Synthesis and characterization of ternary NH4Ln 2F7 (Ln = Y, Ho, Er, Tm, Yb, Lu) nanocages

Article abstract of DOI:10.1002/ejic.200600076

In this paper, a new class of NH₄Ln₂F₇ (Ln = Y, Ho, Er, Tm, Yb, Lu) inorganic nanocages that has been discovered will be presented. A facile template-free synthetic route was developed for one step, high yield, and large scale synthesis of ternary NH₄Ln ₂F₇ (Ln = Y, Ho, Er, Tm, Yb, Lu) nanocages. On the basis of our studies, these nanocages are thermodynamically stable forms of this group of NH₄Ln₂F₇ compounds. The tendency of NH ₄Ln₂F₇ (Ln = Y, Ho, Er, Tm, Yb, Lu) to form these new-type nanostructures is believed to have a close relationship with their inherent layered structures, similar to that of inorganic fullerene-like nanoparticles. This new type of nanocage can be easily doped with other lanthanide ions, which may endow these nanocages with novel properties. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200600076

FULL PAPER
DOI: 10.1002/ejic.200600076
Synthesis and Characterization of Ternary NH4Ln2F7 (Ln = Y, Ho, Er, Tm,
Yb, Lu) Nanocages
Xin Liang,[a] Xun Wang,*[a] Leyu Wang,[a] Ruoxue Yan,[a] Qing Peng,[a] and Yadong Li[a]
Keywords: Nanostructures / Lanthanides
In this paper, a new class of NH4Ln2F7 (Ln = Y, Ho, Er, Tm,
Yb, Lu) inorganic nanocages that has been discovered will
be presented. A facile template-free synthetic route was de-
veloped for one step, high yield, and large scale synthesis of
ternary NH4Ln2F7 (Ln = Y, Ho, Er, Tm, Yb, Lu) nanocages.
On the basis of our studies, these nanocages are thermody-
Lu) to form these new-type nanostructures is believed to
have a close relationship with their inherent layered struc-
tures, similar to that of inorganic fullerene-like nanoparticles.
This new type of nanocage can be easily doped with other
lanthanide ions, which may endow these nanocages with
novel properties.
namically stable forms of this group of NH4Ln2F7 com- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
pounds. The tendency of NH4Ln2F7 (Ln = Y, Ho, Er, Tm, Yb,
Germany, 2006)
els,[21] in solid lasers, and display panels.[22] The tendency
of NH4Ln2F7 to form these new types of nanostructures is
believed to have a close relationship with their inherent lay-
ered structures, similar to inorganic fullerene-like nanopar-
ticles. The synthesis of hollow spheres of such complex
compounds has great importance and is worthy of thought-
ful research.
Herein we report a facile water/ethanol solvothermal
synthetic method for one-step, high-yield preparation of a
novel family of ternary NH4Ln2F7 (Ln = Y, Ho, Er, Tm,
Yb, Lu) nanocages under a temperature of 180 °C. The ten-
dency of NH4Ln2F7 to form these new types of nanostruc-
tures is believed to have a close relationship with their in-
herent layered structures, similar to inorganic fullerene-like
nanoparticles. Since the structures of such materials have
great similarity, especially for the KF-LnF3 and NH4F-
LnF3 systems, synthesis of other MF-LnF3 hollow nano-
structures should be possible and have great significance.
