First-principles Calculations For Titanium Monoxide And Titanium Nickel Clusters

Alloy clusters have received extensive attention due to the novel structures and properties. They provide us with the possibility of controlling the structure and chemical composition of particles at nanoscale level. Recently, transition metal alloy clusters have been attracted considerable theoretical and experimental interests because of its very interesting properties and potential application. However, much less work has been done on titanium alloy clusters. In this thesis, we investigate the structures, stability and electronic properties of titanium-based binary clusters, by the first-principles calculation with Density-Functional Theory (DFT) implemented in the VASP code.We study the geometric and magnetic properties of Ti_nO (n=1-9) clusters by using the VASP package based on plane-wave pseudopotential technique. Firstly, the lowest energy geometries of Ti_nO (n=1-9) clusters are obtained, together with the optimized Ti_n (n=2-9) clusters structures. The resulting geometries show that the oxygen atom remains on the surface of clusters and does not change the geometry of Ti_n significantly. Based on the optimized geometries, we discuss the binding energy and second-order energy differences with the size of clusters. The results show that Ti7O cluster is endowed with special stability. The stability of Ti_nO clusters is validated by the recent time-of-flight mass spectra. And then, we calculate the magnetic moments of Ti_nO and Ti_n clusters. The total magnetic moments for Ti_nO clusters with n=1-4, 8-9 are constant with 2 and drop to zero at n=5-7. Generally, the total magnetic moment of small Ti_nO clusters is lower than the corresponding Ti_n cluster due to the doping oxygen atom. In addition, the local magnetic moment and charge partition of each atom, and the density of states are discussed. The magnetic moment of the Ti_nO is clearly dominated by the localized 3d electrons of Ti atoms while the oxygen atom contributes a very small amount of spin in Ti_nO clusters.The growth behavior and stability of Ti_nNi (n=1-13) clusters are studied. Firstly, we calculate the structures of Ti_nNi clusters. It is found that a structural transition of Ti_nNi clusters from two-dimensional to three-dimensional geometry occurs at n=4. However, the nickel atom remains on the surface of clusters and does not change the geometry of Ti_n significantly. Our calculations demonstrate that the structures of Ti_nNi clusters follow a pentagonal growth pattern. The clusters with a configuration of icosahedron show higher stability than other clusters. It may be attributed to its high compactness. In addition, the size-dependence of cluster properties such as binding energy, second-order energy, HOMO-LUMO gaps are calculated and analyzed. The results show that Ti₇Ni and Ti₁₂Ni clusters have special stability, which may be interpreted with the help of atomic shell model. At last, further calculations are performed to study cluster-cluster interaction between two Ti12Ni clusters. It is found that whether the clusters retain their individual identities depends on the relative orientation of two clusters.