The atomic and electronic structure of metallic glasses: search for a structure-induced minimum in the density of states

The atomic and electronic structures of amorphous and crystalline Mg-Zn alloys are studied by computer simulation, electronic band-structure calculations and photoemission measurements. The spectra for the metallic glasses and for pure crystalline zinc show a narrow band of Zn 3d states centred at a binding energy E_B of about -9.7 eV, overlapping the bottom of a broad sp band. There are indications of a minimum in the electronic density of states at the Fermi level for the glasses and for the pure metals. Molecular dynamics and potential-energy mapping calculations based on pseudopotential-derived interatomic forces are used to construct models for the atomic structure, with no other input than the composition and the atomic numbers and atomic weights of the components. The analysis of these models-which are in reasonable agreement with X-ray and neutron diffraction data-shows that the local topology of the glassy structure is very similar to that of the stable crystalline intermetallic compounds. The glassy structure is best described as a disordered tetrahedral close packing with a weak tendency to chemical short-range order whose precise degree remains to be detailed. The linearised muffin-tin orbital method in the atomic sphere approximation is used to perform self-consistent calculations of the electronic DOS of crystalline Mg and Zn, of the hexagonal Laves phase MgZn₂ and of 'amorphous' supercells (each containing 60 atoms) representing glassy MgZn₂ and Mg₇Zn₃ alloys. In each case the authors find a minimum in the DOS at E_F, and d bands centred at E_B approximately=-7.5 eV. A transition-state calculation shows that the d-band position in the photoemission spectra is shifted relative to the electronic eigenvalue due to self-energy corrections. Photoemission and X-ray emission intensities are calculated from the partial local DOS and the self-consistent potentials in a single-scatterer final-state approximation. The comparison with experimental confirms the validity of the electronic structure calculations. The work represents one of the first ab initio calculations of the atomic and the electronic structure of a metallic glass, and the first confirmation of the existence of a minimum in the electronic DOS at E_F. The relevance of the DOS minimum to the structure-potential relationship and to the stability of the glassy phase is discussed.