Direct low-temperature synthesis of RB6 (R=Ce, Pr, Nd) nanocubes and nanoparticles

Article abstract of DOI:10.1016/j.jssc.2009.08.032

Rare-earth hexaborides (RB₆, R=Ce, Pr, Nd) nanocrystals were prepared by a facile solid state reaction in an autoclave. Single-crystalline RB₆ nanocubes were fabricated at 500 °C starting from B₂O₃, RCl₃

Full text of DOI:10.1016/j.jssc.2009.08.032

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Journal of Solid State Chemistry 182 (2009) 3098–3104
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Direct low-temperature synthesis of RB6 (R=Ce, Pr, Nd) nanocubes
and nanoparticles
Ã
Maofeng Zhang a,b, , Xiaoqing Wang b, Xianwen Zhang b, Pengfei Wang b, Shenglin Xiong b,
ÃÃ
Liang Shi b, Yitai Qian b,
a Section 503, Xi’an High Technique Institute, Xi’an 710025, China
b Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, University of Science and Technology of China, Jinzhai Road 96, Hefei 230026, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Rare-earth hexaborides (RB6, R=Ce, Pr, Nd) nanocrystals were prepared by a facile solid state reaction in
an autoclave. Single-crystalline RB6 nanocubes were fabricated at 500 1C starting from B2O3, RCl3 ꢀ 6H2O
and Mg powder. RB6 nanoflakes and nanoparticles could be obtained around 400 1C using NaBH4 as
boron resource. XRD patterns show that all of the hexaborides are cubic phase with high crystallinity
and high purity and have lattice parameters that are consistent with nearly stoichiometric products.
Raman spectra elucidate the active vibrational modes of the hexaborides. The TEM and FESEM images
clearly show that the nanocubes have an average size of 200 nm and nanoparticles of 30 nm. Our
experiment developed an efficient, simple and low-cost route to prepare RB6, which could be extended
further to the preparation of other rare-earth metal hexaborides.
Received 14 June 2009
Received in revised form
25 August 2009
Accepted 30 August 2009
Available online 9 September 2009
Keywords:
RB6
Low-temperature
Nanocubes
Nanoparticles
& 2009 Elsevier Inc. All rights reserved.
1. Introduction
method of rare-earth oxides with boron at 1700 1C [5,18,19];
high-pressure and high temperature synthesis from rare-earth
oxides and boron at 1600 1C [20]; carbothermal reduction of
lanthanide oxide and boron at 1500 1C [21]; solution method in
molten aluminum at about 1300 1C [22]; chemical vapor deposi-
tion (CVD) method around 1150 1C [7,8,23]; salt electrolysis at
850 1C [24]; pulsed laser deposition (PLD) technique at 850 1C
[25]. Recently, submicron-sized rare-earth hexaborides were
synthesized at a relatively low-temperature of 900 1C using metal
acetate precursors [26]. Since the properties of a nanostructured
material can be significantly changed by quantum effects, it is
expected that nanostructure RB6, such as nanowires [7,8,27] and
nanoobelisks [28], can give rise to exceptional properties. Never-
theless, most previous methods for the preparation of rare-earth
hexaborides need either high-temperature or harsh reaction
conditions. Researchers are exploring low reaction temperature,
low cost, simple and convenient synthetic methods for the
preparation of these compounds.
However, to our knowledge, few papers on the preparation of
rare-earth hexaborides synthesized at the temperature as low as
400 1C have been reported as yet. In the present study, we report a
simple low-temperature synthesis of RB6 (R=Ce, Pr, Nd)
nanocrystals by a facile solid state reactions in an autoclave. It is
worth noting that one feature of this synthesis route is the high
pressure in the autoclave, coming from the produced H2 during
the reaction (about 50 atm in reaction equation (1) and 100 atm in
reaction equation (2), estimated by the ideal gas law). The
Rare-earth hexaborides (RB6) (R=La, Ce, Pr, Nd, etc.) have
attracted much attention because of their various peculiar
properties such as high melting point, high hardness, high
chemical stability, superconductivity [1,2], magnetic properties
[3–5], high efficiency thermionic emission and narrow band
semiconductivity, and so on [6–10]. Therefore, rare-earth
hexaborides possess great technical importance. In the rare-earth
hexaborides family, LaB6 and CeB6 are used as an electron source
with high brightness and longevity. Compared with LaB6 (work
function ꢁ2.72 eV), CeB6 has a lower work function (ꢁ2.5 eV) and
a lower volatility, which means lower operation temperature and
longer service life when used as a thermionic electron emitter
[11,12]. It has been reported that PrB6 (work function ꢁ3.12 eV)
[13] might be a viable cathode material as well as LaB6, which has
substantial practical application [14].
