The Structures And Properties Of Several Two-dimensional Lanthanum Boride:First-principles Study

Boron has been one of the research hotspots in condensed matter physics for a long time because of its special bonding properties,which makes it possess a variety of nanostructures such as two-dimensional nanosheets,one-dimensional nanotubes and zero-dimensional nanometer clusters.In particular,due to its excellent properties such as superconductivity and high hardness,metal boride two-dimensional nanomaterials have potential applications in the fields of catalysts,electronic devices and energy storage.Further development of these novel nanomaterials will promote the innovation of materials science and technology.In this paper,first-principles calculation based on density functional theory and particle swarm optimization algorithm are used to present several stable two-dimensional structures of lanthanum doped boron,and their stability,electronic structure,mechanical properties,hydrogen storage properties and energy storage properties as anode materials for Li-ion batteries are studied.Specific research results are as follows:（1）Several kinds of stable two-dimensional LaB structures were obtained.The kinetic and thermodynamic stability of the system were analyzed by phonon spectrum calculation and Ab initio molecular dynamics simulation.The bonding characteristics,detailed electronic structure information and mechanical properties of the system were further given.For 2D LaB₄ monolayer,the structure can be regarded as a standing wave nested with two columns with phase difference ofπ/2,wherein,La atom is located at the position of the wave node,and B atom is distributed on the envelope of the standing wave.The LaB₄monolayers have high dynamic and thermodynamic stability,and the phonon spectrum does not exist virtual frequency,and the original topological structure can not collapse at the effective temperature of～1000 K.Atom B has obvious characteristics of polycentric bonds,where B atoms form a stable four-membered ring structure.The analysis of electronic properties shows that the LaB₄ monolayer is metallic and has good conductive properties.The Young’s modulus of the two-dimensional system along the x direction and y direction areY_X^2D=37.23 GPanm,Y_Y^2D=30.53 GPanm respectively,The effect of uniaxial strain on the energy band structure of the system is analyzed.The results show that the energy band structure of the system appears Dirac point when a certain stretching or compression ratio is reached.For a 2D LaB₂ monolayer,its structure can be seen as an elongated hexagonal arrangement composed of three La atoms and four B atoms.The structure also has good kinetic and thermodynamic stability,and can maintain the original topological structure at～1000 K effective temperature.Differential charge density analysis and Mulliken charge layout analysis indicate that B atoms form a stable boron single chain structure through 2c-2e bond.The analysis of energy band structure and state density shows that LaB₂ monolayer also has good conductivity.The calculated Young’s modulus of the system isY_X^2D=136.9 GPanm、Y_Y^2D=82.3 GPanm,which indicates that the structure has anisotropic mechanical properties.For LaB and La₂B₂ two-dimensional structures,the two structures also have good stability.The energy band structure and density of states diagram show that these two structures are metallic conductors.The Young’s modulus in x direction and y direction of LaB monolayer isY_X^2D=37.23 GPanm andY_Y^2D=30.53 GPanm,respectively.The Young’s modulus in x direction and y direction of La₂B₂ monolayer isY_X^2D=25.49 GPanm,Y_Y^2D=78.43 GPanm,respectively.The results show that LaB and La₂B₂ structures have anisotropic mechanical properties,and 2D La₂B₂ structures have negative Poisson’s ratio.（2）The hydrogen storage properties of LaB₄ monolayer and typical LaB nanotubes were studied.The results of hydrogen storage in LaB₄monolith show that La atoms in LaB₄monolith can adsorb up to 8 H₂ molecules with an average adsorption energy of 0.128 e V/H₂,and the mass density of hydrogen storage can reach 8.077 wt%.Further,based on the stable LaB₄monolayer,structure and get a series of stable LaB nanotubes,analyzed their structural stability and electronic properties of（5,0）is studied in LaB hydrogen storage properties of nanotubes,the research results show that each（5,0）LaB nanotube structure units,up to 90 H₂adsorption,adsorption on average to 0.101 e V/H₂,the density of hydrogen storage quality can reach 6.182 wt%.In general,both LaB two-dimensional structure and nanotube structure have good hydrogen storage properties,and can be used as potential hydrogen storage materials.（3）The energy storage characteristics of LaB₄ monolayer as anode material for Li ion batteries were studied.Firstly,the adsorption characteristics of the LaB₄ monollayer on Li atoms were analyzed.We selected 16 possible adsorption sites and calculated the results.The results show that a single Li atom tends to be adsorbed on the top of the"La-B square",and the LaB₄ monollayer has a good adsorption effect on Li atoms with an adsorption energy of0.371 e V/H₂.The analysis of electronic structure shows that the LaB₄ monolayer adsorbed Li atom has the conductive property of metal and is suitable for the anode material of ion battery.The study on the diffusion process shows that the lowest diffusion barrier of Li on the single-layer surface of LaB₄ is 0.890 e V,indicating that Li is prone to diffusion on the surface of LaB₄,which ensures that the anode material of lithium ion battery can have a fast charge and discharge rate.Further theoretical calculations show that up to 24 Li atoms can be adsorbed on both sides of the LaB₄ monolayers,and the corresponding maximum theoretical specific capacity is 294.355 m Ah/g.Therefore,LaB₄ monolayer is expected to be a candidate anode material for Li-ion batteries.