Ab initio study of stability and quadrupole coupling constants in borophosphates

The DFT method was used to predict the formation energies and quadrupole coupling constants CQ in a series of borophosphates: Li3BP2O8, Li2NaBP2O8, Na3BP2O8, Li2B3PO8, Na5B2P3O13, LiNa2B5P2O14 and Na3B6PO13 composed of different networks and different amounts of borate and phosphate units. The change in formation energies with increasing number of B atoms in this series is attributed to the multiplicity of boron sites and is explained by density of states calculations. The calculated CQ values of 7Li, 23Na and 11B are correlated with the coordination and distortion of polyhedra to elucidate the influence of local and more distant environments. As for the CQ of 11B, it should be in the ranges of 0.26-0.36, 0.48-0.84 and ∼1 MHz for boron tetrahedral distortion indices of 0.004-0.013, 0.015-0.019 and 0.033, respectively, whereas CQ ∼3.0 MHz corresponds to boron in a triangular site. The obtained numerical relationships make it possible to predict the quadrupole frequencies for these nuclei based only on their local environment, and vice versa, to propose structural models from NMR data. These results provide guidance for studying similar characteristics of other borophosphates, the structure of which varies depending on the initial reaction, composition and temperature.

The Effect of Disorder on Endogenous MAS-DNP: Study of Silicate Glasses and Crystals

In dynamic nuclear polarization nuclear magnetic resonance (DNP-NMR) experiments, the large Boltzmann polarization of unpaired electrons is transferred to surrounding nuclei, leading to a significant increase in the sensitivity of the NMR signal. In order to obtain large polarization gains in the bulk of inorganic samples, paramagnetic metal ions are introduced as minor dopants acting as polarizing agents. While this approach has been shown to be very efficient in crystalline inorganic oxides, significantly lower enhancements have been reported when applying this approach to oxide glasses. In order to rationalize the origin of the difference in the efficiency of DNP in amorphous and crystalline inorganic matrices, we performed a detailed comparison in terms of their magnetic resonance properties. To diminish differences in the DNP performance arising from distinct nuclear interactions, glass and crystal systems of similar compositions were chosen, Li₂OCaO·2SiO₂ and Li₂CaSiO₄, respectively. Using Gd(III) as polarizing agent, DNP provided signal enhancements in the range of 100 for the crystalline sample, while only up to around factor 5 in the glass, for both ⁶Li and ²⁹Si nuclei. We find that the drop in enhancement in glasses can be attributed to three main factors: shorter nuclear and electron relaxation times as well as the dielectric properties of glass and crystal. The amorphous nature of the glass sample is responsible for a high dielectric loss, leading to efficient microwave absorption and consequently lower effective microwave power and an increase in sample temperature which leads to further reduction of the electron relaxation time. These results help rationalize the observed sensitivity enhancements and provide guidance in identifying materials that could benefit from the DNP approach.

Half-integer-spin quadrupolar nuclei in magic-angle spinning paramagnetic NMR: The case of NaMnO2

A combination of solid-state NMR methods for the extraction of ²³Na shift and quadrupolar parameters in the as-synthesized, structurally complex NaMnO₂ Na-ion cathode material, under magic-angle spinning (MAS) is presented. We show that the integration of the Magic-Angle Turning experiment with Rotor-Assisted Population transfer (RAPT) can be used both to identify shifts and to extract a range of magnitudes for their quadrupolar couplings. We also demonstrate the applicability of the two-dimensional one pulse (TOP) based double-sheared Satellite Transition Magic-Angle Spinning (TOP-STMAS) showing how it can yield a spectrum with separated shift and second-order quadrupolar anisotropies, which in turn can be used to analyze a quadrupolar lineshape free of anisotropic bulk magnetic susceptibility (ABMS) induced shift dispersion and determine both isotropic shift and quadrupolar products. Combining all these experiments, the shift and quadrupolar parameters for all observed Na environments were extracted and yielded excellent agreement with the density functional theory (DFT) based models that were reported in previous literature. We expect these methods to open the door for new possibilities for solid-state NMR to probe half-integer quadrupolar nuclei in paramagnetic materials and other systems exhibiting large shift dispersion.

Silicon-29 echo train coherence lifetimes and geminal 2J-couplings in network modified silicate glasses

The natural abundance ²⁹Si echo-train coherence lifetimes in network-modified silicate glasses were examined under static and magic-angle spinning (MAS) conditions. The nuclear magnetic properties of modifier cations were found to play a major role in determining ²⁹Si coherence lifetimes, leading to differences as large as three orders of magnitude. In compositions with abundant NMR active nuclei, such as alkali silicates, the ²⁹Si coherence lifetimes are dominated by coherent dephasing due to residual heteronuclear dipolar couplings, whereas in compositions dilute in NMR active nuclei, such as alkaline earth silicates, the ²⁹Si coherence lifetimes are dominated by incoherent dephasing due to paramagnetic impurities. Expressing the inverse of the coherence lifetime as a residual full width at half maximum (FWHM), we found that increasing rates of both MAS and a π-pulse train are effective in removing the residual ²⁹Si heteronuclear broadenings, with a near-linear relationship between FWHM and MAS rotor period and π-pulse spacing. Based on these results, we conclude that accurate ²⁹Si J coupling measurements will be the most challenging in lithium silicate glasses due to strong homonuclear dipolar couplings among ⁷Li nuclei, requiring MAS speeds up to 100 kHz, and be the least challenging in the alkaline earth silicate glasses. At a modest MAS speed of 14kHz, distributions of geminal J couplings across Si-O-Si linkages were measured in alkali and alkaline earth silicate glasses giving mean values of 4.2Hz and 5.1Hz in 0.4 CaO·0.6 SiO₂ and 0.33 Ba₂O·0.67 SiO₂ glasses, respectively, and 5.2Hz and 5.3Hz in 0.33 Na₂O·0.67 SiO₂ and 0.33 K₂O·0.67 SiO₂ glasses, respectively. We also observe greater variance in the J distributions of alkaline earth silicate glasses consistent with greater structural disorder due to increased modifier cation potential, i.e., the charge-to-radius ratio, Z/r of the cation.

