The structural roles of Sc and Y in aluminosilicate glasses probed by molecular dynamics simulations

The incorporation of rare-earth (RE) elements into an aluminosilicate (AS) glass generally enhances its optical and mechanical properties. Atomistic molecular dynamics (MD) simulations were employed for probing the coordinations of Y ^{3 +} and Sc ^{3 +} in Y ₂O ₃–Al ₂O ₃–SiO ₂ and Sc ₂O ₃–Al ₂O ₃–SiO ₂ glasses of variable Si, Al, and RE contents. YO ₆ and ScO ₅ polyhedra are the most abundant RE species. We explore the trends in the distributions of the various (non)bridging oxygen species and Si/Al/RE cations in the first and second RE coordination spheres, respectively. The lowest RE ^[4] and RE ^[5] coordinations of both Y and Sc exhibit a strong preference for coordinating non-bridging O species, which gradually relaxes for increasing coordination number p. Clear deviations from a statistical Si/Al/RE distribution around the REO _p polyhedra is observed, with preferences for RE–RE contacts relative to RE–Al and (notably) RE–Si. The extents of RE–RE associations are similar for Y and Sc and grow slightly for increasing Si content and/or glass network polymerization. The propensity for RE–Al contacts becomes emphasized in the Sc AS glasses, mainly at the expense of Sc–Si. The YO _p and ScO _p polyhedra connect to their Si/Al/RE neighbors primarily by sharing corners, but a significant extent of edge-sharing is also observed, which increases both with the RE ^[p] coordination number, and along the series Si <Al ^[4] <Al ^[5]/Al ^[6] ≈ RE of neighbors.