SmB6 nanoparticles: Synthesis, valence states, and magnetic properties

Article abstract of DOI:10.1016/j.jallcom.2015.06.086

Nanocrystalline SmB₆ particles were synthesized by the solid-state reaction of Sm₂O₃/SmCl₃ with NaBH₄ in the temperature range of 1000-1200 °C. The phase composition, grain morphology, microstructure and valence states of SmB₆ were investigated by using XRD, FESEM, HRTEM and XANES. It is interestingly found that the SmB₆ nanocrystalline particles with size of 50 nm are easily prepared by using SmCl₃ as raw material. The FFT patterns of HRTEM images reveal that the SmB₆ nanocrystalline particles have a high crystallinity with cubic structure. The XANES results show that the valence state of Sm is more likely Sm³⁺. The magnetic measurement shows that the SmB₆ nanoparticles show paramagnetic behavior, but there is a small anomaly in the paramagnetic state. The present synthesis technique is novel and invaluable for developing highly crystallized SmB₆ nanoparticles.

Full text of DOI:10.1016/j.jallcom.2015.06.086

Accepted Manuscript
SmB Nanoparticles: Synthesis, Valence States, and Magnetic Properties
6
Lihong Bao, Luomeng Chao, Yingjie Li, Ming Ming, B. Yibole, O. Tegus
PII:
S0925-8388(15)30202-4
10.1016/j.jallcom.2015.06.086
JALCOM 34428
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 12 May 2015
Revised Date: 8 June 2015
Accepted Date: 10 June 2015
Please cite this article as: L. Bao, L. Chao, Y. Li, M. Ming, B. Yibole, O. Tegus, SmB Nanoparticles:
6
Synthesis, Valence States, and Magnetic Properties, Journal of Alloys and Compounds (2015), doi:
10.1016/j.jallcom.2015.06.086.
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ACCEPTED MANUSCRIPT
Abstract: Nanocrystalline SmB particles were synthesized by the solid-state
6
o
reaction of Sm O /SmCl with NaBH in the temperature range of 1000–1200 C. The
2
3
3
4
phase composition, grain morphology, microstructure and valence states of SmB6
were investigated by using XRD, FESEM, HRTEM and XANES. It is interestingly
found that the SmB nanocrystalline particles with size of 50 nm are easily prepared
6
by using SmCl as raw material. The FFT patterns of HRTEM images reveal that the
3
SmB nanocrystalline particles have a high crystallinity with cubic structure. The
6
3
+
XANES results show that the valence state of Sm is more likely Sm . The magnetic
measurement shows that the SmB nanoparticles show paramagnetic behavior, but
6
there is a small anomaly in the paramagnetic state. The present synthesis technique is
novel and invaluable for developing highly crystallized SmB nanoparticles.
6

ACCEPTED MANUSCRIPT
SmB Nanoparticles: Synthesis, Valence States, and Magnetic
6
Properties
a
a,b
a
a
c
a*
Lihong Bao , Luomeng Chao , Yingjie Li ，Ming Ming , B. Yibole , O. Tegus
a
Inner Mongolia Key Laboratory for Physics and Chemistry of Functional Materials, Inner Mongolia Normal
University, Hohhot 010022, China
b
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,
2 Wenhua Road, Shenyang 110016, China
7
c
Fundamental Aspects of Materials and Energy (FAME), Faculty of Applied Sciences, Delft University of
Technology, Mekelweg 15, 2629 JB Delft, The Netherlands
Nanocrystalline SmB particles were synthesized by the solid-state reaction of
6
o
Sm O /SmCl with NaBH in the temperature range of 1000–1200 C. The phase
2
3
3
4
composition, grain morphology, microstructure and valence states of SmB were
6
investigated by using XRD, FESEM, HRTEM and XANES. It is interestingly found
that the SmB nanocrystalline particles with size of 50 nm are easily prepared by
6
using SmCl as raw material. The FFT patterns of HRTEM images reveal that the
3
SmB nanocrystalline particles have a high crystallinity with cubic structure. The
6
3
+
XANES results show that the valence state of Sm is more likely Sm . The magnetic
measurement shows that the SmB nanoparticles show paramagnetic behavior, but
6
there is a small anomaly in the paramagnetic state. The present synthesis technique is
novel and invaluable for developing highly crystallized SmB nanoparticles.
6
Key words: SmB , solid-state reaction, valence state
6
PACS number(s): 71.27.+a, 74.25.Jb, 75.70.Tj, 76.75.+i
1
. Introduction
Rare-earth hexaborides (RB ) with cubic CaB -type structure have been
6
6
extensively investigated both theoretically and experimentally for decades [1-2].
Recently, several RB nanowires such as LaB , CeB , PrB , NdB and GdB have
6
6
6
6
6
6
been synthesized, and they are believed to be beneficial to developing field emission
cathodes due to their low work function and a very sharp tip with diameters of less
than hundred nanometers [3-7]. On the other hand, the optical absorption of RB6
nanoparticles has also attracted much attention for the coexistence of high visible light
transmission and strong near infrared absorption [8, 9]. These interesting optical
absorption properties are well satisfied the demand for reducing solar heat for the
windows of automotives and architectures, and have also potential applying for
medical care [10-12]. More recently, the mixed valence SmB as a multi-functional
6
*
E-mail address: tegusph@imnu.edu.cn

