Chemical bonding of transition metal disilicides

Abstract

First-principles molecular orbital calculations of 3d and 4d transition-metal (TM or M) disilicides are conducted using model clusters composed of nearly 20 atoms, with special attention to their chemical bonding. M is positively charged and Si is negatively charged for early TM disilicides; however the direction of charge transfer changes between Fe and Co in the 3d TM disilicides. The net charge of M decreases almost linearly with increasing atomic number except for Sc3Si5 (defective ScSi2). The bond overlap population between M-Si decreases and that between Si-Si increases with rising atomic number of M. The energy distribution of Si 3s and 3p components is found to play an important role in the magnitude of both Si-Si and M-Si bonds. The Si-Si bond strength in Sc3Si5 is exceptionally strong among these disilicides. With regard to net charge and bond overlap population, the results for 4d TM disilicides are qualitatively the same as those for 3d TM disilicides.