement,
approximately 20 (30) mg of ﬁnely ground sample wasplaced in an aluminum (platinum) capsule, and wellcalcined a-Al2O3 was used as reference standard.
The X-ray powder diffraction experiments were
performed using a MXP18 Mac Science diffractometerwith the following operation conditions: 270 KoTo

ARTICLE IN PRESS
T. Nakajima et al. / Journal of Solid State Chemistry 177 (2004) 987–999

3. Results
3.1. Powder X-ray diffraction
Shown in Fig. 2 is the X-ray powder diffraction
pattern in the PI state, measured at 350 K. The obtained
data contain a main product of YBaMn2O6 as well as aminor impurity of BaMnO3d (less than 1.0%). With
regard to YBaMn2O6, we previously assigned the X-ray
diffraction patterns for each phase to the monoclinic
structure with the dimension of ap  bp  2cp [4], where
ap ; bp and cp denote the unit cell for the simple cubic
perovskite. However, careful experiment and re-consideration of the observed peaks resulted in the
monoclinic but a larger unit cell of O2ap  O2bp  2cp
as indexed in Fig. 2. It is to be noted that the 0 0 1
reﬂection at around 2y ¼ 12 indicates the doubling thec parameter with respect to cp and thus ensures the Y/Ba
ordering in the form of layers. The result of the Rietveld
reﬁnement of the crystal structures for this and otherphases will be provided in the proceeding subsections.
In a similar way, re-consideration of the X-ray
diffraction pattern in the PM2 phase led to theconclusion that the crystal structure has the monoclinic
unit cell O2ap  O2bp  2cp ; whereas the PM1 phase wasfound to be the triclinic system with O2ap  O2bp  2cp :-312,-132,132,312-204,024,204

310,-222,222-114,114,131

220
004

-202,022,202
201
003

001

110
002
111,-111

Experiment112

200,020-112

350 K PI

Intensity / a.u.

600 K, 5 o2yo120 with the step size of 0.02 , CuKa
radiation, V ¼ 50 kV and I ¼ 300 mA. The neutron
powder diffraction experiments were conducted for
200 KoTo600 K using the Kinken powder diffractometer for high 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. Neutrons with a wavelengthof 1.8196 Å were obtained by the 331 reﬂection of
the Ge monochromator. The 120 -blank-sample-180
collimation was employed. The data were collected witha step-scan procedure in the range 2y ¼ 10–120 with astep width of 0.01 . The 1.5 g powder sample waswrapped by an Al foil to form a cylindric shape(8 f  15 mm) and then ﬁxed to an Al sample holder
with He gas ﬁlled.
The microscopic structures arising from the CO andOO in the PI phase were probed by means of TEM using
JEM2010 (JEOL) operating at 200 kV at room temperature. The sample was ﬁnely ground under methanol
then dispersed on Cu grids coated with holy carbon
support ﬁlms.
The magnetic properties were studied using a SQUID
magnetometer between 5 and 700 K at a static magnetic
ﬁeld (H) of 0.1 T. The electric resistivity was measured
using a well-sintered pellet for T ¼ 100–620 K by aconventional four-probe technique.

989

Calculation

10

20

30

40

50

60

2θ / °
Fig. 2. (upper) The experimental powder X-ray diffraction pattern of
YBaMn2O6 at 350 K (PI phase), indexed assuming the monoclinic unit
cell with O2ap  O2bp  2cp : (lower) The calculated X-ray diffraction
pattern. The upper and lower vertical marks show positions calculated
for Bragg reﬂections for YBaMn2O6 and BaMnO3d, respectively. The
lower trace is a plot of the difference between calculated and observed
intensities.

We collected the powder X-ray diffraction patterns overa wide T range (300 KoTo560 K), which covers the PI,
PM1 and PM2 phases. After that, the lattice parameters
were determined by the least-squares ﬁtting, selecting
well-isolated peaks at higher 2y: The temperature
variation of the reduced lattice constants (ap ; bp ; cp ),the angles (a; b; g) and the unit-cell volume (V ) isdisplayed in Fig. 3. The result is in general consistent
with the previous result (Fig. 3 in Ref. [4]), although weassume the different crystal system for the PM1 phaseand the different unit-cell dimensions for all phases. It iseasy to recognize discontinuous changes of the lattice
parameters at Tc1 ; suggesting a ﬁrst-order phasetransition. In contrast, within our experimental accuracy
the structural parameters, ap ; bp ; cp ; b (and V ), arecontinuously changed at Tc2 : However, it is noteworthy
that the temperature dependence of V in the PI state isclearly different from that in the PM2 state. Thesebehaviors are consistent with the second-order-like
transition at Tc2 and are attributable to the change inelectronic states by the MI transition.
3.2. Chemical characterizations and thermodynamic
stability
From the TG experiment under H2 ﬂowing, theoxygen content of YBaMn2O6 was estimated to be theideal stoichiometry 6.070.02 per formula unit. Here, wemeasured the weight loss of the specimen after the

ARTICLE IN PRESS
T. Nakajima et al. / Journal of Solid State Chemistry 177 (2004) 987–999

990

Tc2 Tc1

ap
a
ap

3.88

cp
b

3.86
bp

3.84

bp

3.82

γ

91.5
91.0

Tc3
Tc1
Coo

ling

90.5

300

400

500

600

Temperature (K)Fig. 4. Typical example of DSC curves of YBaMn2O6 taken with aheating/cooling rate of 20 K/min.

90.0

α

234.0
3

Tc2

200

β

89.5

Volume (Å )ting

PI PM2 PM1

3.80

Angle (°)

Heat Flow / a.u.

3.90

Hea

exo

3.92

endo

Lattice Constant (Å)

3.94

233.5
233.0
232.5
232.0
300

350

400

450

500

550

600

Temperature (K)Fig. 3. Temperature dependence of the reduced lattice constants,(ap ; bp ; cp ), the angles (a; b; g), and the volume (V ), determined using a
set of powder X-ray diffraction patterns, which were collected withincreasing T: Estimated errors are within the size of each symbol.