Separation of quadrupolar and paramagnetic shift interactions in high-resolution nuclear magnetic resonance of spinning powders

Separation and correlation of the shift anisotropy and the first-order quadrupolar interaction of spin I = 1 nuclei under magic-angle spinning (MAS) are achieved by the phase-adjusted spinning sideband (PASS) nuclear magnetic resonance (NMR) experiment. Compared to methods for static samples, this approach has the benefit of higher sensitivity and resolution. Moreover, the PASS experiment has the advantage over previous MAS sequences in the ability to completely separate the shift anisotropy and first-order quadrupolar interactions. However, the main drawback of the pulse sequence is the lower excitation bandwidth. The sequence is comprehensively evaluated using theoretical calculations and numerical simulations and applied experimentally to the ²H NMR of a range of paramagnetic systems: deuterated nickel(II) acetate tetrahydrate, deuterated copper(II) chloride dihydrate, and two forms of deuterated oxyhydride ion conductor BaTiO_3-xH_y. Our results show that despite the issue with broadband excitation, the extracted shift and quadrupolar interaction tensors and the Euler angles relating the two tensors match well with the NMR parameters obtained with static NMR methods. Therefore, the new application of the PASS experiment is an excellent addition to the arsenal of NMR experiments for ²H and potentially ¹⁴N in paramagnetic solids.

Through-space 11 B-27 Al correlation: Influence of the recoupling channel

Through-space heteronuclear correlation (D-HETCOR) experiments based on heteronuclear multiple-quantum correlation (D-HMQC) and refocused insensitive nuclei enhanced by polarization transfer (D-RINEPT) sequences have been proven to be useful approaches for the detection of the spatial proximity between half-integer quadrupolar nuclei in solids under magic-angle spinning (MAS) conditions. The corresponding pulse sequences employ coherence transfers mediated by heteronuclear dipolar interactions, which are reintroduced under MAS by radiofrequency irradiation of only one of the two correlated nuclei. We investigate herein using numerical simulations of spin dynamics and solid-state NMR experiments on magnesium aluminoborate glass how the choice of the channel to which the heteronuclear dipolar recoupling is applied affects the transfer efficiency of D-HMQC and D-RINEPT sequences between ¹¹ B and ²⁷ Al nuclei. Experimental results show that maximum transfer efficiency is achieved when the recoupling scheme is applied to the channel, for which the spin magnetization is parallel to the B₀ axis in average.

Expanding the chemistry of borates with functional [BO2]- anions

More than 3900 crystalline borates, including borate minerals and synthetic inorganic borates, in addition to a wealth of industrially-important boron-containing glasses, have been discovered and characterized. Of these compounds, 99.9 % contain only the traditional triangular BO₃ and tetrahedral BO₄ units, which polymerize into superstructural motifs. Herein, a mixed metal K₅Ba₂(B₁₀O₁₇)₂(BO₂) with linear BO₂ structural units was obtained, pushing the boundaries of structural diversity and providing a direct strategy toward the maximum thresholds of birefringence for optical materials design. ¹¹B solid-state nuclear magnetic resonance (NMR) is a ubiquitous tool in the study of glasses and optical materials; here, density functional theory-based NMR crystallography guided the direct characterization of BO₂ structural units. The full anisotropic shift and quadrupolar tensors of linear BO₂ were extracted from K₅Ba₂(B₁₀O₁₇)₂(BO₂) containing BO₂, BO₃, and BO₄ and serve as guides to the identification of this powerful moiety in future and, potentially, previously-characterized borate minerals, ceramics, and glasses.

Revealing Spatial Distribution of Al-Coordinated Species in a Phase-Separated Aluminosilicate Glass by STEM-EELS

Structure determination of glass remains an important issue in glass science. The electron microscope with its high spatial resolution and versatile functions has been widely applied to observe phase separation and structural heterogeneity in glass. While elemental analysis such as energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) may provide local compositional information with nanometer-scale resolution, structural information in a glass network cannot be directly obtained. Here, a novel way to probe local coordination is employed using electron energy loss fine structure (ELNES) in the scanning transmission electron microscope (STEM). The method is demonstrated in a phase-separated aluminosilicate glass with multiple Al-coordinated species. With the support of ab initio calculation, two exciton-like peaks in the Al L_2,3-edge at around 77 and 80 eV are attributed to 4-fold and 5,6-fold Al excitations, respectively. Mapping of the relative intensity ratio for two peaks in a phase-separated microstructure reveals a heterogeneous distribution of highly coordinated Al species in real space. The finding is in agreement with previous MD simulation that 5- and 6-fold Al species are favored to form in the Al-rich phase. This work has demonstrated that complex network structure within the phase-separated region can now be studied via STEM-EELS.

Separation of quadrupolar and paramagnetic shift interactions with TOP-STMAS/MQMAS in solid-state lighting phosphors

A new approach for processing satellite-transition magic-angle spinning (STMAS) and multiple-quantum magic-angle spinning (MQMAS) data, based on the two-dimensional one-pulse (TOP) method, which separates the second-rank quadrupolar anisotropy and paramagnetic shift interactions via a double shearing transformation, is described. This method is particularly relevant in paramagnetic systems, where substantial inhomogeneous broadening may broaden the lineshapes. Furthermore, it possesses an advantage over the conventional processing of MQMAS and STMAS spectra because it overcomes the limitation on the spectral width in the indirect dimension imposed by rotor synchronization of the sampling interval. This method was applied experimentally to the 27 Al solid-state nuclear magnetic resonance of a series of yttrium aluminum garnets (YAGs) doped with different lanthanide ions, from which the quadrupolar parameters of paramagnetically shifted and bulk unshifted sites were extracted. These parameters were then compared with density functional theory calculations, which permitted a better understanding of the local structure of Ln substituent ions in the YAG lattice.

