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.