Exploring the electronic structure of elemental lithium: From small molecules to nanoclusters, bulk metal, and surfaces

Clusters of lithium atoms ranging in size from Li₄ to Li₄₀ and bulk metallic solids, including surfaces, are investigated through first principles electronic structure calculations, which are based upon density functional theory and the electron localization function (ELF). It is found that large lithium pπ-type contributions in the electronic wavefunction cause the electrons to localize in interstitial regions, which leads to multicenter bonding for both the clusters and the solids, including their surfaces. For the smaller clusters these stabilizing pπ interactions also lead to short Li-Li interatomic distances, which in conjunction with the longer bonds induces "distance alternation" in the range from 2.45 Å to 3.15 Å. This consequence of the additional pπ interactions is absent in simple solids due to symmetry. The electronic structure of the clusters is topologically insensitive to deformations that do not affect their general shape, but changes significantly upon isomerization. The ramifications upon dynamic properties is that the clusters are quasi-rigid at low temperatures and retain their shape though the distance alternation pattern is suppressed. The picture which emerges for bonding in the bulk solid is that the metallic state arises from the presence of a large number of partially occupied multicenter bonds. For nanoscale clusters only the surface of these clusters exhibits strong localization, whereas their interiors display localization properties similar to the bulk metallic solid. On the other hand, localized states similar to those of the clusters ("dangling bonds") are found on the (001) surface of body-centered cubic (bcc) and face-centered cubic (fcc) lithium solids.