Previously, RB6 has been synthesized by various methods, such
as direct solid phase reaction of lanthanide or its oxides with
elemental boron around 1800 1C [1,15–17]; the floating zone
Ã
Corresponding author at: Hefei National Laboratory for Physical Sciences at
Microscale, Department of Chemistry, University of Science and Technology of
China, Jinzhai Road 96, Hefei 230026, China.
ÃÃ
Corresponding author.
E-mail addresses: mfzhang@ustc.edu.cn (M. Zhang),
ytqian@ustc.edu.cn (Y. Qian).
0022-4596/$ - see front matter & 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2009.08.032

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reaction equations can be described as follows:
RCl3 ꢀ 6H2OðsÞþ3B2O3ðsÞþ16:5MgðsÞ
¼ RB6ðsÞþ1:5MgCl2ðsÞþ15MgOðsÞþ6H2ðgÞ
ð1Þ
ð2Þ
RCl3 ꢀ 6H2OðsÞþ6NaBH4ðsÞþ3MgðsÞ
¼ RB6ðsÞþ3NaClðsÞþ3MgOðsÞþ3NaOHðsÞþ16:5H2ðgÞ
CeB6
PrB6
X-ray diffraction (XRD), energy dispersive X-ray (EDX) spectro-
scopy and Raman spectra were used to confirm the structures and
compositions of RB6. The morphologies of the products were
characterized by field-emission scanning electron microscope
(FESEM) and transmission electron microscope (TEM).
NdB6
2. Experimental section
10
20
30
40
50
60
70
80
2.1. Sample preparation
2θ (degree)
Preparation of RB6 (R=Ce, Pr, Nd) nanocubes: In a typical
procedure, an appropriate amount of RCl3 ꢀ 6H2O (0.003 mol), B2O3
Fig. 1. Typical XRD patterns of the RB6 nanocubes prepared at 500 1C for 12 h.
(0.009 mol), and excessive Mg (0.075 mol) were put into
a
stainless steel autoclave of 25 ml capacity. Then, the autoclave
was sealed under Ar atmosphere and maintained at 500 1C for
12 h, followed by naturally cooling to room temperature. The
product was washed with dilute hydrochloric acid and distilled
water, respectively, to remove MgO, MgCl2 and other impurities.
The final products were vacuum-dried at 60 1C for 4 h.
Preparation of RB6 (R=Ce, Pr, Nd) nanoparticles: In a typical
procedure, an appropriate amount of RCl3 ꢀ 6H2O (0.003 mol),
excessive NaBH4 (0.025 mol), and excessive Mg (0.040 mol) were
put into a stainless steel autoclave of 25 ml capacity. Then, the
autoclave was sealed under Ar atmosphere and maintained
at 400 1C for 48 h, followed by naturally cooling to room
temperature. The products were washed with dilute hydrochloric
acid and distilled water, respectively, to remove NaOH, MgO, NaCl,
and other impurities. The final products were vacuum-dried at
60 1C for 4 h.
CeB6
PrB6
NdB6
10
20
30
40
50
60
70
80
2.2. Characterization
2θ (degree)
The phase and composition of the products were determined
a Rigaku D/Max-gA rotating-anode X-ray diffractometer
Fig. 2. Typical XRD patterns of the RB6 nanoparticles prepared around 400 1C
for 12 h.
by
equipped with monochromatic high-intensity CuK
˚
(l=1.54178 A). The morphologies of the samples were observed
a radiation
characteristic peaks of impurities were detected. The strong and
sharp reflection peaks disclose that the as-prepared nanocrystals
are all well crystallized.
