Atomic order and cluster energetics of a 17 wt% Si-based glass versus the liquid phase.

Aerodynamic levitation of a multicomponent 17 wt% Si glass formed by rapid quenching of the melt phase was studied by high resolution x-ray diffraction (XRD) and reverse Monte Carlo (RMC) modelling. The main local atomic order features comprised interactions between Si, Fe and Mg polyhedra, the stereochemistry of which was on a par with the literature. Both the glass and the liquid state appeared to consist of the same fundamental Si-O, Fe-O and Mg-O clusters, with only the relative number of each varying between the two. Transition from liquid to glass involved a three-fold decrease in uncoordinated O (to within the first minimum of the total g(r)) and a marked increase of Fe-Si-Mg polyhedra bridging O. Octahedral Fe coordination was not suggested by the RMC data. All-electron open-shell density functional theory (DFT) calculations of the most prominent clusters suggested independence between the Fe oxidation state and its polyhedra O-coordination. Of secondary thermodynamic importance were indications of network-forming Fe(2+) and Fe(3+) distorted trigonal and tetrahedral polyhedra. In all occasions, the Fe ferrous and ferric states involved comparable binding energies within similar clusters which indicate a dynamic equilibrium between the two.