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Phil. Trans. R. Soc. A (2010) 368, 1285–1299
doi:10.1098/rsta.2009.0271
REVIEW
Metal clusters inside out
B Y A RNDT S IMON*
Max-Planck-Institute of Solid State Research, Heisenbergstrasse 1,
70569 Stuttgart, Germany
Solid-state chemistry of cluster compounds with metals in the left part of the periodic
table is described. As a function of the metal valence electron concentration (VEC),
characteristic changes occur with stepwise changing features. Strongly metal–metalbonded and ligand-encapsulated clusters exist for large values of VEC. At decreased
values, endohedral atoms stabilize the clusters, and with the lowest values, only clusters
exist with no ligand shell. This general trend holds for systems with discrete, as well as
condensed, clusters as illustrated for d metals, including the lanthanides, alkaline earth
and alkali metals.
Keywords: clusters; condensed clusters; suboxides; subnitrides; hydride halides; rare earths
1. Introduction
Today, the term metal cluster is frequently used for entities containing a limited
number of metal atoms, no matter whether these atoms are bonded to each other.
In the following, the term will be used in its original meaning, as introduced by
Cotton (1964), i.e. referring to units where an excess of metal valence electrons
is involved in metal–metal (M –M ) bonding. Particularly, metals on the lefthand side of the periodic table exhibit a unique solid-state cluster chemistry.
These metals become increasingly less electropositive and valence electron poor
the further left they are, resulting in a stepwise change of cluster character.
Starting with transition metals in groups 7–5, the number of electrons available
for M –M bonding is large, and a plethora of clusters with strongly bonded Mn
cores surrounded by non-metal atoms is known. Also, with metals of groups 4
and 3, including the lanthanides, a rich chemistry of M –M -bonded compounds
exists. Clusters of the rare earth (RE) metals are still entirely encapsulated by
non-metal atoms, and thus enclosed in an ionic shell; however, they need to be
stabilized by endohedral atoms, and the bonding to the endohedral atoms is also
heteropolar. A drastic change comes when we move on to cluster compounds
of groups 2 and 1 metals. Now, the metal atoms arrange around non-metal
atoms, and the encapsulation is lost. Along the line described, the characteristic
features of the clusters change from a metal atom core with heteropolar bonding
*[email protected]
One contribution of 13 to a Theme Issue ‘Metal clusters and nanoparticles’.
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to the outside, via heteropolar bonding to both inside and outside, finally to
heteropolar bonding only to the inside. Thus, M –M bonding is turned from inside
to outside.
So far, the focus has been on discrete clusters and the consequence of
decreasing metal valence electron concentration (VEC). A second systematic
change originates from ligand deficiency. An incomplete encapsulation of the
Mn core by ligands results in cluster condensation via M –M bonds between
adjacent clusters. Such condensed cluster phases comprise oligomers as well as
one-dimensional, two-dimensional and three-dimensional interconnected systems.
An impressive number of metal-rich phases of transition metals with p elements
exhibit structures to be described within this concept (Simon 1981). Whereas
the cluster cores are empty at a sufficiently large VEC, the combined ligand
and electron deficiency leads to an occupation of the cluster cores by endohedral
atoms, as in the case of discrete clusters. One meets the scenario nearly exclusively
in the extended new chemistry of metal-rich halides of the RE elements. It is no
surprise that such condensation also occurs with the bare clusters of metals of
groups 1 and 2.
The ideas outlined above have been illustrated with a multitude of examples
originating from quite different sources (Simon 1995). Here, the short review
focuses on a few representatives selected from a vast number of compounds. The
review covers some textbook knowledge, as well as recent results that corroborate
the above ideas and add new facet to chemistry of discrete and condensed clusters
in solids. The selection is mainly limited to clusters based on or derived from
octahedral M6 units for the sake of simplicity.
