ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 310 (2007) 1337–1339
www.elsevier.com/locate/jmmm

Synchrotron X-ray diffraction study on the square-lattice
antiferromagnets ðCuClÞLaNb2O7 and ðFeClÞLaNb2 O7
Noriaki Obaa, Hiroshi Kageyamaa,, Takashi Saitob, Masaki Azumab, Werner Paulusa,c,Taro Kitanoa, Yoshitami Ajiroa, Kazuyoshi YoshimuraaaDepartment of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japanb
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japanc
University of Rennes 1, Sciences Chimiques de Rennes UMR 6226, Inorganic Materials, Soft Chemistry and Reactivity,
Campus de Beaulieu Bât 10B, Rennes cedex 35042, France
Available online 7 November 2006

AbstractHigh resolution synchrotron X-ray diffraction methods have been used to study the crystal structure of the two-dimensional squarelattice antiferromagnets ðCuClÞLaNb2 O7 and ðFeClÞLaNb2 O7 . While the structure of the former is of tetragonal symmetry with simple
perovskite unit cell dimensions in the a; b plane ða  aÞ, the latter exhibits a tiny orthorhombic distortion with a 2a  2a  csuperstructure, providing a ﬁrst example of modulation in the family of transition-metal oxyhalides prepared from topotactic ionexchange reactions. Possible arrangement in the FeCl layer is proposed.
r 2006 Elsevier B.V. All rights reserved.
PACS: 61.10.i; 74.20.Mn
Keywords: Ion exchange; Perovskite; Spin liquids; ðCuClÞLaNb2 O7 ; ðFeClÞLaNb2 O7 ; Synchrotron X-ray diffraction

1. Introduction
It has recently been recognized that ion-exchange
reaction is a useful tool for designing magnetic materials.
Starting from double- and triple-layered ðn ¼ 2; 3Þ perovskites RbLaB2 O7 and RbA2 B3 O10 (A2þ ¼ Ca2þ , Sr2þ ;B5þ ¼ Nb5þ , Ta5þ ), a new family of two-dimensionalsquare-lattice antiferromagnets expressed as ðMXÞLaB2 O7and ðMXÞA2 B3 O10 (M2þ ¼ Fe2þ , Co2þ , Cu2þ y; X ¼
Cl , Br has been prepared [1–4]. In the ﬁnal products,
the ½MXþ layers are sandwiched by the nonmagnetic
perovskite slabs (Fig. 1(a)). The magnetic propertiesof several materials have been examined so far.
ðFeClÞLaNb2 O7 with S ¼ 2 undergoes a conventional
antiferromagnetic order at 78 K, though there is ananomaly in the magnetic susceptibility at around 8 K [5].
An unusual behavior characterized by the spin liquidCorresponding author. Tel.: +81 75 753 3991; fax: +81 75 753 4000.

E-mail address: kage@kuchem.kyoto-u.ac.jp (H. Kageyama).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2006.10.358

has been found in an S ¼ 12 system ðCuClÞLaNb2 O7 ,where no long-range magnetic order exists even at
0 K and there is an energy gap of 2.3 meV between thespin-singlet ground state and the ﬁrst excited triplet state[6,7]. Such a spin-liquid state on the square lattice isinteresting in connection with the RVB state predicted
by Anderson after the discovery of high T c superconductivity [8].
Previous powder X-ray and neutron diffraction
studies on all the materials in this family revealed thatthe structure was described simply by a tetragonal
symmetry (space group P4=mmm) with an approximate
dimension of a ¼ b3:9 Å and c12 Å for n ¼ 2 andc16 Å for n ¼ 3. However, to understand the nature ofthe spin liquid in ðCuClÞLaNb2 O7 , a more precise
determination of the crystal structure and comparison
with those of related materials is highly desired. Here,
we report on the results of high resolution synchrotron
X-ray diffraction (XRD) for ðCuClÞLaNb2 O7 andðFeClÞLaNb2 O7 .

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N. Oba et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1337–1339

bFig. 1. Schematic view of the crystal structure of ðMClÞLaNb2 O7ðM ¼ Cu; FeÞ. (b) Possible ordered pattern of the FeCl layer inðFeClÞLaNb2 O7 , where open and closed circles represent chlorine andiron atoms. Arrows denote the disordering of irons suggested by Viciu
et al. [5].