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hexaboride is considered the candidate material for topological Kondo insulators
[
13-15] and its optical absorption properties have become a hot research topic [16]. In
addition, the SmB have relatively lower work function of 3.11 eV and that makes it
6
to be a potential thermionic emission material [17]. Therefore, there are great research
prospects for further studying the SmB intrinsic properties.
6
In general, there are many ways to prepare the SmB materials. Hatnean et al
6
have prepared the large sized and high quality SmB single crystal by floating zone
6
method [18]. SmB nanowires are prepared by the liquid Sm (melting point of 1072 � )
6
reacting with BCl gas at a temperature of 1100 � [19]. SmB powders are prepared
3
6
o
by a solid state reaction of Sm sources with B or B C at a high temperature of 1650 C
4
[
20]. However, at this high reaction temperature, the stoichiometry of the products
was hard to be well controlled due to the high volatilization nature of Sm element. In
order to lower the reaction temperature, the Mg or I as an effective catalyst was used
2
to reduce the reaction temperature at a condition of autoclave [21-22].
In this paper, we report a simple and novel method for preparing the SmB6
nanoparticles, using different Sm sources of Sm O /SmCl reacting with NaBH in a
2
3
3
4
continuous evacuating process. The effects of different Sm sources on the phase
composition, grain size, morphology, and the magnetic properties of SmB6
nanoparticles were investigated by XRD, SEM, TEM, and magnetic measurements.
As results the SmB nanoparticles are more easily prepared by using SmCl than
6
3
Sm O under same experimental condition. Meanwhile, present work provides
2
3
evidence through XANES that the valence of Sm is about 2.88, indicating the surface
state is dominant.
2
. Experiment
Sm O /SmCl (purity of 99.99%, Baotou Institute of Rare Earths) and NaBH
4
2
3
3
(purity of 99.0 %, Sigma-Alorich, USA) powders in a molar ratio of 1:12 and 1:6
were mixed in an agate mortar, respectively. Then the mixtures were pressed into
o
pellet and reacted at a temperature between 1000~1200 C for 2~4h. After reaction,
the products were washed by hydrochloric acid and distilled water several times for
removing the SmBO impurity phase, which were formed during the reaction. The
3
phase identification was examined by X-ray diffraction (Cu K radiation, Philips
α
PW1830). The crystal surface morphology was characterized by field emission
scanning electron microscope (FESEM: Hitachi SU-8010). The microstructure and
EDS analysis are characterized by transmission electron microscopy (TEM:
FEI-Tecnai F20 S-Twin 200 kV). The XANES measurement was carried out using
synchrotron radiation at the BL-12A station of Photon Factory, Japan. The magnetic
properties were measured by using SQUID magnetometer (Quantum Design MPMS
5
XL).
3
. Results and discussion
3
.1. Phase Identification of SmB6

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Fig. 1 XRD patterns of SmB powder synthesized at different temperatures, (a) prepared
6
Sm O as raw material, (b) prepared SmCl as raw material
2
3
3
o
Fig.1 (a) shows the XRD patterns of SmB prepared at 1000 and 1100 C for 4 h
6
using Sm O as raw material. It can be seen that all the diffraction peaks at these
2
3
sintering temperatures can be assigned to the CaB -type single phase with space group
6
of Pm-3m without any SmBO impurity phase, confirming high purity of the
3
o
synthesized products. For further increasing reaction temperature to 1150 C, it is
found that there is no powder product obtained after the reaction. This is probably
caused by the high evaporation rate of Sm O at high reaction temperature. By
2
3
comparison with Fig.1 (b), it was found that in the case of using SmCl as raw
3
o
material, even the reaction temperature increases to 1150 and 1200 C, we can obtain
the single phase SmB . Its XRD patterns are well indexed and assigned to the parallel
6
crystal plane of (100), (110), (111), (210) and (211), which is much different from the
results of Sm O as raw material. This means that it is easier to prepare SmB
2
3
6
nanoparticles using SmCl as precursor than using Sm O at same reaction
3
2
3
temperature.
3
.2. Micrograph and microstructure of SmB6
Fig. 2 FESEM images of SmB prepared at different temperatures, using Sm O as raw materials,
6
2
3
o
o
o
(
a) 1000 C for 4 h, (b) 1100 C for 4 h, (c) 1100 C for 20 h
In order to better compare the grain sizes and typical shapes of SmB products
6
synthesized by using different raw materials, the field emission scanning electron

