Chemical synthesis and microstructure of nanocrystalline RB6(R = Ce, Eu)

Article abstract of DOI:10.1016/j.jallcom.2014.06.207

Nanocrystalline RB₆(R = Ce and Eu) have been successfully synthesized by a solid-state reaction of CeO₂and Eu₂O₃with NaBH₄at a temperature range of 900-1200 °C. Phase composition, grain morphology, microstructure and valence states of RB₆were investigated by using XRD, FESEM, HRTEM and XANES measurements. Results show that all the synthesized hexaborides are composed of single-phase nanoparticles with cubic morphology. The FFT patterns of HRTEM images reveal that the hexaborides have a high crystallinity with CaB₆-type cubic structure. The present preparation technique is as a novel and invaluable for the developments of highly crystallized RB₆nanoparticles.

Full text of DOI:10.1016/j.jallcom.2014.06.207

Journal of Alloys and Compounds 617 (2014) 235–239
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Chemical synthesis and microstructure of nanocrystalline RB6
(R = Ce, Eu)
⇑
Bao Lihong, B. Wurentuya, Wei Wei, Li Yingjie, O. Tegus
Inner Mongolia Key Laboratory for Physics and Chemistry of Functional Materials, College of Physics and Electronic Information, Inner Mongolia Normal University,
Hohhot 010022, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 May 2014
Received in revised form 29 June 2014
Accepted 30 June 2014
Available online 9 July 2014
Nanocrystalline RB6 (R = Ce and Eu) have been successfully synthesized by a solid-state reaction of CeO2
and Eu2O3 with NaBH4 at a temperature range of 900–1200 °C. Phase composition, grain morphology,
microstructure and valence states of RB6 were investigated by using XRD, FESEM, HRTEM and XANES
measurements. Results show that all the synthesized hexaborides are composed of single-phase nanopar-
ticles with cubic morphology. The FFT patterns of HRTEM images reveal that the hexaborides have a high
crystallinity with CaB6-type cubic structure. The present preparation technique is as a novel and invalu-
able for the developments of highly crystallized RB6 nanoparticles.
Keywords:
Nanocrystalline
Rare-earth hexaborides
Solid-state reaction
Microstructure
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
nanowires have a very sharp tip with diameters of less than hundred
nanometers, it is very well fit the purpose of higher brightness and
Rare-earth hexaborides (RB6) as a multifunctional novel mate-
rial have attracted many interests owing to their special electronic
and magnetic performance. Among the rare-earth hexaborides,
LaB6 and CeB6 as an excellent thermionic electron emitter were
characterized by high brightness, thermal stability, low volatility
and high mechanical strength [1–4]. SmB6 as a topological Kondo
insulator also has attracted much attention due to the manifesta-
tion of strong electronic correlations which give rise to exotic
ground state [5–8]. NdB6 [9] and EuB6 [10] are extensively studied
due to their complicated magnetic transport properties. In resistiv-
ity and specific-heat measurements, there are two magnetic phase-
transition temperatures observed for EuB6 at Tc ꢀ 15.5 and 12.6 K.
GdB6 [11] was reported to have the lowest work function. How-
ever, because of its relatively poor stability at high working tem-
perature [12], it is more suitable for field emission electron
source with high brightness at room temperature rather than
thermionic cathode. Based on the above mentioned facts, there
are great research prospects for rare-earth hexaborides, especially
regarding to their excellent thermoemission, magnetic and topo-
logical insulator properties.
lower energy spread for field emission transmission and scanning
electron microscope. By comparison with nanowires, the RB6 nano-
powders have shown many superior properties. The various RB6
nanopowders showed different absorption capabilities in near infra-
red rays (NIR) due to the different free electron plasmon energies
such as 1.97 eV for LaB6, 1.96 eV for CeB6, 1.89 eV for PrB6 and
1.90 eV for NdB6, respectively [21]. Takeda et al. have reported that
the LaB6 nanopowder shows a largest NIR absorption among the RB6
nanopowders [22]. Yuan et al. studied the particle size effects on the
optical properties of the composites of LaB6 nanopowder mixing
with polymethyl methacrylate (PMMA) [23]. The result shows that
when the particle size is about 70 nm, it has the best optical property
among the composites. Above mentioned excellent optical property
of RB6 nanopowder will very well meet the unique approach to
reduce solar heat, which also provide a potentially low-cost and
high-productivity solution. More recently, Lai et al. have success-
fully prepared a core–shell structured LaB6@C-SiO2, which shows
excellent NIR photothermal conversion property and bright blue
emission under UV irradiation, and green emission under visible
irradiation [24]. Therefore, the RB6 nanopowder will be widely used
for the commercial applications as transparent solar control sub-
stances such as PET films and PVB interlayers for automotive and
architectural windows. Another important development in the RB6
nanopowder is that a great improvement on the density, hardness
and fracture toughness of the WC-10 Co alloys has been achieved
by adding a small amount of LaB6 nanopowder [25].