2. Ligand-encapsulated clusters
In his pioneering work, Brosset (1945) showed that divalent molybdenum forms a
cluster [Mo6 Cl4+
8 ] that consists of an octahedral Mo6 core surrounded by Cl atoms
above all faces. Some years later, similar hexanuclear clusters, [Nb6 Cl2+
12 ] and
2+
[Ta6 Cl12 ], were identified with all 12 edges of the octahedron being coordinated
by Cl atoms (Vaughan et al. 1950). The two prototypes of hexanuclear clusters
are shown in figure 1a,b.
In the nomenclature of Schäfer & Schnering (1964), the Cl atoms are marked
as X i -type. Additional X a -type atoms coordinate the corners of the octahedron.
A large number of compounds are derived from these characteristic units, e.g. via
ligand exchange. A visual description of chemical bonding was developed early on
by Kettle (1965): with ligands above the eight faces of the octahedron, two-centretwo-electron bonds are formed along the 12 edges, and in the case of 12 ligands
above the edges, three-centre-two-electron bonds into the eight faces exist. As the
M –M bonds share a metal-to-ligand antibonding contribution, the cluster VEC
frequently deviates from the closed-shell configurations of 24 and 16 electrons,
respectively, depending particularly on the type of ligands.
Drastic electron deficiency creates a qualitatively new feature. The cluster
cores incorporate additional atoms forming strong heteropolar bonds with the
surrounding M atoms at the expense of the weak M –M bonding. A first example
(Simon 1967) was found with Nb6 I11 , whose structure is made up of [M6 X8 ]-type
clusters interconnected according to [Nb6 I8 ]I6/2 . The low value of VEC = 19 is
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Figure 1. Clusters with octahedral M6 core: (a) [Mo6 Cl8 ], (b) [Nb6 Cl12 ], (c) [Nb6 I8 H],
(d) [Th6 Br12 H7 ] and (e) [Sc6 I12 (C2 )].
the reason for a reversible incorporation of a hydrogen atom into the cluster
centre, see figure 1c. Formally, the H atom adds one electron to the cluster, but
actually it acts as an acceptor forming H− (Fritsche et al. 1984). The then even
electron number of the cluster changes the paramagnetic behaviour into nonmagnetic at low temperature. The ‘voluntary’ uptake of an atom into the [Nb6 I3+
8 ]
unit becomes a necessary condition for cluster formation with the neighbouring
element zirconium, and a variety of compounds based on [Zr6 X12 Z ] clusters, where
Z comprising atoms of main group and transition elements is known (Corbett
1981). A particularly interesting case is found with the homologue thorium. In
part, owing to its larger atomic size, the [M6 X12 ]-type cluster is overcrowded with
seven hydrogen atoms in [Th6 Br12 H3+
7 ], each providing one electron to the cluster
(Braun & Simon 1996). This reaches the ‘magic’ electron count 16. Recalling
the description of M –M bonding in the [M6 X12 ]-type cluster in terms of eight
electron pairs in the octahedral faces, the H− ions represent a kind of colouring
of these multi-centre bonds, see figure 1d. One molecule H2 per cluster can be
desorbed before the compound [Th6 Br12 H5 ]Br6/2 decomposes into other phases.
Proceeding to the RE metals, the electron deficiency is further increased. Yet,
numerous compounds based on the [M6 X12 ] topology are formed, provided the
units are stabilized by endohedral atoms and additional electron donors outside
the clusters. As a novelty, the cluster centres can also be occupied by molecular
anions like the ethanide ion C6−
2 , e.g. in the Sc6 I12 C2 cluster (Dudis & Corbett
1987) shown in figure 1e. Through backbonding, the C–C distance is shortened
to approximately 140 pm.