ACCEPTED MANUSCRIPT
microscopy (FESEM) images were shown in Fig. 2. It can be seen from Fig. 2 (a) that
o
the SmB prepared by using Sm O at 1000 C is mainly composed of a great deal of
6
2
3
aggregated particles without any regular shapes. When the reaction temperature is
o
1
100 C, it is clearly seen from the Fig. 2(b) that a small amount of cubic shaped
SmB nanocrystals were observed, but there are some large grains with non-cubic
6
morphology around them. For a prolonged reaction time of 20 h, as shown in Fig. 3(c),
an obvious grain-growth behavior and the clear grain morphology of each individual
particle were observed. The largest grain size is up to about 3 ꢀm. This grain growth
is probably caused by that the high specific surface and high diffusion coefficients of
nanoparticles have caused the mass transport through lattice and grain boundaries.
Fig. 3 FESEM images of SmB prepared at different temperatures for 2 h, using SmCl as raw
6
3
o
o
o
material, (a) 1100 C, (b) 1150 C, (c) 1200 C
Fig. 3 shows the FESEM image of SmB nanoparticles prepared by SmCl raw
6
3
materials at different reaction temperatures. It can be found from Fig. 3 (a) there are a
large number of small cubic grains initial formed with a mean size of 50 nm at 1100
o
C, at the same time these crystals adhered together to show a less distributions. While
o
the reaction temperature elevated to 1150 C, some perfect nanocubes with a mean
size of 100 nm formed and small amount of aggregated nanoparticles shown in Fig. 3
o
(
b). Subsequently, with the increasing temperature to 1200 C, all the crystals have
crystallized into perfect cubic shape with average grain size of 200 nm and showed a
good distribution, which is shown in Fig. 3(c). Comparing the FESEM observations
of using Sm O as raw material with SmCl , we have found the SmB crystals
2
3
3
6
prepared by SmCl show an excellent crystallinity and more uniform distribution than
3
Sm O . The reason is maybe the binding energy of Sm and O is stronger than that of
2
3
Sm and Cl. Thus, NaBH is easier to react with SmCl rather than Sm O .
4
3
2
3

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Fig. 4 (a) TEM analyses of SmB nanocrystalline, (b) the HRTEM image (top right), (c)
6
the indexing FFT patterns (down left).
For further study the microstructure of SmB nanoparticles, transmission electron
6
microscope (TEM) is used to characterize the grain morphology and crystallinity. A
o
typical TEM image of SmB nanoparticles prepared at 1100 C is depicted in Fig. 4. It
6
can be seen that the average grain size is around 40–50 nm, which is consistent with
the SEM observation. The top right of HRTEM reveals the single crystalline nature of
SmB and the lattice fringes d = 0.427 nm and d = 0.298 nm, which agree well with
6
the (100) and (110) crystal planes shown in Fig. 4(c), respectively. Up to now, Zhang
et al [21] and Selvan et al [23] have successfully prepared the micron sized SmB6
particle in a high pressure condition of autoclave. But in present work, this synthesis
method shows an advantages of reaction process is easy controllable and the reaction
does not require high pressure.
3
.3. Valence states of SmB nanoparticles
6
Fig. 5 (Color online) The total X-ray absorption spectra of SmB nanoparticles against photon
6

ACCEPTED MANUSCRIPT
energy at 295 K, The inset is XANES spectra around the Sm L edge
3
Fig. 5 displays the total XANES spectra µ(E) around the Sm L edge for SmB
3
6
nanoparticles at 295 K. It can be seen from the whole energy region that there is a
couple of sharp white lines located at the position of 6719 eV and 7313 eV, which are
corresponding to the absorption spectra of Sm L and L edge. From the upper side of
3
2
enlarged image of peak A, it is clearly found there is a weak peak near it (marked in
2
+
arrow), representing the Sm mixed valence states of trivalent (Sm ) and tetravalent
3
+
2+
(
Sm ). The Sm peak comes from the excitation from 2P to 5d orbital with final
3/2
6
3+
state 4f and the Sm peak comes from the excitation from 2P to 5d orbital with
final state 4f 5d . This result is well agreement with the results [24].
3/2
5
1
In order to further study the valence state of SmB in details, the Lorenzian
6
function as following with an arctangent is applied to fit the absorption spectra:
2
2
A (Γ / 2)
A (Γ / 2)

