Lithium clusters

As mentioned above, the tendency of the elements to form covalent bonds reducing their connectivity increases with increasing electronegativity of the element. In the case of the alkali elements, the formation of molecular clusters-in which the metal displays a relatively low coordination number-should be possible only for the lighter group elements. A number of lithium clusters are indeed known. Selected examples of them will be discussed below. 

Lithium as the first element in the Periodic Table is expected to exhibit special properties. It is the most electronegative of the alkali elements but simultaneously the most electropositive of the Period. That is apparent in lithium chemistry, since there is, for instance, a widely used organometallic chemistry and a rich structural chemistry which is not common for the other group elements. On the other hand, high polar species and lithium bonds with great ionic character are almost a constant in lithium chemistry. The chemical 

Ph4Li4 4Et20 Li4C4 skeleton. Phenyl moiety occupying triangular faces of Li4. 7 

Ph2Li2 2TMEDA Li2 with bridging phenyl groups and one chelating diamine at each Li atom. 8 

In Table 4.2, some selected examples of lithium aggregates with structures determined by crystallographic studies are described. 

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A lithium cluster in the micropores of the carbon sample has a very similar environment as lithium atoms in metallic lithium. Hence, we observe long low-voltage plateaus on both discharge and charge for lithium insertion in the microporous carbon. 

From all the applications we have done in recent years [10-12], we review those that show the essence of our methodology. After introducing the VB formalism, we study the four electrons problem, a cluster of hydrogen, in an unusual limit, in order to address the problem of insulator to metal transition in solid hydrogen under pressure. Then we proceed to the applications to neutral and anionic lithium clusters, which are systems with very delocalized bonds. 

STUDY OF THE ELECTRONIC STRUCTURE OF ANIONIC LITHIUM CLUSTERS... 

We apply the VB methodology to the study of the electronic structure of the small anionic lithium clusters Li" (2[Pg.389]

In our calculations we associate to each Li atom a (10s2p) atomic centered gaussian basis set contracted to [4s,2p] (Table 3). We treat only the valence electrons at the VB level and keep the inner shell electrons (LiIs) in a core obtained by HF calculations. Therefore we are neglecting the core-core and core-valence correlation effects, which are small for these small lithium clusters (Fig.5). 

Fig.5. Geometry, point group and electronic state of some small anionic lithium clusters. Up to Li4, the most stable conformations are linear. The most stable conformation of Li5 is planar.

We see, in Fig.8a, that the structures Ei and E2 (ionic structures) for the D ,h Li3- cluster are the most important. This is the first anionic lithium cluster that... 

It is interesting to notice that as the systems become bigger and more compact, a larger number of VB structures becomes necessary to describe them. It means that the metallic character of these systems is increasing (that is why we usually say that small lithium clusters could be prototypes of metals). At the same time, Pauling s structures are showing their importance. From the C2v Li4 ... 

The influence of the extra electron on the shape of the small anionic lithium clusters... 

Fig. 16. Maps of electronic charge density of linear anionic lithium clusters.

Energy (a.u.) of ground and first excited states of the anionic lithium clusters. 

We will investigate the stability of the anionic lithium clusters in Section 5. A relevant quantity is the dependence of the binding energy per atom on the number of atoms, as well as the electron affinity of neutral Lin clusters. From the latter, we evaluate whether these neutral clusters are able to receive an extra electron and to form an anionic system. 

Before starting an investigation of the stability of small anionic lithium clusters, it is necessary to study the conformations of lowest energy of the neutral Li2, Li3,... 

Several works about neutral and positively charged lithium clusters can be found in the literature (see, for instance, Refs. 10, 12, 27-29), while there are not so many references for the anionic clusters. The main reason is that, for the anionic lithium clusters, experimental results are unusual and if they exist, they are related to the physical observables indirectly. 

Fig. 18. Equilibrium geometries of some small neutral lithium clusters. Their lowest energy states and symmetry groups are also shown.

STUDY OF THE STABILITY OF LITHIUM CLUSTERS 5.1. Binding energy per atom... 

Binding energy per atom EJn (kcal/mol) of the small neutral lithium clusters. 

For the lithium clusters, we have seen that the Pauling s structures can have the highest weights in the VB wave function, e.g. the C2v Li4" and C2v Li5 clusters. The structures are certainly very important when three-body bonds are present. According to our results, a VB description of the anionic or neutral lithium clusters without the Pauling s structures would be incomplete or would result in a less compact wave function. 

Lithium clusters have been a popular model for the calculation of metal properties because of their low atomic number. Lasarov and Markov (49) used a Hiickel procedure to determine the properties of a 48-atom Li crystal. They found a transition to metal properties with the binding energy per atom approaching 1.8 eV at 30 atoms. The ionization potential approached the bulk value since some electrons occupy antibonding molecular orbitals, as observed for Ag clusters. The calculated properties of the largest cluster were not those of a bulk metal. 

Regarding the reversible capacity of disordered carbons, we found that lithium is mainly in a quasi-metallic state, another minor part being in a state comparable to intercalation. If we consider that the lithium density in the intercalated domains is comparable to that of graphite, we can conclude that the enhanced reversible capacity of the disordered carbons is related with the quasi-metallic state of lithium. In this paper, we postulate that the electron density of the quasi-metallic lithium clusters is related with the size of the neighbour carbon layers. In future work, we plan to investigate different materials in order to determine if there is any relationship between the reversible capacity and the fringe length. 

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