Journal of the Physical Society of Japan
Vol. 72, No. 3, March, 2003, pp. 694–697#2003 The Physical Society of Japan

Crystal Structure and Magnetic Properties of the Quasi-One-Dimensional
Quantum Spin System Cu2 Cl4 H8 C4 SO2Masashi FUJISAWA, Jun-Ichi Y AMAURA1 , Hidekazu TANAKA2 , Hiroshi K AGEYAMA1 ,Yasuo N ARUMI3;4 and Koichi K INDO3;4
Department of Physics, Tokyo Institute of Technology, Oh-okayama, Meguro-ku, Tokyo 152-8551
Institute for Solid State Physics, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8581
2
Research Center for Low Temperature Physics, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551
3
CREST, Japan Science and Technology, Kawaguchi, Saitama 332-0012
4
Research Center for Materials Science at Extreme Conditions, Osaka University, Toyonaka, Osaka 560-0043
1(Received October 2, 2002)

The crystal structure of Cu2 Cl4 H8 C4 SO2 was analyzed by single-crystal X-ray diﬀraction. This
compound consists of the double chain of the edge-sharing octahedra CuCl5 O. With decreasing
temperature, the magnetic susceptibility decreases steeply toward zero below 10 K after exhibiting abroad maximum at 65 K, which is characteristic of the one-dimensional antiferromagnet. The magnetic
properties of the present system can be described by the double spin chain model with dominant
exchange interactions along the leg. The observed small excitation gap =kB  5:8 K is attributed to theweak exchange alternation along the leg or the weak zig-zag rung interactions. The magnetization curvemeasured at 1.3 K for the magnetic ﬁeld parallel to the c-axis shows a discontinuous jump at Ht  22:8 T
indicative of the phase transition of the ﬁrst order.
KEYWORDS: Cu2 Cl4 H8 C4 SO2 , crystal structure, double chain, singlet ground state, excitation gap, bondalternation, magnetic susceptibility, high-field magnetization process, ESR
DOI: 10.1143/JPSJ.72.694

1.

Introduction

Table I. Crystal data for Cu2 Cl4 H8 C4 SO2 .

The crystal structure of Cu3 Cl6 (H2 O)2 2H8 C4 SO2
was reported by Swank and Willet.1) The crystal
structure has an inﬁnite chain composed of Cu3 Cl6trimers. Ishii et al.2) presented the magnetic properties of
Cu3 Cl6 (H2 O)2 2H8 C4 SO2 . The compound has a singlet
ground state with a small excitation gap =kB  5:2 K.
They interpreted the gapped ground state in terms of a trimer
chain model. However, the temperature dependence of themagnetic susceptibilities and the angular dependence of theg-factor observed could not be understood in terms of thetrimer chain model3) and the crystal structure, respectively.
Thus, we re-examined the crystal structure by X-ray
diﬀraction using single crystals. It was found that thecompound assumed to be Cu3 Cl6 (H2 O)2 2H8 C4 SO2 by Ishii
et al.2) was actually Cu2 Cl4 H8 C4 SO2 . Cu2 Cl4 H8 C4 SO2 is a
new compound that has not been reported to date. In thispaper, we report its crystal structure and magnetic properties
studied by X-ray diﬀraction, magnetization and ESR
measurements.
2.

Chemical formulaSpace group

a (A)

b (A)

Cu2 Cl4 H8 C4 SO2
P1
9.425(1)10.793(2)c (A)6.620(1) (deg)98.869(8) (deg)95.25(2) (deg) 3)
V (A120.850(7)Z
No. of observed reﬂections

