Theoretical Study On Structures,Segregation Effect And Magnetic Properties Of Transition Mental Mixed Atomic Clusters

Bimetallic clusters have potentially applications because of their unique optical,magnetic and catalytic properties.The properties of bimetallic clusters closely depend on their structures.The cluster structures closely depend on strength of atomic interactions.Theoretical researches show evident segregation effects in Co-Cu cluster system,which is entirely in contrast with Ag-Pt cluster system.To understand relation between structure of bimetallic clusters and stability of cluster,it is necessary to obtain more isomers.Searching for isomers of bimetallic clusters is important topic.Bimetallic clusters with same geometrical structures useily have more isomers,and the energy difference between isomers are often very small.Therefore,it is necessary to make clusters converse quickly and effectively in molecular dynamics simulations.Moreover,with increasing cluster size,stable alloy clusters often have evident core-shell structural characteristic.Theoretical researches often apply genetic algorithm to search for stable core-shell clusters.However,computational times will increase with increasing cluster size.How to quickly and effectively search stable core-shell clusters is also an important investigative problem.In this paper,we presented new computational methods,two of which are an effective global sampling method for extracting isomers of bimetallic clusters,and a core-shell cluster evolutionary method.Apply our new methods and first-principle calculations approach,we investigate the structures and properties of icosahedral PdXAg13-x(x = 0-13)and CuXAg13-x(x=0-13)clusters,respectively.We also investigated the structures and magnetic properties of new core-shell B@Fe8@Mg10 and B@Mn8@Mg10 clusters.Applying them as structure element,we designed and calculated structures and magnetic properties of Mg2BN-1(Fe4Mg4)N and Mg2BN-i(Mn4Mg4)N(N=2-8).nanochain.The main research contents are as follows:We present a modified Velocity-Verlet algorithm,and by combining it into molecular dynamics simulations,cluster system converges accurately and rapidly.On this basis,we develop an effective global sampling method for extracting isomers of bimetallic clusters.Using this method,we investigated icosahedral PdXAg13-x(x = 0-13)clusters.Further,after first-principle calculations based on density functional theory(DFT)calculations,we find each isomer still remains icosahedral structure because of strong s-d orbital hybridization,and the cluster is more stable when the Pd atom is in the center of the cluster.Whether for the clusters with central Pd atom or for the clusters with central Ag atom,the clusters are more stable if the mixture degree of Pd and Ag atoms is high,which dues to strong Pd-Ag interaction from strong s-d hybridization.There exists a large binding energy difference for Pd-Ag cluster because of strong interaction of Pd-Pd and Pd-Ag when the Pd atom is in the center of the cluster.Through calculating the atomic-projected density of states(PDOS),we reveal that the magnetic moment of Pd,Agi3-x mainly comes from s electrons of Ag when 0<x<5 while d electrons of Pd when 5<x<13.With our modified Velocity-Verlet algorithm and global sampling method for extracting isomers of bimetallic clusters,we also investigated icosahedral CuXAgi3-x(x=0-13)clusters.Most of Cu-Ag clusters remains icosahedron after DFT calculations.Moreover,the icosahedral Cu-Ag cluster is more stable when the Cu atom is in the center of the cluster.Whether the Cu or Ag atom is in the cluster center,the clusters are more stable if Cu and Ag atoms have high segregation extent.There exists a large binding energy difference for Cu-Ag cluster because of strong Cu-Cu interactions.The HOMO-LUMO gap(energy difference between the highest occupied molecular orbital and the lowest unoccupied molecular orbital)of the cluster greatly depends on the composition ratio of Cu and Ag,which is closely related to the average bond length and the bond ratio in the cluster.An optimized icosahedral Cu-Ag cluster,i.e.,Cu7Ag6 in which Cu atoms have the largest segregation degree,is found to be the largest HOMO-LUMO gap.We presented a sequential micro-motion press-expand model,and further develop an effective core-shell cluster evolutionary method.This method accomplishes quick and effective evolutionary of core-shell clusters with over hundres size from arbitrary initial structure.The computational process is very flexible,because the method only has two operators.Many tests of stable clusters reveal that the structure of stable cluster is atoms optimal shell-atomic array according to potential field of core clusters.On this basis,we obtain a new core-shell A@B8@C10 cluster.We investigate structures and magnetic properties of B@Fe8@Mg10 and B@Mn8@Mg10.The most stable state is interface layer(Fe/Mn)has axial paramagnetic moments,in contrast with the magnetic moment orientations inner layer(B)and external layer(Mg)atoms.The total magnetic moments of B@Fe8@Mg10 cluster is 1 7?B,and the local magnetic moments of Fe atoms are 2.1?2.44?B.The total magnetic moments of B@Mn8@Mg10 cluster is 23?B,and the local magnetic moments of Fe atoms are 2.689?B3.028?B.We apply A@B8@C10 cluster as structure element,we design Mg2BN-1(Fe4Mg4)N and Mg2BN-1(Mn4Mg4)N(N=2-8).nanochains when Fe/Mn atoms have axial paramagnetic moments,which have contain well structural symmetries after DFT calculations.Total magnetic moments of both nanochains almost increase linearly with structure elements.