Magnetic field dependence of the rare earth B12 icosahedral cluster compound GdB18Si5

Article abstract of DOI:10.1016/j.poly.2005.03.162

Magnetic field dependence of the rare earth B₁₂ icosahedral cluster compound GdB₁₈Si₅ was investigated. GdB ₁₈Si₅ is the only system in the rare earth boron icosahedral cluster compounds in which 3 dimensional long range order has been observed. GdB₁₈Si₅ exhibits an antiferromagnetic transition at T_N = 3.2 K, with the spins antiferromagnetically aligned in-plane, perpendicular to [0 0 1]. The magnetic phase diagram was determined for the field applied along the c-axis. The general magnitude of field corresponds to the temperature scale. An interesting dependence at low magnetic fields was observed when the field was varied in-plane, with a reorientation of the spin, a spin flip, appearing to occur at fields below 300 G.

Full text of DOI:10.1016/j.poly.2005.03.162

Polyhedron 24 (2005) 2803–2807
www.elsevier.com/locate/poly
Magnetic field dependence of the rare earth B12 icosahedral
cluster compound GdB18Si5
*
Takao Mori
National Institute for Materials Science, Advanced Materials Laboratory, Namiki 1-1, Tsukuba 305-0044, Japan
PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama, Japan
Received 7 October 2004; accepted 11 January 2005
Available online 17 October 2005
Abstract
Magnetic field dependence of the rare earth B12 icosahedral cluster compound GdB18Si5 was investigated. GdB18Si5 is the only
system in the rare earth boron icosahedral cluster compounds in which 3 dimensional long range order has been observed. GdB18Si5
exhibits an antiferromagnetic transition at TN = 3.2 K, with the spins antiferromagnetically aligned in-plane, perpendicular to [001].
The magnetic phase diagram was determined for the field applied along the c-axis. The general magnitude of field corresponds to the
temperature scale. An interesting dependence at low magnetic fields was observed when the field was varied in-plane, with a reori-
entation of the spin, a spin flip, appearing to occur at fields below 300 G.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Boron icosahedra; Rare earth borides; Antiferromagnetic, f electron
1. Introduction
cluster is supplying a new function to the compound,
i.e., magnetism.
The subject of how magnetism manifests in cluster
compounds is the focus of extensive research today
[1,2]. Despite being insulating, magnetically dilute f-
electron systems, magnetic transitions at moderate
temperatures have been discovered for rare earth B12
icosahedral cluster compounds (e.g., Tf = 29 K for
HoB17CN [3]). A wide range of behavior such as 1D
dimer-like magnetic behavior in TbB50-type com-
pounds [4,5] or 2D spin glass behavior in a layered
series of RE-B-C(N) compounds [3,6,7] has been dis-
covered. Interestingly, it has been indicated that the
B12 icosahedral clusters play an important role in
mediating the magnetic interaction which is a novel
phenomenon. In other words, the boron icosahedral
Recently, interesting new borosilicide compounds
like Gd5Si2B8 [8], Ce5Si3ꢀxBx [9], and Tb1.8C2Si8B36
[10] have also been synthesized, but this is the most bor-
on-rich system. GdB18Si5 is of especial interest since it is
the only rare earth B12 icosahedral boron cluster com-
pound in which 3D long range order has been discov-
ered [11]. It is also of note because despite exhibiting
long range order, the gadolinium atomic sites in
GdB18Si5 actually have a partial occupancy of 0.68 com-
pared to fully occupied sites like in the GdB44Si2 com-
pound which exhibits short range order [12]. The
results of magnetic susceptibility and specific heat previ-
ously reported indicate that a typical antiferromagnetic
transition is occurring at TN = 3.2 K, with the spins
antiferromagnetically aligned in-plane, perpendicular
to [001] [11]. The magnetic field dependence of the
transition in GdB18Si5 was investigated in this work to
further make clear the nature of the transition.
*
Tel.: +81298604323; fax: +81298516280.
E-mail address: mori.takao@nims.go.jp.
0277-5387/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.03.162

