Geometric, electronic properties and the thermodynamics of pure and Al--doped Li clusters

The first–principles density functional molecular dynamics simulations have been carried out to investigate the geometric, the electronic, and the finite temperature properties of pure Li clusters (Li${}_{10}$, Li${}_{12}$) and Al–doped Li clusters (Li${}_{10}$Al, Li${}_{10}$Al${}_2$). We find that addition of two Al impurities in Li${}_{10}$ results in a substantial structural change, while the addition of one Al impurity causes a rearrangement of atoms. Introduction of Al–impurities in Li${}_{10}$ establishes a polar bond between Li and nearby Al atom(s), leading to a multicentered bonding, which weakens the Li–Li metallic bonds in the system. These weakened Li–Li bonds lead to a premelting feature to occur at lower temperatures in Al–doped clusters. In Li${}_{10}$Al${}_2$, Al atoms also form a weak covalent bond, resulting into their dimer like behavior. This causes Al atoms not to melt' till 800 K, in contrast to the Li atoms which show a complete diffusive behavior above 400 K. Thus, although one Al impurity in Li$_{10}$ cluster does not change its melting characteristics significantly, two impurities results in surface melting’ of Li atoms whose motions are confined around Al dimer.