1
 E − E 
1
2
0
µ
(E) =
+
+ 0.5+ arctan


2
2
2
2

(
E − E ) + (Γ / 2) (E − E ) + (Γ / 2)
π

Γ / 2

1
2

where E and E is the energy at which the transition from 2P to unoccupied 5d
1
2
3/2
states occurs for trivalent and the tetravalent Sm, Γ is the core hole lifetime for the
transition, A and A are the areas of the Lorenzian peaks. The arctangent function
1
2
represents an infinite sum of Lorenzian functions characterized by the energy E of
0
the onset of the continuum. The parameters are summarized in Table 1 and the fitting
curves are shown in Fig. 6. It is estimated the Sm valence from the areas A and A
1
2
given in Table 1 is about 2.88, which is consistent with the results of Chazalviel et al
25].
[
Fig. 6 (Color online) The fitting results of normalized Sm L edge XANES spectra of SmB
3
6
Table 1 The parameters obtained from fitting the Sm L3 edge XANES spectra to Lorenzian function

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E0 (eV)
710
E1 (eV)
E -E (eV)
E2 (eV)
E -E (eV)
Γ
(eV)
4.8
A1
0.18
A2
1
0
2
0
6
6711.5
1.5
6719
9
1.35
3
.4 Magnetic properties of SmB6
Fig.7 shows the magnetization as a function of temperature. The data were
collected under zero-field-cooled (ZFC) and field-cooled (FC) conditions in an
applied field of 10 mT. There is an opening between the ZFC and FC data, indicating
irreversible or history dependent magnetism in SmB . A dome shaped magnetization
6
curve is observed in the M(T) data below 70 K, which does not show the typical
behavior of paramagnet. The inset in Fig. 7 shows the full M(H) loop of SmB6,
collected at 2.5 K. There is no observable hysteresis in the M(H) loop. However, a
clear deviation from a linear paramagnetic response is observed at low fields. These
behaviors are almost same as those observed in bulk samples [26].
Fig. 7 (Color online) Temperature dependence of the magnetization of SmB nanoparticles
6
measured under ZFC and FC conditions in an applied field of 10 mT. Inset shows the full M(H)
loop of SmB measured at 2.5 K.
6
3
.5. Reaction mechanism of synthesis SmB6
Based on the above mentioned experimental analysis, the NaBH has played an
4
important role for a reducing agent and as a boron source for whole synthesis
procedure at the same time. Thus, at initial chemical reaction procedure, following
o
reaction must be preceding at about 500 C [27-28] regardless of Sm O or SmCl as
2
3
3
raw materials as shown in Eq. (1).
NaBH (S) → NaH(S) + BH (S)
(1)
4
3
At second step, Sm O or SmCl reacts with BH to form the SmB when the
2
3
3
3
6
o
reaction temperature is higher than 1000 C as described in Eq. (2), which is fully
confirmed by the XRD analysis.

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Sm O (S) or SmCl (S) + BH (S)→ SmB (S) + H (g)
(2)
2
3
3
3
6
2
At last, the gaseous H were removed by pumping and the impurity phase
2
SmBO was removed by hydrochloric acid washing.
3
4
. Conclusions
In summary, a novel synthesis method for SmB has been successfully
6
demonstrated in present work. The advantages of the method are that the reaction
process is easy controllable and the reaction does not require high pressure. It is easier
to obtain the SmB nanoparticles by using SmCl as raw material than using Sm O .
6
3
2
3
Furthermore, the grain size and morphology of SmB nanocrystalline are very
6
sensitive to the reaction temperature. The valence of Sm in the SmB nanoparticles is
6
2
.88, which is very close to the valence in the surface state. The magnetic properties
of SmB nanoparticles are same as those of bulk materials. The present preparation
6
route is significantly important for developing new rare-earth nanomaterials with a
wide range of possible applications.
Acknowledgements
This work is supported by Fundamental Research Open Project of Inner
Mongolia (Grant No. 20130902), National Natural Science Foundation of China (No.
5
1302129) and Program for Young Talents of Science and Technology in Universities
of Inner Mongolia (Grant No. NJYT-14-B03).
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Highlights
1
. Single-phased nanocrystalline SmB are prepared by a solid-state reaction in a
6
continuous evacuating process.
2
. The reaction temperature has a significant effect on the grain size and
morphology.
3
+
3
. The valence state of Sm in nanocrystalline SmB is more likely Sm .
6