Theoretical Study On Mg/Na Cage Structures With Endohedral Icosahedron B₁₂

The discovery of carbon fullerene cages and their solids has opened up a new way to construct new materials.Stable cage clusters can be used as"artificial atoms"or"super atoms"to imitate the process of atomic cluster construction.However,cage clusters formed by other elements are usually unstable.Ab initio calculations in 2001show that highly stable Zr@Si₁₆ fullerenes and Frank-Kasper polyhedral Ti@Si₁₆clusters with large HOMO-LUMO energy gaps can be obtained by embedding Zr and Ti atoms.In 2002,Zr@Ge₁₆ was proved to be able to form Frank-Kasper polyhedron,indicating that new clusters can be designed by adjusting embedded atoms and cage atoms.These results were subsequently confirmed by experiments.In the past twenty years,a lot of experiments and theoretical studies have been carried out on different clusters,and many very stable and abundant cage clusters have been found through embedded doping.Boron is adjacent to carbon in the periodic table of elements,which shows unusual physical properties because of its electron deficiency and highly delocalized covalent bonds.Many borides contain hollow icosahedral structure of B₁₂,but the lowest energy structure of B₁₂ clusters is planar.In the structural search of boron clusters,the thermodynamically stable icosahedral-B₁₂ filled core-shell structure was found,and the study showed that the filling of icosahedral-B₁₂ effectively improved the structural stability of core-shell boron clusters.In this paper,a series of different sizes of core-shell structures with icosahedral-B₁₂ as embedded clusters and Mg/Na atoms as cage atoms are constructed,in order to explore the metal cage structure that can just wrap icosahedral-B₁₂.The main conclusions are as follows:（1）Based on the calculation of density-functional theory,it is found that the cage structure composed of 22 Mg atoms just wraps an I_h-B₁₂ icosahedron,thus obtaining a B₁₂@Mg₂₂ core-shell structure with T₂₃ symmetry.Through molecular dynamics simulation and vibration frequency analysis,it is found that B₁₂@Mg₂₂ has good thermodynamic stability and kinetic stability.The analysis of electron density and bonding shows that the charge is distributed throughout the structure,and 40 multi-center two-electron bonds play an important role in the stability of the structure.By studying the hydrogen storage of B₁₂@Mg₂₂,it is found that the structure can adsorb about 72 hydrogen molecules,and the calculated the gravimetric hydrogen density is17.82 wt%.The reduced density gradient and noncovalent interactions analysis show that there is a weak interaction between B₁₂@Mg₂₂ structure and hydrogen molecules.（2）A core-shell structure B₁₂@Na₁₆ with T₂₃ symmetry and I_h-B₁₂ icosahedron as the core is proposed,in which Na₁₆ cage can just wrap the I_h-B₁₂ icosahedron.In the molecular dynamics simulation of NVT ensemble at 800 K,it is found that the structure has not been seriously deformed,which shows that the structure has good thermodynamic stability.Vibration frequency analysis shows that there is no imaginary frequency,which proves that the structure has good kinetic stability.The electron density analysis shows that the charge is distributed throughout the structure.The results of bonding analysis show that there are 26 multi-center two-electron bonds in the structure,which plays a very key role in the stability of the structure.In the application of hydrogen storage,B₁₂@Na₁₆ can adsorb about 71 hydrogen molecules,and the gravimetric hydrogen density is 22.20 wt%.In addition,the reduced density gradient and noncovalent interactions analysis prove that there is a weak interaction between B₁₂@Na₁₆ structure and hydrogen molecules.