Theoretical Study On BC Superhard Phases And Polyhedral Cluster Of Au₆B₈N₂₄

Carbon is one of the important elements that make up life on earth.At the same time,it can form a variety of hybrid types,and then form the allotrophic structure of rich and colorful carbon,such as diamond,graphite and fullerene.Diamonds are the hardest substances found in many of the natural beings on Earth that made up of carbon.We refer to materials that are comparable to diamonds as superhard materials.Theoretically,the hardness of superhard materials is greater than or equal to 40 GPa.Use light elements such as boron,carbon,nitrogen,oxygen to form various compounds is a traditional idea for making superhard materials.People focus their attention on synthetic materials,the most representative being diamond and c-BN.Based on the DFT,we predict the superhard structure of 10 carbon-boron three-dimensional crystal compounds,and carry out geometric optimization and correlation analysis in the third chapter.The calculated results show that the average density of the ten structures is about 48%to 83%of the diamond density,the bulk modulus is about 53%to81%of the diamond,and the hardness is 43%to 66%of the diamond.The theoretical hardness of these structures greater than 40 GPa may be a potential superhard material.Analysis of the structural energy band diagram shows that the four structures B₂C₃-I,B₃C₄,B₄C₃,and BC₄ are indirect band gap semiconductors,and the six structures B₃C₂-I,B₄C,B₅C₃,B₃C₅,B₃C₂-II,and B₂C₃ are conductors.The simulated moral X-ray diffraction spectrum provides a theoretical basis for experimental observation and preparation in the future.Since the experimental synthesis of C₆₀,the study of cluster structures with high symmetry has attracted wide attention.It has been found that the stability of cluster molecules is correspondingly improved after doping different elements.Due to the presence of transition metal elements,such clusters present a variety of new physical and chemical properties,in many transitional metal elements.The Au element is widely used in the construction of polyhedral cluster structures because of its unique physical and chemical properties.In the fourth chapter of this thesis,the cluster structure Au₆B₈N₂₄4 was optimized to calculate and analyze its stability and electronic structure.The optimized Au₆B₈N₂₄4 cluster structure can be regarded as a five-membered ring composed of one Au atom,three N atoms and one B atom,and a total of 24 five-membered rings are connected.The vibration frequency analysis and molecular dynamics simulation of the structure show that the structure has good kinetics and thermodynamic stability.Analyze the electronic properties of the structure,the Au and N atoms lose electrons in the structure,the B atoms get electrons,the Au atom has obvious d orbital characteristics,and the Au atom and the N atom form a?bond.A?bond and a?bond are formed between the N atom and the N atom,and a?bond is formed between the B atom and the N atom.