21277 (I > 3 ðIÞ)R; wR

0.079; 0.121

559.6(2)The crystal data are listed in Table I. The chemical formula
was found to be Cu2 Cl4 H8 C4 SO2 from the results ofstructural and chemical analyses. Since the crystal is very
hygroscopic, the surface of the crystal was gradually
damaged during the X-ray measurement, which gives riseto the somewhat large R-factor (¼ 0:079). However, thevalue of the R-factor is suﬃcient to show that the structure iscorrectly solved, although some modiﬁcation of the atomic
parameters will be needed.
The susceptibilities were measured using a SQUID
magnetometer (Quantum Design MPMS XL). Since thecrystals were highly hygroscopic, we treated them in a glove
box ﬁlled with dry nitrogen. High-ﬁeld magnetization
measurement was performed using an induction methodwith a multilayer pulse magnet, at the Research Center for
Materials Science at Extreme Conditions, Osaka University.
A magnetic ﬁeld was applied along the long axis c. The
magnetization data were collected at 1.3 K in magnetic ﬁelds
of up to 68 T. ESR spectrum was measured at room
temperature using X (9 GHz) band frequency.

Experimental Procedures

Single crystals of Cu2 Cl4 H8 C4 SO2 were prepared bydissolving CuCl2 and H8 C4 SO2 in ethyl alcohol. The
solution was evaporated at 60 C over six weeks. Long
platelet crystals were obtained.
In the X-ray crystal structure analysis, the intensity data
were collected using an imaging plate type Weissenberg
camera (Mac Science DIP 320 V) with graphite monochromated Mo K radiation (2 < 60 ). Absorption and decay
were corrected in the data merging process. The structure
was solved by the direct method4) and reﬁned by the fullmatrix least-squares method using teXsan program package.
E-mail: mercy@lee.phys.titech.ac.jp.
694

J. Phys. Soc. Jpn., Vol. 72, No. 3, March, 2003

3.

M. F UJISAWA et al.

Results and Discussion

3.1 Crystal structure
The atomic parameters are listed in Table II. Figure 1(a)shows the crystal structure of Cu2 Cl4 H8 C4 SO2 . This
Table II. Fractional atomic coordinates and equivalent isotropic displace 2 ). Hydrogen atoms are omitted because we adopt thement parameters (Acalculated positions for them.
Atom

x

y

z

Ueq

Cu(1)

0.0824(1)0.37333(10)0.8518(2)0.0277(4)Cu(2)

0.0780(1)0.3725(1)0.3523(2)0.0287(5)Cl(1)

0.2772(3)0.4513(2)1.1483(4)0.0333(7)Cl(2)

0:1159ð3Þ

0.3163(2)0.5644(4)0.0305(7)Cl(3)

0.2776(3)0.4555(2)0.6442(4)0.0335(7)

Cl(4)
S(1)

0:1148ð3Þ
0.1287(3)0.3153(2)0.0459(2)0.0597(4)0:2225ð3Þ

0.0297(7)0.0350(7)0.036(2)O(1)

0.0549(9)0.1349(6)0:2149ð10Þ

O(2)0.0061(9)0:1123ð6Þ

0:279ð1Þ

0.040(2)C(1)

0.273(1)0.096(1)0.017(2)0.047(3)C(2)

0.439(2)0.176(2)0:029ð2Þ

0.096(6)C(3)

0.433(2)0.109(1)0:261ð2Þ

0.068(4)C(4)