Statistical learning of NMR tensors from 2D isotropic/anisotropic correlation nuclear magnetic resonance spectra

Many linear inversion problems involving Fredholm integrals of the first kind are frequently encountered in the field of magnetic resonance. One important application is the direct inversion of a solid-state nuclear magnetic resonance (NMR) spectrum containing multiple overlapping anisotropic subspectra to obtain a distribution of the tensor parameters. Because of the ill-conditioned nature of this inverse problem, we investigate the use of the truncated singular value decomposition and the smooth least absolute shrinkage and selection operator based regularization methods, which (a) stabilize the solution and (b) promote sparsity and smoothness in the solution. We also propose an unambiguous representation for the anisotropy parameters using a piecewise polar coordinate system to minimize rank deficiency in the inversion kernel. To obtain the optimum tensor parameter distribution, we implement the k-fold cross-validation, a statistical learning method, to determine the hyperparameters of the regularized inverse problem. In this article, we provide the details of the linear-inversion method along with numerous illustrative applications on purely anisotropic NMR spectra, both synthetic and experimental two-dimensional spectra correlating the isotropic and anisotropic frequencies.

Local Structure of Glassy Lithium Phosphorus Oxynitride Thin Films: A Combined Experimental and Ab Initio Approach

Lithium phosphorus oxynitride (LiPON) is an amorphous solid-state lithium ion conductor displaying exemplary cyclability against lithium metal anodes. There is no definitive explanation for this stability due to the limited understanding of the structure of LiPON. Herein, we provide a structural model of RF-sputtered LiPON. Information about the short-range structure results from 1D and 2D solid-state NMR experiments. These results are compared with first principles chemical shielding calculations of Li-P-O/N crystals and ab initio molecular dynamics-generated amorphous LiPON models to unequivocally identify the glassy structure as primarily isolated phosphate monomers with N incorporated in both apical and as bridging sites in phosphate dimers. Structural results suggest LiPON's stability is a result of its glassy character. Free-standing LiPON films are produced that exhibit a high degree of flexibility, highlighting the unique mechanical properties of glassy materials.

Bioactive Glasses: A Promising Therapeutic Ion Release Strategy for Enhancing Wound Healing

The morbidity, mortality, and burden of burn victims and patients with severe diabetic wounds are still high, which leads to an extensively growing demand for novel treatments with high clinical efficacy. Biomaterial-based wound treatment approaches have progressed over time from simple cotton wool dressings to advanced skin substitutes containing cells and growth factors; however, no wound care approach is yet completely satisfying. Bioactive glasses are materials with potential in many areas that exhibit unique features in biomedical applications. Today, bioactive glasses are not only amorphous solid structures that can be used as a substitute in hard tissue but also are promising materials for soft tissue regeneration and wound healing applications. Biologically active elements such as Ag, B, Ca, Ce, Co, Cu, Ga, Mg, Se, Sr, and Zn can be incorporated in glass networks; hence, the superiority of these multifunctional materials over current materials results from their ability to release multiple therapeutic ions in the wound environment, which target different stages of the wound healing process. Bioactive glasses and their dissolution products have high potency for inducing angiogenesis and exerting several biological impacts on cell functions, which are involved in wound healing and some other features that are valuable in wound healing applications, namely hemostatic and antibacterial properties. In this review, we focus on skin structure, the dynamic process of wound healing in injured skin, and existing wound care approaches. The basic concepts of bioactive glasses are reviewed to better understand the relationship between glass structure and its properties. We illustrate the active role of bioactive glasses in wound repair and regeneration. Finally, research studies that have used bioactive glasses in wound healing applications are summarized and the future trends in this field are elaborated.

Structural origins of the Mixed Alkali Effect in Alkali Aluminosilicate Glasses: Molecular Dynamics Study and its Assessment

The comprehension of the nonlinear effects provided by mixed alkali effect (MAE) in oxide glasses is useful to optimize glass compositions to achieve specific properties that depend on the mobility of ions, such as the chemical durability, glass transition temperature, viscosity and ionic conductivity. Although molecular dynamics (MD) simulations have already been applied to investigate the MAE on silicates, less effort has been devoted to study such phenomenon in mixed alkali aluminosilicate glasses where alkali cations can act both as modifiers, forming non-bridging oxygens and percolation channels, and as charge compensator of the AlO4- units present in the network. Moreover, the ionic conductivity has not been computed yet; thus, the accuracy of the atomistic simulations in reproducing the MAE on the property is still open to question. In this work, we have validated five major interatomic potentials for the classical MD simulations by modelling the structure, density, glass transition temperature and ionic conductivity for three aluminosilicate glasses, (25 - x)Na2O - x(K2O) - 10(Al2O3) - 65(SiO2) (x = 0, 12.5, 25). It was observed that only the core-shell (CS) polarizable force field well reproduces the experimentally measured MAE on Tg and the ionic conductivity as well as the higher conductivity of single sodium aluminosilicate glass at low temperature and the higher conductivity of single potassium aluminosilicate glass at high temperature. The MAE is related to the suppression of jump events of the alkaline ions between dissimilar sites in the percolation channels consisting of both sodium and potassium ions as in the case of alkaline silicates. The superior reproducibility of the CS potential is originated from the larger and the flexible ring structures due to the smaller Si-O-Si inter-tetrahedra angle, creating appropriate percolation channels for ion conductivity. We also report detailed assessments for using the potential models including the CS potential for investigating MAE on aluminosilicates.

Dynamics in inorganic glass-forming liquids by NMR spectroscopy

Dynamical NMR spectroscopy provides unique mechanistic understanding of the transport and relaxation processes in glass-forming liquids over timescales typically ranging from ~10^-9 s to ~10² s, and thus has been used extensively in the past to study the dynamical behavior of polymeric and organic glass-forming liquids. However, reports in the literature of similar studies on inorganic glass-forming liquids have remained somewhat limited due to the experimental challenges. In this contribution we present a review of the high-temperature NMR spectroscopic studies of atomic and molecular dynamics in a wide variety of inorganic glass-forming liquids including oxides, halides and chalcogenides as well as select ionic liquids and molten salts. The significance of these dynamical processes in understanding the nature of the liquid-to-glass transition and their connection with the macroscopic transport properties of these liquids are discussed.