In Fig. 4, we represent a typical example of the DSC
curve measured with the heating and cooling rate of
20 K/min. Upon heating, three endothermic reactions
appear at 520, 480 and 195 K, nearly corresponding to
the transition temperatures Tc1 ; Tc2 and Tc3 : Upon
cooling, we observed respective three exothermic peaks.
Reﬂecting the ﬁrst-order phase transitions at Tc1 andTc3 ; the DSC peaks are observed at somewhat lower
temperatures of 500 and 175 K than those in heatingprocess, while the observed peak at 475 K is almost
identical to that in heating process, indicating thistransition to be nearly second order. These behaviors
are in good accordance with the susceptibility andresistivity experiments to be mentioned later.

3.3. Magnetic and transport properties
following reaction: YBaMn2O6+6H2(g)-(1/2)Y2O3+
BaO+2MnO+(3/2)H2O(g). Complemental pieces of
information were obtained by DTA and X-ray diffraction experiments, which clariﬁed that annealing thespecimen at relatively high temperatures in air destabilizes the metal-ordering in YBaMn2O6. Namely, theanti-site disordering between the Y3+ and Ba2+ sites
occurs at around 1100 K. The resultant phase,
Y0.5Ba0.5MnO3 with the A-sites being randomly distributed, irreversibly decomposes into YMnO3 and
BaMnO3 at B1300 K. When heated in turn under Ar
gas ﬂowing (PO2 o1:5  104 torr), the stoichiometricsample starts to release its oxygen at around 600 K, andﬁnally reaches to yB0:5 (YBaMn2O5.5) at 1000 K, asdetected by the TGA experiment. Moreover, during theTEM experiments, where an extremely high vacuum(PO2 o3  108 torr) is achieved, we happened to ﬁndthat the loss of oxygen in YBaMn2O6 starts at much
lower temperature B380 K. For this reason, it was
unfortunately impossible to perform TEM to probe
superstructures in the PM1 and PM2 phases.

We show the magnetic susceptibility and resistivity of
YBaMn2O6 taken upon heating in Fig. 5 as solid curves.
The w exhibits a steep drop at Tc1 ; whereas a change in r
at Tc1 is subtle. In contrast, the r shows a drastic change
at Tc2 ; while the w is nearly unchanged across thistemperature.
In the PM1 and PI regimes, the w behaves mostly
in line with the Curie–Weiss law. The least square ﬁtting
to the Curie–Weiss formula in the PM1 regime between
600 and 700 K gave the effective Bohr magneton
Peff ¼ 6:235ð8Þ mB and the Weiss temperature
y ¼ 271:4ð8Þ K (see the dotted line in Fig. 5). The
obtained value of Peff is close to the theoretical value(P2eff (Mn3+)+Peff 2(Mn4+))1/2=6.240 mB, where weassume the Mn3+ (3d4) ions to take the high-spin
conﬁguration due to the stronger Hund coupling thanthe crystal ﬁeld stabilization energy. The ferromagnetic
spin–spin correlations derived from the positive ysupports the metallic conductivity associated with theso-called double exchange interaction.

ARTICLE IN PRESS
T. Nakajima et al. / Journal of Solid State Chemistry 177 (2004) 987–999
Tc2 Tc1
10

6(1 1 0)(0 0 2)Tc3
1.2

1000 Oe109915(a) 350 K

PI(b) 500 K

PM2(c) 580 K

PM1

1.0

0.6

2

-2

10

3

AFI

0.4

PM2

PI

PM1

10

10
0.2
Heating
Cooling
0.0

0

100

200

10

300

400

500

600

10
700

1

0

-1

26

28

2θ / °

30(1 1 0)(0 0 2)

10

24

Intensity / a.u.

0.8

4

Resistivity (Ω cm)

χ ( x10 emu/mol Mn)

10

24

26

28

2θ / °

30-2

Temperature (K)(-1 1 0)(0 0 2)(1 1 0)Fig. 5. Magnetic susceptibility and electrical resistivity of thepolycrystalline sample of YBaMn2O6 as a function of T: Solid andbroken curves represent, respectively, heating and cooling scans. The
dotted lines represent the results of the Curie–Weiss ﬁtting, performedin the PI and PM1 regions (for details, see text).
24

In the PI state, we performed the Curie–Weiss ﬁtting
between 330 and 400 K, which gave a negative value of y(=122.5(9) K), indicative of antiferromagnetic interactions. The antiferromagnetic spin–spin correlations
below Tc2 results in the Neél ordering at Tc3 ; at which asudden reduction is observed in the w2T curve. Our
neutron diffraction experiment [7] indeed revealed Bragg
reﬂections of magnetic origin, the intensities of which
begin to glow at Tc3 :Broken curves in Fig. 5 represent the magnetic
susceptibility and resistivity of YBaMn2O6 taken uponcooling. One ﬁnds distinct hystereses at Tc1 and Tc3 bothin the w2T and r2T curves, indicating these transition
to be ﬁrst order. Around Tc2 ; the cooling and heatingprocesses coincide with each other within the experimental accuracy, suggesting the second-order-like transition. The w shows a small upturn below 50 K,
accompanied by small hysteresis. Since the neutron
diffraction patterns do not show any change below andabove this temperature, the magnetic structure must be
almost identical and only a slight canting of theantiferromagnetically ordered moments may occur
below Tc3 :3.4. Rietveld refinement
We show in Fig. 6(a) the neutron powder diffraction
pattern measured at 350 K in the PI phase. All the peaks
responsible for YBaMn2O6 can be successfully indexed,

10

26

28

30

30

40

2θ / °

20

50

60

70

2θ / °

80

90

100 110 120

Fig. 6. Powder neutron diffraction proﬁles for YBaMn2O6 at: (a)350 K, (b) 500 K and (c) 580 K. The calculated and observed
diffraction proﬁles are shown at the top with the solid line and thecross markers, 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. The second
vertical marks for the impurity phase BaMnO3d. The signals from the
Al foil wrapping the sample and/or the Al sample holder were excludedin the 2y region. Insets: the enlarged plots for 24 o2yo30 , to indicatethat the use of the triclinic P1 space group is needed for (c).

