Molecular orbital interactions in glass-forming Zr70Cu30 liquid quasicrystals

• SRO in a Zr70Cu30 liquid was due to Cu- and Zr-centered clusters of between 12 and 14 coordination.
• Intramolecular stability was due to surface Zr atoms, in excess of 90% of the HOMO and LUMO density.
• Spin mixing was almost exclusively limited to interactions between Zr-center and Zr-surface atoms.
• Cluster cohesion was due to surface Zr atoms with secondary contributions from Zr centers.
• Cu centers formed interacted with surface atoms of the same species, mainly through σ bonding.

Based on RMC analysis, approximately 40% of the SRO in a Zr70Cu30 melt, studied by aerodynamic levitation, revealed a possibility of ICOS or near-ICOS ordering. The main constituents of such structure were both Cu- and Zr-centered clusters for which center atom coordination was between 12 and 14, with Cu surface atoms ranging between 3 and 5. Intramolecular stability was almost exclusively due to surface Zr atoms, in excess of 90% of the HOMO and LUMO density. In the case of Zr-centered clusters, the center contributed 3–5% of its d density towards frontier orbitals with the surface Zr and Cu atoms largely responsible for cluster cohesion. In the case of the Cu-centered clusters, the center atom was found to contribute by as much as 20% towards mainly bonding orbitals with surface atoms of the same species. Spin mixing was almost exclusively limited to interactions between Zr-center and Zr-surface atoms. Both the lowest energy Zr- and Cu-centered clusters comprised a 13-coordinated metal center and revealed the lowest spin mixing. On increasing molecular weight of the clusters studied, two separate stability modes were detected as a function of the center atom: in the case of the Cu center, sequential addition of surface Cu atoms shifted the center’s affinity towards orbitals with surface Zr atoms. In the case of the Zr center, addition of a single Zr surface atom did not affect orbital populations, while a further addition of a surface Cu atom caused a dissociation of the metal center and an increase of the interaction between different surface species.