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T. Mori / Polyhedron 24 (2005) 2803–2807
2. Experimental
hydrofluoric acid and nitric acid to obtain crystals of
GdB18Si5. Pieces of crystals were broken off and crushed
into powder which was then measured with a high reso-
lution powder X-ray diffractometer (Rigaku Co.,
RINT2000) with Cu Ka radiation which confirmed the
single phase of the samples.
The structure of GdB18Si5 is rhombohedral (space
˚
group R3m) with lattice constants of a = b = 10.07 A,
˚
c = 16.45 A. Views of the structure are depicted in
Preparation of crystals of GdB18Si5 which were mea-
sured in this work were carried out in the following way.
First of all, GdB6 powder was prepared by the borother-
mal reduction method of rare earth oxide under vacuum
using BN crucibles:
Gd2O3 þ 15B ! 2GdB6 þ 3BO
ð1Þ
The Si flux method was used to obtain crystals, with a
starting composition of Gd:B:Si of around 1:20:100 in
molar ratio. The desired amounts of boron and silicon
were added to GdB6. The mixture was heated to a tem-
perature of around 1923 K for 10 h, then cooled to
1573 K at a rate of 50 K/h, after which the heat was
switched off and the crucible cooled to room tempera-
ture. The excess silicon was removed by a mixture of
Fig. 1. The structure of rare earth boron-rich icosahe-
dral compounds can basically be understood in that
the boron icosahedra form a particular arrangement
with rare earth atoms occupying sites in spaces among
the icoasahedra. In the case of GdB18Si5, in the short-
est metal–metal spacing, the gadolinium sites form a
zigzag chain along the [110] axis with a separation
˚
of 5.04 A. The [001] alignment of the crystals was
Fig. 1. Crystal structure of GdB18Si5, projected onto the (a) [110] plane and (b) [001] plane. The polyhedrons are B12 icosahedra, small gray circles
indicate silicon atoms, and the large black circles indicate gadolinium atoms.

T. Mori / Polyhedron 24 (2005) 2803–2807
2805
successfully determined with the [001] plane forming
large faces, however, we could not assign the explicit
in-plane alignment of the crystals by Laue photogra-
phy techniques. Such difficulty has been noticed for
other boron-rich systems like MgAlB14 and TbB50-type
compounds [13] and is generally attributed to the
complicated structure and weakness of reflection of
boron-rich compounds. However, in the case of the
GdB18Si5 crystals, it appears that there are multigrains
within the in-plane alignment which are aligned in
different directions in-plane (whereas these grains are
uniformly ordered along the [001] direction with the
[001] plane forming orderly). This is indicated from
magnetic measurements where the susceptibility below
TN did not differ largely despite incrementally rotating
the magnetic field direction in-plane, i.e., the spins are
antiferromagnetically aligned in-plane in a certain
direction [11] but due to the multigrains aligned in dif-
ferent directions in-plane, the observed susceptibility is
a sum of parallel and also perpendicular susceptibility
components. Despite not being able to resolve the
particular in-plane alignment of a single crystal piece,
the in-plane measurements carried out in this work
are still important because they obviously contain a
component of the parallel susceptibility and reflect its
behavior under magnetic field.
Fig. 2. Temperature dependence of the c-axis magnetic susceptibility
of GdB18Si5 for 16 kG (open circles), 14 kG (closed circles), 12 kG
(open triangles), 10 kG (closed squares), 8 kG (open diamonds), 6 kG
(closed triangles), 4 kG (crosses), and 2 kG (open squares). The lines
are guides to the eye.
Magnetic susceptibility was measured by using a
Quantum Design MPMS-XL SQUID magnetometer
from 1.8 to 300 K, with fields up to 70 kG. The magne-
tization was measured with the magnetic field applied
along the [001] c-axis and in-plane.
3. Results and discussion
3.1. Magnetic field applied along the c-axis
Fig. 3. Magnetic phase diagram of GdB18Si5 with the field applied
along the c-axis. AF indicates the antiferromagnetic phase. The line is
a guide to the eye.
The temperature dependence of the magnetic sus-
ceptibility with the field applied along [001] for vari-
ous fields is plotted in Fig. 2. The c-axis is
perpendicular to the direction the spins are aligned
[11] and accordingly little temperature dependence of
the c-axis susceptibility is observed below the transi-
tion temperature. As is typical for antiferromagnetic
transitions, the transition temperature shifts to lower
temperatures as the magnetic field is increased. We
define TN as the intercept of the lower and higher
temperature region extrapolations. The magnetic
phase diagram is plotted in Fig. 3. The magnitude
of the magnetic field corresponds well with the tem-
perature scale observed here.
saturates to reach the full moment expected for the
8S7/2 ground state of Gd3+
.
3.2. Magnetic field applied in-plane
As noted in Section 2, the crystals of GdB18Si5 ap-
peared to be composed of multigrains which were not
uniformly aligned along a particular direction in the
in-plane direction (whereas the alignment of these
grains along the c-axis was uniformly very good owing
to large crystal planes of [001]). Due to the various
alignment of the multigrains leading to a summation
of both parallel and perpendicular components, a
rotation of the magnetic field within the in-plane
The magnetization curve at 1.8 K with field applied
along the c-axis is shown in Fig. 4. The magnetization
increases linearly as the field is increased until it reaches
a critical field HC around 15 kG (as can be estimated
from Fig. 3) after which the magnetization rapidly