using the monoclinic system with the same unit cell
O2ap  O2bp  2cp as that assumed in analyzing thepowder X-ray diffraction pattern (Section 3.1). No
extinction reﬂections both in the X-ray and neutron
diffraction data allow us to arrive at a conclusion that
only P2; Pm and P2=m are possible among monoclinic
space groups.
For oxides in general, the use of the neutron
diffraction method has an advantage over the X-ray
diffraction method in that the former can determine thepositions of oxygen ions more precisely. This pointshould be particularly emphasized for the manganite
perovskites, since oxygen bridging angles (+Mn–O–Mn)

ARTICLE IN PRESS
T. Nakajima et al. / Journal of Solid State Chemistry 177 (2004) 987–999

992

between neighboring MnO6 octahedra are known toplay a crucial role in the physical properties [1]. The
reﬁnement of the crystal structure was therefore
performed ﬁrst by using the neutron diffraction data.
The Rietveld method, implemented in the computer
program RIETAN2000 developed by Izumi and Ikeda[8], was used. YBaMn2O6 was assumed to have thespace group, either P2; Pm or P2=m; and BaMnO3d,detected as the impurity, was introduced in thecalculations as the secondary phase. For each possible
space group, we started the reﬁnement with the initialatomic positions, referring to those of the metal-ordered
but tetragonal system LaBaMn2O6 [3]. The R factors aredeﬁned as the following:
"
#1=2
X
X
2
2
Rwp ¼
wðjFio j  jFic jÞ =wjFio j;i"i

Re ¼ ðN  PÞ=X#1=2
wjFio j2

;

i

RI ¼

X

jIKo  IKc j=

K

RF ¼

X
K

X

IKo ;

K
1=2

1=2

jIKo  IKc j=

X 1=2
IKo ;
K

where w is the weighing factor, Fio and Fic are theobserved and calculated intensities, N is the total
number of Fio data when the background is reﬁned, P
is the number of adjusted parameters, IKo and IKc are
the observed and calculated integrated intensities.
In the course of the reﬁnement, the isotropic
temperature factors for each element were varied but
constrained to be equal, in order to avoid correlation
between the thermal and fractional occupancies. Setting
oxygen occupancies as free parameters did not improve
the result, so the oxygen ions are ﬁxed to be fully
occupied. This assumption is veriﬁed by the TG
observation that the oxygen content is determined as
6.070.02. Consideration of fractional occupancies andanti-site disordering between the Ba2+ and Y3+ ions did
not improve the result as well, which is understood inlight of a signiﬁcant difference in ionic radius between
Y3+ and Ba2+. The high quality and homogeneity ofthe specimen with the A-site ordering are supported by
the distinct 7.5 Å modulation along the [001]p spreading,
without any notable fault, over a wide range of the[100]p-zone TEM image (Fig. 2(a) in Ref. [7]). All
attempt to reﬁne in Pm and P2=m never results in thereliability factors better than 12%, and the obtained
parameters such as bond lengths and angles were quite
unrealistic. However, P2 quickly converged and gave
reasonable structure as shown later on, providing by far
the best reliability factors as follows: Rwp ¼ 9:87%,
Re ¼ 6:55%, RI ¼ 4:96% and RF ¼ 2:90%. The observed, calculated and difference patterns from theRietveld reﬁnement are displayed in Fig. 6(a) and thereﬁned unit-cells parameters, positional coordinates,
displacement parameters and reﬁnement statistics are
summarized in Table 1. Table 2 demonstrates selectedbond distances and angles. The unit-cell parameters,
a ¼ 5:5193ð3Þ Å, b ¼ 5:5131ð3Þ Å, c ¼ 7:6135ð2Þ Å andb ¼ 90:295ð3Þ , agree well with those determined fromthe X-ray diffraction experiment. As a matter of fact,
the X-ray diffraction pattern calculated using theparameters in Table 1 satisfactorily reproduce theexperimental one (see Fig. 2).
There are crystallographically two inequivalent Mnsites, Mn(1) and Mn(2), and eight oxygen sites,
O(1)BO(8). The oxygen arrangement around Mn(1)and Mn(2) is illustrated in Fig. 7(a). Two kinds of MnO6octahedron show a large distribution in +Mn–O–Mn
angles and a remarkable size difference as evidenced by
the average Mn–O distances (1.96(2) Å for Mn(1) and1.93(2) Å for Mn(2)) This fact suggests that the Mn(1)and Mn(2) ions take, respectively, trivalent and tetravalent. Namely, the occurrence of the CO state likecheckerboard in the ab plane is strongly anticipated at
the MI transition temperature Tc2 : This view is further
supported by the bond valence sum calculation [9],which provides Mn(1)=+3.56, Mn(2)=+3.80, andwill be conﬁrmed by the TEM in Section 3.5.
The powder neutron diffraction data at 500 K (in thePM2 phase) shown in Fig. 6(b) were successfully
characterized, as in the case of the PI phase, by the
monoclinic O2ap  O2bp  2cp cell without any extinct
reﬂections, and the Rietveld analysis gave the bestﬁt (Rwp ¼ 10:15%, Re ¼ 6:43%, RI ¼ 5:07% andRF ¼ 3:06%) when one assumed P2 among P2, Pm
and P2=m: The experimental and theoretical neutron
diffraction patterns are compared in this ﬁgure, and thereﬁned structural parameters are listed in Tables 1 and2. The lattice parameters and the angle at 500 K solved
are as follows: a ¼ 5:5181ð4Þ Å, b ¼ 5:5142ð4Þ Å,c ¼ 7:6443ð3Þ Å, b ¼ 90:267ð4Þ . The crystal structureof the PM2 phase is similar to that of the PI phase in
principle. However, the former phase differs from thelatter in that the Mn(1)O6 and Mn(2)O6 octahedra havenearly the same size with the average Mn–O distances of
1.96(2) and 1.95(2) Å, for Mn(1) and Mn(2), respectively, despite a rather large distribution in +Mn–O–
Mn angles in each octahedron. This observation is
supported by the metallic conductivity above Tc2 ; where
both Mn sites are supposed to take the valence of +3.5.
It appears that the lack of change in symmetry at Tc2 isagainst crystallographic phase transition. However, as
revealed by TEM (see Section 3.5), there is a translational symmetry breaking at Tc2 due to the formation of
the CO/OO superstructure, enlarging the unit cell of the
PI phase to 2O2ap  O2bp  4cp : The shifts of atomic
positions relevant to the structural modulations inducedby the CO/OO of the CE type would be too small to be

