Theoretical Study On The Al₆Na₂ Cluster And Its Assembled Structures

As a new dimension of material structures,the cluster has attracted extensive attention.Due to the particularity of cluster size,researches on clusters are considered as a bridge to understand the transformation from atoms and molecules to bulk materials.The research fields of clusters is gradually extended from traditional physics and chemistry to materials science and biomedical technology.How to use the novel properties of clusters is one of the hot topics of current concerns.Using bottom-up assembling,the basic characteristics of the clusters can be preserved in the assembled materials,and the properties of the assembled materials can be tuned by the cluster compositions.Therefore,the study of cluster assembled materials has a practical significance for the use of clusters.In this thesis,the density functional theory is used to study the aluminum-sodium binary cluster Al₆Na₂.Al₆Na₂ is a magic number cluster with 20 valence electrons forming a closed shell structure.The large HOMO-LUMO gap indicates that it is an ideal cluster assembly unit.In this cluster,Al₆^2-forms an octahedron core,and the two Na⁺cations are ionically bound on the peripheries,Based on this feature,we design two Zintl phase crystals with space group Pn-3m and Fd-3m.The geometry and electronic structure of the Al₆Na₂ clusters are well preserved in the assembled crystals.Under the PAW-PBE method,the interaction energies of Al₆Na₂ units in Pn-3m and Fd-3m assembly structures are 1.68 eV and 1.62 eV,respectively.The calculation of the elastic constant and the phonon spectrum indicate that the two assembled structures are stable.The band gaps of the Pn-3m and Fd-3m structures are 0.89 eV and 1.40 eV,respectively.The results show that the two assembly structures are semiconductors composed of simple metals Al and Na.In addition,the two crystals are ultralight materials,the densities are 0.45 g/cm³ and 0.92 g/cm³ for the Pn-3m and Fd-3m structures respectively.