Recently, rare-earth hexaborides with nanostructure such as
nanowires [13–16] and nanopowders [17–20] have attracted great
interests in both scientific and technological areas. Since the
⇑
Corresponding author.
E-mail address: tegusph@imnu.edu.cn (O. Tegus).
http://dx.doi.org/10.1016/j.jallcom.2014.06.207
0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

236
B. Lihong et al. / Journal of Alloys and Compounds 617 (2014) 235–239
Up to now, many synthesis methods [26–30] have been devel-
(111), (210) and (211), which belongs to the cubic crystal system
with the Pm-3m space group.
oped to prepare the RB6 nanopowder for a low reaction tempera-
ture, easy to handle precursors, simple process controlled and
low cost. Here, we report a simple and novel method for preparing
the nanocrystalline RB6 (R = Ce, Eu) by a solid-state reaction of
R2O3 with NaBH4 in a continuous evacuating conditions. In this
way, we have found the trivalent CeB6 can be obtained using the
mixed valence of CeO2 as raw materials, which is confirmed by
the study of X-ray absorption near edge structure (XANES). The
effects of reaction temperature on the crystal phase, size and mor-
phology were characterized by using XRD, FESEM and HRTEM.
3.3. The morphology and microstructure of RB6 nanocrystals
In order to better obtain the information of grain sizes and typ-
ical shapes of as-synthesized RB6 products grown under different
experimental conditions, the field emission scanning electron
microscopy (FESEM) are used to observe the grain morphology. It
can be seen from Fig. 3(a) that the CeB6 prepared at 1000 °C is com-
posed of a great deal of aggregated nanoparticle and a small num-
ber of cubic body with a size of 10–20 nm shown in the upper side
of magnified SEM. When the reaction temperature is increased to
1100 °C, the nanoparticle convert into small crystalline nanocube
with a size of 50–100 nm shown in Fig. 3(b). But it is also found
that the CeB6 crystals are adhered together to show a less distribu-
tions and other some amounts of large-sized non-cubic morphol-
ogy crystals observed in this reaction temperature of 1100 °C.
The reason is might be the aggregations raw material of CeO2
formed the non-cubic morphology of large grains, which lead to
the prepared powder less uniformed and distributions.
2. Experiments
The CeO2 (99.99% purity), Eu2O3 (99.9% purity) and NaBH4 99.0 purity) powder
in a fixed molar ratios were mixed in an agate mortar for an hour. Then the mix-
tures were put into a quartz tube and placed in the resistance furnace at a reaction
temperature in the range from 900 to 1200 °C for 2 h. Whole reaction was kept
under a vacuum of 2 ꢁ 10ꢂ2 bar. The phase identification was examined by X-ray
diffraction (Cu Ka radiation, Philips PW1830). The crystal morphology was charac-
terized by field emission scanning electron microscope (FESEM: Hitachi SU-8010)
and the microstructure is 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.
Fig. 4 shows the FESEM image of EuB6 prepared at 1000–1200 °C.
It is interestingly found from Fig. 4(a) that there are a large number
of small cubic grains not nanoparticle, which grain size in the range
of 10–20 nm at the reaction temperature of 1000 °C. Subsequently,
while the reaction temperature elevated to 1100 °C, the perfect
small nanocubes maintained the grain size of 30 nm and cubic mor-
phology. However, when the reaction temperature increasing to
1100 and 1200 °C, there shows an obvious grain-growth behavior,
which grain size increased to 50 and 100 nm respectively. One of
the important factors for the grains growth is the high specific sur-
face and high diffusion coefficients of nanocube have cause to mass
transport through lattice and grain boundaries to grain growth.
Comparing the SEM observations of EuB6 with CeB6, we have found
the EuB6 crystals show a more uniform distribution than CeB6. This
is due to raw materials of Eu2O3 is more uniform than CeB6.