[M6 X12 Z ]-type clusters are frequently present in metal-rich RE halides,
but may not be immediately recognizable. The crystal structure of the
‘Al-contaminated monoiodide LaIAlx ’ can be refined to R = 0.04 with an
NaCl-type structure on the basis of the Bragg reflections ignoring diffuse
scattering effects. However, taking the observed spheres of diffuse scattering
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(a)
(b)
(c)
Figure 2. (a) Experimental X-ray diffraction images of LaIAlx , (b) calculated images of diffuse
scattering effects with a model and (c) of disordered interconnected [La6 AlI12 ] clusters.
(a)
(b)
Figure 3. (a) Crystal structure of [Nb6 F12 ]F6/2. (b) The shading visualizes a dispersion of
sub-nano-sized metal particles in a dielectric medium.
around the Bragg reflections into account, shown in figure 2a, together with the
calculated intensities in figure 2b, a structure of Al-centred [La6 I12 Al] clusters in a
uniform (not random) distribution is derived (figure 2c) and refined on the basis
of measured intensities in the entire reciprocal space (Oeckler et al. 2005).
As long as the M6 unit in a cluster compound is fully encapsulated by ligands
above edges/faces and corners, the general feature of such an arrangement is that
of sub-nano-sized pieces of a metal distributed in a dielectric matrix. This feature
is schematically illustrated for the structure of Nb6 F15 = [Nb6 F12 ]F6/2 (Schäfer
et al. 1965) in figure 3. The loading of the Th6 octahedra by hydrogen in the
isostructural compound Th6 Br15 H7 (Braun et al. 1996) thus reminds us of the
chemical behaviour of the bulk metal thorium, which can absorb a huge amount
of the gas.
So far, entirely ligand-encapsulated clusters have been discussed. The M –M bonded cores in the compounds are well separated, and hence semiconducting
or insulating behaviour results. As a first step towards ligand deficiency,
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Figure 4. (a–c) Oligomeric [Mo4n+2 O6n+6 ] clusters for n = 2, 3, 5, as found in reduced oxomolybdates and (d,e) same type of clusters based on C2 -centred octahedra in La carbide iodides.
one can remove the atoms in front of the corners of the M6 octahedron.
As long as the closed electronic configuration is preserved, the compounds
are still semiconducting; however, electron deficiency may lead to electronic
delocalization between clusters and metallic properties via charge transfer
through appropriate ligands. Chevrel phases and the various phases based on
the molybdenum chalcogenide cluster [Mo6 X8 ] serve as examples. A critical
next step is the interconnection of adjacent M6 cores via M –M bonding
as a consequence of further ligand deficiency. The mode of interconnection
varies from corner, via edge to face sharing. Corner sharing allows for onedimensional, two-dimensional and three-dimensional M –M -bonded systems and
occurs most abundantly with reduced Nb oxides (Köhler et al. 1992; Svensson
et al. 1999). Edge sharing frequently occurs in trans-position leading to onedimensional systems as in molybdenum oxides (McCarley 1990), but also twodimensional systems are known with the examples of zirconium monohalides
(Corbett 1981). An obvious way of interconnection via opposite faces is also
frequently found in ternary molybdenum chalcogenides (Chevrel & Sergent 1986).
Intermediates comprise oligomeric clusters formed from varying numbers of
condensed M6 units. It is amazing to see extremely close structural relationships
in the chemistry of seemingly very different metals like, e.g. molybdenum and
RE metals.
Figure 4 depicts some anionic oligomeric clusters as they are found in reduced
oxomolybdates. They are based on the [M6 X12 ]-type cluster and are the starting
members of a general series M4n+2 X6n+6 , which contains linear oligomeric clusters,
the largest observed being composed of five condensed Mo6 octahedra, and
which ends with the infinite polyanion [Mo4 O−
6 ]. Taking into account the loss
of three valence electrons when moving from Mo to RE, the formation of
topologically similar clusters calls for a replacement of the oxygen by less
electron demanding halogen atoms as ligands and stabilization of the clusters
by endohedral atoms. Indeed, numerous carbide halides have been synthesized
and structurally characterized, which contain the discrete [RE6 X12 C2 ] cluster, as
well as oligomers formed from up to three such discrete units, which all contain C2
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Figure 5. High-resolution transmission electron microscopy image of a sample with nominal
composition La14 (C2 )3 I20 , showing an intergrowth of clusters from the homologous series
La4n+2 (C2 )n I5n+5 . Scale bar, 5 nm.
units (Mattausch et al. 2006). Even the kind of connection between the clusters
via bridging non-metal atoms is, in part, identical in these compound classes of
Mo and RE.