Introduction
In recent years, novel nano-sized morphologies of inor-
ganic materials have become a new focus of research as they
may offer new opportunities for application in various areas
of nanotechnology, such as optical, biology, and catalysis,
etc. Among these different morphologies, hollow nano-
structures, as competitive building blocks for future nano-
devices, attract considerable interest due to their high surface
area and low-density characteristics as well as their other
shape-dependent properties.[1–7] Great effort has been fo-
cused on the fabrication of different kinds of hollow nano-
structures for this is a crucial step in the research and appli-
cation of hollow nanostructures. Tenne and co-workers suc-
cessfully synthesized a series of transition metal sulfide ful-
lerene-like hollow nanostructures, such as MoS2, WS2, and
ReS2.[8–11] Many other hollow nanostructures (e.g., TiO2,
SnO2, CdTe) have been prepared by using layer-by-layer
self-assembly, sonochemical deposition, chemical-bath de-
position, and other template-dependent methods.[12–19] On
the basis of the colloidal chemical template-free hydrother-
mal method, binary lanthanide fluoride and hydroxide ful-
lerene-like hollow spheres have been synthesized.[20] How-
ever, to the best of our knowledge, there are few reports
concerning a colloidal chemistry-based template-free one-
step synthesis of hollow nanostructures with small dia-
meters for ternary (quaternary) lanthanide fluoride com-
pounds. As is well known, MF-LnF3 is a big family of com-
pounds including many useful materials, such as KY3F10
and NaYF4, which have great potential as biological lab-
Results and Discussion
NH4Ln2F7 (Ln = Y, Ho, Er, Tm, Yb, Lu) crystallized in
a cubic structure. The XRD pattern of NH4Y2F7 shown in
part a of Figure 1 can be indexed as arising from the pure
cubic phase (JCPDS 43-0842) with lattice constant a =
13.4 Å. Similar XRD patterns of a cubic phase were also
observed from NH4Ho2F7 (JCPDS 43-0842) to NH4Lu2F7
(JCPDS 43-0842). The peak positions and relative inten-
sities in those samples were almost identical to those ob-
served in NH4Y2F7, which indicated that this class of com-
pounds shares a similar crystal structure. By careful obser-
vation a tiny difference can be found in the 2θ values of
[a] Department of Chemistry, Tsinghua University,
Beijing 100084, P. R. China
E-mail: wangxun@mail.tsinghua.edu.cn
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
2186
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 2186–2191

Ternary NH4Ln2F7 Nanocages
FULL PAPER
the XRD peaks, which gradually increase on going from
On the basis of our former work, we realized that for
NH4Ho2F7 to NH4Lu2F7, indicating the gradual shrinkage families of lanthanide compounds that share a similar com-
[24]
of crystal structures according to the atomic number. The position, such as LnF3
and LnPO4,[25] both the crystal
diffraction patterns for NH4Ln2F7 (Ln = Y, Ho, Er, Tm, structures and physical properties may vary gradually on
Yb, Lu) group compounds reveal an internal consistency ascending the series as a result of the lanthanide contrac-
associated with structural isotypes. The work of Krishnan tion. For our system, the ionic radii of lanthanide elements
Rajeshwar and Etalo A. Secco et al., showed that the struc- were found to be greatly responsible for the phase forma-
ture of NH4Ln2F7 (Ln = Y, Ho, Er, Tm, Yb, Lu) can be tion. For all the lanthanide members, the resulting products
visualized as layers of LnF6 octahedron (built up by mutual derived under the same synthetic conditions were of dif-
sharing of corners and edges) with interleaving layers of ferent structure. The variation in the structure is believed to
NH4 groups,[23] which is somewhat similar to that of inor- be caused by the contraction of ionic radii of Ln3+
ganic fullerene-like nanoparticles.
(Table 1). The members from Lu to Ho, ionic radius 0.85–
0.89 Å, formed NH4Ln2F7 compounds; the members from
La to Dy, whose ionic radii range 1.06–0.91 Å, adopt other
crystal structures that are different from the layered struc-
tures of NH4Ln2F7. The XRD patterns of other lanthanide
categories from La–Dy can be seen in Figure 1 (parts b, c).
For the larger lanthanide ions (Ln = La, Ce, Nd, Pr, Sm,
Eu), LnF3 structures were obtained (Ln = La–Sm, the re-
sulting products are pure hexagonal structures; Ln = Eu,
a mixture of hexagonal and orthorhombic structures were
found). For the intermediate-sized lanthanide ions (Ln =
Gd, Tb, Dy) NH4Ln3F10 was formed.[26]
The morphology and microstructural details can be ob-
served from the TEM, HRTEM observations. The typical
low-magnification images of the as-prepared NH4Ln2F7
(Ln = Y, Ho, Er, Tm, Yb, Lu) nanocages (here Y, Ho, and
Tm have been chosen as examples) are shown in Figure 2.