Fig. 2 shows the XRD patterns of the RB6 nanoparticles
prepared by Eq. (2). All the reflection peaks of the products can
be easily indexed to cubic structure with calculated lattice
with
a transmission electron microscope (JEOL-2010) with
an attached energy dispersive X-ray spectroscopy system, having
an accelerating voltage of 200 kV with a tungsten filament.
Field-emission scanning electron microscope images were taken
on a JEOL JSM-6300F SEM. Raman spectra were measured on a
LABRAM-HR Raman spectrophotometer. The 514.5-nm laser was
used as an excitation light source.
˚
˚
constants a=4.13570.001 A (CeB6); 4.12670.001 A (PrB6); and
˚
4.11970.001 A (NdB6), which are in good agreement with the
literature values (JCPDS card nos.: 11-0670; 25-1455; 11-0087).
No other impurity peaks were detected, indicating the high purity
of as-obtained samples.
3. Results and discussion
Energy dispersive X-ray (EDX) spectroscopy was used to
characterize the composition of the products. EDX analysis of
CeB6 samples reveals the presence of Ce and B for CeB6 nanocubes
(Fig. 3a) and CeB6 nanoparticles (Fig. 3b), and the elements of
Ce and B are dominant in the materials. The other elements
detected in the samples originate from copper grids and the
absorption of oxygen on the powder surface. Similar results are
found in samples of RB6 (R=Pr, Nd) nanocubes and nanoparticles.
The morphologies and microstructure of the samples were
examined by FESEM and TEM. Fig. 4a–c shows the FESEM images
3.1. Morphology and structure of the samples
XRD analysis was used to determine the structure and phase of
the samples. Fig. 1 shows XRD patterns of the RB6 nanocubes
obtained by Eq. (1). All the peaks can be readily indexed to cubic
crystal system [space group: Pm3m (221)] of RB6 with calculated
˚
˚
lattice constants a=4.13670.001 A (CeB6); 4.12570.001 A (PrB6);
˚
and 4.11970.001 A (NdB6), which is very close to the reported
data (JCPDS card nos.: 79-2163; 25-1455; 11-0087). No

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of nanocrystalline RB6 prepared at 500 1C for 12 h. It is evident
that some particles have the shape of rectangular prisms, others
as cubes. The particle size distribution ranges from 160 to 220 nm
by measuring the size of a large number of well defined particles.
The average particle size is about 200 nm. It is named as
‘‘nanocubes’’ in the present work. Take CeB6 for example, TEM
images of CeB6 nanocubes obtained at different reaction
temperatures and times are shown in Fig. 5a–c. CeB6 nanocubes
were formed at 500 1C for 12 h (Fig. 5a). The inset ED pattern of
the selected area taken along the o0014 zone axis indicates its
single-crystalline nature of individual CeB6 nanocube. Varying
reaction time between 12 and 48 h did not significantly affect the
size and morphology of CeB6 nanocubes (Fig. 5b). If reaction time
was shorter than 4 h, the reaction became very incomplete and
the crystallinity was very poor due to too short reaction time.
At 450 1C, only a small amount of RB6 could be obtained. However,
if the reaction temperature increases to 600 1C, the particle
size increases (Fig. 5c). The optimum reaction temperature is
500 1C and large-amount of RB6 could be obtained under this
temperature.
The crystal growth mechanisms of the hexaboride nanocubes
have been explained on the basis of TEM images and XRD
observations. At initial stage, it produces amorphous product.
This amorphous product acts as the nucleation center for the
formation of hexaboride cubes. It converts into small crystalline
primary particles at the elevated temperature. Finally, the
crystalline primary particles are joined together to form
nano-sized crystals, followed by a 3D-oriented attachment [29].
Ultimately, the cubic crystals are obtained at 500 1C. The
explanation is based on the following: The final shape of the
particles is decided by two different crystalline phases, {100} and
{111}, where the {111} phase has a higher surface energy than the
{100} phase. Therefore, at higher temperatures the {111} phase
has gained more thermal energy, which leads to the formation of
the most stable phase of the cubic morphology.