ARTICLE IN PRESS
T. Nakajima et al. / Journal of Solid State Chemistry 177 (2004) 987–999

993

Table 1z

Biso (Å2)

0.041(5)0.526(5)00.527(5)0.005(5)0.518(6)0.017(5)0.575(6)0.268(7)0.271(7)0.771(7)0.798(6)0.051(6)0.494(5)

0
0
0.5
0.5
0.242(2)0.244(2)000.195(1)0.212(1)0.237(2)0.247(2)0.5
0.5

0.9(1)
0.9(1)
0.4(1)
0.4(1)
0.3(1)
0.3(1)
1.25(5)1.25(5)1.25(5)1.25(5)1.25(5)1.25(5)1.25(5)1.25(5)

0.5
0
0.5
0
0.026(4)0.492(5)00.50.267(4)0.766(3)0.247(4)0.715(3)00.50.000(7)0.486(7)00.518(6)0.005(8)0.503(9)0.072(9)0.569(9)0.249(8)0.261(8)0.752(8)0.781(8)0.016(8)0.468(8)

0
0
0.5
0.5
0.243(4)0.242(4)000.193(1)0.215(1)0.240(2)0.244(2)0.5
0.5

1.0(1)
1.0(1)
0.4(1)
0.4(1)
0.3(1)
0.3(1)
1.32(6)1.32(6)1.32(6)1.32(6)1.32(6)1.32(6)1.32(6)1.32(6)0.480(6)0.001(7)0.5
0.014(6)0.005(8)0.488(9)0.002(9)0.476(9)0.041(8)0.432(8)0.214(7)0.255(6)0.718(7)0.749(6)0.021(6)0.516(7)0.254(7)0.229(8)0.751(7)0.733(7)0.005(8)0.488(7)00.511(7)0.001(8)0.506(9)0.005(8)0.496(9)0.009(7)0.469(8)0.270(8)0.774(7)0.239(7)0.739(8)0.009(6)0.533(7)0.235(8)0.743(8)0.290(7)0.780(8)0.003(5)0.000(6)0.5
0.502(5)0.244(7)0.235(6)0.747(7)0.752(7)0.011(5)0.000(6)0.269(6)0.233(5)0.226(6)0.201(5)0.495(5)0.495(5)0.756(5)0.772(5)0.802(6)0.801(4)1.1(1)
1.1(1)
0.6(2)
0.6(2)
0.4(2)
0.4(2)
0.4(2)
0.4(2)
1.49(8)1.49(8)1.49(8)1.49(8)1.49(8)1.49(8)1.49(8)1.49(8)1.49(8)1.49(8)1.49(8)1.49(8)

Atom

Site

x

y

Temperature: 350 K
Symmetry: monoclinic
Space group: P2 (No. 3)
a ¼ 5:5193ð3Þ Å
b ¼ 5:5131ð3Þ Å
c ¼ 7:6135ð2Þ Å
b ¼ 90:295ð3Þ
V ¼ 231:67ð2Þ Å3
Rwp ¼ 9:87%
Re ¼ 6:55%
RI ¼ 4:96%
RF ¼ 2:90%

Y(1)
Y(2)
Ba(1)
Ba(2)
Mn(1)
Mn(2)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
O(7)
O(8)

1c
1a
1d
1b
2e
2e
1a
1c
2e
2e
2e
2e
1b
1d

0.5
0
0.5
0
0.018(3)0.481(3)00.50.279(2)0.765(2)0.247(2)0.709(2)00.5Temperature: 500 K
Symmetry: monoclinic
Space group: P2 (No. 3)
a ¼ 5:5181ð4Þ Å
b ¼ 5:5142ð4Þ Å
c ¼ 7:6443ð3Þ Å
b ¼ 90:267ð4Þ
V ¼ 232:60ð2Þ Å3
Rwp ¼ 10:15%
Re ¼ 6:43%
RI ¼ 5:07%
RF ¼ 3:06%

Y(1)
Y(2)
Ba(1)
Ba(2)
Mn(1)
Mn(2)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
O(7)
O(8)

1c
1a
1d
1b
2e
2e
1a
1c
2e
2e
2e
2e
1b
1d

Temperature: 580 K
Symmetry: triclinic
Space group: P1 (No. 1)
a ¼ 5:4948ð15Þ Å
b ¼ 5:4920ð14Þ Å
c ¼ 7:7174ð4Þ Å
a ¼ 89:804ð20Þ
b ¼ 90:173ð20Þg ¼ 91:160ð4ÞV=232.83(8) Å3
Rwp ¼ 9:12%
Re ¼ 6:22%
RI ¼ 3:21%
RF ¼ 1:78%

Y(1)
Y(2)
Ba(1)
Ba(2)
Mn(1)
Mn(2)
Mn(3)
Mn(4)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
O(7)
O(8)
O(9)
O(10)
O(11)
O(12)

1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a

Positional parameters for YBaMn2O6.

observed by powder X-ray and neutron diffraction
experiments.
The Rietveld reﬁnement for the neutron diffraction
data at 580 K (in the PM1 state) clariﬁed that thestructure has the triclinic P1 symmetry with theO2ap  O2bp  2cp unit cell. The reﬁnement statistics
and unit-cell parameters are Rwp ¼ 9:12%, Re ¼ 6:22%,
RI ¼ 3:21%,
RF ¼ 1:78%,
a ¼ 5:4948ð15Þ Å,b ¼ 5:4920ð14Þ Å, c ¼ 7:7174ð4Þ Å, a ¼ 89:804ð20Þ ,b ¼ 90:173ð20Þ , g ¼ 91:160ð4Þ . The obtained parameters are summarized in Tables 1 and 2, and theexperimental and calculated neutron diffraction patterns
are shown in Fig. 6(c). The four inequivalent MnO6octahedra have the following average Mn–O distances:1.93(8) Å (Mn(1)), 1.94(8) Å (Mn(2)), 1.96(8) Å (Mn(3)),
and 1.96(3) Å (Mn(4)). Although the neutron diffraction
patterns in Figs. 6(a)–(c) look similar to each other, aclose look at the patterns, e.g., those around 2y=27

ARTICLE IN PRESS
T. Nakajima et al. / Journal of Solid State Chemistry 177 (2004) 987–999