The final member of the series of RE compounds, the infinite chain, e.g.
in Gd4 I5 C2 , is well characterized. However, in figure 5, the high-resolution
transmission electron microscopy image of a sample of related La carbide iodides
(Kienle et al. 2008) exhibits a typical disorder that occurs quite frequently in this
kind of phases, and which may be described as an epitaxial intergrowth of chains
and oligomeric clusters of a general composition La4n+2 (C2 )n I5n+5 and evidences
a source of varying clusters yet to be isolated in discrete well-ordered compounds.
3. Bare-metal clusters
Moving from the transition metals to the alkaline earth and alkali metals
primarily changes the number and kind of valence electrons. However, these
metals exhibit a rich and still growing cluster chemistry in the subnitrides and
suboxides (Simon 1979, 1997, 2004), respectively. The structural features can be
extrapolated from the criteria presented previously. Owing to the lower values of
VEC, the clusters need to be stabilized by endohedral non-metal atoms, and, as
a qualitatively new feature, the clusters have lost their ligand shell.
Alkali metal suboxides were discovered a century ago in admirable
investigations by Rengade (1909). In spite of several attempts, their chemical
nature was disclosed only many decades later. Early hints to the existence of Ba
subnitrides came from the work of Addison et al. (1975); however, again their
compositions and structures were unknown for several decades. The structures of
Rb and Cs suboxides represent textbook knowledge and, hence, will be treated
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Figure 6. Suboxide clusters (a) [Rb9 O2 ] and (b) [Cs11 O3 ] formed from two and three M6 O
octahedra, respectively; subnitride clusters (c) [Ba14 CaN6 ] composed of six condensed [Ba5 CaN]
units with Ca in the centre; (d) [Ba12 Li8 N6 ] cluster containing six condensed [Ba3 Li3 N] units; (e)
[Ba6 Li12 N6 ] cluster formed from condensed trigonal bipyramids [Ba3 Li2 N] indicated by shading;
and (f ) [Li26 N] cluster formed from four interpenetrating Li12 icosahedra (one shaded) occupied
by N in the central tetrahedron (shaded).
only very briefly in order to pave the way for the more complicated subnitrides.
[Rb9 O2 ] and [Cs11 O3 ] clusters formed from two and three face-condensed M6
octahedra around O2− ions are shown in figure 6a,b.
2− −
The surplus of metal valence electrons according to [Rb+
9 O2 ]e5 and
2− −
[Cs+
11 O3 ]e5 is used in M –M bonding. Owing to the absence of a ligand shell,
M –M bonding is not confined to the discrete cluster, but also involves intercluster bonding. Detailed measurements of electrical and thermal properties
indicate that the bonding in the solids Rb9 O2 and Cs11 O3 is like that in the
elemental alkali metals, the single atoms being replaced by the corresponding
clusters. Indeed, the clusters behave like ‘giant’ alkali metal atoms and
combine with excess of metal to form a kind of intermetallic compounds.
