Theoretical Study Of The Magnetic Properties And Hydrogenation Of Bimetallic Clusters

Along with the rapid development of computational methods and computer technology, computational physics has become more and more important in modern physics. Due to its moderate computational consume and high precision, density functional theory based first-principles calculation is among the most popular and versatile methods available in condensed-matter physics, quantum chemistry and material science. In this thesis, we investigate the geometric structures, electronic properties and magnetic properties of ScnAl and TMAln clusters, and the electronic properties of hydrogenated Al12 and MAl12 clusters by using density functional theory. The main contents are presented as the followings:In the first chapter, we give a brief introduction to clusters. With a size beween those of atoms and macroscopical systems, clusters have many unique properties, and attract many experimental and theoretical research attentions. Firstly, we introduce the basic properties of clusters. Secondly, some common methods in experimental and theoretical studies on clusters are discussed. Following, we highlight some of the characteristics of metal clusters. On this basis, we summarize and look forward to the research on clusters. Last but not least, we simply describe the purpose and results of our work on clusters.In the second chapter, we introduce chiefly the basic concept and progress of density functional theory. First, we briefly reviewed the development of quantum chemistry. Then, we introduce breiefly the development process of density functional theory, and two exchange-correlation functioals. Finally, we pay more attention to the simulation package Dmol3 used in the work.In the third chapter, we investigate the growth behaviors, stabilities, electronic and magnetic properties of the ScnAl, (n = 1–14) by density functional theory with the generalized gradient approximation. All the calculated results are summarized as follows. (1) In the ground-state structures of ScnAl clusters, the aluminum atom remains on the surface of clusters for n < 9, but is trapped within the cage of Sc host atoms for n≥9. (2) The stability analysis is carried out by calculating the average binding energy and the second-order energy differences. The obtained results show that the doping of the Al atom improves the stability of scandium clusters, and ScnAl clusters at n = 3, 6, 10 and 12 possess relatively higher stability. (3) The HOMO–LUMO gaps of ScnAl clusters are discussed. It is found that the HOMO–LUMO gaps of ScnAl clusters containing more than six atoms are very small, and all the HOMO and LUMO states are nondegenerate and delocalized. (4) This magnetism study reveals that the Al atom is seen to induce significant changes in the magnetic property of the host cluster. The doping of the Al atom reduces the magnetic moment of the host clusters except for Sc9 and Sc15 clusters. Especially, the total magnetic moment of the Sc12Al cluster is only 5μb (but 15μb for Sc13), and the magnetic moment of the ScnAl cluster is quenched for n = 5, 7, 9 and 11. NPA shows that the 3d electrons play a dominant role for the magnetism of the system.In the fourth chapter, we investigate the growth behavior, stability, and electronic and magnetic properties of TMAln (TM=Cr,Mn,Fe,Co,Ni, n= 1–7,12) clusters using density functional theory based on generalized gradient approximation. All the calculated results are summarized as follows. (1) TMAln clusters have similar geometries as that of Aln+1 clusters, where the TM atom can be thought of as a substitutional impurity in the Aln+1 clusters with a slight distortion on the whole. For TMAl12 cage-like clusters, substitutional atoms Cr, Mn, and Fe prefer to reside at the cluster surface, while Co and Ni dopant atoms prefer to reside at the cluster center. (2) The stability analysis in relation to the calculation of the average binding energy and second-order energy differences shows that TMAl3 clusters possess relatively higher stability. Besides, the analysis of the HOMO–LUMO gaps of TMAln clusters indicates that TMAl12 clusters have very small HOMO–LUMO gaps, and all the HOMO and LUMO states are non-degenerate and delocalized. (3) The result of NPA shows the charge transfer mainly happens between TM 4s, 3d, and 4p and Al 3s and 3p states. So there exits sd-p hybridization in TM atoms and s-p hybridization in Al atoms, and there also exists strong hybridization between TM 4s, 3d, and 4p and Al 3s and 3p states. (4) This magnetism study reveals that their magnetic behavior of TMAl and TMAl2 clusters, which are the planar structures, can be interpreted in light of the valence-bond theory. Their magnetic behavior of MAl3 (C3v) and MAl4 (C4v) clusters, which are the three-dimensional structures, can be interpreted in light of the spherical jellium model. For TMAln (n > 4) clusters, their geometrical shape basically deviates from spherical structure, and the energy level should also change significantly. At this time, hybridization between the atomic orbitals of the guest atom TM and host atom Al play a decisive role on their magnetic moment.In the fifth chapter, we investigate structural, energetic, and electronic properties of hydrogenated Al12 and MAl12 (M=Al,Li,Na,K) clusters using density functional theory based on generalized gradient approximation. All the calculated results are summarized as follows. (1) The most stable Al12H12 and MAl12H12 (M=Al,Li,Na,K) clusters possess icosaheral symmetry, with each hydrogen atom bonded to an Al atom in atop position. (2)We compare the ground state structure properties of bare clusters with hydrogenated clusters, and find Al12H12 expands upon hydrogen adsorption on its surface in contrast to the remaining cluster. (3) The stability analysis in relation to the calculation of the average binding energy, vertical ionization potential and HOMO-LUMO gap, shows that hydrogenated clusters enhances the stability of the aluminum clusters, and LiAl12H12 cluster has a relatively high hydrogen storage capacity. (4) The analysis of the charge density of the HOMO show most of the charge of the additional electron is localized at the on-top sites on the surfaces of the bare clusters, thus suggesting their preference toward hydrogen absorption. The analysis of the deformation electron density, show the higher electron density on the H atoms indicates partial electron transfer from the Al atoms towards hydrogen upon adsorption.