A convenient co-reduction route to nanocrystalline neodymium disilicide

Article abstract of DOI:10.1016/j.ssc.2004.09.062

Nanocrystalline neodymium disilicide was synthesized by co-reducing anhydrous neodymium trichloride and sodium fluorosilicate with metallic sodium in an autoclave at 650 °C. The as-prepared product was characterized and studied by X-ray powder diffraction, transmission electron microscopy and thermogravimetric analysis.

Full text of DOI:10.1016/j.ssc.2004.09.062

Solid State Communications 132 (2004) 743–745
www.elsevier.com/locate/ssc
A convenient co-reduction route to nanocrystalline
neodymium disilicide
a,
Jianhua Ma , Yunle Gu , Liang Shi , Luyang Chen , Yitai Qian
b
b
b
b
*
aWenzhou University, Wenzhou, Zhejiang 325035, People’s Republic of China
bStructure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026,
People’s Republic of China
Received 12 May 2004; accepted 20 September 2004 by T.T.M. Palstra
Available online 19 October 2004
Abstract
Nanocrystalline neodymium disilicide was synthesized by co-reducing anhydrous neodymium trichloride and sodium
fluorosilicate with metallic sodium in an autoclave at 650 8C. The as-prepared product was characterized and studied by X-ray
powder diffraction, transmission electron microscopy and thermogravimetric analysis.
q 2004 Elsevier Ltd. All rights reserved.
PACS: 71.20.Ps
Keywords: A. Nanostructures; B. Chemical synthesis; C. Transmission electron microscopy; C. X-ray diffraction
1. Introduction
[7] by mixing and heating rare earth metal and silicon
powders under protective atmosphere, arc-melting synthesis
[8] of the constituent elements under a high-purity argon
atmosphere, MEVVA (metal vapor vacuum arc) ion source
method of thin film followed by rapid thermal annealing
from 900 to 1200 8C. However, there are few reports on the
synthesis of nanocrystalline neodymium disilicide.
In this paper, we have developed a route to prepare
nanocrystalline neodymium disilicide via co-reducing
anhydrous neodymium trichloride and sodium fluorosilicate
by metallic sodium in an autoclave at 650 8C. This reaction
can be described as follows
Rare earth (RE) silicides have attracted interest during
the last few years for both fundamental and technological
properties [1], such as their metallic resistivity, a small
lattice mismatch (K2.55–C0.83%) [2] and very low
Schottky barrier (w0.3 eV) on n-type silicon [3]. Since,
Tu et al. [4] and Norde et al. [5] found the potential device
applications in infrared detectors, rare earth silicides
became very attractive for electronic applications [6] in
the field of ohmic contacts. As one of this kind of rare earth
silicides, neodymium silicides also have these unique
properties. Furthermore, Nd5Si4 and NdSi1.4 exhibit mag-
netic behavior, while NdSi2Kx exhibits an antiferromagnetic
NdCl3 C2Na2SiF6 C11Na/NdSi2 C12NaF C3NaCl
(1)
´
transition at a Neel temperature of TNZ10 K.
Traditionally, neodymium silicide can be prepared by
different methods, such as a mechanical alloying (MA) route
2. Experimental
All the manipulations were carried out in a dry glove box
filled with N2. Typically, 0.01 mol anhydrous NdCl3 and
0.02 mol Na2SiF6 were placed in a quartz inner-liner tube.
* Corresponding author. Tel.: C86 577 86689505; fax: C86 577
86689508.
E-mail address: mjh820@ustc.edu (J. Ma).
0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2004.09.062