994
Table 2

T ¼ 350 K

T ¼ 500 K

Distances (Å)
Mn(1)–O(1)Mn(1)–O(3)Mn(1)–O(4)Mn(1)–O(5)Mn(1)–O(6)Mn(1)–O(7)Mn(2)–O(2)Mn(2)–O(3)Mn(2)–O(4)Mn(2)–O(5)Mn(2)–O(6)Mn(2)–O(8)Mn(1)–O (average)
Mn(2)–O (average)
Angles (deg)Mn(1)–O(1)–Mn(1)Mn(2)–O(2)–Mn(2)Mn(1)–O(3)–Mn(2)Mn(1)–O(4)–Mn(2)Mn(1)–O(5)–Mn(2)Mn(1)–O(6)–Mn(2)Mn(1)–O(7)–Mn(1)Mn(2)–O(8)–Mn(2)1.85(2)  1
2.08(2)  1
2.03(2)  1
1.81(2)  1
2.05(2)  1
1.98(2)  1
1.88(2)  1
1.80(2)  1
2.09(2)  1
1.90(3)  1
1.99(3)  1
1.95(2)  1
1.96(2)
1.93(2)172.4(7)161.8(8)157.5(8)165.6(8)176.8(8)162.9(8)165.1(7)172.3(8)1.90(2)  1
1.97(3)  1
2.06(3)  1
1.81(3)  1
2.07(2)  1
1.96(3)  1
1.89(3)  1
1.91(3)  1
2.03(3)  1
1.93(2)  1
1.97(3)  1
1.98(2)  1
1.96(2)
1.95(2)

157.6(8)157.6(8)157.4(7)167.2(7)177.6(8)163.2(7)173.0(8)169.0(8)T ¼ 580 K
Distances (Å)
Mn(1)–O(1)Mn(1)–O(3)Mn(1)–O(4)Mn(1)–O(5)Mn(1)–O(6)Mn(1)–O(7)Mn(2)–O(2)Mn(2)–O(3)Mn(2)–O(4)Mn(2)–O(5)Mn(2)–O(6)Mn(2)–O(8)Mn(3)–O(1)Mn(3)–O(7)Mn(3)–O(9)Mn(3)–O(10)Mn(3)–O(11)Mn(3)–O(12)Mn(4)–O(2)Mn(4)–O(8)Mn(4)–O(9)Mn(4)–O(10)Mn(4)–O(11)Mn(4)–O(12)1.82(8)  1
1.91(8)  1
1.91(9)  1
2.04(8)  1
1.97(8)  1
1.94(8)  1
1.85(8)  1
1.98(9)  1
1.96(8)  1
1.95(8)  1
1.92(9)  1
2.02(8)  1
2.03(8)  1
1.94(9)  1
1.90(8)  1
1.90(9)  1
2.15(8)  1
1.89(8)  1
1.93(8)  1
2.00(8)  1
1.85(8)  1
1.94(9)  1
1.94(8)  1
2.12(8)  1

Mn(1)–O (average)
Mn(2)–O (average)
Mn(3)–O (average)
Mn(4)–O (average)

1.93(8)
1.94(8)1.96(8)1.96(8)Angles (deg)Mn(1)–O(1)–Mn(3)Mn(2)–O(2)–Mn(4)Mn(1)–O(3)–Mn(2)Mn(1)–O(4)–Mn(2)Mn(1)–O(5)–Mn(2)Mn(1)–O(6)–Mn(2)Mn(1)–O(7)–Mn(3)Mn(2)–O(8)–Mn(4)Mn(3)–O(9)–Mn(4)Mn(3)–O(10)–Mn(4)Mn(3)–O(11)–Mn(4)Mn(3)–O(12)–Mn(4)164.6(9)160.4(11)162.0(10)171.5(9)170.1(9)162.0(10)173.9(9)165.3(11)172.7(10)169.6(9)153.6(11)155.4(10)Selected bond distances and angles for YBaMn2O6.

(compare the insets of Fig. 6) recognizes the necessity toemploy the P1 space group for (c). In fact, the R factors
never became better than 20, when the P2 symmetry was
assumed.
Determination of the crystal structure below Tc3 is
difﬁcult and still in progress partly because the transition involves the antiferromagnetic ordering with a fourfold periodicity along the c-axis (accompanied by thereformation of the CO/OO superstructure) [7]. Preliminary analysis indicates that the structure is also of
monoclinic symmetry without any extinct reﬂections,giving P2; Pm and P2=m as possible space groups.
3.5. Transmission electron microscope

Fig. 7. The arrangement of oxygen ions around Mn at (a) 350 K and(b) 500 K, showing the Mn–O bond distances.

In Figs. 8(a)–(d), we represent the [001]p-zone TEM
image containing microtwinstructures and corresponding electron diffraction (ED) patterns, taken at room
temperature (PI phase). The ED patterns taken from theareas A and C, include the satellite reﬂections with thecommensurate structural modulations (q), written as
q1 =(1/4,1/4,0)p and q2 =(1/4,1/4,0)p, respectively. The
ED patterns taken from the area B can be understood interms of the superimposition of the two vectors. The
formation of the twin structures is also visible in theimage, which shows the contrasts of the corresponding
superlattices. As one can see from Figs. 8(e)–(g), the ED
patterns simulated by means of Digital Micrograph(Gatan) for the areas A, B and C nicely reproduce theexperimental ED patterns.

ARTICLE IN PRESS
T. Nakajima et al. / Journal of Solid State Chemistry 177 (2004) 987–999

995

Fig. 8. (a) The [001]p zone TEM image at room temperature (PI phase), indicating the CE-type CO/OO ordering and the twin formation. (b)–(d) The
observed ED patterns taken from the area A, B, and C. (e)–(g) The calculated ED patterns for the area A, B, and C.

Fig. 9(a) shows the [100]p-zone ED pattern, containing strong superlattice reﬂections with a two-foldperiodicity along the c axis with respect to cp : Thisoriginates from the Y/Ba ordering. In addition, we
observed considerably weaker reﬂections with a fourfold periodicity along the same direction, enlarging theunit cell to 2O2ap  O2bp  4cp : ED patterns are taken
along various directions including [011]p, [021]p and[111]p (see Figs. 9(b)–(d)), and the observed superreﬂections are plotted on the reciprocal lattice in
Fig. 9(e). As we will demonstrate later, the observed
superreﬂections are attributable to the CO and OO
of a new kind.