Fig. 6a–c shows typical TEM images of the RB6 (R=Ce, Pr, Nd)
samples prepared by using reaction equation (2). Nanoflakes were
prepared at 420 1C for CeB6, PrB6, and NdB6. The inset selected-
area electron diffraction (SAED) pattern obtained from a random
aggregate of nanocrystalline CeB6 indicates its high crystallinity.
These concentric rings can all be assigned to the cubic phase of
CeB6, which is consistent with XRD pattern. In our previous work
[30], however, LaB6 nanoparticles with average particle size of
30 nm could be obtained at 400 1C for 12 h.
3000
Cu
Ce
Ce
Ce
2000
1000
O
Ce
Cu
B
Ce
Ce
0
4000
3000
2000
1000
0
In the present preparation process, it is found that the reaction
temperature and time play important roles in the formation
of different morphologies of RB6, such as nanoflakes and
nanoparticles. To understand the formation mechanism of these
morphologies, we take CeB6 samples for example. Fig. 7a–c shows
the TEM images and ED patterns of the nanocrystalline CeB6
prepared at 420 1C for 8, 24 and 48 h, respectively. At initial stage,
nanocrystallites with a mean grain size of 3 nm were formed
Ce
0
1
2
3
4
5
6
7
8
9
10
Energy (keV)
Fig. 3. EDX spectra of (a) CeB6 nanoparticles and (b) CeB6 nanocubes on TEM
copper grid.
Fig. 4. FESEM images of RB6 nanocubes prepared at 500 1C for 12 h (a) CeB6, (b) PrB6, and (c) NdB6.

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Fig. 5. TEM images of the CeB6 nanocubes prepared at (a) 500 1C for 12 h (inset: SAED pattern), (b) 500 1C for 24 h, (c) 600 1C for 12 h.
Fig. 6. TEM images and corresponding ED patterns of nanocrystalline RB6 prepared at 420 1C for 12 h (a) CeB6, (b) PrB6, and (c) NdB6.

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Fig. 7. TEM images and corresponding ED patterns of the nanocrystalline CeB6 prepared at (a) 420 1C for 8 h, (b) 420 1C for 24 h, (c) 420 1C for 48 h, (d) 500 1C for 12 h, and
(e) 600 1C for 12 h.
(Fig. 7a). After reacted for 12 h, nanoflakes were obtained (Fig. 6b).
When reaction time was extended to 24 h, as shown in Fig. 7b,
nanoflakes almost disappeared and turned into nanoparticles.
After 48 h (Fig. 7c), CeB6 nanoparticles with an average size
of about 30 nm were fabricated. Form Fig. 7a–c, it was found
that the corresponding ED patterns indicating the crystallinity
of the samples was improved gradually due to the increases of
reaction duration. On the other hand, a higher temperature
favored the formation of CeB6 nanoparticles. At 500 1C, the
dominant products were nanoparticles and only a small amount
of nanoflakes could be observed, as shown in Fig. 7d. At
600 1C, nanoflakes disappeared and CeB6 nanoparticles were
formed (Fig. 7e). Nevertheless, no CeB6 could be obtained
below 350 1C.
used instead of B2O3. It is found that the reactants whether in
the liquid or gaseous state play a crucial role in determining the
shape of the final product. In the preparation of RB6 nanocubes, as
Eq. (1) described, B2O3 begins to turn into liquid (melting point
450 1C) in the heating process. With the increase of temperature,
more liquid B2O3 coats the RCl3 and produces RB6 nanocubes. In
the process to RB6 nanoparticles by reacting RCl3 with NaBH4, as
Eq. (2) described. The reaction (Eq. (2)) is thermodynamically
spontaneous and exothermic. When the heating temperature is
420 1C, the inner temperature in the autoclave may be higher than
500 1C. NaBH4 begins to decompose acutely when the tempera-
ture is above 500 1C as expressed in Eq. (3). The gaseous BH3 and
NaH react further with RCl3 and produce RB6 nanoparticles.
