Sub-Nanometer-Range Structural Effects From Mg²⁺ Incorporation in Na-Based Borosilicate Glasses Revealed by...

Magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) experiments and molecular dynamics (MD) simulations were employed to investigate Na₂O-B₂O₃-SiO₂ and MgO-Na₂O-B₂O₃-SiO₂ glass structures up to ≈0.3 nm. This encompassed the {Na^[}, {Mg^[}, and {B^[3], B^[4]} speciations and the {Si, B^[, M^[}-BO and {Si, B^[, M^[}-NBO interatomic distances to the bridging oxygen (BO) and nonbridging oxygen (NBO) species, where the superscript indicates the coordination number. The MD simulations revealed the dominance of Mg^[5] coordinations, as mirrored in average Mg²⁺ coordination numbers in the 5.2-5.5 range, which are slightly lower than those of the larger Na⁺ cation but with a narrower coordination distribution stemming from the higher cation field strength (CFS) of the smaller divalent Mg²⁺ ion. We particularly aimed to elucidate such Na⁺/Mg²⁺ CFS effects, which primarily govern the short-range structure but also the borosilicate (BS) glass network order, where both MD simulations and heteronuclear double-resonance ¹¹B/²⁹Si NMR experiments revealed a reduction of B^[4]-O-Si linkages relative to B^[3]-O-Si upon Mg²⁺-for-Na⁺ substitution. These effects were quantified and discussed in relation to previous literature on BS glasses, encompassing the implications for simplified structural models and descriptions thereof.