Bergman-type medium-range order in rapidly quenched Ag0.74Ge0.26 eutectic alloy studied by ab initio molecular dynamics simulation

The structure of liquid and rapidly quenched amorphous Ag0.74Ge0.26 alloy at the eutectic composition was studied by ab initio molecular dynamics (MD) simulations. The local structural properties were systematically investigated from the liquid at 1123 K to amorphous solid at 300 K. The pair-correlation function at 976 K from the MD simulations agrees well with the experimental data. The local structures were also analyzed using Honeycutt-Andersen (HA) indices, Voronoi tessellation and the atomic cluster alignment (ACA) method. The HA indices analysis reveals that there is a high population of pentagonal bipyramid structure which become more predominant upon solidification. Voronoi tessellation analysis indicates strong icosahedral short-range order (SRO) in the liquid and amorphous samples generated by the MD simulations. Using the ACA method, the development of icosahedral SRO upon cooling is further confirmed and a Bergman medium-range order is also observed. The analysis of structural properties and chemical short-range order suggests that Ag atoms tend to have a Bergman-like packing, while the dispersed Ge atoms prevent the alloy from forming a long-range order upon cooling. The Bergman medium-range order is also confirmed by the constrained reversed Monte Carlo results.