Atomic-Scale Origin of the Cation Field Strength Dependence of Mechanical Properties in Divalent-Cation Aluminosilicate Glasses

Aluminosilicate glasses are technologically important materials known for their high mechanical strength. However, the atomistic origins of their composition-dependent mechanical properties remain incompletely understood. Here, 50SiO2–25Al2O3–25RO glasses (R = Be, Mg, Ca, Sr, or Ba) were investigated by combining solid-state 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR), ultrasonic pulse-echo measurements, and machine-learning molecular dynamics (MLMD) simulations. Machine-learning interatomic potentials trained on r2SCAN-level density functional theory reference data reproduced the experimental elastic trends substantially better than PBE-based potentials. The 27Al MAS and triple-quantum (3Q) MAS NMR spectra, together with the MLMD simulations, show that the fractions of Al[5] and Al[6] increase systematically with the ionic field strength (IFS) of the divalent cation, consistent with the established IFS dependence of Al coordination. This trend is most pronounced for 50SiO2–25Al2O3–25BeO, for which 27Al NMR data are reported here, thereby extending the established IFS systematics to the high-field-strength limit represented by Be2+. The simulations further show that these changes in Al coordination are accompanied by an increase in the fraction of three-bonded oxygen (TBO), which provides additional cross-linking sites within the glass network. Athermal quasi-static shear simulations were used to decompose the shear response by element and local coordination environment. TBO was identified as the stiffest oxygen species, exhibiting the largest per-atom shear contribution and the smallest nonaffine squared displacement, Dmin2. The present coordination-resolved mechanical decomposition provides a framework for linking atomic-scale structural motifs to macroscopic elastic properties in multicomponent oxide glasses.