Structure?Stability And Magnetism Of V₂B_n,Fe₂B_n And Co₂B_n Clusters

Clusters, as a transition state between molecules and macroscopic solid materials, have attracted considerable attention. Its unique spatial structures and physicochemical properties change along with the clusters size, and present a certain change rule. Earlier scientists have begun to study the pure boron clusters, then appeared the theoretical study of doping boron clusters. Over the last decades, both the experimental and theoretical researchers have always been keen to study the doped clusters, especially in catalysis, magnetism, organic chemistry and nanoscale research. For transition metal?TM? doped clusters, the structures and properties are obviously changed by the doping atoms. Hence, we guess that the addition of magnetic metal will cause some unexpected changes in nature of boron clusters. People hope that the change law of the structures and properties of the doped clusters can be figured out through theoretical calculation, which can guide the experiment effectively. In this dissertation, density functional theory?DFT? calculations have been used to study the structures, magnetism and electronic properties of transition metal doped Bn clusters. In this process, we use the Gaussian and VASP computing softwares. The main results are as following:Firstly, we used Gaussian software to calculate the V₂B_n clusters. All of the geometries for the V₂B_n?n=1–10? isomers that we considered were initially optimized using the B₃ LYP method under the framework of density functional theory?DFT? using the 6-311+G?d? basis set. The frequency calculations were carried out to confirm that all of the clusters reported in the text are geometrically stable. When n=2-4, the V₂B_n clusters prefer an incomplete bipyramidal structure. For the V₂B_n?n=6-10? clusters, the bipyramidal structures were confirmed as the lowest-energy structures. Based on an analysis of the second-order difference of energies, average binding energies of V atoms, dissociation energies of the B atom, vertical ionization potentials, vertical electron affinities and the chemical hardness, we confirmed that the V2B6 bipyramid was thermodynamically stable, and might be observed in a future experiment. Detailed orbital analysis of the V₂B₆ cluster was performed to infer a rational interpretation of its stability.For Fe₂B_n?n=1–10? clusters, all the calculations within density-functional theory?DFT? were performed by means of ab initio pseudopotential plane-wave method, as implemented in the Vienna Ab initio Simulation Package?VASP?. We chose the Perdew–Burke–Ernzerh?PBE? exchange–correlation functional for the spin-polarized generalized gradient approximation?GGA?. Different from the stereo structures of the previous study for V₂B_n cluster, Fe2B5 favor plane structure due to the small radius of Fe atom. The structures for Fe₂B_n clusters?n?6? are the ones that prefer to be bipyramid structure and the two Fe atoms remain on the apex. The stability analysis is performed by calculating the average binding energies, average dissociation energies of Fe atoms and second-order differences of energies, which shows that clusters with n=4 and 7 have higher stability than their neighboring clusters. Through the study of vertical ionization potentials, vertical electron affinities and the chemical hardness, we have further understanding of the electronic properties of Fe₂B_n?n=1–10? clusters. In addition, the analysis of DOS, spin density and the HOMO-LUMO gaps for Fe₂B_n?n=1–10? clusters are carried out. The results show that the d shell electrons of Fe atoms play a major responsibility for the determination of the magnetism for Fe₂B_n?n=1–10? clusters and there is sp-d hybridization in those clusters. Furthermore, obvious charge transfers can be found from Fe to the B site through the bader charge analysis.For Co₂B_n?n=1–10? clusters in this research, we adopted the same method and software with Fe₂B_n. The most stable configuration of Co₂B₅ is a planar structure. When n?6, the Co₂B_n clusters prefer to be bipyramid structure and the two Co atoms remain on the apex. By calculating the average binding energies, dissociation energies and the second-order differences of energies, Co₂B₇ cluster was inferred as the magic number cluster for Co₂B_n?n=1–10? clusters. Similarly, the electronic properties of Co₂B_n?n=1–10? clusters are discussed, from the vertical ionization potentials, vertical electron affinity and chemical hardness of these three areas. Through the analysis of DOS, spin density and the HOMO-LUMO gaps for Co₂B_n?n=1–10? clusters, we can see that most magnetism of Co₂B_n?n=1–10? clusters is from the d shell electrons of Co atoms. There is sp-d hybridization in those clusters. The analysis of Bader charge shows that B atoms act as an electron donor. This is related to their close electronegative properties.