DOI: 10.1002/anie.201007483
Clathrates
Extension of the Clathrate Family: The Type X Clathrate
Ge P S Te **
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Maria A. Kirsanova, Andrei V. Olenev, Artem M. Abakumov, Mikhail A. Bykov, and
Andrei V. Shevelkov*
Clathrates present a unique class of compounds whose
structures consist of a host framework and isolated guest
atoms located in the large polyhedral cages formed by
framework atoms. The clathrates attract much interest
owing to their intriguing physical properties, especially
The compound Ge P S Te was obtained as a single
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phase from pure elements. The stoichiometric mixture of the
elements was first heated at 6008C and then annealed at
5508C in sealed silica ampoules for 72 h. Ge P S Te is
[
1]
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stable in moist air at room temperature. The results of
differential scanning calorimetry (DSC) show that
Ge P S Te melts incongruently above 5708C, producing
[
2,3]
thermoelectricity and superconductivity.
To date there
[
1]
are only nine clathrate structure types, which differ by the
shape of the constituent polyhedral cages and their spatial
arrangement. In contrast to clathrate gas hydrates, there is a
large group of semiconducting or intermetallic clathrates,
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Ge P Te and elemental sulfur. The low decomposition
3
0
16
8
temperature of Ge P S Te imposes kinetic limitations on
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the solid-state synthesis owing to the low reactivity of
germanium. We activated elemental germanium by trans-
forming it into the more reactive and volatile compound
[4]
which crystallize in only four structure types. The over-
[4]
whelming majority of known clathrates are of type I. Their
frameworks are based on the Group 14 elements (Si, Ge, or
Sn), and additionally contain the elements of Groups 13 and
5 (with exception of Ge40.0Te I ). The peculiar exception
5.3 8
to this rule is Li17.7Mg36.8Cu21.5Ga , which is a rare type IV
GeS . This method allows the efficient activation of elements
2
with low reactivity, such as germanium and silicon, and
provides a promising route to clathrates with poor thermal
stability.
[
5]
1
[
6]
66
clathrate and does not contain Group 14 elements. The
framework of a typical semiconducting clathrate bares
negative charge, while alkaline or alkaline earth metals
occupy the guest positions. In clathrates with inverse polarity,
also known as cationic clathrates, the guest atoms are usually
Ge P S Te crystallizes in the rhombohedral space
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ꢀ
6
group R3m with unit cell parameters a = 17.120(3) ꢀ, c =
10.608(2) ꢀ, and Z = 1. It should be noted that the unit cell
has pseudocubic metrics. To differentiate between the cubic
and trigonal symmetry, an electron diffraction investigation
was undertaken. The selected area electron diffraction
[4]
halogen anions, but several type I cationic clathrates with
[
7]
[8,2c]
guest Te atoms, such as Ge P Te , Si P Te
,
and
(SAED) patterns of Ge P S Te (Figure S1 of the Support-
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3
0
16
8
46ꢀx
x
8ꢀy
[9]
Si2 Te7+x have also been reported. There is a limited
ing Information) can be completely indexed with both
trigonal and cubic unit cells. The geometry of the SAED
patterns does not allow us to differentiate between these two
possibilities. Convergent beam electron diffraction (CBED)
clearly demonstrates the difference. The CBED patterns
taken along h111i directions of the cubic structure should
demonstrate either sixfold or threefold symmetry. In the
rhombohedral structure, such symmetry is possible only for
the [001] pattern, whereas it should be absent for the patterns
corresponding to other h111i cubic directions, such as the
trigonal [211] zone. CBED patterns along the [001] and [211]
zone axes are shown in Figure 1. The sixfold symmetry is
clearly visible on the [001] pattern, and it is definitely absent
on the [211] pattern. This result rules out the cubic symmetry
for Ge P S Te and confirms the choice of the R-centered
0ꢀx
variety of clathrate-forming polyhedra and restricted possi-
bilities of their spatial arrangement, and the discovery of a
new clathrate structure type opens a way for further
[
10]
expansion of the clathrate family,
probably leading to
new properties. Herein we report on the synthesis and crystal
structure of a novel compound Ge P S Te belonging to the
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new type of clathrate structure.
[
*] M. A. Kirsanova, M. A. Bykov, Prof. Dr. A. V. Shevelkov
Chemistry Department, Lomonosov Moscow State University
Leninskie Gory 1-3, Moscow 119991 (Russia)
Fax: (+7)495-939-4788
E-mail: shev@inorg.chem.msu.ru
Dr. A. V. Olenev
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“
SineTheta” Ltd., scientific park of Lomonosov Moscow State
trigonal unit cell.
University
The structure was solved using the single-crystal X-ray
data (R = 0.030, see the Supporting Information). The general
view of the crystal structure is shown in Figure 2. Germanium
and phosphorus atoms form the framework, whereas tellu-
rium atoms occupy the guest positions. There is only one
Leninskie Gory, 1-77, Moscow (Russia)
Dr. A. M. Abakumov
EMAT, University of Antwerp
Groenenborgerlaan 171, 2020 Antwerp (Belgium)
[
**] We thank Dr. P. E. Kazin and Dr. V. A. Kulbachinskii for their help
with physical property measurements. The Russian Foundation for
Basic Research is acknowledged for the support, grant no 10-03-
1
2 2
polyhedron type, [5 6 ], composed of three- and four-
[
11]
coordinate germanium and phosphorus atoms.
The unit
1
2 2
00277.
cell contains six [5 6 ] tetrakaidecahedra, which are substan-
tially distorted on account of the fact that the distances
between the guest Te and framework atoms vary in a wide
Angew. Chem. Int. Ed. 2011, 50, 2371 –2374
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2371