0.280(2)0.095(1)0:385ð2Þ

0.054(4)compound is composed of the inﬁnite Cu double chain andthe H8 C4 SO2 molecule with short Cu  O contacts between
them. Figure 1(b) shows a schematic Cu double chain, where
the magnetic Cu2þ ion is surrounded octahedrally by ﬁve
chlorine ions and an oxygen atom of H8 C4 SO2 . The CuCl5 O
octahedra are elongated along the Cl0 (4)–Cu(1)–O(1) andCl0 (2)–Cu(2)–O(2) axes which are perpendicular to the chain
and parallel to the b-axis. The elongation should be ascribedto the Jahn–Teller eﬀect. In one chain of the double chain,
the distance between Cu2þ ions is slightly alternating, i.e., and the other is 3.318(2) A.
 Therefore, theone is 3.302(2) A
exchange interactions should be weakly alternating along thechain.
The long direction of the obtained single crystals is the cdirection. The crystals are easily cleaved along a plane
including the c-axis. The g-factor was measured by ESR.
The maximum g-factor was obtained when the magnetic
ﬁeld is parallel to the cleavage plane and perpendicular tothe c-axis. The g-factor becomes a maximum when themagnetic ﬁeld is parallel to the elongated axis of the CuCl5 O
octahedron. Thus, the cleavage plane was deduced to beð1; 0; 0Þ. The g-factors for H k a , b and c-axes were
determined as ga ¼ 2:03, gb ¼ 2:24 and gc ¼ 2:03, respectively. These ﬁeld directions are almost perpendicular to one
another. The angular dependence of the g factor iscompatible with the crystal structure determined in thepresent work.
3.2 Magnetic Properties
The magnetic susceptibilities for H k a , b and c-axes areshown in Fig. 2 as a function of temperature. The susceptibilities exhibit the same temperature dependence. The
diﬀerence between the absolute values of the three susceptibilities is due to the anisotropy of the g-factor, because thethree susceptibilities almost coincide with one another whennormalized by the g-factor. The susceptibilities have a broad
maximum at around 65 K, which is characteristic of theantiferromagnetic Heisenberg chain. Below 10 K, the susceptibility decreases rapidly toward zero with decreasing
temperature. This result indicates that the ground state is aspin singlet with a small excitation gap. Since no anomaly

b
a
c

Cl

695

O Cu C S(a)7
H || a*H || b
H || c54-3

M/H (10 emu/mol)

6

c

3
2
Cu2Cl4 . H8C4SO2
H=1T

1
Cl

O

Cu

(b)
Fig. 1. (a) Crystal structure of Cu2 Cl4 H8 C4 SO2 . (b) Double chain
structure composed of edge-sharing CuCl5 O octahedra and the holeorbitals dðx2 y2 Þ of Cu2þ ions.

0
0

50

100

150
T (K)

200

250

300

Fig. 2. Temperature dependence of magnetic susceptibilities
Cu2 Cl4 H8 C4 SO2 measured at H ¼ 1 T for H k a , b and c-axes.

inJ. Phys. Soc. Jpn., Vol. 72, No. 3, March, 2003

M. F UJISAWA et al.

J’J

Fig. 3. Exchange interactions in the double chain. Thick solid and dashedlines denote the strong interactions along the leg and thin solid, dashedand dotted lines denote weak zig-zag rang interactions.

7

α=1.0
J/kB=104.6 K

5

0.98
105.6

4

Cu2Cl4 . H8C4SO2

3
Cu2Cl4 . H8C4SO2
H || b

0.96
106.6

2
1
0
0

20

40

60

80

100

T (K)
Fig. 4. Observed and calculated magnetic susceptibilities for H k b asfunctions of temperature. Solid lines denote the results of calculations
based on the alternating chain model.

H || c T = 1.3 K

0.3

0.2

0.1

Hc

Ht

0.0
0

10

20

30
H (T)

40

50

60

Fig. 5. Magnetization curve for H k c measured at 1.3 K.
0deﬁned by
P  ¼ J =J, and the exchange constants are deﬁned
as H ¼ hi;ji Jij ðSi  Sj Þ. The susceptibility calculated with ¼ 0:98 and J=kB ¼ 105:6 K is close to the experimental
one. However, there is a signiﬁcant diﬀerence between theexperimental and calculated susceptibilities below 50 K.
For the S ¼ 1=2 antiferromagnetic spin ladder, it is known
that the rung interaction can produce the excitation gap, nomatter how small it may be. Thus, there is a possibility thatthe small excitation gap in Cu2 Cl4 H8 C4 SO2 arises from theweak zig-zag rung interactions denoted by thin solid, dottedand dashed lines in Fig. 3. The gapped ground state in thepresent system should be attributed to the exchange alternation along the leg and the zig-zag rung interactions. To thebest of the authors’ knowledge, the spin model shown in
Fig. 3 has not been studied theoretically.
Figure 5 shows the magnetization curve measured at
1.3 K in magnetic ﬁelds of up to 54 T for H k c. A phasetransition is observed at Hc  4:3 T, where the excitation
gap closes. The ground state becomes gapless for H > Hc .
From the value of Hc , the magnitude of the gap  isevaluated as =kB ¼ gc B Hc =kB  5:8 K with gc ¼ 2:03
obtained by ESR measurement.
Another phase transition with a small magnetization jump
can be observed at Ht  22:8 T. The magnitude of themagnetization jump M at Ht is M ¼ 0:011 B . The
phase transition is considered to be of the ﬁrst order. At
present, the mechanism leading to the ﬁeld-induced phasetransition is unknown. We also measured magnetization inmagnetic ﬁelds of up to 68 T. No anomaly indicative of theadditional phase transition was observed above Ht .