ˆ
The binary phases Rb9 O2 and [Rb9 O2 ]Rb3 =Rb
6 O have been structurally
ˆ 4 O and [Cs11 O3 ]Cs10 =Cs
ˆ 7 O.
characterized, as well as Cs11 O3 , [Cs11 O3 ]Cs=Cs
Interestingly, the very similar metals Rb and Cs are sufficiently different that
they separate in space when forming ternary compounds such as [Cs11 O3 ]Rb,
[Cs11 O3 ]Rb2 and [Cs11 O3 ]Rb7 . The crystal structure of the latter, as well as
that containing one Rb atom per cluster is depicted in figure 7. The atoms
between the clusters have no bonding to the O2− ions, but constitute a purely
metallic matrix.
In the preceding section, the bonding in Nb6 F15 based on ligand-encapsulated
clusters was schematized in terms of sub-nano-sized particles of a metal regularly
dispersed in a dielectric matrix (figure 3). In the case of Cs11 O3 , the bonding
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(a)
(b)
Figure 7. Crystal structures of: (a) [Cs11 O3 ]Rb7 and (b) [Cs11 O3 ]Rb.
(a)
(b)
Figure 8. (a) Crystal structure of Cs11 O3 . (b) The shading shows the metallic bonding between
the clusters and the exclusion of the conduction electrons from the cluster centre.
situation is reversed. Now, the presence of Coulomb bubbles of 3 × O2− ions
inside the clusters leads to a confinement of the conduction electrons to the space
between the clusters, as indicated in figure 8.
The structure corresponds to a void metal exhibiting a regular array of regions
that are forbidden for the conduction electrons. This general structure principle
becomes even more explicit with subnitrides. The most obvious analogy to
the structures of alkali metal suboxides is found with [Ba6 N]Na16 : bare Ba6 N
octohedra are surrounded by pure sodium (Snyder & Simon 1994). However, such
simple arrangements are rather the exception with subnitrides and more complex
clusters are the rule.
Figure 6c shows the characteristic cluster [Ba14 CaN6 ] present in numerous
subnitride phases (Steinbrenner & Simon 1996). It is formed from six facesharing Ba5 Ca octahedra, each centred by an N3− ion with Ca in the middle. The
2+ 3−
core of the cluster, [Ba8 CaN6 ], according to [Ba2+
8 Ca N6 ] is electroneutral and
structurally represents one unit cell of (anti)perovskite. Such a sub-nano-sized
piece of salt is surrounded by six Ba atoms that provide electrons for M –M
2+ 3− −
bonding according to [Ba2+
14 Ca N6 ]e12 and a metallic skin to the cluster,
as in the case of the alkali metal suboxides. The analogy is even closer, as
stoichiometric compounds exist where the clusters are arranged in a matrix of
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Figure 9. Crystal structure of [Ba14 CaN6 ]Na17 .
alkali metal. A whole series of phases [Ba14 CaN6 ]Nan has been characterized
via single-crystal-structure investigations, where n takes the values 7, 8, 14,
17, 21 and 22 (Vajenine & Simon 2001). They all represent regular dispersions
of sub-nano-sized pieces of a salt in a metal. As an example, the structure of
[Ba14 CaN6 ]Na17 is depicted in figure 9 (Simon & Steinbrenner 1996).
All attempts failed to substitute the sodium by heavier alkali metals. The
cluster does not bind to K, Rb and Cs, very much like elemental Ba that does
not mix or form compounds with these metals. Of course, one could have expected
a partial substitution within the alkali metal substructure, as binary compounds
such as Na2 K and Na2 Rb are known. However, an unexpected reaction occurs:
in contact with K, a part of Na is extracted from the subnitrides to form liquid
Na/K alloy. The reaction can be used for the systematic preparation of phases
with low values of n by performing the nitridation reaction in an Na/K melt of
appropriately chosen composition.
In contrast to the heavy alkali metals, the light homologue Li participates in
subnitride formation, and it does it in a dual way. Li has affinity to both Ba and
N, as indicated by the existence of Ba/Li intermetallic compounds and the very
stable nitride Li3 N. It should therefore be able to enter the subnitride cluster, as
well as the purely metallic region between the clusters. Indeed, [Ba14 LiN6 ]Na14
can be synthesized, which is isotypic, with the cubic Ca-containing compound,
and only the central Ca atom being replaced by Li. When the relative amount of
Li is increased, the metal also becomes part of the region between the clusters.
