Homogeneous glass formation in binary rare-earth silicate systems has thus far been precluded due to the presence of extensive liquid-liquid immiscibility and a strong tendency of these liquids toward crystallization. In this study, we demonstrate homogeneous glass formation in the Sc2O3-SiO2 binary system within a narrow compositional window (37-39 mol% Sc2O3) near a deep eutectic between the compounds Sc2Si2O7 and Sc2SiO5, using containerless laser melting under aerodynamic levitation. The atomic structure of these unusual glasses is investigated using multinuclear (29Si, 45Sc, 17O) solid-state nuclear magnetic resonance (NMR) and Raman spectroscopy. The spectroscopic results, when taken together, provide a comprehensive picture of the structure of these glasses characterized by pyrosilicate [Si2O7]6- anionic units interconnected by Sc cations in ScO6 coordination polyhedra, via Si-O-Sc linkages. A significant fraction (similar to 6%) of the oxygen atoms in the structure is present as free oxide (FO) ions in Sc-O-Sc linkages, providing connectivity between the ScO6 polyhedra. The formation of the FO species via oxygen disproportionation reaction is promoted by the uniquely high field strength of the Sc3+ ions, and the resulting structural frustration is hypothesized to suppress crystallization of the stable pyrosilicate phase in these liquids, enabling glass formation in an otherwise non-glass-forming binary system. These findings highlight the critical role of rare-earth cation field strength in controlling oxygen speciation, structure, and glass-forming ability in this binary silicate system.
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