In order to further study the microstructure of RB6 nanocrystal at
different reaction temperature, the TEM as an effective character-
ization method to be used to observe the grain morphology and
crystallinity. Herein, the EuB6 as an example to be given the analyses
in Fig. 5 with the selected area electron diffraction (SAED) pattern
and the high resolution TEM (HRTEM) image. It can be seen from
images that it is mainly composed by the large amounts of spherical
nanoparticle and small amount of nanocube with a mean size of
10 nm, which insert magnified TEM fully confirmed the degree of
crystallinity. Its corresponding SAED pattern can be indexed to
(200) and (210) planes, indicating at initial temperature of 900 °C
the ultrafine nanoparticles and nanocrystal are coexist. While the
reaction temperature increase from 900 to 1100 °C, the nanoparticle
3. Results and discussion
3.1. Characterization of the raw materials
It is necessary to study the microstructure of raw materials
because the particle sizes of raw materials have influence on the
grain size of final products in solid-state reaction. Fig. 1(a) shows
the particle morphology of CeO2 powders, it displays a nearly
spherical morphology with a size in the range 50–100 nm except
for a small number of aggregations. Comparing with Fig. 1(b), it
can be found the Eu2O3 powder displays a uniform distributions
and spherical morphology with a size of 40 nm, some of them
are adhered together to show a liner shape.
3.2. Phase identification of RB6 nanocrystals
Fig. 2(a) shows the XRD patterns of CeB6 prepared at 1000–
1150 °C holding for 2 h. The diffraction peaks of prepared at 1000
and 1150 °C can be assigned to the CaB6 main phase and small
amounts of LaBO3 impurity phase detected, which is might be
needed more acid-washing to exclude them. There is also small
amounts of impurity is detected from Fig. 2(b) when the EuB6 pre-
pared at 1100 °C. Comparing Fig. 2(a) with (b) it can be concluded
that the optimization reaction temperature of CeB6 and EuB6 are
1100 and 1150 °C, respectively. Their diffraction peaks are well
indexed and assigned to the parallel crystal plane of (100), (110),
Fig. 1. FESEM image of raw materials, (a) CeO2 and (b) Eu2O3.

B. Lihong et al. / Journal of Alloys and Compounds 617 (2014) 235–239
237
Fig. 2. XRD patterns of RB6 nanopowders prepared at different temperature for (a) CeB6 and (b) EuB6 after acid-washing.
Fig. 3. FESEM images of CeB6 prepared at (a) for 1000 °C and (b) for 1100 °C.
Fig. 4. FESEM images of EuB6 prepared at (a) 1000 °C, (b) 1100 °C, (c) 1150 °C and (d) 1200 °C.
have transformed into nanocube with the mean size of 20 nm
shown in Fig. 6. Its corresponding HRTEM image shows the EuB6
nanocube displays a high crystallinity at 1100 °C, where the top
right of FFT patterns reveals that the EuB6 single crystal is mainly
composed of (100) and (110) crystal planes. Fig. 7 shows the TEM
images of EuB6 prepared at 1150 °C. It can be seen that the grain size

238
B. Lihong et al. / Journal of Alloys and Compounds 617 (2014) 235–239
Fig. 5. TEM analyses of EuB6 prepared at 900 °C for 2 h with the SAED pattern of the powder and its indexing (down left) and the HRTEM image (top right).
Fig. 7. TEM analyses of EuB6 prepared at 1150 °C for 2 h with the HRTEM image of
nanocube (top left) and the FFT patterns (top right).
Fig. 6. TEM analyses of EuB6 prepared at 1100 °C for 2 h with the HRTEM image of
nanocube (top left) and the FFT patterns (top right).
edge structure (XANES) technique was used to obtain the informa-
tion on the electronic state of element such as valence and coordi-
nation environment. Fig. 8 describes the normalized XANES spectra
obviously increase to 50 nm, which is agreement with SEM results,
and FFT patterns confirm the cubic structure. Based on the above
HRTEM analysis, it can be concluded that, the EuB6 ultrafine nano-
particles transformed into nanocube when the reaction tempera-
ture increase from 900 to 1150 °C.
l(E) around the rare earth (R) L3 edge for raw materials and reac-
tion products, respectively, where the date of LaB6 and NdB6 is just
for comparison with CeB6. It can be seen from the upper layer in
Fig. 8 that there is obvious one extra peak at the position of
5487 eV and 6212 eV for La2O3 and Nd2O3, respectively. Based on
the investigation of Aritani et al. [31] and Choi et al. [32], these
peaks representing the La and Nd located in the trivalent state,
which come from the excitation from 2P3/2 to 5d orbitals. However,
the two peaks locate in the position of 5728 eV and 5736 eV from
the CeO2 spectra, indicating a characteristic of mixed valence state.
