Geometry, electronic properties, and 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 (${\mathrm{Li}}_{10}$, ${\mathrm{Li}}_{12}$) and Al-doped Li clusters (${\mathrm{Li}}_{10}\mathrm{Al}$, ${\mathrm{Li}}_{10}{\mathrm{Al}}_2$). We find that the addition of two Al impurities in ${\mathrm{Li}}_{10}$ results in a substantial structural change, while the addition of one Al impurity causes a rearrangement of atoms. Introduction of Al impurities in ${\mathrm{Li}}_{10}$ 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 ${\mathrm{Li}}_{10}{\mathrm{Al}}_2$, Al atoms also form a weak covalent bond, resulting in their dimerlike behavior. This causes Al atoms not to “melt” until $800\mathrm{K}$, in contrast to the Li atoms which show a complete diffusive behavior above $400\mathrm{K}$. Thus, although one Al impurity in ${\mathrm{Li}}_{10}$ cluster does not change its melting characteristics significantly, two impurities results in “surface melting” of Li atoms whose motions are confined around an Al dimer.