The hollow interior features of NH4Ln2F7 nanocages were
confirmed by the TEM observations. It can be seen from
Figure 2 (a–c) that nearly all the nanoparticles in the images
are of hollow interiors and the nanoparticles are uniform
with an average size of 20–30 nm. Careful TEM observa-
tions were also carried out on both the LnF3 (Ln = La–Eu)
and NH4Ln3F10 (Ln = Gd–Dy) series (TEM images are
found in the supporting information). However, only a few
hollow-sphere-type structures were found. This morphology
difference between NH4Ln2F7 (Ln = Y, Ho, Er, Tm, Yb,
Lu), LnF3 (Ln = La–Eu), and NH4Ln3F10 (Ln = Gd–Dy)
indicates that the formation of the hollow interior forms
is unique to the NH4Ln2F7 series and it also excludes the
possibility that the nanocages were formed by the Maran-
goni effect. That is where the concentrated solutes diffuse
out into the droplet-vapor interface leaving the pure solvent
behind in the center of the nanoparticle. At this point the
solvent evaporates leaving the polycrystalline material to
deposit on the outer circumference and leaving a hollow
core in the center. According to this assumption, nanocages
Figure 1. XRD patterns of (a) NH4Ln2F7 (Ln = Y, Ho, Er, Tm,
Yb, Lu); (b) NH4Ln3F10 (Ln = Gd, Dy, Tb); (c) LnF3 (Ln = La–
Eu).
Table 1. Crystal structure and chemical composition of lanthanide compounds obtained under the identical synthetic condition (h&o is
short for hexagonal and orthorhombic).
Ln3+
La
Ce
Nd
Pr
Sm
Eu
Gd
Tb
Dy
Ho
Y
Er
Tm
Yb
Lu
R(Ln3+) [Å]
Products
Crystal structures hexagonal
1.06
LnF3
1.03
1.01
1.00
0.96
0.95 0.94
NH4Ln3F10
h&o cubic
0.92
0.91
0.89
NH4Ln2F7
cubic
0.89
0.88
0.87
0.86
0.85
Eur. J. Inorg. Chem. 2006, 2186–2191
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2187

X. Liang, X. Wang, L. Wang, R. Yan, Q. Peng, Y. Li
FULL PAPER
would form in other lanthanide categories from La to which proved the formation of hollow sphere nanostruc-
Dy, because of the similar environments in the growth tures under current experimental conditions. The distance
procedure.
between the inner and outer layers was calculated to be ap-
proximately 0.32 nm for NH4Y2F7 nanocages, correspond-
ing to the crystal plane of (410), and the other categories
show a close value of about 0.32 nm, indicating a similar
formation mode under adopted conditions. Though the
crystal lattices were somewhat distorted compared with the
typical onion-like lattices of fullerene-like nanoparticles, the
lattice striations displayed a nearly close-caged configura-
tion with high crystallinity. The electronic diffraction of the
selected area (insert in Figure 3, a), where the diffractive
dots of single crystal can be clearly seen, further confirmed
the single crystal features of the nanocages. It is interesting
to note that the inner spaces are usually quite different in
shape, including all kinds of irregular polygons, indicating
that the growth of these structures might be rather sensitive
to fluctuations from the outer environment. Nevertheless,
the nanoparticles with hollow interiors can be easily ob-
tained under designed synthetic conditions, and from the
TEM images we can see that almost all of the final products
are hollow nanostructures. The HRTEM observations con-
firmed that the formation of the nanocages may have a
close relationship with the inherent layered structure of
NH4Ln2F7.
The thermal gravimetric analysis was carried out to study
the thermal stability of these nanocages. NH4Ln2F7 (Ln =
Y, Ho, Er, Tm, Yb, Lu) decomposes at high temperature.
The TGA curves for NH4Ln2F7 (Ln = Y, Ho, Tm) powders
are shown in Figure 4. From the TGA curves, two weight-
loss steps can be observed. The first step is from room tem-
perature to 250 °C, in which, the mild weight loss may be
caused by desorption of water and other solvents such as
ethanol on the NH4Ln2F7 surface. The main weight loss
occurs in the temperature range from 250 to 360 °C. In this
step, NH4Ln2F7 converted to LnF3. The experimental
weight loss is about 11.10 wt.-% for NH4Y2F7, 7.35 wt.-%
for NH4Ho2F7, and 6.84 wt.-% for NH4Tm2F7, respectively,
which agrees well with the theoretical weight loss of the
decomposition of NH4Ln2F7 to LnF3 (11.25 wt.-% for
NH4Y2F7, 7.69 wt.-% for NH4Ho2F7, and 7.57 wt.-% for
NH4Tm2F7). The chemical reaction that occurs in the pro-
cess of this main weight loss can be expressed as follows:
NH4Ln2F7 Ǟ LnF3 + NH3 Ȇ + HF Ȇ
The decomposition temperature of these three samples is
quite similar. It is about 356 °C for NH4Y2F7, 352 °C for
NH4Ho2F7, and 361 °C for NH4Tm2F7.
