Structural Evolution Of Transition Metal Co,Rh,Ir,Ni,Pd And Pt Clusters

As aggregates of finite number of atoms at the microscopic level, clusters exhibit novel geometrical, electronic and magnetic properties as well as widely potential applications. Research on clusters will not only help in-depth understanding of the characteristics, but also help to better understand the physical nature of the bulk. First of all, in theoretical study of atomic clusters, one must determine the ground state geometries of atomic clusters. Atomic clusters have a lot of isomers, which increase fast as the increasing of clusters size, because of the fast increasing of the local minimum (exponential growth rather than series). Since the cluster which consisted of dozens of atoms has a large number of isomers, so it's difficult to find its ground-state structure. For transition metal clusters, because of the delocalization of d electrons and the existence of the large number of low-energy states, it's hard to determine the ground state geometric structure.Experimental studies show that the transition metal cluster has many unique and interesting characters and a lot of outstanding characters which are not discovered in bulk, and it is expected to play an important role on the synthesis of nano-materials and devices, so the transition metal clusters are widely concerned. Although the experimental methods can be adopted to get some information of clusters, they can not identify the ground state geometric structures of free clusters, so all kinds of theoretical methods are widely applied.The structural evolution of transition metal (Co, Rh, Ir, Ni, Pd and Pt) clusters containing from 13 to 60000 atoms are investigated by means of the Gupta potential combined with the steepest descent method. Four typical structural motifs are considered: icosahedron (Ih), decahedron (Dh), face centered cubic octahedron (Fcc-oct), and hexagonal close-packed (Hcp). Our conclusions show as follow:(1) For the clusters of small size (compared with Co, Rh, Ir, the family Ni, Pd, Pt clusters have the smaller size), the four typical structures show the same sequence in the cluster stabilities, their structural stabilities from strong to weak are: Ih> Dh> Hcp> Fcc-oct.(2) Within 1200 atoms, Ih structures of Co, Rh, Ir, have the highest stability, the range of clusters size in which Ih structure dominates, decreases with the increment of the atomic weight, and the Ni, Pd, Pt clusters show the similar properties in a smaller size (Ih structure is the most stable structure within 3800, 1800 and 1200 atoms for the Co, Rh, Ir clusters, respectively. And for the Ni, Pd and Pt clusters, Ih structure is the most stable structure within 500,100 and 40 atoms, respectively).(3) With the increase in cluster size, theΔcurves of Ih and Dh structures diverge (Dh structure diverges faster) and their stabilities decrease, while theΔcurves of Hcp and Fcc-oct structures tend to converge and their stabilities increase.(4) When the cluster size becomes large, the main competition exists between structure of Hcp and structure of Fcc-oct, and both structures are stable.(5) In the process of evolution to crystal of the six metal clusters, the structures of RhN cluster with 1800 atoms and NiN clusters with 600 atoms have shown crystal structure, while the Co, Ir, Pd and Pt clusters have not yet reached their crystal structures within about 60,000 atoms.