2. Experiments
Polycrystalline samples of ðMClÞLaNb2 O7 were prepared by ion-exchange reaction between RbLaNb2 O7 andMCl2 at 320  C for M ¼ Cu and 350  C for M ¼ Fe in asealed, evacuated Pyrex tube for one week. The ﬁnal
products were washed with water to eliminate unreacted
MCl2 and RbCl byproduct. Synchrotron X-ray powder
diffraction experiments were carried out at 300 K on thelarge Debye-Scherrer camera installed at BL02B2 in
SPring-8. The sample powder with homogeneous granularity prepared by precipitation method was sealed in a
glass capillary of 0.2 mm in diameter.
3. Results and discussion
Fig. 2 shows the synchrotron XRD proﬁles forðCuClÞLaNb2 O7 and ðFeClÞLaNb2 O7 , where the wavelength
was calibrated at 0:776532 Å. The observed peaks for bothcompounds are fairly sharp and nearly symmetric. This is
an indication of successful ion-exchange reactions andgood crystallinity of the specimens. All the peaks forðCuClÞLaNb2 O7 are indexed by a tetragonal unit cell andfrom the least-square ﬁtting the lattice constants forðCuClÞLaNb2 O7 are determined as a ¼ 3:88005ð6Þ Å andc ¼ 11:7337ð3Þ Å, consistent well with those reported previously [1]. For ðFeClÞLaNb2 O7 , observed peaks basicallycoincide with those for ðCuClÞLaNb2 O7 . However, a closer
look at the data revealed the splitting of some reﬂections
together with the appearance of several new discrete peaks. Asmall peak shown in the inset of Fig. 2, for example, is thesuperstructure reﬂection that is indexed as ð12 12 0Þ. The
analysis led us to the conclusion that the crystal structure isorthorhombic with cell parameters being a ¼ 7:76213ð8Þ Å,b ¼ 7:74218ð8Þ Å and c ¼ 11:8508ð1Þ Å. Note that this is the113
201

b400

a(FeCl)LaNb2O7
1/2 1/20

a002

Intensity (arb. units)

[MCl]

202
114, 005

102, 003, 110
111

Lac(CuCl)LaNb2O7112, 103

NbO6004

1338

7.0

8.0

510

23.0

15202523.2

30

2 θ (deg)
Fig. 2. Synchrotron X-ray diffraction patterns of (upper) ðCuClÞLaNb2 O7and (lower) ðFeClÞLaNb2 O7 . The insets show the enlarged plots forðFeClÞLaNb2 O7 , highlighting the superreﬂection assigned as ð12 12 0Þ andthe splitting of the (4 0 0) reﬂection.

ﬁrst experimental observation of nontetragonal symmetry
and the superstructure in the family of ðMXÞLaB2 O7 andðMXÞA2 B3 O10 .
The observed 2a  2a  c modulation in the present
study must be related to the structural mismatch between
perovskite slabs and FeCl layers. A plausible model is thatirons in the FeCl layer are ordered as shown in Fig. 1(b). Infact, Viciu et al. claimed based on the data taken by alaboratory X-ray diffractometer that the ions are statistically disordered being located at the 4o site in the spacegroup P4=mmm [5], resulting in an averaged structuraldescription. It is worth noting that such ordering could beaccompanied by the tilting and/or distortion of the NbO6octahedra.
4. SummaryHigh resolution of the SPring-8 X-ray diffractometerallows us to detect a tiny orthorhombic distortion inðFeClÞLaNb2 O7 . A possible model to explain the 2a 2a  c superstructure of the FeCl layer was proposed. On
the contrary, no superstructure reﬂection was found forðCuClÞLaNb2 O7 within the experimental accuracy. The
reﬁnements of both materials are in progress. From theviewpoint of inorganic structural chemistry, this study
offers an interesting subject to investigate structuralmodulation and instability as a function of M, X, A,
B and n.

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N. Oba et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1337–1339

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Acknowledgement

References

The synchrotron radiation experiments were performedat the BL02B2 in the SPring-8 with the approval of theJapan Synchrotron Radiation Research Institute (JASRI)(Proposal No. 2006A1062). This work has been supported
by Grants-in-Aid for Young Scientists (A) (No. 17684018)
and Science Research on Priority Area (No. 16076210)
from the Ministry of Education, Culture, Sports, Scienceand Technology of Japan. WP’s visit to Kyoto wassupported by the 21st century COE Program, Kyoto
University Alliance for Chemistry.

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