NH4Y2F7 samples were annealed at 200 and 400 °C for
3 h to investigate this conversion process. XRD confirmed
that the sample annealed at 200 °C remained as NH4Y2F7;
in contrast the sample annealed at 400 °C converted to YF3
completely. TEM images of the samples derived at different
annealing temperatures are shown in Figure 5 from which
Figure 2. TEM images of (a) NH4Y2F7 nanocages; (b) NH4Tm2F7
nanocages; (c) the Tem images of NH4Ho2F7 nanocages.
HRTEM characterization (Figure 3, a–f) provides us we can see that most of the NH4Y2F7 nanoparticles remain
with further insight into the structural details of NH4Ln2F7 intact with hollow interiors and have no apparent aggrega-
nanocages. It can be seen from the HRTEM images that tion after being annealed at 200 °C for 3 h, in contrast to
most of the nanoparticles usually had a hollow interior, the nanoparticles annealed at 400 °C that have aggregated
2188
www.eurjic.org
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 2186–2191

Ternary NH4Ln2F7 Nanocages
FULL PAPER
Figure 3. HRTEM image of (a) an individual NH4Y2F7 nanocage with diameter about 22 nm, the inset is an EDS analysis of the selected
area; (b) three conterminous NH4Y2F7 nanocages; (c) an individual NH4Y2F7 nanocage with diameter ca. 20 nm; (d) an individual
globular NH4Tm2F7 nanocage; (e) an individual NH4Yb2F7 nanocage; (f) NH4Ho2F7 nanocage.
NH4Y2F7 converts to YF3 at higher temperature. As the
literature has reported, the structure of NH4Y2F7 can be
visualized as layers of YF6 octahedron with interleaving lay-
ers of NH4 groups and the hydrogen bonding effect of
+
NH4 is a critical factor responsible for the stability of am-
monium fluorometallates.[27–28] The layered structure of
NH4Y2F7 would collapse if the interleaving layers of NH4
groups were lost. So it is believed that ammonium loss in
the process of conversion of NH4Y2F7 to YF3 is an impor-
tant factor in the collapse of the nanocages. These phenom-
ena show that the inner structure of NH4Y2F7 is very im-
portant for the stability of these hollow interior forms.
Y3+, Ho3+, Er3+, Tm3+, Yb3+, and Lu3+ can crystallize
in homogeneous phase NH4Ln2F7 and their ionic radii are
Figure 4. TGA curves of NH4Ln2F7 (Ln = Y, Ho, Tm).
to a large extent with no remaining nanocages. These results similar, thus making it convenient to dope NH4Ln2F7 nan-
show that the nanocages can be maintained at the relatively ocages with these lanthanide ions, which may then endow
high temperature of 200 °C before the composition of this class of nanocage with novel properties. The most fre-
NH4Y2F7 and the hollow interior forms collapse as quently used up-conversion ion Er3+ and the sensitizer ion
Eur. J. Inorg. Chem. 2006, 2186–2191
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2189

X. Liang, X. Wang, L. Wang, R. Yan, Q. Peng, Y. Li
FULL PAPER
YF3 (revealed by the XRD analysis). It can be seen that the
up-conversion emission is stronger and can be observed by
the naked eye. The UC spectrum exhibits the characteristic
peaks of an Er3+- and Yb3+-doped YF3 system (Figure 6).
This consequence proves the successful doping of Er3+
Yb3+ into NH4Y2F7.