Improved Magnetization Transfers among Quadrupolar Nuclei in Two-Dimensional Homonuclear Correlation NMR Experiments Applied to Inorganic Network Structures

We demonstrate that supercycles of previously introduced two-fold symmetry dipolar recoupling schemes may be utilized successfully in homonuclear correlation nuclear magnetic resonance (NMR) spectroscopy for probing proximities among half-integer spin quadrupolar nuclei in network materials undergoing magic-angle-spinning (MAS). These (SR221)M, (SR241)M, and (SR281)M recoupling sequences with M=3 and M=4 offer comparably efficient magnetization transfers in single-quantum–single-quantum (1Q–1Q) correlation NMR experiments under moderately fast MAS conditions, as demonstrated at 14.1 T and 24 kHz MAS in the contexts of 11B NMR on a Na2O–CaO–B2O3–SiO2 glass and 27Al NMR on the open framework aluminophosphate AlPO-CJ19 [(NH4)2Al4(PO4)4HPO4·H2O]. Numerically simulated magnetization transfers in spin–3/2 pairs revealed a progressively enhanced tolerance to resonance offsets and rf-amplitude errors of the recoupling pulses along the series (SR221)M< (SR241)M< (SR281)M for increasing differences in chemical shifts between the two nuclei. Nonetheless, for scenarios of a relatively minor chemical-shift dispersions (≲3 kHz), the (SR221)M supercycles perform best both experimentally and in simulations.

Recent advances and future perspectives of sol–gel derived porous bioactive glasses: a review

Sol–gel derived bioactive glasses have been extensively explored as a promising and highly porous scaffold materials for bone tissue regeneration applications owing to their exceptional osteoconductivity, osteostimulation and degradation rates.

Improved sensitivity and quantification for 29Si NMR experiments on solids using UDEFT (Uniform Driven Equilibrium Fourier Transform)

We demonstrate the possibility to use UDEFT (Uniform Driven Equilibrium Fourier Transform) technique in order to improve the sensitivity and the quantification of one-dimensional ²⁹Si NMR experiments under magic-angle spinning (MAS). We derive an analytical expression of the signal-to-noise ratios of UDEFT and single-pulse (SP) experiments subsuming the contributions of transient and steady-state regimes. Using numerical spin dynamics simulations and experiments on ²⁹Si-enriched amorphous silica and borosilicate glass, we show that 59₁₈₀298₀59₁₈₀ refocusing composite π-pulse and the adiabatic inversion using tanh/tan modulation improve the robustness of UDEFT technique to rf-inhomogeneity, offset, and chemical shift anisotropy. These pulses combined with a two-step phase cycle limit the pulse imperfections and the artifacts produced by stimulated echoes. The sensitivity of SP, UDEFT and CPMG (Carr-Purcell-Meiboom-Gill) techniques are experimentally compared on functionalized and non-functionalized mesoporous silica. Furthermore, experiments on a flame retardant material prove that UDEFT technique provides a better quantification of ²⁹Si sites with higher sensitivity than SP method.

Statistical Mechanical Modeling of Borate Glass Structure and Topology: Prediction of Superstructural Units and Glass Transition Temperature

Predicting the compositional evolution of the atomic-scale structure and properties of oxide glasses is important for designing new materials for advanced applications. A statistical mechanics-based approach has recently been applied to predict the composition-structure evolution in binary phosphate glasses, while topological constraint theory (TCT) has been applied in the last decade to predict the structure-property evolution in various oxide and nonoxide glass systems. In this work, we couple these two approaches to enable quantitative predictions of the compositional dependence of glass transition temperature and the population of superstructural units. The object of the study is the lithium borate glass system because they feature interesting structural characteristics (e.g., boron anomaly), and ample structure and property data are available. In these glasses, the average coordination number of boron first increases when lithium modifiers are added and then later decreases accompanied by network depolymerization. First, on the basis of ¹⁰B nuclear magnetic resonance spectroscopy data from literature, we present a statistical description of the structural evolution in lithium borate glasses by accounting for the relative enthalpic and entropic contributions to the bonding preferences. We show that the entire glass structure evolution (both short- and intermediate-range) can be predicted based on experimental structural information for only a few glass compositions. We then show that the developed structural model can be combined with a previously established TCT model to predict the compositional evolution of the glass transition temperature. This work thus opens a new avenue for the computational design of glasses with tailored properties.

Modifier clustering and avoidance principle in borosilicate glasses: A molecular dynamics study

Oxide glasses are typically described as having a random, disordered skeleton of network-forming polyhedra that are depolymerized by network-modifying cations. However, the existence of local heterogeneity or clustering within the network-forming and network-modifying species remains unclear. Here, based on molecular dynamics simulations, we investigate the atomic structure of a series of borosilicate glasses. We show that the network-modifying cations exhibit some level of clustering that depends on composition-in agreement with Greaves' modified random network model. In addition, we demonstrate the existence of some mutual avoidance among network-forming atoms, which echoes the Loewenstein avoidance principle typically observed in aluminosilicate phases. Importantly, we demonstrate that the degree of heterogeneity in the spatial distribution of the network modifiers is controlled by the level of ordering in the interconnectivity of the network formers. Specifically, the mutual avoidance of network formers is found to decrease the propensity for modifier clustering.

Quantitative determination of the phosphorus environment in lithium aluminosilicate glasses using solid-state NMR techniques

The phosphorus environment is determined quantitatively using ³¹P Magic Angle Spinning NMR constrained by ³¹P–²⁷Al Multiple-Quantum Coherence-based NMR techniques.

NMR shifts in aluminosilicate glasses via machine learning

Machine learning (ML) approaches are investigated for the prediction of nuclear magnetic resonance (NMR) shifts in aluminosilicate glasses, for which NMR has proven to be a cutting-edge method over the last decade.

