Theoretical Study On The Structures And Properties Of （LiF）_n（n=2-18） And Li_n+1Fn（n=2-17） Clusters

Cluster is the stable aggregate composed of several atoms,molecules or ions.How the structures of clusters gradually evolves with size,which sizes of clusters are particularly stable in the evolution process and can be regarded as magic-number clusters,and what unique physical and chemical properties of these magic clusters have are the most concerned issues in cluster research.LiF is a typical ionic crystal,which is widely used in the fields of biology,chemical industry,medicine and energy.Early studies have shown that non-stoichiometric LiF clusters may form super-alkali.Therefore,we systematically studied the evolution behavior of the structures and properties of（LiF）_n（n=2-18）and Li_n+1F_n（n=2-17）clusters,obtained some stable magic clusters,and explored their electronic structures,bonding properties,spectral characteristics and reactivity.It is found that non-stoichiometric Li₁₄F₁₃and Li₁₇F₁₆have extremely low ionization potential and extremely high hyperpolarizability,which can be regarded as super-alkali clusters.The main conclusions are as follows:The stable structures of（LiF）_n（n=2-18）were optimized by using genetic algorithm combined with density functional theory method.Not only the stable isomers（n=2-9）reported before were obtained,but also some new ground-state structures（n≥10）were found.The structural evolution behavior of（LiF）_n（n=2-18）clusters is:small-sized（LiF）_n（n=2-3）clusters are all planar structures,and from n=4,the three-dimensional structures（3D）are more stable.For n=4-12,the ground-state structures of the clusters are dominated by cage structures,while（LiF）₉and（LiF）₁₅form tubular structures,（LiF）₁₆and（LiF）₁₈are rock-salt structures.（LiF）_n（n=2,4,6,9,12 and 15）are magic-number clusters and particularly stable.The electronic structure analysis shows that Li atom transfers its 2s electron to the 2p orbital of F atom,forming a strong Li-F ionic bond.Because both Li and F atoms form a closed-shell structure,the（LiF）_ncluster has a smaller affinity potential（0.27 e V-0.56 e V）and a larger ionization potential（10.91e V-12.27 e V）and energy gap（8.23 e V-9.50 e V）.The IR,Raman and UV-Vis spectra of（LiF）_nmagic-number clusters were simulated.The electrostatic potential and double descriptor analysis show that F and Li atoms are the active sites of nucleophilic reaction and electrophilic reaction,respectively.The stable structures of Li_n+1F_n（n=2-17）clusters were optimized at the same computational level as（LiF）_n.The results show that the most stable structures of Li_n+1F_nare configurations with a tail Li atom or F atom vacancy,and the structural evolution rule is:only Li₃F₂is a planar structure,and the most stable structures of the clusters are all 3D structures from n=3.With the increasing of cluster size,the ground-state structures of Li_n+1F_ngradually evolved from cubic structure and tubular structure to cage structure,and the rock-salt structure was formed at n=13.The clusters with sizes of n=3,5,8,11,13 and 15 are particularly stable,which are magic-number clusters of Li_n+1F_n.The electronic structure analysis shows that Li-F in Li_n+1F_nare still ionic bonds.However,due to the weak binding of some Li atoms to valence electrons,Li_n+1F_nhas a smaller ionization potential.The vertical ionization potential and adiabatic ionization potential are 2.75 e V-5.85 e V and 2.68 e V-4.75 e V,respectively.In particular,the adiabatic ionization potential/vertical ionization potential of Li₁₄F₁₃and Li₁₇F₁₆are particularly small（2.68/2.75 e V and 2.77/2.97 e V,respectively）,which are lower than3.89 e V of Cs atom.They are super-alkali clusters and have great hyperpolarizability（3237157.14 a.u.and 716489.10 a.u.,respectively）.We also simulated the IR,Raman and UV-Vis spectra of Li_n+1F_nmagic-number clusters and predicted their reactivity.