Tight-binding study of relaxation in RhN and PdN clusters (9≤N≤165)

We have used a tight-binding total-energy expression to study the electronic, magnetic, and structural properties of relatively large Rh-N and Pd-N clusters (9 less than or equal to N less than or equal to 165). An important feature of our approach is that it treats simultaneously, and in a consistent way, both atomic and electronic structures by performing a relaxation process using molecular dynamics based on the method of Car and Parrinello. The clusters are built by adding successive atomic shells around a central atom. These shells are allowed to relax independently and obey symmetry constraints. It is found that although an overall contraction occurs, the relaxed structure is far from being homothetical to the unrelaxed one. For small RhN clusters (N less than or equal to 75) multiple magnetic solutions are obtained and in all cases the local magnetic moments M(i) present a remarkable size and environment dependence. The relation between the observed average magnetization and the cluster geometry is also analyzed. Perspectives of extensions of this study are discussed.