We present a comprehensive molecular dynamics (MD) simulation study of composition–structure trends in a set of 25 glasses of widely spanning compositions from the following four systems of increasing complexity: Na₂O–B₂O₃, Na₂O–B₂O₃–SiO₂, Na₂O–CaO–SiO₂–P₂O₅, and Na₂O–CaO–B₂O₃–SiO₂–P₂O₅. The simulations involved new B–O and P–O potential parameters developed within the polarizable shell-model framework, thereby combining the beneficial features of an overall high accuracy and excellent transferability among different glass systems and compositions: this was confirmed by the good accordance with experimental data on the relative BO₃/BO₄ populations in borate and boro(phospho)silicate networks, as well as with the orthophosphate fractions in bioactive (boro)phosphosilicate glasses, which is believed to strongly influence their bone-bonding properties. The bearing of the simulated melt-cooling rate on the borate/phosphate speciations is discussed. Each local {BO₃, BO₄, SiO₄, PO₄} coordination environment remained independent of the precise set of co-existing network formers, while all trends observed in bond-lengths/angles mainly reflected the glass-network polymerization, i.e., the relative amounts of bridging oxygen (BO) and non-bridging oxygen (NBO) species. The structural roles of the Na⁺/Ca²⁺ cations were also probed, targeting their local coordination environments and their relative preferences to associate with the various borate, silicate, and phosphate moieties. We evaluate and discuss the common classification of alkali/alkaline-earth metal ions as charge-compensators of either BO₄ tetrahedra or NBO anions in borosilicate glasses, also encompassing the less explored NBO-rich regime: the Na⁺/Ca²⁺ cations mainly associate with BO/NBO species of SiO₄/BO₃ groups, with significant relative Na–BO₄ contacts only observed in B-rich glass networks devoid of NBO species, whereas NBO-rich glass networks also reveal substantial amounts of NBO-bearing BO₄ tetrahedra.

The short and intermediate range structures of a large series of bioactive borophosphosilicate (BPS) glasses were probed by solid-state nuclear magnetic resonance (NMR) spectroscopy and atomistic molecular dynamics (MD) simulations. Two BPS glass series were designed by gradually substituting SiO₂ by B₂O₃ in the respective phosphosilicate base compositions 24.1Na₂O–23.3CaO–48.6SiO₂–4.0P₂O₅ (“S49”) and 24.6Na₂O–26.7CaO–46.1SiO₂–2.6P₂O₅ (“S46”), the latter constituting the “45S5 Bioglass” utilized for bone grafting applications. The BPS glass networks are built by interconnected SiO₄, BO₄, and BO₃ moieties, whereas P exists mainly as orthophosphate anions, except for a minor network-associated portion involving P–O–Si and P–O–B^[4] motifs, whose populations were estimated by heteronuclear ³¹P{¹¹B} NMR experimentation. The high Na⁺/Ca²⁺ contents give fragmented glass networks with large amounts of nonbridging oxygen (NBO) anions. The MD-generated glass models reveal an increasing propensity for NBO accommodation among the network units according to BO₄ < SiO₄ < BO₃ ≪ PO₄. The BO₄/BO₃ intermixing was examined by double-quantum–single-quantum correlation ¹¹B NMR experiments, which evidenced the presence of all three BO₃–BO₃, BO₃–BO₄, and BO₄–BO₄ connectivities, with B^[3]–O–B^[4] bridges dominating. Notwithstanding that B^[4]–O–B^[4] linkages are disfavored, both NMR spectroscopy and MD simulations established their presence in these modifier-rich BPS glasses, along with non-negligible B^[4]–NBO contacts, at odds with the conventional structural view of borosilicate glasses. We discuss the relative propensities for intermixing of the Si/B/P network formers. Despite the absence of pronounced preferences for Si–O–Si bond formation, the glass models manifest subtle subnanometer-sized structural inhomogeneities, where SiO₄ tetrahedra tend to self-associate into small chain/ring motifs embedded in BO₃/BO₄-dominated domains.