4. Discussions
4.1. Heavily distorted MnO6 octahedron
For conventional perovskites (ABO3), wherein the Asites are occupied by the single element and so are theB-sites, the BO6 octahedra are almost always assumed
to be rigid. Keeping its ideal shape, octahedra tilt so as
to relax the size mismatch between the AO and BO2
layers, typically when f o1: More explicitly, sort andmagnitude of the octahedral tilting depend not only on

the mismatch but also on coordination preference,covalency, etc. However, the obtained crystal structureof YBaMn2O6 cannot be described by such a simpliﬁed
picture, as a result of an ultimate ionic size difference
between Y3+ and Ba2+ ions. Considering the tolerance
factors, f1 ¼ 1:07 for {A=Ba2+ and B=Mn+3.5} andf2 ¼ 0:92 for {A=Y3+ and B=Mn+3.5} [10], each MnO2
layer in YBaMn2O6 should feel strain forces with the
opposite signs (or a structural frustration) from theadjacent YO and BaO layers. Having this frustration,
one can not achieve an adequate compensation for thisstructural mismatch solely from the tilting of the rigid
MnO6 octahedra. As a consequence, the shape of the
MnO6 octahedron deviates heavily from the ideal
octahedron.
Fig. 10 demonstrates the [110]p projection of thecrystal structure at 350 K. Reﬂecting the Y3+/Ba2+ordering in the form of layers, the averaged Y–O
distance (2.592(9) Å) becomes shorter than the averaged
Ba–O distance (2.875(9) Å). In other words, the MnO2
planar oxygen ions (i.e., O(3), O(4), O(5), O(6)) aredisplaced largely from the ideal position z ¼ 0:25 toward
the YO layer. For a given octahedron, the averaged
distances between oxygen ions of MnO2 and YO layers
are shortened (/dO(pl)–O(ap)S=2.62(3) Å for Mn(1) and2.59(3) Å for Mn(2)), while those of MnO2 and BaOARTICLE IN PRESS
T. Nakajima et al. / Journal of Solid State Chemistry 177 (2004) 987–999

996

[100]

oxygen ions (/dO(pl)–O(pl)S=2.80(2) Å for Mn(1) and2.74(2) Å for Mn(2)). The Mn ions shift from the ideal
position (z ¼ 0:25) as well toward the YO layer. Similar
structural feature, that is, the deviation from the rigid
MnO6 octahedron, has been observed for the A-siteordered LaBaMn2O6 which has the space group
P4=mmm [3]. However, LaBaMn2O6 has only one
crystallographically equivalent site in Mn and apicaland planar oxygen and each octahedron is much lessdistorted.

[011]
000

001
011

010

100
000

(a)

(b)

[021]

[111]
011

012

000

100 000

4.2. Anomalous octahedral tilting
110

(d)

(c)

For the purpose of dictating the octahedral tilting
systems along with corresponding space groups, it isconvenient to use Glazer’s notation [11–13]. Fig. 11(a)shows that the MnO6 octahedra in the PI phase are

001*

010*
100*

(e)
Fig. 9. The ED patterns at room temperature taken along the zone
axes: (a) [100]p, (b) [011]p, (c) [021]p and (d) [111]p. (e) the fundamental
spots derived from the primitive cubic perovskite (solid circles) and thesuperspots derived from the CO/OO (open circles), plotted on thereciprocal lattice.

Fig. 10. A [110]p view of YBaMn2O6 at 350 K, where polyhedra
represent MnO6 octahedra, open circles Y3+, and shaded circles Ba2+,
demonstrating the signiﬁcant distortion of the octahedra.

layers are elongated (/dO(pl)–O(ap)S=2.89(3) Å forMn(1) and 2.87(3) Å for Mn(2)). Intermediate values
are obtained for the mean distances between the planar

Fig. 11. The octahedral tilting of YBaMn2O6 projected along ap ; bp
and cp ; indicating (a) abc at 350 K and (b) a0bc at 580 K.

ARTICLE IN PRESS
T. Nakajima et al. / Journal of Solid State Chemistry 177 (2004) 987–999

staggered when viewed along any direction, [100]p,[010]p, [001]p. It follows that one can approximately
write it as abc, namely a three-tilt system, although
the octahedron itself is heavily distorted as discussed
above and the space group may not be reproduced. The
octahedral tilting in the PM2 phase is approximately
described as abc as well. This is naturally understood because structural parameters for the PM2 phase
are almost similar to those for the PI phase, except thedifference in size between the inequivalent MnO6octahedra. Interestingly, as shown in Fig. 11(b), theoctahedra in the PM1 phase are connected in the transconﬁguration along the bp - and cp -axis, while thosealong the ap -axis are connected in the cis-conﬁguration.
Accordingly the tilting system can be approximately
regarded as a0bc.
It is well known for ordinary perovskite system(ABO3) that tilting systems vary solely with f ; namely,
the size mismatch between the AO and BO2 layers.
According to the experimental and theoretical considerations [12,13], non-zero tilting is realized in generalwhen f is smaller than 1. In other words, the absence ofoctahedral tilting (the zero tilting system) is seen forf 41: In the present system, the MnO2 layers are
sandwiched by the BaO and YO layers, resulting in acompetition between two possible tilting systems responsible for f1 ¼ 1:07 and f2 ¼ 0:92; respectively.
Therefore, the observed tilting systems of YBaMn2O6,a0bc for Tc1 oT and abc for ToTc1 ; should bedominated by the role of YO layers over the BaO layers.
It is interesting to note that the zero tilting (a0a0c0) is
obtained when the apical oxygen deﬁciency is introduced in the YO layers of YBaMn2O5 [5] or the Y3+
ions are replaced by the La3+ ions with a larger ionicradius [3]. In these two cases, the BaO layers should takethe initiative in determining the tilting system.
The modiﬁcation of the tilting system from a0bc to  a b c at Tc1 clearly explains why the structuraltransition from the pseudo-orthorhombic (PM1) to thepseudo-tetragonal system (PM2) is observed. The
pseudo-orthorhombicity (ap abp acp ) above Tc1 is explained in terms of the tilting anisotropy along the ap
axis together with the anisotropy caused by the metalordering along the c-axis. The pseudo-tetragonal symmetry (ap Dbp acp ) below Tc1 can be naturally explainedby the trans-conﬁgurations along all primitive axes. The
temperature-induced transformation of octahedral tilting systems has been often observed for perovskites andlayered perovskites as in CaTiO3 [14] and CaGeO3 [15],and discussed in connection with the theoretical possible
sequence of space group (groups–subgroup relationship)as demonstrated by Aleksandrov [16] and Howard andStokes [17]. It is commonly found that the tilting
transformation is such that the number of tilt increases
upon cooling. This tendency is also seen in YBaMn2O6,
where two-tilt to three-tilt occurs upon cooling,997