On the Transient Decomposition and Reaction Kinetics of Zinc Dialkyldithiophosphate

Despite the ubiquitous use of the zinc dialkyldithiophosphate (ZDDP) as an antiwear additive, no complete information is yet available on its exact decomposition reactions and kinetics to form triboreactive protective films on contacting surfaces. This hinders the replacement of ZDDP with more environmentally friendly additives of similar antiwear capabilities. Using a multitechnique approach, this study shows that before the formation of a phosphate-rich protective film, the decomposition of ZDDP proceeds by forming intermediate zinc sulfide and sulfate species, which can be mechanically mixed with the iron oxides on the rubbing steel surfaces. The mixed sulfur-oxide layer can play different vital roles including binding the subsequently formed phosphate layers with the metal surface. These layers consist mainly of zinc thiophosphate of initially short chains, which are formed due to the excess concentration of metal oxide on the surface. As the concentration of the oxide decreases in the subsequent layers, the short chains start to polymerize into longer ones. The polymerization process follows first-order reaction kinetics with two distinctive phases. The first one is a fast transient burst phase near the steel surface, whereas the second phase dominates the formation process of the layers away from the substrate and is characterized by slow kinetics. The findings of this study provide new insights into the decomposition mechanisms of the currently most widely used antiwear additive and open future opportunities to find green alternatives with similar superior antiwear properties.

Bioactive Glasses and Glass-Ceramics for Healthcare Applications in Bone Regeneration and Tissue Engineering

The discovery of bioactive glasses (BGs) in the late 1960s by Larry Hench et al. was driven by the need for implant materials with an ability to bond to living tissues, which were intended to replace inert metal and plastic implants that were not well tolerated by the body. Among a number of tested compositions, the one that later became designated by the well-known trademark of 45S5 Bioglass® excelled in its ability to bond to bone and soft tissues. Bonding to living tissues was mediated through the formation of an interfacial bone-like hydroxyapatite layer when the bioglass was put in contact with biological fluids in vivo. This feature represented a remarkable milestone, and has inspired many other investigations aiming at further exploring the in vitro and in vivo performances of this and other related BG compositions. This paradigmatic example of a target-oriented research is certainly one of the most valuable contributions that one can learn from Larry Hench. Such a goal-oriented approach needs to be continuously stimulated, aiming at finding out better performing materials to overcome the limitations of the existing ones, including the 45S5 Bioglass®. Its well-known that its main limitations include: (i) the high pH environment that is created by its high sodium content could turn it cytotoxic; (ii) and the poor sintering ability makes the fabrication of porous three-dimensional (3D) scaffolds difficult. All of these relevant features strongly depend on a number of interrelated factors that need to be well compromised. The selected chemical composition strongly determines the glass structure, the biocompatibility, the degradation rate, and the ease of processing (scaffolds fabrication and sintering). This manuscript presents a first general appraisal of the scientific output in the interrelated areas of bioactive glasses and glass-ceramics, scaffolds, implant coatings, and tissue engineering. Then, it gives an overview of the critical issues that need to be considered when developing bioactive glasses for healthcare applications. The aim is to provide knowledge-based tools towards guiding young researchers in the design of new bioactive glass compositions, taking into account the desired functional properties.

Predicting Q-Speciation in Binary Phosphate Glasses Using Statistical Mechanics

Predicting the compositional evolution of the atomic-scale structure of oxide glasses is important for developing quantitative composition-property models. In binary phosphate glasses, the addition of network modifiers generally leads to depolymerization of the networks as described by the Q-speciation, where Q ⁿ denotes PO₄ tetrahedra with n number (between 0 and 3) of bridging P-O-P linkages per tetrahedron. Upon the initial creation of nonbridging oxygens and thus partly depolymerized Q species, a variety of network former-modifier interactions exist. Here, on the basis of ³¹P magic angle spinning nuclear magnetic resonance spectroscopy data from the literature, we present a statistical description of the compositional evolution of Q-speciation in these glasses by accounting for the relative enthalpic and entropic contributions to the bonding preferences. We show that the entire glass structure evolution can be predicted based on experimental structural information for only a few glass compositions in each series. The model also captures the differences in bonding preferences in glasses with different field strengths (charge-to-size ratio) of the modifier cations.

Structure of Ancient Glass by 29 Si Magic Angle Spinning NMR Spectroscopy

²⁹ Si magic angle spinning (MAS) NMR spectroscopy has been applied for the first time to the structural analysis of ancient glass samples obtained from archaeological excavations. The results show that it is possible to establish the distribution of Si environments in ancient glass by ²⁹ Si MAS NMR, so long as the concentrations of magnetic impurities, such as Mn and Fe oxides, are low. In general, good agreement has been obtained with compositions determined by means of electron probe microanalysis. In addition, the ²⁹ Si MAS NMR data reveal structural differences between glasses manufactured at separate ancient sites.

Structure and Crystallization of Alkaline-Earth Aluminosilicate Glasses: Prevention of the Alumina-Avoidance Principle

Aluminosilicate glasses are considered to follow the Al-avoidance principle, which states that Al-O-Al linkages are energetically less favorable, such that, if there is a possibility for Si-O-Al linkages to occur in a glass composition, Al-O-Al linkages are not formed. The current paper shows that breaching of the Al-avoidance principle is essential for understanding the distribution of network-forming AlO₄ and SiO₄ structural units in alkaline-earth aluminosilicate glasses. The present study proposes a new modified random network (NMRN) model, which accepts Al-O-Al linkages for aluminosilicate glasses. The NMRN model consists of two regions, a network structure region (NS-Region) composed of well-separated homonuclear and heteronuclear framework species and a channel region (C-Region) of nonbridging oxygens (NBOs) and nonframework cations. The NMRN model accounts for the structural changes and devitrification behavior of aluminosilicate glasses. A parent Ca- and Al-rich melilite-based CaO-MgO-Al₂O₃-SiO₂ (CMAS) glass composition was modified by substituting MgO for CaO and SiO₂ for Al₂O₃ to understand variations in the distribution of network-forming structural units in the NS-region and devitrification behavior upon heat treating. The structural features of the glass and glass-ceramics (GCs) were meticulously assessed by advanced characterization techniques including neutron diffraction (ND), powder X-ray diffraction (XRD), ²⁹Si and ²⁷Al magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and in situ Raman spectroscopy. ND revealed the formation of SiO₄ and AlO₄ tetrahedral units in all the glass compositions. Simulations of chemical glass compositions based on deconvolution of ²⁹Si MAS NMR spectral analysis indicate the preferred formation of Si-O-Al over Si-O-Si and Al-O-Al linkages and the presence of a high concentration of nonbridging oxygens leading to the formation of a separate NS-region containing both SiO₄ and AlO₄ tetrahedra (Si/Al) (heteronuclear) in addition to the presence of Al_[4]-O-Al_[4] bonds; this region coexists with a predominantly SiO₄-containing (homonuclear) NS-region. In GCs, obtained after heat treatment at 850 °C for 250 h, the formation of crystalline phases, as revealed from Rietveld refinement of XRD data, may be understood on the basis of the distribution of SiO₄ and AlO₄ structural units in the NS-region. The in situ Raman spectra of the GCs confirmed the formation of a Si/Al structural region, as well as indicating interaction between the Al/Si region and SiO₄-rich region at higher temperatures, leading to the formation of additional crystalline phases.

