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Li26 Clusters in the Compound Li13Na29Ba19.

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Icosahedral Clusters
DOI: 10.1002/anie.200602137
Li26 Clusters in the Compound Li13Na29Ba19**
Volodymyr Smetana, Volodymyr Babizhetskyy,
Grigori V. Vajenine, and Arndt Simon*
In recent years, numerous metal-rich nitrides of alkalineearth metals, particularly of Ba in combination with Na, have
been discovered.[1] Extending this chemistry to the heavier
homologues of Na proved unsuccessful; however, Li emerged
as the most promising candidate for further development of
the field, as we found several new subnitride phases of this
element. A detailed knowledge of the relevant intermetallic
systems is needed to open up this field of research.
In the Na–Ba system, two binary compounds are known,[2]
Na2Ba[3] and NaBa[4] , and in the Li–Ba system, one compound
is known, BaLi4.[5] In contrast, no miscibility exists in the
solid-state Li–Na system.[6] Ternary compounds were previously unknown in the Na–Li–Ba system. Herein, we report
the first such compound, which, furthermore, exhibits quite
unusual structural features. Single crystals of the ternary
compound were obtained during an attempt to prepare Licontaining subnitrides. After a composition of Li13Na29Ba19
was determined from the crystal-structure analysis, the
compound was also obtained in high yield from an appropriate combination of the elements.[7]
Li13Na29Ba19 crystallizes in a new structure type of cubic
symmetry.[8] The main structural feature of this compound is a
Li26 cluster formed from four interpenetrating icosahedra
(Figure 1). In this cluster, a central Li4 tetrahedron is capped
by 4, 6, and 4 6 3 Li atoms above its faces, edges, and vertices,
respectively. Such anti-Mackay-type[9, 10] Li26 clusters have
been predicted to be stable in the gas phase,[11] and
isostructural Ar26[12] and Ba26[13] species have been postulated
on the basis of cluster distributions in mass spectra of the
gaseous elements. In the solid state, the M26 cluster of g-brass
(Cu5Zn8) is well known,[14] but is composed of different atom
types. Similarly, the M26 cluster of Ag4Li9 contains both Ag
and Li atoms.[15] To our knowledge, Li13Na29Ba19 is the first
example of a solid with homoatomic M26 clusters.
The 4 + 4 atoms at the center of the Li26 cluster define
what is known as a tetrahedral star (TS; Figure 1 a).[16]
Additional capping of the TS by 6 Li atoms and 4 Li3
[*] Dipl.-Chem. V. Smetana, Dr. V. Babizhetskyy, Dr. G. V. Vajenine,
Prof. Dr. A. Simon
Max-Planck-Institut f7r Festk9rperforschung
Heisenbergstrasse 1, 70569 Stuttgart (Germany)
Fax: (+ 49) 711-689-1642
Dr. G. V. Vajenine
Institut f7r Anorganische Chemie
UniversitBt Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
[**] The authors thank C. Hoch for collecting the single-crystal
diffraction data and J. K9hler for helpful discussions.
Angew. Chem. Int. Ed. 2006, 45, 6051 –6053
Figure 1. a) Tetrahedral star (TS) and b) icosahedral (I) fragments of
c) the Li26 cluster in the intermetallic phase Li13Na29Ba19.
triangles results in four interpenetrating centered Li13 icosahedra, (I; Figure 1 b). The structures of numerous crystalline
and quasicrystalline intermetallic phases can be systematized
on the basis of TS[17] and I.[18] Icosahedral Li13 fragments are
found in the binary compound BaLi4, where they share faces
to form infinite chains. Distorted I surround Na atoms in the
structures of Na2Ba and NaBa. The structure of NaBa can also
be elegantly described with a TS framework.[17]
The TS is composed of 5, and I of 20 (slightly distorted)
face-sharing tetrahedra. The Li26 cluster contains 57 closepacked tetrahedra. This type of close packing of tetrahedra in
intermetallic structures was recognized by Frank and Kasper
early on. Their approach can also be used to describe the local
environments of the Na and Ba atoms in Li13Na29Ba19. While
the Na atoms are icosahedrally coordinated, larger FrankKasper-type polyhedra[19] with 15–17 vertices surround the Ba
atoms. In the crystal structure of Li13Na29Ba19 (Figure 2), the
Na and Ba atoms lie between the Li26 clusters, which form a
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Table 1: Crystal volumes per formula unit at room temperature for Li, Na,
Ba, and their compounds.
V = Vcell/Z [G3]
DV[a] [%]
+ 4.1
this work
[a] The percent volume change for LixNayBaz is defined as DV =
100 [V(LixNayBaz)x V(Li)y V(Na)z V(Ba)]/[x V(Li) + y V(Na) + z V(Ba)].
Figure 2. Face-centered cubic arrangement of Li26 clusters (light gray)
in Li13Na29Ba19. The octahedral and half of the tetrahedral voids are
occupied by BaBa4Na12 polyhedra (dark gray). The remaining Na and
Ba atoms are omitted for clarity. The cubic unit cell of Li13Na29Ba19 is
face-centered cubic array (A sites). All of the octahedral
(O sites) and half of the tetrahedral (T sites) voids
are filled by BaBa4Na12 polyhedra, as in the LiAlSi
structure type,[20, 21] corresponding to a filled zinc blende
or rock salt structure. The resulting formulation of
(Li26)A4 (BaBa4Na12)O
4 (BaBa4Na12)4 ffiLi13Na12Ba5 accounts for
only 5 of 19 Ba atoms and 12 of 29 Na atoms per formula unit.
