Electronic structure of icosahedral and cubo-octahedral Fe13clusters

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Abstract
Recent work has emphasised the importance of icosahedral coordination as an 'ideal' reference structure for models of short-range order (SRO) in metallic glasses. To test the presumed local stability of icosahedral coordination by a first-principles method, self-consistent-field scattered-wave local-spin-density-functional calculations have been performed for free clusters of thirteen iron atoms in icosahedral and cubo-octahedral geometries. Converged solutions for four different electronic configurations corresponding to nonmagnetic (NM), ferromagnetic (FM), and two different anti-ferromagnetic (AFM) orderings of the atomic magnetic moments were obtained over a range of central atom to first-nearest-neighbour shell distances (d=4.4-5.25 au). The densities of states, spin densities, atomic magnetic moments, and relative binding energies of these clusters are compared and discussed. The spin-polarised configurations (FM, AFM) are substantially more stable than the non-spin-polarised (NM) configuration, with the ground state being FM, and have large magnetic moments ( mu approximately 2.8 mu B) on the 'surface' atoms. However the magnitude and relative orientation of the magnetic moment on the central atom is very sensitive to d, and also varies somewhat with the coordination geometry. For each electronic configuration, the relative stabilities of the icosahedral and cubo-octahedral geometries differ by less than 0.2 eV per atom (for a given d), with the latter predicted to be the more stable geometry, contrary to some earlier work. This study represents only a first step towards the implementation of first-principles methods capable of dealing explicitly with the effects of SRO on the electronic structure of metallic glasses, and possible extensions of the method in future work are discussed.