744
J. Ma et al. / Solid State Communications 132 (2004) 743–745
Then about 0.11 mol metallic sodium was added in the tube.
After that, the liner tube was put into a stainless steel
autoclave and sealed under argon atmosphere. The auto-
clave was heated at 650 8C for 10 h, followed by cooling to
room temperature in the furnace. The obtained products
from the tube were washed several times with dilute alkali
solution, distilled water and absolute ethanol to remove
impurities. The final product was vacuum-dried at 60 8C for
12 h.
The powder product was analyzed by powder X-ray
diffraction (XRD) on an X-ray differactiometer (Rigaku rA)
˚
using Cu Ka radiation (wavelength lZ1.54178 A) and
transmission electron microscopy (TEM) on a Hitachi 800
transmission electron microscope. The thermogravimetric
analysis was performed on a thermal analyzer (Model: TA-
50) below 1000 8C in air at a rate of 10 8C minK1 to study its
oxidation behavior.
Fig. 2. TEM image of the neodymium disilicide prepared via a co-
reduction route.
3. Results and discussion
Fig. 1 shows the XRD pattern of the as-prepared
neodymium disilicide sample. All the ten diffraction peaks
at the different d-spacing can be indexed as tetragonal phase
of neodymium disilicide ((101), (004), (103), (112), (105),
(200), (211), (116), (204 and 107) and (213)). After
refinement, the lattice parameters are calculated to be aZ
an average size of 25 nm in diameter. But it exhibits slightly
agglomerated particle morphology.
The oxidation behavior of the as-prepared nanocrystal-
line NdSi2 was studied below 1000 8C by TGA, as shown in
Fig. 3. Below 550 8C, the weight of the sample decreases
slightly. This is because the sample adsorbs some water on
the surface of the particles. Meanwhile hydration phenom-
enon might take place on the surface due to the activity of
NdSi2. This can also be confirmed by the TEM image in Fig.
2. The image shows that the particles have out layers due to
the hydration of neodymium disilicide. So, on the TGA
curve, dehydration of absorbed water occurs around 100 8C
˚
˚
4.104 A, cZ13.49 A, which are in good agreement with the
˚
˚
values (aZ4.111 A, cZ13.56 A) found in the literature [9].
No evidences of impurities such as Nd, Si, Nd2O3 and SiO2,
can be found in the XRD pattern.
Fig. 2 shows the transmission electron microscope
(TEM) image of as-prepared neodymium disilicide. From
the image, we can find the sample consists of particles with
Fig. 1. XRD pattern of the neodymium disilicide prepared via a co-reduction route.

J. Ma et al. / Solid State Communications 132 (2004) 743–745
745
salt system. The molten salt helps to form the nanocrystal-
line neodymium disilicide. So we can conclude that the
formation steps of nanocrystalline neodymium disilicide are
as follows: decomposition of the sodium fluorosilicate, co-
reduction of silicon tetrafluoride and anhydrous neodymium
trichloride and formation of nanocrystalline neodymium
disilicide in molten salt system.
4. Conclusion
In summary, nanocrystalline tetragonal neodymium
disilicide has been successfully prepared via a co-reduction
route by the reaction of metallic sodium with neodymium
trichloride and sodium fluorosilicate in an autoclave at
650 8C. The molten salt system serves as reaction medium to
control the reaction speed and particle size. The particles
have an average size of 25 nm in diameter. The TGA curve
shows that the oxidation begins at 550 8C. So the sample has
good oxidation resistance below 550 8C.
Fig. 3. TGA curves heated in flowing air for the neodymium
disilicide.
and dehydration of hydration-water occurs at 300 8C. From
550 to 800 8C, the weight of the powders increases rapidly
which means that the oxidation of the sample begins at
550 8C. So from this curve, we can conclude that the as-
prepared nanocrystalline neodymium disilicide has good
oxidation resistance below 550 8C.
In this route, sodium fluorosilicate can decompose into
sodium fluoride and gaseous silicon tetrafluoride above
400 8C. So it is believed that the synthetic reaction of
neodymium disilicide is based on the co-reduction of
anhydrous neodymium trichloride and silicon tetrafluoride
by metallic sodium. The formation process of neodymium
disilicide can be described as follows:
Acknowledgements
Financial support from the Chinese National Foundation
of Natural Science Research is gratefully acknowledged.
References
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reaction takes place, the formed heat can make the system in
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¨
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