Solid-state nuclear magnetic resonance (NMR) spectroscopy offers an array of options for exploring structural order of amorphous materials both over short (≲0.3 nm) and medium (≲1 nm) ranges. We review the advances reported in the literature for characterizing the structures of oxide-based glasses over roughly the past 15 years. Besides describing the current understanding of basic short-range structural aspects of binary and ternary glass systems that typically involve a single glass modifier and one (silicates and phosphates) or two (e.g., aluminosilicates, aluminophosphates and silicophosphates) network formers, we focus on illustrating the progress made for revealing intermediate-range structural order, such as extracting information about connectivities among the basic building blocks in the glass networks and how more extended structural motifs may be identified. By assuming only relatively basic background in solid-state NMR and/or the structure of glasses, we hope this review will appeal to newcomers of either area.

By exploiting ¹H and ³¹P magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy, we explore the proton and orthophosphate environments in biomimetic amorphous calcium phosphate (ACP) and hydroxy-apatite (HA), as grown in vitro at the surface of a 10CaO–85SiO₂–5P₂O₅ mesoporous bioactive glass (MBG) in either a simulated body fluid or buffered water. Transmission electron microscopy confirmed the presence of a calcium phosphate layer comprising nanocrystalline HA. Two-dimensional ¹H–³¹P heteronuclear correlation NMR established predominantly ¹H₂O↔³¹PO₄^3– and O¹H↔³¹PO₄^3– contacts in the amorphous and crystalline component, respectively, of the MBG surface-layer; these two pairs exhibit distinctly different ¹H→³¹P cross-polarization dynamics, revealing a twice as large squared effective ¹H–³¹P dipolar coupling constant in ACP compared with HA. These respective observations are mirrored in synthetic (well-crystalline) HA, and the amorphous calcium orthophosphate (CaP) clusters that are present in the pristine MBG pore walls: besides highlighting very similar local ¹H and ³¹P environments in synthetic and biomimetic HA, our findings evidence closely related NMR characteristics, and thereby similar local structures, of the CaP clusters in the pristine MBG relative to biomimetic ACP.

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.

The structures of 15 La–Al–Si–O glasses, whose compositions span 11–28 mol% La₂O₃, 11–30 mol% Al₂O₃, and 45–78 mol% SiO₂, are explored over both short and intermediate length-scales by using a combination of solid-state ²⁷Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations. MAS NMR reveals Al speciations dominated by AlO₄ groups, with minor but significant fractions of AlO₅ (5–10%) and AlO₆ (≲3%) polyhedra present in all La₂O₃–Al₂O₃–SiO₂ glasses; the amounts of Al^[5] and Al^[6] coordinations increase for decreasing molar fraction of Si. The MD simulations reproduce this compositional trend, with the fractional populations of AlO_p groups (p = 4, 5, 6) according well with the experimental results. The modeled La speciations mainly involve LaO₆ and LaO₇ polyhedra, giving a range of average La³⁺ coordination numbers between 6.0 and 6.6; the latter increases slightly for decreasing Si content of the sample. Besides the expected bridging and non-bridging O species, minor contributions of oxygen triclusters (≤9%) and free O²⁻ ions (≤4%) are observed in all MD data. The glass structures exhibit a pronounced Al/Si disorder; the MD simulations reveal essentially random SiO₄–SiO₄, SiO₄–AlO_p and AlO_p–AlO_q (p, q = 4, 5, 6) associations, including significant amounts of AlO₄–AlO₄ contacts, regardless of the n_Al/n_Si molar ratio of the glass. The strong violation of Al^[4]–Al^[4] avoidance is verified by 2D ²⁷Al NMR experimentation that correlates double-quantum and single-quantum coherences, here applied for the first time to aluminosilicate glasses, and evidencing AlO_p–AlO_q connectivities dominated by AlO₄–AlO₄ and AlO₄–AlO₅ pairs. The potential bearings from distinct fictive temperatures of the experimental and modeled glass structures are discussed.