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T. Mori / Polyhedron 24 (2005) 2803–2807
Fig. 6. Temperature dependence of the magnetic susceptibility of
GdB18Si5 with field applied in-plane for 600 G (open triangles), 400 G
(closed circles), 200 G (crosses), 160 G (open squares), 130 G (closed
triangles), and 100 G (closed squares). The lines are guides to the eye.
Fig. 4. Magnetization curve of GdB18Si5 with the field applied along
the c-axis at 1.8 K. The arrow indicates the critical field of 15 kG. The
line is a guide to the eye.
behavior can be seen more clearly in the temperature
dependence of in-plane susceptibility given for various
fields in Fig. 6. The susceptibility above 300 G is similar
to the perpendicular (c-axis) susceptibility. It appears
that a reorientation of the spin direction, a spin flip is
achieved at these low fields. We do not think this is a spu-
rious result since the sample is rigidly fixed and the move-
ment of the sample at such low fields (and with this small
susceptibility difference) is extremely unlikely. Further-
more, after measurements, the sample was always
checked and the physical orientation observed not to
have altered at all. The magnetization curve shown in
Fig. 5 was measured with a maximum magnetic field of
1 kG (not shown) and there was no hysteresis.
To summarize, the results indicate that a very low
field dependence of the parallel susceptibility exists in
the GdB18Si5 system with a reorientation of the spin-axis
appearing to occur below 300 G. The unusual points of
this behavior of the in-plane susceptibility (the parallel
susceptibility) are the gradual nature of the shift to high-
er magnetization and the low field dependence. The ori-
gin of this behavior is likely due to the round spin of the
gadolinium ions, but the actual observation of a spin flip
at such low magnetic fields is rare.
Obtaining more information on the explicit magnetic
ordering of GdB18Si5 is desirable. Neutron scattering is
a powerful technique to investigate the magnetic struc-
ture, but due to the extremely large cross sections of
both natural boron and gadolinium this is impossible
for the as-prepared samples. We have previously suc-
ceeded in synthesizing extremely 11B isotopic pure sam-
ples of the even more boron-rich TbB44Si2 sample and
carried out neutron diffraction measurements [14]. The
procurement of isotope gadolinium is presently being
investigated. We are also considering alternate methods
using hot neutrons.
Fig. 5. Low field magnetization curve of GdB18Si5 with the field
applied in-plane at 1.8 K. The arrow indicates the critical field of
15 kG. Both field increasing and decreasing data points are plotted
together.
direction did not cause a large difference in observed
susceptibility. Therefore, although it was not possible
to directly measure only the parallel susceptibility,
the in-plane measurement was carried out by fixing
the magnetic field in a certain direction of the crystal
perpendicular to [001] and varying the magnitude.
Since the measured susceptibility can be observed to
obviously contain a component of the parallel suscep-
tibility, this gives a measure of the parallel susceptibil-
ity behavior under magnetic field.
First of all, the magnetization curve of the in-plane
magnetization at low fields is shown in Fig. 5. An inter-
esting dependence at low fields is observed. As field is
increased above 50 G the magnetization shows an upper
curvature and shifts to a larger regime above 300 G. This

T. Mori / Polyhedron 24 (2005) 2803–2807
2807
4. Conclusions
range order has been observed, and interestingly has a
relatively low partial occupancy of the gadolinium sites.
The magnetic field dependence of the antiferromag-
netic transition in the rare earth B12 icosahedral cluster
compound GdB18Si5 was investigated. Below TN = 3.2
the spins are antiferromagnetically aligned in-plane,
perpendicular to the c-axis. The magnetic phase diagram
was determined with the magnetic field applied along the
c-axis. Typical dependence of an antiferromagnetic sys-
tem was observed, with TN shifting to lower tempera-
tures as field is increased. The magnitude of magnetic
field corresponds well to the temperature scale. Rapid
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