although we have to admit that there is symmetry
increase.
The symmetry increase upon cooling is quite rare. An
example is found in CdPS3, whose monoclinic C2/m
space group at room temperature changes into arhombohedral R3 space group below 228 K, accompanied by the change in the stacking mode [18]. They argue
that this transition occurs due to a complex interplay
between the layers, taking into account van der Waals
forces, short-range repulsions and HOMO/LUMO
interactions. In the present compound, such unusualtransition with tilting change would come from theinternal strain induced by the A-site ordering and fromthe kinetic energy of the eg electrons stabilized by thedx2 y2 ordering (see Section 4.3).
4.3. Charge and orbital ordering
As addressed in Section 3.5, the PI phase has
superreﬂections with the q modulation. In addition,
the four-fold periodicity has been observed along the cdirection. Since the q modulation has been frequently
obtained for A1xA0 xMnO3 with x being close to 0.5[19,20], it is straightforward to ascribe it to the CO andOO of the CE type. However, the observed quadruplication along the c-axis is incompatible with the original
CE structure, in which the CE-type layers uniformly
stack along the c-axis. We thus propose a new superstructure, a modified CE ordering pattern, where the CEtype layers stack with the sequence [aabby] as
illustrated in Fig. 12(a). The elaboration of this superstructure and the reformation of the CO/OO pattern
induced by the spin-ordering below Tc3 have been given
in Ref. [7]. It is interesting to address that other groups
have claimed similar CO/OO structures in TbBaMn2O6[21,22] and SmBaMn2O6 [22]. The proposed ordering
patterns are different from each other as to how the
CE-type layers are built up, presumably due to thedifference in ionic radius between Y and Tb (Sm), and/or to slight difference in synthetic conditions. The
systematic understanding of the CO/OO patterns as wellas controling the synthetic conditions is needed.
One of the most characteristic features in YBaMn2O6
is the presence of the PM2 phase. In the ordinary solid
solution of A0.5A0 0.5MnO3, the MI transition to the CO/
OO state with the CE structure is always accompaniedby the drastic ﬁrst-order structural phase transition [1].
In YBaMn2O6, however, the metallic behavior isretained in the PM2 phase despite the drastic ﬁrst-order
structural phase transition at Tc1 ; where the magnetic
interactions abruptly change as derived from the w  T
curve. Add to this, the unusual structural transition at
Tc1 from the low-T pseudo-tetragonal (monoclinic) tothe high-T pseudo-orthorhombic (triclinic), suggest thata hidden parameter may play a role in compensating forthe lattice entropy. We propose here the ab-plane OO inARTICLE IN PRESS
998

T. Nakajima et al. / Journal of Solid State Chemistry 177 (2004) 987–999

Fig. 12. (a) Modified CE-type charge/OO pattern with a four-fold periodicity (aabb) along the c-axis for the PI state. The lobes indicate thed3x2 r2 =d3y2 r2 type orbital at the Mn3+ sites, while the circles denote the Mn4+ sites. (b) Proposed dx2 y2 type OO pattern for the PM2 state.

the PM2 phase, in which not the d3x2 r2 =d3y2 r2 orbitals(responsible for the insulating CE structure below Tc2 )but the dx2 y2 orbitals are occupied as illustrated in Fig.
12(b). The increase of ðap þ bp Þ=2 at Tc1 and thedecrease of cp (see Fig. 3) is favorable to this OO state,
and the peculiar lattice distortion in the PM1 phasearising both from the layered structure and octahedral
tilting may be relaxed by the OO. The formation of thetwo-dimensional dx2 y2 band in the PM2 phase would be
ascribed to kinetic energy gain of the eg electrons andpredict a signiﬁcant anisotropy between in-plane andout-of-plane electric resistivity, although single crystals
have not been available yet.
The dx2 y2 OO (accompanied by the A-type spinordering) has been reported for A1xA0 xMnO3, where x
is close to 0.5 and f is slightly smaller than 1.
Nd1xSrxMnO3 is a typical example [23,24]. It should
be pointed out that in the metal-disordered systems thisstate tends to coexist with the insulating d3x2 r2 =d3y2 r2
OO state of the CE type. The coexistence of the twostates has been experimentally detected by the neutron
diffraction experiment, and the volume fraction of eachstate is reported to change with T: On the contrary, ourneutron diffraction study indicates no sign of theelectronic phase separation. Instead, each phase appears
independently as a function of T: It is possible that,
because of the absence of the lattice disorder andinhomogeneity in YBaMn2O6, the electronic phaseseparation, typically observed in the metal-disorderedsystems, is completely suppressed but replaced by thedistinct and successive transitions.
4.4. Extremely high MI transition temperature
Finally, we would like to discuss why the MI
transition
temperature(TMI ¼ Tc2 ¼ 480 K)
in
YBaMn2 O6 is extremely high, in comparison withhalf-doped metal-disordered system A0.5A0 0.5MnO3. It
is known that f is structurally correlated with the oxygen

