Icosahedral Short-Range Order in Amorphous Alloys

We have characterized the icosahedral short-range order in amorphous solids using local environment probes. Such topological local order is pronounced even in an amorphous alloy that does not form quasicrystalline phases upon crystallization, as demonstrated by the extended x-ray absorption fine structure and x-ray absorption near-edge structure of a Ni-Ag amorphous alloy analyzed through reverse Monte Carlo simulations.

Figure 1

Upper panel: Solid lines are the experimental XRD and Fourier-transformed (FT) EXAFS data for the $\mathrm{N}{\mathrm{i}}_{\mathrm{60}}\mathrm{A}{\mathrm{g}}_{\mathrm{40}}$ amorphous alloy. The dashed line is the RMC simulated XRD. Lower panel: the solid lines present the inverse Fourier transforms (IFT) of the first peaks in the FT EXAFS spectra, in comparison with the RMC simulation (circles).

Figure 2

Ni $K$-edge (a) and Ag $K$-edge (b) XANES spectra calculated for Ni (or Ag) in the RMC-simulated amorphous $\mathrm{N}{\mathrm{i}}_{\mathrm{60}}\mathrm{A}{\mathrm{g}}_{\mathrm{40}}$. The calculations averaged XANES of 2400 Ni-centered clusters and 1600 Ag-centered clusters, respectively, within the RMC configuration. The XANES features are consistent with those measured of the $\mathrm{N}{\mathrm{i}}_{\mathrm{60}}\mathrm{A}{\mathrm{g}}_{\mathrm{40}}$ amorphous alloy, but different from those of experimental or simulated fcc bulk Ni (or Ag), and from those calculated for artificially constructed ordered Ni-Ag fcc clusters (cluster I for $\mathrm{L}{1}_0$ and cluster II for $\mathrm{L}{1}_2$).

Figure 3

Normalized common-neighbor pairs of the $\mathrm{N}{\mathrm{i}}_{\mathrm{60}}\mathrm{A}{\mathrm{g}}_{\mathrm{40}}$ amorphous alloy. The insets show the 3D RMC configuration, and its total (line 1) and partial RDFs (lines 2 to 4 for Ni-Ni, Ni-Ag or Ag-Ni, and Ag-Ag, respectively, weighted by their contributions to the total RDF), as well as the 48 perfect icosahedra (the center atom forms twelve 555 pairs with the neighbors), involving 13% of the atoms. The dark spheres are Ni atoms.