3.4. Valence state characterizations of RB6 nanocrystals
It is well known that the valence state of rare-earth element in
raw materials is a key factor for whether obtaining the RB6 prod-
ucts in the reduction reaction. In order to clarify the valence state
of raw materials and reaction products, the X-ray absorption near

B. Lihong et al. / Journal of Alloys and Compounds 617 (2014) 235–239
239
single step, low cost and grain size controlled. The nanocrystalline
RB6 has perfect cubic grain morphology and the reaction tempera-
ture has a significant effect on the grain size and morphology. For
EuB6 crystal, while the reaction temperature increasing from 900
to 1150 °C, the ultrafine-nanoparticles transformed into nanocube.
XANES spectra show that the rare-earth elements are in the state of
trivalent state in the hexaborides and the CeB6 also can be pre-
pared by tetravalent raw materials. Thus, in present work, this
new preparation route is significantly important for the 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 (Nos. 51161017 and 51302129).
Fig. 8. Normalized XANES spectra around the R L3 edge for raw materials and RB6.
References
According to the work of Li et al. [33], the two peaks of CeO2 are
attributed by the Ce trivalent and tetravalent states, which come
from the excitation from 2P3/2 to 5d orbitals with final state 4f1.
Comparing with the XANES spectra in the lower layer of Fig. 8, it
is found that there are single spectra peak observed at 5487,
5728 and 6212 eV for LaB6, CeB6 and NdB6, respectively. It indi-
cates the rare-earth elements are in the trivalent state in these
hexaborides. Therefore, based on the above analysis, it can be con-
firmed that the rare-earth hexaborides such as CeB6 not only can
be synthesized by trivalent raw materials, but also by tetravalent
raw materials, which opens a new preparation route for nanoma-
terials with a wide range of possible applications.
[1] J.Q. Xu, Y.M. Zhao, Q.Y. Zhang, J. Appl. Phys. 104 (2008) 124306.
[2] X.J. Wang, Y.D. Jiang, Z.L. Lin, K.C. Qi, B.L. Wang, J. Phys. D: Appl. Phys. 42 (2009)
055409.
[3] K. Togawa, T. Shintake, T. Inagaki, K. Onoe, T. Tanaka, Phys. Rev. ST Accel.
Beams 10 (2007) 020703.
[4] J.M. Lafferty, J. Appl. Phys. 22 (1951) 299.
[5] M. Dzero, K. Sun, P. Coleman, V. Galitski, Phys. Rev. B 85 (2012) 045130.
[6] T. Caldwell, A.P. Reyes, W.G. Moulton, P.L. Kuhns, M.J.R. Hoch, P. Schlottmann,
Z. Fisk, Phys. Rev. B 75 (2007) 075106.
[7] E. Frantzeskakis, N. de Jong, B. Zwartsenberg, Y.K. Huang, Y. Pan, X. Zhang, J.X.
Zhang, F.X. Zhang, L.H. Bao, O. Tegus, A. Varykhalov, A. de Visser, M.S. Golden,
Phys. Rev. X 3 (2013) 041024.
[8] A. Barla, J. Derr, J.P. Sanchez, B. Salce, G. Lapertot, B.P. Doyle, R. Rüffer, R.
Lengsdorf, M.M. Abd-Elmeguid, J. Flouquet, Phys. Rev. Lett. 94 (2005)
166401.
[9] Jolanta Stankiewicz, Marco Evangelisti, Zachary Fisk, Phys. Rev. B 83 (2011)
113108.
[10] J.S. Rhyee, C.A. Kim, B.K. Cho, H.C. Ri, Phys. Rev. B 83 (2002) 113108.
[11] H. Zhang, Q. Zhang, G.P. Zhao, J. Tang, O. Zhou, L.C. Qin, J. Am. Chem. Soc. 127
(2005) 13120.
[12] E.K. Storms, B.A. Mueller, J. Appl. Phys. 52 (1981) 2966.
[13] H. Zhang, Q. Zhang, J. Tang, L. Chang Qin, J. Am. Chem. Soc. 127 (2005) 2862.
[14] H. Zhang, Q. Zhang, G.P. Zhao, J. Tang, O. Zhou, L.C. Qin, Adv. Mater. 18 (2006)
87.
3.5. The reaction mechanism for RB6 nanocrystals
In the whole synthesis procedure, it is believed that the nano-
crystalline RB6 were obtained by a solid-state reactions of rare-
earth oxide with NaBH4, where must be a strong reducing agent.
