Sub-Nanometer-Range Structural Effects From Mg(2+) Incorporation in Na-Based Borosilicate Glasses Revealed by Heteronuclear NMR and MD Simulations.

Magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) experiments and molecular dynamics (MD) simulations were employed to investigate Na(2)O-B(2)O(3)-SiO(2) and MgO-Na(2)O-B(2)O(3)-SiO(2) glass structures up to ≈0.3 nm. This encompassed the {Na([p])}, {Mg([p])}, and {B([3]), B([4])} speciations and the {Si, B([p]), M([p])}-BO and {Si, B([p]), M([p])}-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 Mg(2+) 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 Mg(2+) ion. We particularly aimed to elucidate such Na(+)/Mg(2+) 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 (11)B/(29)Si NMR experiments revealed a reduction of B([4])-O-Si linkages relative to B([3])-O-Si upon Mg(2+)-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.