Investigations On Novel Metal Boride Clusters And Nanostructures

Boron-based nanostructures have high melting points,high hardness,low density,and high chemical stability,with wide potential applications in superconducting,thermal conductivity,cathode field emissions,light electrode,energy storage,and other fields.The microstructure of the substance determines its macroscopic properties.Clusters,which lie between atoms,molecules,and macroscopic solid matter,serve as the building blocks of condensed matters.Our group have performed a systematic investigation on bare boron nanoclusters and metal-doped boron nanomaterials in recent years.This thesis mainly focuses on theoretic investigations on Ta-doped metallo-borospherenes Ta₃&B₁₂^-and Ta₄B₁₈.We also experimentally synthesized and characterized La-lean lanthanum boride nanomaterials.The main contents of this thesis are as follows:1.From inverse sandwich Ta₂&B₇⁺and Ta₂&B₈ to spherical trihedral Ta₃&B^-₁₂:prediction of the smallest metallo-borosphereneTransition-metal-doped boron nanoclusters exhibit interesting structures and bonding.Inspired by the experimentally discovered inverse sandwich D_6h Ta₂&B₆ and spherical trihedral D_3h La₃&B₁₈ and based on extensive first-principles theory calculations,we predict herein the structural transition from the perfect di-metal-doped inverse sandwich D_7hTa₂&B₇⁺(1)and D_8h Ta₂&B₈(2)to tri-Ta-doped spherical trihedral D_3hTa₃&B^–₁₂(3).As the smallest metallo-borospherene reported to date,Ta₃&B^–₁₂(3)contains three octa-coordinate Ta atoms as integral parts of the cage surface coordinated in three equivalent?⁸-B₈ rings which share two eclipsed equilateral B₃ triangles on the top and bottom interconnected by three B₂ units on the waist.Detailed orbital and bonding analyses indicate that both Ta₂&B₇⁺(1)and Ta₂&B₈(2)possess?+?dual aromaticity,while Ta₃&B^-₁₂(3)is?+?+?triply aromatic in nature.The IR,Raman,and UV-vis or photoelectron spectra of the concerned species are computationally simulated to facilitate their future spectroscopic characterizations.2.Perfect spherical tetrahedral metallo-borospherene Ta₄B₁₈ as a superatom following the 18-electron ruleCage-like metallo-borospherenes exhibit unique structures and bonding.Inspired by the newly reported smallest spherical trihedral metallo-borospherene D_3h Ta₃B^–₁₂(1)which contains two equivalent B₃triangles interconnected by three B₂ units on the cage surface,we present herein a first-principles theory prediction of the perfect spherical tetrahedral metallo-borospherene T_d Ta₄B₁₈(2)which possesses four equivalent B₃ triangles interconnected by six B atoms,with four equivalent nonacoordinate Ta centers in four?⁹-B₉ rings as integrated parts of the cage surface.As the well-defined global minimum of the neutral,Ta₄B₁₈(2)possesses 4 10c–2e B₉(?)–Ta(d_?)and 8 10c–2e B₉(?)–Ta(d_?)coordination bonds evenly distributed over four Ta-centered Ta@B₉ nonagons,with the remaining 18 valence electrons in 9 22c–2e totally delocalized bonds following the 18-electron principle(1S²1P⁶1D¹⁰)of a superatom.Such a bonding pattern renders spherical aromaticity to the tetrahedral complex which can be used as building blocks to form the face-centered cubic crystal Ta₄B₁₅(3).3.Experimental study on periodical lanthanum-lean lanthanum boride systemLanthanum hexaboride is a rare earth metal boride with high strength,high melting point,low electron escape function,excellent electrical conductivity,etc.The existence of lanthanum vacancy defects in La B₆material changes its physical and chemical properties,thereby affects its performance.Although there were experimental investigations on the La-lean lanthanum boride systems 60 years ago,there have been no definite conclusions formed to date on the formation mechanisms,structures,properties of the systems.In this work,periodic La-lean lanthanum boride nanomaterials were synthesized by DC arc discharge method and subsequently characterized using high-resolution electron microscopes,energy spectrum analyses,and powder diffraction patterns.It is found that the thus–synthesized La-lean system exhibits periodic lattice morphologies.The boron framework of the La-lean system appears to be basically the same as that in the stoichiometric La B₆,but large portions of vacant La sites were observed in the system,forming La-lean nanocrystals with the lattice spacing increased by approximately five times in(111)direction.Periodic La-lean lanthanum boride nanostructures can exist stably under ambient conditions.The special vacancy patterns of the La-lean nanomaterials may possess important applications in catalysis.