/
Conclusion
In this work, a novel class of ammonium lanthanide fluo-
ride nanocage was successfully synthesized. The tendency
of NH4Ln2F7 to form nanocages is believed to have a close
relationship with their inner structure. Our study presents
the first example of ternary nanocages to the best of our
knowledge. We believe that this method could be expanded
to the synthesis of other rare-earth hollow nanostructures,
especially the MF-LnF3 categories, and thus brings new op-
portunities to these fast expanding research fields.
Experimental Section
In a general preparation, EDTA chelated with Ln3+ to form a clear
solution at pH 2–3 by adding diluted HCl and NH3·H2O solution.
And then NH4F, which was dissolved into water/ethanol mixed sol-
vents, was added. The mixture was transferred to an autoclave and
was treated at 180 °C for 20 h under solvothermal conditions. The
autoclave was then cooled to room temperature. The precipitate
was then filtered, washed with water to remove ions possibly rem-
nant in the final products, and dried at 80 °C in air. Following the
above procedures, NH4Ln2F7 (Ln = Y, Ho, Er, Tm, Yb, Lu) nano-
cages with high yield (90%) could be obtained.
NH4+ and EDTA are found to be responsible for the phase forma-
tion of NH4Y2F7. To investigate the influence of NH4+, EDTA was
dissolved with NaOH instead of NH3·H2O, and then NaYF4 was
prepared and no hollow-sphere structures were found in the sam-
ples. This result confirms that it is easy to induce other cations to
take the place of ammonium in the products. The concentration of
EDTA greatly affects the phase purity of NH4Y2F7. Keeping the
amount of Y3+ (0.2 mmol) unchanged, when the amount of EDTA
is less than 0.2 g, the resulting products are mixtures of flakes and
hollow-spheres. When the amount of EDTA is more than 1.0 g, the
nanocages were elongated to an irregular form. Similar phenomena
have been observed on the samples of other NH4Ln2F7 (Ln = Ho,
Er, Tm, Yb, Lu). When the amount of EDTA is kept at 0.2–1.0 g,
regular nanocages can be easily obtained on a large scale.
Figure 5. TEM images of samples annealed at (a) 200 °C; (b)
400 °C.
Yb3+ were co-doped into NH4Y2F7 to denote the feasibility
of doping lanthanide ions into NH4Ln2F7. Doped samples
were prepared by the same synthetic procedure, except for
adding 5% (total molar ration) YbCl3 and ErCl3 in YCl3 in
the first stage. The up-conversion fluorescence property of
the final product was measured by a 980-nm IR laser. The
up-conversion emission of doped NH4Y2F7 sample is weak
due to the quenching effect of ammonium. After being an-
nealed at 500 °C for 3 h, the NH4Y2F7 can convert into
The obtained sample was characterized by a Bruker D8-Advance
X-ray powder diffractometer with Cu-Ka radiation (λ = 1.5418 Å).
The size and morphology of the IF nanoparticles were determined
at 200 kV by a Hitachi H-800 transmission electron microscope
(TEM) and a JEOL JEM-2010F high-resolution transmission elec-
tron microscope. Fluorescent spectra were recorded with a Hitachi
F-4500 Fluorescence Spectrophotometer. Up-conversion fluores-
cent spectra were obtained with the LS-50B fluorescence spectro-
photometer (Perkin–Elmer Corp., Forster City, CA) with an exter-
nal 0–800 mW adjustable laser (980 nm, Beijing Hi-Tech Optoelec-
tronic Co., China) as the excitation source, instead of the Xenon
source in the spectrophotometer, and with an optic fiber accessory.
The thermal decomposition behavior of the NH4Ln2F7 (Ln = Y,
Ho, Tm) samples was studied by thermogravimetric analysis (TGA)
with a TGA2050 thermal analysis device (American TA Corpora-
Figure 6. UC emission spectra of YF3 (Er3+, Yb3+).
2190
www.eurjic.org
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 2186–2191

Ternary NH4Ln2F7 Nanocages
FULL PAPER
[11] Y. R. Hacohen, E. Grunbaum, R. Tenne, J. Sloan, J. L. Hutchi-
son, Nature 1998, 395, 336–337.
[12] T. Nakashima, N. Kimizuka, J. Am. Chem. Soc. 2003, 125,
6386–6387.
tion). TGA determination was carried out in nitrogen at a heating
rate of 20 °C/min in a range of room temperature to 900 °C.
