Physica B 329–333 (2003) 844–845

Structures and physical properties of metal-orderedmanganites RBaMn2O6 (R : Y and rare earth elements)
T. Nakajiama*, H. Kageyama, Y. Ueda
Materials Design and Characterization Laboratory, Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa,
Chiba 277-8581, Japan

Abstract
New metal-ordered manganites RBaMn2 O6 with a successive stacking of RO–MnO2 –BaO–MnO2 –RO have beensynthesized and investigated in the structures and electromagnetic properties. Comparing to the metal-disorderedmanganites, two remarkable features are recognized in RBaMn2 O6 : (1) relatively high charge-order transition
temperature and (2) the presence of structural transition without any charge and magnetic ordering. These features areclosely related to the peculiar structure that the MnO2 square-lattice is sandwiched by the rock-salt layers of two kinds;
RO and BaO with different lattice sizes.
r 2003 Elsevier Science B.V. All rights reserved.
Keywords: Metal-ordered manganites; Electromagnetic properties; Phase transitions

Recently, magnetic and electrical properties of perovskite-type manganites with a general formula2þ
ðR3þ
1x Ax ÞMnO3 (R ¼ rare earth elements and A ¼
alkaline earth elements) have been extensively investigated (see for reviews Ref. [1]). Almost all the works
devoted to a series of perovskite-type manganites so far
are on the disordered manganites with the R3þ and A2þ
ions being randomly distributed. Since the physicalproperties of the perovskite-type manganites are quitesensitive to even a tiny change in lattice distortion, it isimportant to employ a compound without A-site
disorder in order to make clear the effect of A-site
randomness.
Very recently we successfully synthesized a metalordered manganite YBaMn2 O6 with a successive stacking of YO–MnO2 –BaO–MnO2 –YO (see the inset inFig. 2) and observed following three-phase transitions: astructural transition at TS ¼ 520 K; a charge-order (CO)transition at TCO ¼ 480 K and an antiferromagnetictransition at TN ¼ 195 K [2]. Across the phase transition
at TS the resistivity shows little change, whereas themagnetic susceptibility drops signiﬁcantly and the*Corresponding author. Tel./fax: +81-471-36-3436.
E-mail address: t-nakaji@issp.u-tokyo.ac.jp(T. Nakajiama).

magnetic interaction which estimates from the Curie–
Weiss ﬁtting varies from ferromagnetic above TS toantiferromagnetic below TS : Such transition was ﬁrst
observed in the perovskite-type manganites. In thispaper, we report the synthesis, structures and physicalproperties of new metal-ordered manganites RBaMn2 O6(R : Y and rare earth elements).
Powder samples were prepared by a solid-state
reaction of R2 O3 ; BaCO3 and MnO2 [2]. The synthesisof compounds with Ce, Yb and Lu was completelyfailed. The Er- and Tm-compounds included a signiﬁcant amount of impurity phase.
The crystal structures were determined by powder
X-ray diffraction using Cu Ka radiation. The magneticproperties were studied using a SQUID magnetometer.
The electric resistivity of the sintered pellets wasmeasured by a conventional four-probe technique.
RBaMn2 O6 can be classiﬁed into three groups fromthe electromagnetic properties. The ﬁrst group includes
the compounds with R ¼ Tb3þ ; Dy3þ and Ho3þ whoseionic radii are close to Y3þ : These compounds showsuccessive three-phase transitions as observed inYBaMn2 O6 : The magnetic susceptibility and resistivityof DyBaMn2 O6 are shown in Fig. 1. Although thereduction of magnetic susceptibility across TS is clearlyobserved, it is difﬁcult to read ferromagnetic to0921-4526/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0921-4526(02)02545-0

T. Nakajiama et al. / Physica B 329–333 (2003) 844–845

Fig. 1. Magnetic susceptibility and electrical resistivity as afunction of temperature for DyBaMn2 O6 :Fig. 2. Phase diagram for metal-ordered manganites
RBaMn2 O6 (R ¼ Y and rare earth elements). PM: paramagnetic metal, FM: ferromagnetic metal, AFM(A): A-type
antiferromagnetic metal, COI(CE): CE-type charge- andorbital-ordered insulator, AFI(CE): CE-type antiferromagneticinsulator.

antiferromagnetic change of magnetic interaction between Mn ions, because of the signiﬁcant contributionof magnetic rare earth ions to the total magnetic
susceptibility. However, the distinct transitions at TSand TCO are commonly observed in this series. The
second group RBaMn2 O6 with R ¼ La; Pr and Nd has
metallic ferromagnetic transition at TC followed by theA-type antiferromagnetic transition in PrBaMn2 O6 andNdBaMn2 O6 : The third group consists of the intermediate R ¼ Sm; Eu and Gd, and exhibits the structural
and CO transitions simultaneously, followed by themagnetic long-range ordering.
The results are summarized in Fig. 2 as a phase
diagram. It is very interesting to compare the obtainedphase diagram to the generalized phase diagram of the8452þ
metal-disordered ðR3þ
0:5 A0:5 ÞMnO3 which is expressed asa function of the tolerance factor ðf Þ [1]. The generalized
phase diagram is expressed as a function of the tolerance
factor (f Þ; where the ferromagnetic metallic (FM) stateappears near f B1; the CO state is most stabilized forf o0:975 and the middle region ðf B0:975Þ is responsible
for various phenomena including CMR.
In the metal-ordered RBaMn2 O6 ; however, thetolerance factor cannot be deﬁned because the MnO2
square-lattice is sandwiched by the rock-salt layers of
two kinds: RO and BaO with different lattice sizes.
Hence we express the phase diagram as a function of theratio of ionic radius: rR3þ =rBa2þ :The phase diagram of RBaMn2 O6 is similar to thegeneralized phase diagram as a whole. There exist thecharacteristic phases such as FM phase due to doubleexchange interaction and CE-type CO phase. However,there are two striking features in the phase diagram ofRBaMn2 O6 : (1) the high TCO and (2) the presence ofstructural transition at TS above TCO : The FM stateappears around R ¼ La and the CO state is more stablefor Nd and later rare earth. The TCO increases withdecreasing the ratio of rR3þ =rBa2þ and reaches thechampion record TCO ¼ 480 K in YBaMn2 O6 : The
relatively high TCO in the metal-ordered manganites iseasily understood because the metal order at A-site isfavorable for the charge ordering of Mn3þ =Mn4þ : Thephase transition at TS is not accompanied by any chargeordering and magnetic ordering but by the reduction ofmagnetic susceptibility and the change of magnetic
interaction from ferromagnetic to antiferromagnetic.
This transition is characteristic of the compounds withsmall ionic radii of R3þ in which the MnO2 squarelattice is sandwiched by two rock-salt layers with theextremely different lattice sizes. This situation introduces a strong frustration to the MnO2 sub-lattice andas a result MnO6 octahedron itself is distorted leading to
a complex structural deformation (triclinic or monoclinic). Such deformation must give a new perturbation
to the competition of multi-degrees of freedom, that is,spin, charge, orbital and lattice. We propose a possible
orbital ordering, presumably dx2 y2 -type orbital ordering, at TS : The freezing of the orbital, charge and spin
degrees of freedom at the independent temperatures
might be closely related to the peculiar structure of themetal-ordered perovskite-type manganites.

References[1] C.N.R. Rao, B. Raveau, Colossal Magnetoresistance,
Charge Ordering and Related Properties of Manganese
Oxides, World Scientiﬁc, Singapore, 1998.
[2] T. Nakajima, H. Kageyama, Y. Ueda, J. Phys. Chem. Solids
63 (2002) 913.