4.

-3

M/H (10 emu/mol)

6

0.4

2+

indicative of the phase transition such as spin–Peierls
transition is seen in the susceptibility data, the rapid
decrease of the susceptibility is attributed to the crossoverfrom the thermally induced magnetic state to the gappedground state.
From the crystal structure of Cu2 Cl4 H8 C4 SO2 , we can
expect that the hole orbitals dðx2 y2 Þ lie in a plane
including the c-axis, and are linked together along the chain,
as shown in Fig. 1(b). The conﬁguration of the hole orbitals
is similar to that in CuCl2 ,5) which displays magnetic
susceptibility typical of the S ¼ 1=2 one-dimensional (1D)
Heisenberg antiferromagnet.6) For CuCl2 , the broad susceptibility maximum is observed at around 70 K.
Cu2 Cl4 H8 C4 SO2 exhibits almost the same magnetic susceptibility as that of CuCl2 above 30 K. This susceptibility
behavior is consistent with the crystal structure. Below 30 K,
the susceptibilities of Cu2 Cl4 H8 C4 SO2 and CuCl2 behave
diﬀerently. The latter undergoes magnetic ordering at
TN  24 K.7)
The exchange interactions in the double chain are thought
to be described as shown in Fig. 3. Since the hole orbitals
dðx2 y2 Þ are linked together along the chain, the exchangeinteractions J and J 0 along the leg of the double chain should
be dominant. This can be conﬁrmed from that the present
system exhibits magnetic susceptibility typical of the S ¼
1=2 1D antiferromagnet above 30 K. Since the distance
between Cu2þ ions along a leg is slightly alternating, J 6¼ J 0 ,although they should be very close to each other. Thus, we
compare ﬁrst the experimental susceptibility with that
calculated for the S ¼ 1=2 alternating Heisenberg chain.8)
Experimental and calculated susceptibilities for H k b areplotted in Fig. 4, where  is the alternation parameterM ( µB/Cu )696

Conclusion

The new spin gap system reported by Ishii et al.2) was
found
to
be
Cu2 Cl4 H8 C4 SO2andnot
Cu3 Cl6 (H2 O)2 2H8 C4 SO2 . We determined the crystal structure of Cu2 Cl4 H8 C4 SO2 and investigated its magnetic
properties. The present system can be described magnetically by the double spin chain model shown in Fig. 3 withthe strong exchange interactions J and J 0 along the leg. The
temperature dependence of the magnetic susceptibility andthe angular dependence of the g-factor are compatible withthe crystal structure. The ground state of Cu2 Cl4 H8 C4 SO2 isthe spin singlet with the excitation gap =kB  5:8 K. The

J. Phys. Soc. Jpn., Vol. 72, No. 3, March, 2003

origin of the small gap should be attributed to the weakexchange alternation along the leg or the zig-zag runginteractions. The magnetization curve measured at 1.3 K forthe magnetic ﬁeld parallel to the c-axis shows a discontinuous jump at Ht  22:8 T indicative of the ﬁrst order phasetransition.
Acknowledgements
The authors would like to thank T. Tonegawa, K.
Okamoto, K. Takano and A. Honecker for stimulating
discussions. This work was supported by Toray Science
Foundation and a Grant-in-Aid for Scientiﬁc Research on
Priority Areas (B) from the Ministry of Education, Culture,
Sports, Science and Technology of Japan.

M. F UJISAWA et al.

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