Theoretical Study On The Geometric And Electronic Structures Of Al-Na Clusters

We studied the geometries,stabilities and electronic properties of AlNa clusters by theoretical calculation.The global low-energy structures of Al_nNa_m?n=5,6;n+m?10?clusters are searched by using genetic algorithm combined with MOPAC program package,the low energy structures are further investigated by density functional theory?DFT?method and MP perturbation theory.The stabilities and the electronic structures have been analyzed in detail.Except for the reported structures,the new low energy isomers were also obtained.The results indicate that there exist a few isomers with very close energies for Al_nNa_m clusters and the potential energy surface is quit flat.The Na atoms are located at the outside of the clusters.In the Al-Na clusters,the Al-Na bond length is larger than Al-Al bond length,the Al-Al interaction is stronger than Al-Na interaction,doping more Na atoms will make the cluster less stable.The analysis of the electronic structure include AIM charge,the valence molecular orbital,the density of states?DOS?and the electron localization function?ELF?.AIM charge indicates that sodium transfer electrons to aluminum,the charge on Na atoms is 0.70-0.80,the charge on Al atoms is decided by the number of connected Na atoms and their location in the cluster.The valence molecular orbitals of Al₅Na₅ and Al₆Na₂ clusters with 20 valence electrons are 1S²1P⁶1D¹⁰2S²,the orbital types agree well with those predicted by the jellium model.Analysis on the density of states indicates Na atoms transfer their 3s electrons to Al atoms and exist as+1 cations.It is observed that Al₅Na₅ and Al₆Na₂ exhibit especially large gaps,the energy gaps in other clusters are in the range 1.69¹.85 eV.The electron localization function indicates the valence electrons reside at the outside of the Al³⁺cores,and the Na⁺cores are exposed at the outside of the valence electrons.All the results confirm that Al_n moieties in the Al-Na binary clusters form Zintl anions and the Na⁺cations are ionically bonded to them.It suggests Al-Na clusters can be used as building blocks to form cluster-assembled Zintl-phase materials.