Supporting Information (for details see the footnote on the first
page of this article): XRD patterns of samples that were annealed
at 200 °C and 400 °C, respectively, and the TEM images of CeF3,
SmF3, and NH4Tb3F10.
[13] N. A. Dhas, K. S. Suslick, J. Am. Chem. Soc. 2005, 127, 2368–
2369.
[14] X. M. Sun, Y. D. Li, Angew. Chem. Int. Ed. 2004, 43, 3827–
3831.
[15] P. Selvakannan, M. Sastry, Chem. Commun. 2005, 1684–1686.
[16] H. Liang, Y. Guo, H. Zhang, J. Hu, L. Wan, C. Bai, Chem.
Commun. 2004, 1496–1497.
[17] S. Lei, K. Tang, Q. Yang, H. Zheng, Eur. J. Inorg. Chem. 2005,
20, 4124–4128.
[18] S. Xu, H. Wang, J. J. Zhu, X. Q. Xin, H. Y. Chen, Eur. J. Inorg.
Chem. 2004, 23, 4653–4659.
[19] G. Zou, Z. P. Liu, D. B. Wang, C. L. Jiang, Y. T. Qian, Eur. J.
Inorg. Chem. 2004, 22, 4521–4524.
[20] X. Wang, Y. D. Li, Angew. Chem. Int. Ed. 2003, 42, 3497–3500.
[21] L. Y. Wang, R. X. Yan, Z. Y. Huo, L. Wang, J. Z. Hui, J. Bao,
X. Wang, Q. Peng, Y. D. Li, Angew. Chem. Int. Ed. 2005, 44,
6054–6057.
[22] J. H. Zeng, J. Su, Z. H. Li, R. X. Yan, Y. D. Li, Adv. Mater.
2005, 17, 2119–2123.
[23] K. Rajeshwar, E. A. Secco, Can. J. Chem. 1977, 55, 2620–2627.
[24] a) X. Wang, Y. D. Li, Chem. Eur. J. 2003, 9, 5627–5635; b) X.
Wang, X. M. Sun, D. P. Yu, B. S. Zhou, Y. D. Li, Adv. Mater.
2003, 15, 1442–1445.
[25] R. X. Yan, X. M. Sun, X. Wang, Q. Peng, Y. D. Li, Chem. Eur.
J. 2005, 11, 2183–2195.
Acknowledgments
This work was supported by NSFC (20501013, 50372030,
20131030, 90406003), the Foundation for the Author of National
Excellent Doctoral Dissertation of P. R. China and the State Key
Project of Fundamental Research for Nanomaterials and Nano-
structures (2003CB716901).
[1] Y. N. Xia, B. Gates, Y. D. Yin, Y. Lu, Adv. Mater. 2000, 12,
693–713.
[2] M. Prato, M. Maggini, Acc. Chem. Res. 1998, 31, 519–526.
[3] A. L. Balch, M. M. Olmstead, Chem. Rev. 1998, 98, 2123–2165.
[4] F. Caruso, R. A. Caruso, H. Möhwald, Science 1998, 282,
1111–1114.
[5] C. G. Göltner, Angew. Chem. Int. Ed. 1999, 38, 3155–3156.
[6] Y. G. Sun, Y. N. Xia, Science 2002, 298, 2176–2179.
[7] Y. G. Sun, B. Mayers, Y. N. Xia, Adv. Mater. 2003, 15, 641–
646.
[26] Z. J. Kang, Y. X. Wang, F. T. You, J. H. Lin, J. Solid State
Chem. 2001, 158, 358–362.
[27] R. E. Thoma, Inorg. Chem. 1962, 1, 220–226.
[28] A. Tressaud, J. Portier, R. Depage, P. Hagenmuller, J. Solid
State Chem. 1970, 2, 269–275.
[8] R. Tenne, L. Margulis, M. Genut, G. Hodes, Nature 1992, 360,
444–446.
[9] L. Margulis, G. Salitra, R. Tenne, M. Talianker, Nature 1993,
365, 113–114.
[10] Y. Feldman, E. Wasserman, D. A. Srolovitz, R. Tenne, Science
1995, 267, 222–225.
Received: January 25, 2006
Published Online: March 31, 2006
Eur. J. Inorg. Chem. 2006, 2186–2191
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2191