[Ba14 LiN6 ]Li4 Na11 and [Ba14 LiN6 ]Li5 Na10 crystallize in different well-ordered
structures which are closely related to that of [Ba14 LiN6 ]Na14 , in spite of the
deviation in alkali metal content (Smetana et al. 2008a,b). It is remarkable that,
in these mixed metal phases, bonding between Li and Na occurs, although the
binary Li/Na system exhibits an extended range of immiscibility. Obviously, the
neighbourhood of the Ba atoms has a subtle influence on the bonding capability of
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these alkali metals: in the ternary phase [Ba19 Li13 ]Na29 , a short Li–Na distance of
only 319 pm clearly indicates a chemical bond. The compound will be addressed
once more later.
In the cluster [Ba14 LiN6 ], the octahedra surrounding the N atoms contain
one Li atom. Increasing the Li : Ba ratio results in the new cluster [Ba12 Li8 N6 ]
shown in figure 6d (Smetana et al. 2007a,b). Here, the N atoms are surrounded
by Ba3 Li3 octahedra, which are condensed in a way that the Li atoms collect
in the inner part of the cluster, and the Na atoms are arranged outside.
Following the structural principle of the suboxides and subnitrides presented
so far, these clusters are again dispersed in a metallic matrix according to
[Ba12 Li8 N6 ]Na15 .
For the sake of simplicity, the topology of an M6 octahedron was chosen
throughout this review to outline the ideas about encapsulation, stabilization
by endohedral atoms, cluster condensation and loss of encapsulation. Of course,
there are many more closely related systems; however, these are based on
different topologies. In the following, two examples are briefly mentioned to be
more complete. In the large structure of Ba39 Li80 N9 , two types of subnitride
clusters have been found (Smetana et al. 2006). The first, [Ba6 N] octahedra,
is known from the structure of [Ba6 N]Na16 . The second cluster, [Ba6 Li12 N6 ], is
composed of six distorted condensed trigonal bipyramids [Ba3 Li2 N], see figure 6e.
A particulary interesting case is related to the ternary phase Ba19 Li13 Na29
mentioned earlier. The complex structure contains Li26 Mackay clusters formed
from four tetrahedrally arranged interpenetrating icosahedra. This type of cluster,
though consisting of different atoms, is well known from g-brass. There is
experimental evidence that the central Li4 tetrahedron is empty; however, detailed
investigations show that it can also be occupied by, for example, N. The entity
[Li26 N] as the most metal-rich cluster discussed in the context of this review is
shown in figure 6f (Smetana et al. 2008a,b).
The last section started from single M6 octahedra encapsulated by ligands.
Cluster condensation resulted in oligomers and finally infinite arrays of varying
dimensionality. The structural chemistry of bare-metal clusters described so
far closely follows this general pathway, and only the final step is left to
be demonstrated. As examples, chain compounds formed from face-sharing
octahedral units are chosen.
In figure 10, the structures of In[Mo3 Se3 ] (Hönle et al. 1980) and [Ba3 N]Na
(Rauch & Simon 1992) are compared. Both exhibit hexagonal rod packings of
such chains. Whereas the electron-rich Mo6 clusters are entirely encapsulated by
anions bonding to additional cations between the chains, the electron-poor Ba
forms bare chains of octahedra centred by N3− ions and bonded to metallic Na
between the chains, as in the structures of suboxides and subnitrides with discrete
clusters illustrated before. Similar to the latter, the chain compound also allows
a variation of the extension of the metallic region, e.g. with [Ba3 N]Na5 (Snyder &
Simon 1995), as well as Ba3 N (Steinbrenner & Simon 1998), which only contains
[Ba3 N] chains in metallic bonding.