NMR Spectroscopy in Glass Science: A Review of the Elements

The study of inorganic glass structure is critically important for basic glass science and especially the commercial development of glasses for a variety of technological uses. One of the best means by which to achieve this understanding is through application of solid-state nuclear magnetic resonance (NMR) spectroscopy, which has a long and interesting history. This technique is element specific, but highly complex, and thus, one of the many inquiries made by non-NMR specialists working in glass science is what type of information and which elements can be studied by this method. This review presents a summary of the different elements that are amenable to the study of glasses by NMR spectroscopy and provides examples of the type of atomic level structural information that can be achieved. It serves to inform the non-specialist working in glass science and technology about some of the benefits and challenges involved in the study of inorganic glass structure using modern, readily-available NMR methods.

NMR interaction tensors of 51V and 207Pb in vanadinite, Pb5(VO4)3Cl, determined from DFT calculations and single-crystal NMR measurements, using only one general rotation axis

Orientation-dependent NMR spectra of a single crystal of the mineral vanadinite, Pb₅(VO₄)₃Cl, were acquired using only one rotation axis with a general orientation in the hexagonal crystal lattice (space group P6₃/m). The chemical shift (CS) tensors for the ²⁰⁷Pb on Wyckoff positions 6h and 4f, and both CS and quadrupole coupling tensor Q for ⁵¹V at the positions 6h were determined by including the NMR response of symmetry-related atoms in the unit cell (and in case of ²⁰⁷Pb at 4f, also the isotropic shift from MAS NMR spectra). This previously suggested 'single rotation method' greatly reduces the necessary amount of data acquisition and analysis. The precise orientation of the rotation axis could not be found by X-ray diffraction experiments because of the high linear absorption coefficient of vanadinite, which is chiefly due to its high lead content. The axis orientation was therefore included into the multi-parameter data fit routine. This NMR-based approach is widely applicable, and offers an alternative way of orienting single crystals. The NMR parameters derived from the tensor eigenvalues are δ_iso=(-1729±9) ppm, Δδ=(-1071±5) ppm, η_CS=0.362±0.008 for ²⁰⁷Pb at positions 6h, and δ_iso=(-1619±2) ppm, Δδ=(-780±58) ppm, η_CS=0.06±0.08 for positions 4f. For ⁵¹V, δ_iso=(-509±3) ppm, Δδ=(-37±2) ppm, η_CS=0.78±0.09, with the quadrupolar coupling described by χ=(2.52±0.01) MHz and η_Q=0.047±0.003. In contrast to the precisely determined tensor eigenvalues, the orientation of the eigenvectors in the crystal ab -plane of the vanadinite system could only be resolved by resorting to data obtained from density functional theory (DFT) calculations.

Structure–composition trends in multicomponent borosilicate-based glasses deduced from molecular dynamics simulations with improved B–O and P–O force fields

Improved B–O and P–O force fields provide accurate molecular dynamics simulations of multicomponent glasses.

Medium-Range Structural Organization of Phosphorus-Bearing Borosilicate Glasses Revealed by Advanced Solid-State NMR Experiments and MD Simulations: Consequences of B/Si Substitutions

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.

Diffusive, Displacive Deformations and Local Phase Transformation Govern the Mechanics of Layered Crystals: The Case Study of Tobermorite

Understanding the deformation mechanisms underlying the mechanical behavior of materials is the key to fundamental and engineering advances in materials' performance. Herein, we focus on crystalline calcium-silicate-hydrates (C-S-H) as a model system with applications in cementitious materials, bone-tissue engineering, drug delivery and refractory materials, and use molecular dynamics simulation to investigate its loading geometry dependent mechanical properties. By comparing various conventional (e.g. shear, compression and tension) and nano-indentation loading geometries, our findings demonstrate that the former loading leads to size-independent mechanical properties while the latter results in size-dependent mechanical properties at the nanometer scales. We found three key mechanisms govern the deformation and thus mechanics of the layered C-S-H: diffusive-controlled and displacive-controlled deformation mechanisms, and strain gradient with local phase transformations. Together, these elaborately classified mechanisms provide deep fundamental understanding and new insights on the relationship between the macro-scale mechanical properties and underlying molecular deformations, providing new opportunities to control and tune the mechanics of layered crystals and other complex materials such as glassy C-S-H, natural composite structures, and manmade laminated structures.

Satellite and central transitions selective 1H/{27Al} D-HMQC experiments at very fast MAS for quadrupolar couplings determination

Recent study has demonstrated the application of the proton-detected heteronuclear multi-quantum coherence (HMQC) at ultrafast Magic Angle Spinning (MAS) to probe quadrupolar nuclei including ¹⁴N and ³⁵Cl. In addition, for half-integer quadrupolar nucleus like ³⁵Cl, the quadrupolar product can be calculated based on the shift difference between the center band of satellite transition (ST) and the central transition (CT) peaks. The applicability of this technique is further investigated on spin I=5/2, namely ²⁷Al nucleus, and kaolin is chosen as the testing sample. However this study is not straightforward owing to a spin quantum number I=5/2 of ²⁷Al nucleus and a small quadrupolar coupling of kaolin. Furthermore, very fast MAS, which is mandatory for proton-detected experiment to suppress ¹H-¹H homonuclear dipolar interactions, introduces additional complexities. It induces the partial overlap of CT and the center band of inner ST (ST1) resonance in addition to the insufficiency of CT-selective excitation by soft-pulse irradiation. In the current work, we employ the constant-time D-HMQC experiment, in which the duration between two recoupling blocks is fixed to a constant value and the arbitrary t₁ increment can be used within this duration. This constant-time D-HMQC enables the precise determination of CT and ST resonance shifts through CT- and ST-selective observations by the indirect spectral width (i) with asynchronized sampling to the top of rotational-echoes for STs and (ii) three times larger than the spinning frequency, respectively. We also numerically and experimentally develop a satellite-selective excitation technique, in which the radio frequency field is applied to the first spinning sideband of ST1 resonance. The satellite-selective 1D single pulse and 2D conventional D-HMQC experiments are demonstrated. The quadrupolar product of ²⁷Al nucleus extracted from the resulting spectra is in good agreement with the literature.

