Local electronic structure of amorphous metal alloys using cluster models. Evidence for specific metalloid-metal interactions

Although considerable effort has been expended over the past ten years on constructing structural models of amorphous metal alloys, mostly through the application of "dense random packing of hard spheres" ideas and extensions thereof, almost no theoretical effort has been devoted to understanding the local electronic structure and chemical interactions among the various components in such alloys. The present work is part of a substantial effort which is being made to rectify this situation. Detailed quantum-mechanical calculations are carried out on clusters of atoms which simulate possible local environments in amorphous metal alloys. The clusters used are based on simple Bernal polyhedra and distortions of these polyhedra. The materials being studied are the (Fe, Ni, B, P) alloys. In this paper, the simplest Bernal polyhedron, the tetrahedron, is considered as a first model for the local environment in ${\mathrm{Fe}}_{40}$ ${\mathrm{Ni}}_{40}$ ${\mathrm{B}}_{20}$ and ${\mathrm{Fe}}_{40}$ ${\mathrm{Ni}}_{40}$ ${\mathrm{P}}_{20}$ alloys. The tetrahedron contains two Fe atoms and two Ni atoms at the vertices with either a B atom or a P atom at the center. Spin-polarized self-consistent-field—$X\alpha$—scattered-wave calculations have been employed to investigate the electronic structure. It is found that there are considerable bonding interactions between the metalloid atoms (B or P) and the metal atoms of the clusters. Furthermore a detailed analysis of the charge densities of these models leads to the conclusion that the B atom should have a stronger interaction with the Ni atoms than with the Fe atoms, whereas the P atom should have a stronger interaction with the Fe atoms than with the Ni atoms. A brief discussion of recent experimental data which supports this conclusion is presented.