Systematic calculations on planar clusters and monolayers of lithium are performed to study geometries and stabilities of the clusters as well as their convergence behavior with increasing cluster size. The calculations are based on ab initio methods using density-functional theory within the localspin-density approximation for exchange and correlation. The optimized nearest-neighbor distances d» of the Li"clusters, n =1, . . . , 25, of both hexagonal and square geometry increase with cluster size, converging quite rapidly towards the monolayer results. Further, the cluster cohesive energies E, increase with cluster size and converge towards the respective monolayer values that form upper bounds.Clusters of hexagonal geometry are found to be more stable than square clusters of comparable size, consistent with the monolayer results. The size dependence of the cluster cohesive energies can be described approximately by a coordination model based on the concept of pairwise additive nearest-neighbor binding. This indicates that the average binding in the Li"clusters and their relative stabilities can be explained by simple geometric effects which derive from the nearest-neighbor coordination.
LEED experiments show that Li adsorbed at Cu(100) surfaces at room temperature induces a (2×1) missing row substrate reconstruction while adsorption at lower temperatures, T =180 K, results in an unreconstructed Cu(100)+c(2×2)-Li overlayer structure. Substrate reconstruction has not been observed for Na nor for K adsorption. In order to study the specific reconstruction behavior of the Li adsorbate ab initio DFT calculations have been performed on Cu(100)+Ad, Ad = Li, Na, K systems at coverages Θ Ad = 0.25 − 0.5 with and without reconstruction. The calculations show that the (2×1) MR reconstructed surface lies energetically above the ideal (1×1) surface by 0.2 eV per unit cell. However, alkali binding is stronger in the MR geometry as compared to that of the ideal surface where the increase in bond strength becomes smaller in going from Li to Na to K. As a result, the MR reconstructed and the overlayer adsorbate systems are energetically very close for Cu(100)+Li while for Na and K the overlayer geometry is always favored.
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