bridging angle +Mn–O–Mn, and the deviation of+Mn–O–Mn from 180 destabilizes the FM state.
According to the phase diagram plotted against f for
A0.5A0 0.5MnO3, the CO/OO state of the CE type appears
for f o0:975 [1] and TMI increases with decreasing f :However, this view can not be directly applied to
YBaMn2O6, since the structure has the averaged angles
of /+Mn–O(pl)–MnS=165.7(8) and /+Mn–O(ap)–
MnS=167.9(7) (in the PI state), which are much larger
than +Mn–O(pl)–Mn=161.8(1) and +Mn–O(ap)–
Mn=158.0(3) in La0.5Ca0.5MnO3 having much smaller
TMI B170 K [25].
The ﬁrst possible reason for the extremely high TMI is
the absence of the randomness in the lattice owing to the
Y/Ba ordering. It is probable in metal-disordered systemthat the structural randomness suppresses the correlation length for the CO/OO state and descends TMI :Second, the heavily distorted and tilted MnO6 octahedra
should play a key role. In the case of disorderedmanganese perovskite, one usually obtains only one
crystallographically site for Mn, O(pl) and O(ap), giving
rise to unique +Mn–O(pl)–Mn and +Mn–O(ap)–Mn.
This is not the case for YBaMn2O6, where the +Mn–O–
Mn angles take various values, ranging from 157.5(8)
to 176.8(8) in the PI state (see Table 2). The
destabilization of the FM state may be understood interms of the percolation model, taking some metallic
Mn–Mn bonds (with higher angles) and other insulating
Mn–Mn bonds (with lower angles) into consideration.
In other words, the large distribution of angles results innarrowing the effective band width w in YBaMn2O6,
thus destabilizing the FM state and alternatively
stabilizing the CO/OO state.

5. Summary
Relation between structures and physical propertiesof the metal-ordered manganite perovskite YBaMn2O6

ARTICLE IN PRESS
T. Nakajima et al. / Journal of Solid State Chemistry 177 (2004) 987–999

was investigated. A high-temperature metallic phase(Tc1 oT) was solved in the triclinic space group P1: The
MnO6 octahedra are tilted like a0bc. At the ﬁrst ordertransition Tc1 ; the structure becomes the monoclinic P2
with the octahedral tilting system of abc. The dx2 y2
OO is proposed for the PM2 phase. While themonoclinic space group P2 is retained across Tc2 ; twocrystallographically inequivalent MnO6 octahedra show
a distinct volume difference. This is in agreement withthe resistivity measurement that shows the MI transition, accompanied by the CO with the checkerboard
pattern in the ab plane. TEM further revealed themodified CE type of the CO/OO. The extremely high MI
transition temperature can be accounted by the absenceof the randomness in the lattice and the heavily distorted
and tilted octahedra.
Acknowledgments
This work is partly supported by Grants-in-Aid for
Scientiﬁc Research Nos. 40302640, 407 and 758 and for
Creative Scientiﬁc Research (No. 13NP0201) from the
Ministry of Education, Culture, Sports, Science andTechnology, Japan. The authors thank Prof. A.N.
Vasil’ev, Dr. J. Yamaura, Dr. C. Michioka and Dr.
Matsushita for valuable discussions.
References[1] C.N.R. Rao, B. Raveau (Ed.), Colossal Magnetoresistance,
Charge Ordering and Related Properties of Manganese Oxides,
World Scientiﬁc, Singapore, 1998.

999[2] B.M. Letfulov, J.K. Freericks, Phys. Rev. B 64 (2001) 174409.
[3] F. Millange, V. Caignaert, B. Domengés, B. Raveau, E. Suard,
Chem. Mater. 10 (1999) 1974.
[4] T. Nakajima, H. Kageyama, Y. Ueda, J. Phys. Chem. Solids 63(2002) 913.
[5] T. Nakajima, H. Kageyama, H. Yoshizawa, Y. Ueda, J. Phys.
Soc. Jpn. 71 (2002) 2843.
[6] F. Millange, E. Suard, V. Caignaert, B. Raveau, Mater. Res. Bull.
34 (1999) 1.
[7] H. Kageyama, T. Nakajima, M. Ichihara, Y. Ueda, H.
Yoshizawa, K. Ohoyama, J. Phys. Soc. Jpn. 72 (2003) 241.
[8] F. Izumi, T. Ikeda, Mater. Sci. Forum 321–324 (2000) 198.
[9] I.D. Brown, D. Altermatt, Acta Crystallogr. B 41 (1985) 244.
[10] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. B 25 (1969) 925.
[11] A.M. Glazer, Acta Crystallogr. B 28 (1972) 3384.
[12] P.M. Woodward, Acta Crystallogr. B 53 (1997) 32.
[13] P.M. Woodward, Acta Crystallogr. B 53 (1997) 44.
[14] B.J. Kennedy, C.J. Howard, B.C. Chakoumakos, J. Phys.:
Condens. Matter 11 (1999) 1479.
[15] X. Liu, Y. Wang, R.C. Liebermann, P.D. Maniar, A. Norvotsky,
Phys. Chem. Minerals 18 (1991) 224.
[16] K.S. Aleksandrov, Ferroelectrics 14 (1976) 801.
[17] C.J. Howard, H.T. Stokes, Acta Crystalallogr. B 54 (1998) 782.
[18] F. Boucher, M. Evain, R. Brec, Acta Crystalallogr. B 51 (1995)
952.
[19] Y.Q. Wang, X.F. Duan, Z.H. Wang, B.G. Shen, Mater. Sci. Eng.
A 333 (2002) 80.
[20] N. Fukumoto, S. Mori, N. Yamamoto, Y. Moritomo, T.
Katsufuji, C.H. Chen, S-W. Cheong, Phys. Rev. B 60 (1999)
12963.
[21] A.J. Williams, J.P. Attﬁeld, Phys. Rev. B 66 (2002) 220405.
[22] T. Arima, D. Akahoshi, K. Oikawa, T. Kamiyama, M. Uchida,
Y. Matsui, Y. Tokura, Phys. Rev. B 66 (2002) 140408.
[23] R. Kajimoto, H. Yoshizawa, H. Kawano, Y. Tokura, K.
Ohoyama, M. Ohashi, Phys. Rev. B 60 (1999) 9506.
[24] Y. Moritomo, T. Akimoto, A. Nakamura, K. Ohoyama,
M. Ohashi, Phys. Rev. B 58 (1998) 5544.
[25] P.M. Woodward, T. Vogt, D.E. Cox, A. Arulraj, C.N.R. Rao,
P. Karen, A.K. Cheetham, Chem. Mater. 10 (1998) 3652.