Theoretical Structures Design And Properties Study Of Several Metal Boride Nanometer Materials

Boron,is the first element that has only one 2p electron in periodic table,located at a special position between metal and non-metal.The exceptional structures and abnormal chemical bindings of both in boron's elemental form and its many chemical compounds,are caused by boron's electron deficiency,small covalent radius,sp² hybrid orbital and three-center bonding,making boron and boride a hot topic in nanometer materials research.The key to study physical and chemical properties of a nanometer material is structure.However,there exist difficulties and challenges in determining the material structure experimentally in current time.The theoretical simulation method base on high-performance computer avoids the difficulty in the synthesis of pure sample and the restriction in characterization,which is an important means to search the structures and properties of nanometer materials economically and effectively.In this dissertation,we systematically investigate the structures and properties of metal boride nanometer materials by means of the swarm-intelligence CALYPSO?Crystal structure AnaLYsis by Particle Swarm Optimization?structure search method and first-principles calculations.The major works are as follows:?1?2D Ti B₄ monolayer was designed with novel planar hypercoordinate structure.It is a long-standing hot topic in searching for the circumstance which can form planar hypercoordinate atoms.Unlike most of previous works focusing on isolated system such as clusters and molecules,we explore some more challenging materials,which is 2D periodic sheet.The 2D boride with transition metal?TM?is selected to modify the structures and properties due to the abundant chemical bonding pattern of B,under situation of fixing the transition metal boride stoichiometry of TMB₄,a new2D material,namely the TiB₄ monolayer is predicted by using CALYPSO structure search method and exchanging all transition metal except lanthanides and actinides.The TiB₄ monolayer consists of edge-sharing Ti?B₈ wheels with planar octacoordinate Ti atoms located at the center of 8-membered B rings,and each boron atom bind with3 ambient boron atoms.Cohesive energy?6.42 eV per?,phonon dispersion calculations and Ab initio molecular dynamics simulations?2500K?demonstrate the superior thermodynamic and dynamic stabilities of the predicted TiB₄ monolayer.Accorfing to the electron localization function?ELF?calculation,there exists a strong covalent binding interaction in B-B bond,featuring sp² hybridization similar to that in graphene.It is hard for B atom to form stable sp² hybridization which is similar to that of carbon due to its defiecient nature.However,ELF and Bader charge analysis shows that each Ti transfers electron to the B network,which makes it realizable to feature B-B sp²hybridization like graphene,leading to a stable TiB₄ monolayer.Band structure and projected density of states show that the TiB₄ monolayer is metallic,absorption and diffusion of lithium on the TiB₄ monolayer indicate it has a small diffusion energy barrier of 0.18 eV,a high theoretical capacity of 588 mAhg^-1 and a moderate open circuit voltage of 0.23 V,which represent a promising anode material for Li-ion batteries.A series of buckled structures with quasi-planar octacoordinate motifs in VB₄,CrB₄,MoB₄,WB₄ and OsB₄,which highlight the unique role of the Ti atom in stabilizing this planar structure.To the best of our knowledge,this is the first example of a 2D graphene-like material containing completely planar octacoordinate TM atoms,8 is the highest coordinate number in 2D periodic materials so far.?2?Structure and property evolution of chromium-doped small-size boron clusters are systematically explored.Various pattern and marvelous chemical bonding make boron cluster a well system for cluster research.With the changing of boron atom number,the planar or quasi-planar,tube,cage-like,bilayer and core-shell structures et al.are energy dominated.Theoretical study indicate that it is a wonderful avenue to modify the cluster pattern and improve its stability by encapsulating atom or molecule.On the basis of our previous work,we try to stabilize small-size boron fullerene with doping atom.We systematically investigated the structural and electronic properties of CrB_n clusters with n=8,10,12,14,18,20 and 22 through extensive swarm-intelligent CALYPSO structure searches and first-principles calculations.It is found that Cr doping has significantly modified the structure evolution of B cluster.Intriguing transition from half-sandwich to drum-like and then to endohedral cage-like structures has been revealed with the increasing number of B atoms increasing.CrB₈,CrB₁₀ and CrB₁₂ cluster exhibit half-sandwich structure with quasi-planar B moieties similar to the bare B cluster,indicating that small-sized B clusters are promising inorganic ligands.Drum-like structure is formed at CrB₁₆ clusters,while endohedral cage structures emerge at larger Cr B₂₀ and CrB₂₂2 clusters.The endohedral CrB₂₀0 cage has a high symmetry of D_2d and the largest HOMO-LUMO gap among CrB_n in the current work,indicating its high chemical stability,which is attributed to the geometric fit between the size of the Cr atom and the void of the B cage as well as the formation of the 18-electron configuration.The current results proclaim that it is an effective method to design novel boride nanometer materials by doping metal atoms.