Because of gaseous BH3 reacting with RCl3, it first forms 3 nm
grains (Fig. 7a). With increases of reaction time from 12 to 48 h,
the products converts from 3 nm grains into nanoflakes, and
The main question remaining is why the crystals grow in the
form of nanoparticles rather than nanocubes when NaBH4 was

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symmetry. These three prominent peaks are additional
confirmation for the formation of hexaborides because the
peaks obey the selection rule for the hexaborides’ cubic
symmetry. A broad peak around 1400 cm-1, labeled as ’, is
commonly observed with a relatively strong intensity for trivalent
and intermediate-valent crystals [35]. For trivalent case, twice the
energy of T2g is slightly lower than the peak energy, however, the
peak energy systematically follows twice the energy of T2g. In
LaB6, the peak energy is far from twice the energy of T2u and that
of T2g is very close to the peak energy. For SmB6 the energy
coincidence is very good. Thus it can be concluded that the peak
observed in the trivalent case originates from the second-order
A1g
T2g
Eg
CeB6
PrB6
process of T2g
.
NdB6
400
4. Conclusions
200
600
800
1000 1200 1400 1600 1800
Raman shift (cm-1)
In summary, this work presents
a simple and efficient
approach for the preparation of RB6 nanocubes at 500 1C and
RB6 nanoparticles around 400 1C. The XRD patterns, EDX spectra
and Raman spectra confirm the high crystallinity, high purity and
the single phase of the products. The TEM and FESEM images
clearly show that the products are nanocubes with an average size
of 200 nm, and nanoparticles with an average size of 30 nm.
Compared with previous routes, the present route has the
advantages of mildness, simplicity and low cost. This method
opens the prospect of using cheap reactants for the synthesis of
other metal hexaborides.
A1g
Eg
T2g
CeB6
PrB6
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (no. 20431020), the 973 Project of China
(no. 2005CB623601), the China Postdoctoral Science Foundation
Funded Project (no. 20080440708), and the Natural Science Basic
Research Plan in Shanxi Province of China (no. 2009JQ2012).
NdB6
400
200
600
800
1000 1200 1400 1600 1800
Raman shift (cm-1)
References
Fig. 8. Raman spectra of (a) RB6 nanocubes and (b) RB6 nanoparticles measured at
room temperature in ambient.
[1] K. Hiebl, M.J. Sienko, Inorg. Chem. 19 (1980) 2179.
[2] I. Bat’Ko, M. Bat’Kova, K. Flachbart, J. Alloys Compd. 217 (1995) 11.
[3] H. Hacker, Y. Shimada, K.S. Chung, Phys. Status Solidi A 4 (1970–1996) 459.
[4] J.R. Shannon, M.J. Sienko, Inorg. Chem. 11 (1972) 904.
[5] K. Takahashi, S. Kunii, J. Solid State Chem. 133 (1997) 198.
[6] E.L. Kerley, C.D. Hanson, D.H. Russell, Anal. Chem. 62 (1990) 409.
[7] H. Zhang, Q. Zhang, J. Tang, L.C. Qin, J. Am. Chem. Soc. 127 (2005) 2862.
[8] H. Zhang, J. Tang, Q. Zhang, G. Zhao, G. Yang, J. Zhang, O. Zhou, L.C. Qin, Adv.
Mater. 18 (2006) 87.
ultimately size of 30 nm nanoparticles:
NaBH4-NaHþBH3
ð3Þ
3.2. Raman spectra of the products
[9] A.E. Baranovskiy, G.E. Grechnev, A.V. Logosha, I.V. Svechkarev, V.B. Filippov,
N.Y. Shitsevalova, O.J. Zoga", O. Eriksson, Phys. Status Solidi C 3 (2006) 229.
[10] M. Takeda, T. Fukuda, F. Domingo, T. Miura, J. Solid State Chem. 177 (2004)
471.
[11] L.W. Swanson, D.R. McNeely, Surf. Sci. 83 (1979) 11.
[12] P.R. Davis, M.A. Gesley, G.A. Schwind, L.W. Swanson, Appl. Surf. Sci.
(1989) 381.
It is well-known that Raman spectra are an important tool for
identifying the lattice dynamics and the crystal electric field
splitting of rare-earth hexaborides. Fig. 8 shows Raman spectra of
RB6 nanocubes and nanoparticles, respectively. According to group
theory [31], the hexaborides structure has
a Pm3m cubic
[13] S.V. Meschel, O.J. Kleppa, J. Alloys Compd. 221 (1995) 37.
