Theoretical Study On The Magic Number Clusters Al₆CM₄?M=Li,Na,K?and Its Assembled Crystal

In the present work,evolutionary algorithm combined with density functional theory PAW-PBE is used to search the low-energy isomers of Al₆CM_n?M=Li,Na,K;n=2,4,6?clusters.The low-energy isomers are reoptimized by B3LYP and QCISD methods,and their stabilities and electronic structures are analyzed.The results show that C-doping enhances considerably the stabilities of the clusters.The shapes of the molecular orbitals of Al₆CM_n are consistent with those predicted by the Jellium model.However,the strong attraction of the C⁴⁺core to the valence electrons depresses the local potential,which makes the 2S and 2P orbitals are occupied prior to the 1D and1F respectively.The Al₆CM₄?M=Li,Na,K?clusters have tetrahedral structures with C-centered octahedral Al₆C core,and their 26 valence electrons form closed1S²1P⁶2S²1D¹⁰2P⁶ shells.In Al₆CM_n clusters,the Al₆C cores having nearly all of the valence electrons form Al₆C^n-Zintl anions,and the alkali metal cations are bonded by ionic bonding.Although the ionization energy of Li is largest among the alkali atoms,the small size of Li atom makes it interact strongest with the anion core.Therefore,the Li atoms have largest positive charges in Al₆CM₄.The electronic structure of Al₆C^4-core can be considered as the combination of the octet rule and the 18-electron rule,and its electronic structure also conforms to the Wade-Mingos rule.It means that Al₆C^4-can be regarded as a superatomic anion.The Al₆C^4-octahedral core can also be seen as a geometric magic structure,and its interaction with alkali metals will be conducive to the assembled Zintl phase materials.In this paper,we selected the Al₆CNa₄ cluster as buliding blocks.Based on the geometric and electronic structures of Al₆CNa₄,a cluster assembled crystal is built?space groupP 43n?.The electronic structures of the isolated cluster are well retained and the assembled crystal is a superlight?1.10 g/cm³?Zintl phase material.The interaction energy among the clusters reaches 3.12 eV per unit,and the elastic constant and phonon spectrum confirm the assembled structure is dynamically stable.Analyses on the bonding natures show that C-doping contributes significantly to the stability of the cluster and the Al-C and Al-Al interactions are enhanced in the crystal.The assembled crystal is a wide gap semiconductor and the band gap calculated by the hybrid functional HSE06is 2.26 eV.