The remaining Na and Ba atoms fill the rest of the space in the
unit cell, resulting in a polytetrahedral packing.
The Li26 clusters are surrounded by a larger cage of 28 Ba
atoms, which consists of 16 triangles and 12 pentagons. All of
the pentagons are capped from the inside by peripheral Li
atoms of the Li26 cluster and from the outside by Na atoms.
These capping Na and Li atoms form the only LiNa contact
in the structure, in which the remaining Li and Na atoms are
completely separated, consistent with the immiscibility of the
two elements in solid state.
The LiLi (2.94(7)–3.34(2) >) and NaNa (3.54(1)–
3.78(1) >) distances are somewhat shorter than the sums of
the respective metallic radii, but fall within the range of bond
lengths observed for BaLi4[5] and Na2Ba.[3] The only close Li
Na contact (3.19(3) >) is considerably shorter than the sum of
the metallic radii (3.37 >). All other bonding contacts, LiBa
(3.84(3)–4.07(3) >), NaBa (3.98(1)–4.48(1) >), and BaBa
(4.405(1)–4.461(1) >), are comparable with the sums of the
respective metallic radii. As is typical for alloys of electropositive elements, the volume of Li13Na29Ba19 is only slightly
smaller (3 %) than the sum of the volumes of its component
elements (Table 1).
The question remains whether the Li26 clusters in
Li13Na29Ba19 represent the upper limit of icosahedrally
based Li clusters. In principle, polytetrahedral packing could
be continued to yield larger anti-Mackay-type clusters, such
as a Li45 cluster comprising 13 interpenetrating Li13 icosahedra,[9, 10] analogous to the M45 clusters in Mg2Al5Cu6 and
Mg2Zn11.[23–25] According to calculations for gas-phase Li
clusters, structural strain increases with cluster size, such that
Mackay-type icosahedral clusters, followed by a face-centered cubic structure,[11] and finally the body-centered cubic
structure of metallic Li are predicted to form.
Received: May 29, 2006
Published online: August 9, 2006
Keywords: barium · cluster compounds · intermetallic phases ·
lithium · sodium
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(Eds.: M. Driess, N. NCth), Wiley-VCH, Weinheim, 2004,
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[7] Sample A: Ba metal (476 mg; Merck, 99 %; distilled twice with
intermediate heating at 1270 K in vacuum to remove trace H),
Ca metal (12 mg; Merck, 99 %; distilled twice), Ba(N3)2 (129 mg;
recrystallized and dried under vacuum), Li metal (28 mg; Merck,
99 %), and Na–K alloy (69 at % Na; 1200 mg) were placed in a
Ta ampoule in a glove box under an Ar atmosphere. The
ampoule was closed by arc welding under an inert atmosphere
and sealed inside a Duran glass ampoule. To decompose the
azide, the reaction mixture was heated to 670 K at a rate of
10 K h1 and kept at this temperature for 5 days. Then it was
cooled to 390 K at 1 K h1 and annealed at this temperature for
1 month. The resulting product, dispersed in excess liquid Na–K
alloy, was transferred into a press, in which the greater part of the
liquid was extruded at 5 kbar through a small hole. The brittle
metallic product isolated consisted of approximately equal
amounts of Li13Na29Ba19 and Na14Ba14CaN6,[26] according to
powder X-ray diffraction analysis. Sample B: Li metal, Na metal
(Merck, 99 %), and Ba metal were mixed in a 1:3:2 stoichiometry, sealed in Ta and Duran ampoules under Ar (as described
above), heated at 670 K for 10 days, and then cooled to room
temperature at 1 K h1. According to powder X-ray diffraction
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 6051 –6053
analysis, the bulk product contained approximately 60 wt %
Li13Na29Ba19, along with NaBa. All handling of the reactants and
products was performed under purified Ar in a glove box.
According to a temperature-dependant Guinier measurement,[27] Li13Ba29Na19 begins to decompose at 410 K.
The structure of Li13Na29Ba19 was determined from X-ray
diffraction data for a single crystal from sample A; data were
collected on a STOE IPDS diffractometer; crystal size 0.12 6
0.13 6 0.14 mm3, cubic, space group F4̄3m, a = 27.335(2) >, V =
20 424(3) >3, Z = 8, 1calcd = 2.189 g cm3, 2qmax = 42.08, AgKa
radiation, l = 0.56086 >, w–q scan, 293 K, 25 544 collected and
1321 unique reflections, empirical absorption correction, m =
3.872 mm1; the crystal structure was solved using direct
methods (SHELXS-97[28]), and a full-matrix least-squares refinement on F2 (SHELXL-97[29] ; anisotropic for the Ba and Na
atoms) was carried out; 65 parameters, R1 (I>2s(I)) = 0.0307,
wR2 (I>2s(I)) = 0.0483, R1 (all data) = 0.0484, wR2 (all data) =
0.0513, GOF = 1.006. Further details on the crystal structure
investigations may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany
(fax: (+ 49) 7247-808-666; e-mail:,
on quoting the depository number CSD-416631.
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Angew. Chem. Int. Ed. 2006, 45, 6051 –6053
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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