Condensation of [Ba6 N] octahedra is found in the structure of Ba2 N, which
consists of Ba–N–Ba layers with metallic bonding between them (Reckeweg &
DiSalvo 2002). The structure of the new compound [Ba2 LiN] (Smetana et al.
2007a,b) exhibits layerwise cross-linked [Ba5 LiN] units (figure 11), which have
been mentioned earlier as the constituent of the [Ba14 LiN6 ] cluster. These few
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Figure 10. Crystal structures of: (a) In[Mo3 Se3 ] with anionic chains of condensed Mo6 octahedra
encapsulated by Se atoms and cations between them and (b) [Ba3 N]Na with bare [Ba3 N] chains
and metallic bonding between the chains and additional Na.
(a)
(b)
Figure 11. Crystal structure of: (a) Ba2 N formed from condensed [Ba6 N] units and (b) Ba2 LiN
containing layers of crosswise condensed [Ba5 LiN] units.
examples give some impression of an extended chemistry of metal-rich compounds
yet to be explored. The recent discovery of a new class of suboxides may underline
this remark.
Elemental Cs reacts with oxides A2 O3 (A = Al, Ga, In, Fe) at moderate
temperature around 200◦ C to form the isostructural compounds Cs9 AO4 (Hoch
et al. 2009). It is amazing to see that the very stable high-temperature ceramic
material Al2 O3 dissolves under such mild conditions, the large excess of Cs with
respect to a normal valence compound is remarkable and, even more, the existence
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(b)
(a)
Figure 12. (a) [Cs12 (InO4 )] cluster and (b) crystal structure of Cs9 InO4 with these clusters
condensed into chains and metallic bonding between chains and additional Cs atoms.
of Fe3+ in an environment of highly reducing Cs is exciting. The structure depicted
in figure 12 reveals a general arrangement that is closely related to, for example,
the structure of [Ba3 N]Na. Clusters of composition [Cs12 AO4 ] shown in figure 12a
−
5−
are condensed into chains according to [Cs8/2 Cs4 AO4 ] = [Cs+
8 (AO4 ) ] e3 with
M –M bonding within the chains, between them and to additional Cs atoms that
have no bonding contacts to the anions. The phases Cs9 AO4 extend the spectrum
of metal-rich compounds from those containing monoatomic entities such as H,
O, N, etc. in metal clusters via diatomic species such as C2 to complex groups
such as AO4 . These phases could represent the tip of an iceberg with many more
examples to follow.
4. Conclusions and final remarks
This short review covers a part of cluster chemistry and its development over a
long period, from its origin to most recent work. Mainly structural aspects have
been addressed. Details of the structures give insight into a significant variability
with many more results to come. Beyond the details, general features have been
briefly touched, which ascribe the structures to regular dispersions of sub-nanosized pieces of a metal in a salt-like matrix and, vice versa, pieces of a salt in a
metallic matrix. Such general features relate to interesting physical properties as
exemplified for the alkali metal suboxides, which show a very low work function
owing to the electronic confinement and concomitant quantum size effects.
It is interesting to note that the chosen examples Nb6 F15 and Cs11 O3
exhibit similarity with much investigated electronic devices, the so-called
quantum dot and anti-quantum dot arrays, respectively. The first consist
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of arrays of discrete nanoscopic pieces of highly conducting semiconductor
material, the latter of conducting layers with an ordered pattern of holes.
Confinement of the electrons to the dots or to the region between the
holes leads to a bunch of fascinating properties. To use these terms,
Cs11 O3 represents a self-organized three-dimensional anti-quantum dot array,
as Nb6 F15 may be viewed as the corresponding quantum dot array. The
main difference lies in the scale. The arrays in the compounds are nearly
two orders of magnitude smaller than those constructed from semiconductors.
Perhaps, they will become a target of interest when miniaturization enters
this limit.
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++
−−
++
++
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