NMR investigations unveil phase composition-property correlations in Sr0.55Na0.45SiO2.775 fast ion conductor

This paper reports results of ²³Na and ²⁹Si solid-state NMR investigations carried out on sodium strontium silicate ion conductor, Sr_0.55Na_0.45SiO_2.775 and presents the first experimental evidence to show that different synthesis conditions induce multiple devitrified phases. Along with 1-dimensional NMR, ²³Na MQMAS spectra have been used to identify the phases corresponding to polymorphs of Na₂Si₂O₅, in addition to the crystalline SrSiO₃ and the glass/amorphous Na₂Si₂O₅ phases. The surprising observation of about an order of magnitude higher ionic conductivity achieved in devitrified samples is attributed to the growth of the crystalline δ-Na₂Si₂O₅ phase within the amorphous Na₂Si₂O₅ phase domains, identified using NMR. Together with XRD and conductivity measurement data, the study leads to the identification of the chemical phase composition and an understanding of the composition-property-structure correlation in this material. Present findings, while do not show any evidence of Na doping in the SrSiO₃ phase confirming earlier reports, explain the large discrepancy in the conductivity reported in the literature.

In-Situ Studies of Structure Transformation and Al Coordination of KAl(MoO₄)₂ during Heating by High Temperature Raman and 27Al NMR Spectroscopies

Recent interest in optimizing composition and synthesis conditions of functional crystals, and the further exploration of new possible candidates for tunable solid-state lasers, has led to significant research on compounds in this family MIMIII(MVIO₄)₂ (MI = alkali metal, MIII = Al, In, Sc, Fe, Bi, lanthanide; MVI = Mo, W). The vibrational modes, structure transformation, and Al coordination of crystalline, glassy, and molten states of KAl(MoO₄)₂ have been investigated by in-situ high temperature Raman scattering and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, together with first principles density functional simulation of room temperature Raman spectrum. The results showed that, under the present fast quenching conditions, Al is present predominantly in [AlO₆] octahedra in both KAl(MoO₄)₂ glass and melt, with the tetrahedrally coordinated Al being minor at approximately 2.7%. The effect of K⁺, from ordered arrangement in the crystal to random distribution in the melt, on the local chemical environment of Al, was also revealed. The distribution and quantitative analysis of different Al coordination subspecies are final discussed and found to be dependent on the thermal history of the glass samples.

Statistics of silicate units in binary glasses

In this paper, we derive a new model to determine the distribution of silicate units in binary glasses (or liquids). The model is based on statistical mechanics and assumes grand canonical ensemble of silicate units which exchange energy and network modifiers from the reservoir. This model complements experimental techniques, which measure short range order in glasses such as nuclear magnetic resonance (NMR) spectroscopy. The model has potential in calculating the amounts of liquid-liquid phase segregation and crystal nucleation, and it can be easily extended to more complicated compositions. The structural relaxation of the glass as probed by NMR spectroscopy is also reported, where the model could find its usefulness.

Modifier cation effects on (29)Si nuclear shielding anisotropies in silicate glasses

We have examined variations in the (29)Si nuclear shielding tensor parameters of SiO4 tetrahedra in a series of seven alkali and alkaline earth silicate glass compositions, Cs2O·4.81 SiO2, Rb2O·3.96 SiO2, Rb2O·2.25 SiO2, K2O·4.48 SiO2, Na2O·4.74 SiO2, BaO·2.64 SiO2, and SrO·2.36 SiO2, using natural abundance (29)Si two-dimensional magic-angle flipping (MAF) experiments. Our analyses of these 2D spectra reveal a linear dependence of the (29)Si nuclear shielding anisotropy of Q((3)) sites on the Si-non-bridging oxygen bond length, which in turn depends on the cation potential and coordination of modifier cations to the non-bridging oxygen. We also demonstrate how a combination of Cu(2+) as a paramagnetic dopant combined with echo train acquisition can reduce the total experiment time of (29)Si 2D NMR measurements by two orders of magnitude, enabling higher throughput 2D NMR studies of glass structure.

Self-healing capacity of nuclear glass observed by NMR spectroscopy

Safe management of high level nuclear waste is a worldwide significant issue for which vitrification has been selected by many countries. There exists a crucial need for improving our understanding of the ageing of the glass under irradiation. While external irradiation by ions provides a rapid simulation of damage induced by alpha decays, short lived actinide doping is more representative of the reality. Here, we report radiological NMR experiments to compare the damage in International Simplified Glass (ISG) when irradiated by these two methods. In the 0.1 mole percent ²⁴⁴Cm doped glass, accumulation of high alpha decay only shows small modifications of the local structure, in sharp contrast to heavy ion irradiation. These results reveal the ability of the alpha particle to partially repair the damage generated by the heavy recoil nuclei highlighting the radiation resistance of nuclear glass and the difficulty to accurately simulate its behaviour by single ion beam irradiations.

Quantitative structure parameters from the NMR spectroscopy of quadrupolar nuclei

Nuclear magnetic resonance (NMR) spectroscopy is one of the most important characterization tools in chemistry, however, 3/4 of the NMR active nuclei are underutilized due to their quadrupolar nature. This short review centers on the development of methods that use solid-state NMR of quadrupolar nuclei for obtaining quantitative structural information. Namely, techniques using dipolar recoupling as well as the resolution afforded by double-rotation are presented for the measurement of spin–spin coupling between quadrupoles, enabling the measurement of internuclear distances and connectivities. Two-dimensional J-resolved-type experiments are then presented for the measurement of dipolar and J coupling, between spin-1/2 and quadrupolar nuclei as well as in pairs of quadrupolar nuclei. Select examples utilizing these techniques for the extraction of structural information are given. Techniques are then described that enable the fine refinement of crystalline structures using solely the electric field gradient tensor, measured using NMR, as a constraint. These approaches enable the solution of crystal structures, from polycrystalline compounds, that are of comparable quality to those solved using single-crystal diffraction.