[14] W.D. Richard, W.S. Dean, J. Appl. Phys. 26 (1955) 1004.
[15] J.M. Lafferty, J. Appl. Phys. 22 (1951) 299.
[16] B.J. Curtis, H. Graffenberger, Mater. Res. Bull. 1 (1966) 27.
[17] J.R. Rea, E. Kostiner, J. Cryst. Growth 11 (1971) 110.
[18] S. Otani, H. Nakagawa, Y. Nishi, N. Kieda, J. Solid State Chem. 154 (2000)
238.
[19] S. Otani, M.M. Korsukova, T. Mitsuhashi, N. Kieda, J. Cryst. Growth
(2000) 378.
[20] X.D. Zhao, X.Y. Liu, F. Lin, W.N. Liu, W.H. Su, J. Alloys Compd. 249 (1997)
247.
[21] B. Post, D. Moskowitz, F.W. Glaser, J. Am. Chem. Soc. 78 (1956) 1800.
[22] V.V. Gurin, M.M. Korsukova, S.P. Nikanorov, I.A. Smirnov, N.N. Stepanov,
S.G. Shulman, J. Less-Common Met. 67 (1979) 115.
[23] Q.Y. Zhang, J.Q. Xu, Y.M. Zhao, H.J. Xiao, P.L. Shu, Adv. Funct. Mater.
(2009) 5.
symmetry, which contains A1g+Eg+T1g+T2g+2T1u+T2u vibrational
modes, where A1g, Eg, and T2g are Raman active phonons. The
A1g and Eg modes originate from the stretching vibration of
B–B bonds and T2g is due to the valence angle bending of B–B–B in
the boron lattice. It can be seen that these spectra are shifted
along the vertical direction, and the order corresponds to the
decrease of the lattice constants of RB6 from the top to bottom.
However, the peaks of Raman spectra of the same compound of
RB6 nanocubes and nanoparticles remain unchanged in the
present experiment. The three expected main peaks are
observed around 680 cmꢂ1 (T2g), 1140 cmꢂ1 (Eg) and 1260 cmꢂ1
(A1g), which are in agreement with earlier reports [32–34], and
they completely satisfy the polarization selection rule in the cubic
[24] M. Kamaludeen, I. Selvaraj, A. Visuvasama, R. Jayavel, J. Mater. Chem. 8 (1998)
2205.

ARTICLE IN PRESS
3104
M. Zhang et al. / Journal of Solid State Chemistry 182 (2009) 3098–3104
[25] V. Craciun, D. Craciun, Appl. Surf. Sci. 247 (2005) 384.
[26] R.K. Selvan, I. Genish, I. Perelshtein, J.M.C. Moreno, A. Gedanken, J. Phys.
Chem. C 112 (2008) 1795.
[27] H. Zhang, Q. Zhang, G.P. Zhao, J. Tang, O. Zhou, L.C. Qin, J. Am. Chem. Soc. 127
(2005) 13120.
[31] N. Ogita, S. Nagai, N. Okamoto, F. Iga, A. Kunii, J. Akimitsu, M. Udagawa,
Physica B 328 (2003) 131.
[32] P. Teredesai, D.V.S. Muthu, N. Chandrabhas, S. Meenakshi, V. Vijayakumar,
P. Modak, R.S. Rao, B.K. Godwal, S.K. Sikka, A.K. Sood, Solid State Commun. 129
(2004) 791.
[28] J.R. Brewer, N. Deo, Y.M. Wang, C.L. Cheung, Chem. Mater. 19 (2007) 6379.
[29] J.J. Teo, Y. Chang, H.C. Zeng, Langmuir 22 (2006) 7369.
[30] 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.
[33] Z. Yahia, S. Turrell, G. Turrell, J.P. Mercurio, J. Mol. Struct. 224 (1990) 303.
[34] Z. Yahia, S. Turrell, J.P. Mercurio, G. Turrell, J. Raman Spectrosc. 24 (1993) 207.
[35] N. Ogita, S. Nagai, N. Okamoto, M. Udagawa, J. Akimitsu, S. Kunii, Phys. Rev. B
68 (2003) 224305.