Thus, the proposed reaction mechanism for forming RB6 is con-
cluded as follows:
[15] J.Q. Xu, G.H. Hou, T. Mori, H.Q. Li, Y.R. Wang, Y.Y. Chang, Y.S. Luo, B.H. Yu, Y. Ma,
T.Y. Zhai, Adv. Funct. Mater. 23 (2013) 5038.
[16] J.R. Brewer, R.M. Jacobberger, D.R. Diercks, C.L. Cheung, Chem. Mater. 23
(2011) 2606.
[17] Ramakrishnan Kalai Selvan, Isaschar Genish, Ilana Perelshtein, Jose M.
Calderon Moreno, Aharon Gedanken, J. Phys. Chem. C 112 (2008) 1795.
[18] Y.F. Yuan, L. Zhang, L.M. Liang, K. He, R. Liu, G.H. Min, Ceram. Int. 37 (2011)
2891.
[19] M.F. Zhang, Y. Jia, G.G. Xu, P.F. Wang, X.Q. Wang, S.L. Xiong, X.J. Wang, Y. T
Qian, Eur. J. Inorg. Chem. (2010) 1289.
[20] M.F. Zhang, L. Yuan, X.Q. Wang, H. Fan, X. Y Wang, X.Y. Wu, H.Z. Wang, Y.T.
Qian, J. Solid State Chem. 181 (2008) 294.
NaBH4ðSÞ ¼ NaðSÞ þ BðSÞ þ 2H2ðgÞ
ð1Þ
At initial chemical reaction procedure, the NaBH4 is decom-
posed to H2 gas, B and Na when increasing temperature to around
900 °C as shown in Eq. (1). Due to the boiling point of Na is about
882.9 °C, so it evaporated at 900 °C and then deposited at the lower
temperature side, which is observed after the reaction.
2R2O3ðSÞ þ 3H2ðgÞ þ 18BðSÞ ¼ 3RB6ðSÞ þ 3H2OðgÞ þ RBO3ðSÞ ð2Þ
[21] S. Kimura, T. Nanba, S. Kunii, T. Suzuki, T. Kasuya, Solid State Commun. 75
(1990) 717.
At second step, the rare-earth oxides are reduced by the strong
reducing agent of H2, and then the H2 gas reacts with O to form
H2O at an elevated reaction temperature as described in Eq. (2).
The gaseous H2O is removed by vacuum pump and the impurity
RBO3 phase is removed by hydrochloric acid washing. While the
mixed valences of CeO2 as raw materials, there is an additional
reducing reactions as follows:
[22] H. Takeda, H. Kuno, K. Adachiw, J. Am. Ceram. Soc. 91 (2008) 2897.
[23] Y.F. Yuan, L. Zhang, L.J. Hu, W. Wang, G.H. Min, J. Solid State Chem. 184 (2011)
3364.
[24] B.H. Lai, D.H. Chen, Acta Biomaterialia. 9 (2013) 7556.
[25] T.T. Shen, D.H. Xiao, X.Q. Ou, M. Song, Y.H. He, N. Lin, D.F. Zhang, J. Alloys
Comp. 509 (2011) 1236.
[26] M. Hasan, H. Sugo, E. Kisi, J. Alloys Comp. 578 (2013) 176.
[27] A. Aprea, A. Maspero, N. Masciocchi, A. Guagliardi, A.F. Albisetti, G. Giunchi,
Solid State Sci. 21 (2013) 32.
[28] S.Q. Zheng, G.H. Min, Z.D. Zou, H.S. Yu, J.D. Han, J. Am. Ceram. Soc. 84 (2001)
2725.
[29] M. Aksu, E. Aydln, J. Ceram. Process. Res. 14 (2013) 4.
[30] H. Zhang, Q. Zhang, J. Tang, L.C. Qin, J. Am. Chem. Soc. 127 (2005) 8002.
[31] H. Aritani, H. Yamada, T. Yamamoto, T. Tanaka, S. Imamura, J. Synchrotron Rad.
8 (2001) 593.
Ce4þ þ H2 þ B ! CeB6 þ Hþ
ð3Þ
4. Conclusions
[32] Y.G. Choi, K.A. Lee, K.S. Lee, Met. Mater. Int. 4 (2007) 269.
[33] Y.J. Li, O. Naoyuki, N. Ikuo, N. Yusuke, M. Shigeyuki, J. Phys. Soc. Jpn. 78 (2009)
094717.
In summary, a new preparation method for RB6 nanocrytals has
been successfully developed. The method shows the advantages of