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

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.