Structure–composition relationships of bioactive borophosphosilicate glasses probed by multinuclear 11B, 29Si, and 31P solid state NMR

By combining ¹¹B, ²⁹Si, and ³¹P nuclear magnetic resonance (NMR) experimental results, we present the first comprehensive structural investigation of 15 borophosphosilicate (BPS) glasses of the Na₂O–CaO–B₂O₃–SiO₂–P₂O₅ system.

Low-power broadband homonuclear dipolar recoupling in MAS NMR by two-fold symmetry pulse schemes for magnetization transfers and double-quantum excitation

We provide an experimental, numerical, and high-order average Hamiltonian evaluation of an open-ended series of homonuclear dipolar recoupling sequences, SR [Formula: see text] with p=1,2,3,…. While operating at a very low radio-frequency (rf) power, corresponding to a nutation frequency of 1/2 of the magic-angle spinning (MAS) rate (ωnut=ωr/2), these recursively generated double-quantum (2Q) dipolar recoupling schemes offer a progressively improved compensation to resonance offsets and rf inhomogeneity for increasing pulse-sequence order p. The excellent recoupling robustness to these experimental obstacles, as well as to CSA, is demonstrated for 2Q filtering (2QF) experiments and for driving magnetization transfers in 2D NMR correlation spectroscopy, where the sequences may provide either double or zero quantum dipolar Hamiltonians during mixing. Experimental and numerical demonstrations, which mostly target conditions of "ultra-fast" MAS (≳50kHz) and high magnetic fields, are provided for recoupling of (13)C across a wide range of isotropic and anisotropic chemical shifts, as well as dipolar coupling constants, encompassing [2,3-(13)C2]alanine, [1,3-(13)C2]alanine, diammonium [1,4-(13)C2]fumarate, and [U-(13)C]tyrosine. When compared at equal power levels, a superior performance is observed for the SR [Formula: see text] sequences with p⩾3 relative to existing and well-established 2Q recoupling techniques. At ultra-fast MAS, proton decoupling is redundant during the homonuclear dipolar recoupling of dilute spins in organic solids, which renders the family of SR [Formula: see text] schemes the first efficient 2Q recoupling option for general applications, such as 2Q-1Q correlation NMR and high-order multiple-quantum excitation, under truly low-power rf conditions.

Calcium environment in silicate and aluminosilicate glasses probed by ⁴³Ca MQMAS NMR experiments and MD-GIPAW calculations

⁴³Ca MQMAS NMR spectra of three silica-based glasses in which Ca²⁺ ions play different structural roles have been collected and processed in order to extract the underlying NMR parameter distributions. The NMR parameters have been interpreted with the help of molecular dynamics simulations and DFT-GIPAW calculations. This synergetic experimental-computational approach has allowed us to investigate the Ca environment, to estimate Ca coordination numbers from MD-derived models, and to push further the discussion about ⁴³Ca NMR sensitivity to the first and second coordination spheres: ⁴³Ca δiso and Ca-O distance can be successfully correlated as a function of Ca coordination number.

Na/Ca Intermixing around Silicate and Phosphate Groups in Bioactive Phosphosilicate Glasses Revealed by Heteronuclear Solid-State NMR and Molecular Dynamics Simulations

We characterize the intermixing of network-modifying Na(+)/Ca(2+) ions around the silicate (QSi(n)) and phosphate (QP(n)) tetrahedra in a series of 16 Na2O–CaO–SiO2–P2O5 glasses, whose P content and silicate network connectivity were varied independently. The set includes both bioactive and bioinactive compositions and also encompasses two soda-lime-silicate members devoid of P, as well as two CaO–SiO2 glasses and one Na2O–SiO2–P2O5 glass. The various Si/P↔Na/Ca contacts were probed by molecular dynamics (MD) simulations together with heteronuclear magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) experimentation utilizing (23)Na{(31)P} and (23)Na{(29)Si} REDOR, as well as (31)P{ (23)Na} and (29)Si{(23)Na} REAPDOR. We introduce an approach for quantifying the extent of Na(+)/Ca(2+) ordering around a given QP(n) or QSi(n) group, encoded by the preference factor 0⩽ PM ⩽ 1 conveying the relative weights of a random cation intermixing (PM = 0) and complete preference/ordering (PM = 1) for one of the species M, which represents either Na(+) or Ca(2+). The MD-derived preference factors reveal phosphate and silicate species surrounded by Na(+)/Ca(2+) ions intermixed nearly randomly (PM ≲ 0.15), except for the QSi(4) and QSi(1) groups, which manifest more significant cation ordering with preference for Na+ and Ca2+, respectively. The overall weak preferences are essentially independent of the Si and P contents of the glass, whereas PM primarily correlates with the total amount of network modifiers: as the latter is increased, the Na/Ca distribution around the {QP(0), QSi(1), QSi(2)} groups with preference for Ca2(+ )tend to randomize (i.e., PCa decreases), while the PNa-values grow slightly for the {QP(1), QSi(3), QSi(4)} species already preferring coordination of Na. The set of experimental preference factors {PCa} for the orthophosphate (QP(0)) groups extracted from (31)P{(23)Na} REAPDOR NMR-derived M2(P–Na) dipolar second moments agrees well with the MD-generated counterparts. Our results on the Na/Ca intermixing in soda-lime-silicate glasses are discussed in relation to previous reports, highlighting the dependence of the conclusion on the approach to data evaluation.

Direct 17O NMR experimental evidence for Al–NBO bonds in Si-rich and highly polymerized aluminosilicate glasses

Double-resonance ¹⁷O{²⁷Al} NMR unambiguously evidences Al–NBO bonds in rare-earth aluminosilicate glasses.