Germanosilicate and alkali germanosilicate glass structure: New insights from high-resolution oxygen-17 NMR

We present here triple-quantum, magic-angle spinning (3QMAS) NMR spectra for ¹⁷O in a SiO₂–GeO₂ binary glass, and for two sodium germanosilicate glasses, all with Si/Ge ratios of 1. In the binary germanosilicate, three NMR peaks are partially resolved, and correspond to the three types of bridging oxygens, Si–O–Si, Si–O–Ge, and Ge–O–Ge. Peak areas indicate that the relative abundances of these species are close to those expected for random mixing of the Si and Ge in the network. In a sodium germanosilicate glass with a relatively low Na content (Na₂O ≈ 8 mol%), the spectra demonstrate the formation of significant fractions of both nonbridging oxygens bonded to Si, and of oxygens bonded to Ge in five- or six-coordination. At higher Na content (Na₂O ≈ 31%), most or all Ge is four-coordinated and network modification is dominated by the formation of NBO on Si and on Ge. Models of physical properties of alkali germanosilicates, in which modifier oxides are distributed between the Si and Ge components of the network in proportion to the Si/Ge ratio, are thus supported, as is extensive mixing of Si and Ge.     

Effects of cation field strength on the structure of aluminoborosilicate glasses: High-resolution B, Al and Na MAS NMR

Among the most important adjustable compositional variables in controlling glass and glass-melt properties are the relative proportions of network modifiers with varying cation field strength (ratio of charge to radius). Here we determine the details of structural changes caused by variations in the ratio CaO/Na₂O in two series of aluminoborosilicate glasses with different contents of boron oxide. Using high-resolution, high field ¹¹B and ²⁷Al MAS NMR, we report precise values of contents of three- and four-coordinated boron (N₄) and of four- and five-coordinated aluminum (^[5]Al), and calculate fractions of non-bridging oxygens (NBOs). Increasing CaO/Na₂O dramatically lowers N₄ and increases NBO and ^[5]Al, but effects are non-linear with composition. Boron content affects these trends because of energetic constraints of mixing of various network cations. ²³Na spectra reveal slight but systematic increases in the mean Na-O distance with increasing CaO/Na₂O, suggesting that in Ca-rich glasses, Na⁺ has a higher ratio of bridging to non-bridging oxygens in its coordination shell. All of these changes can be understood by the tendency of higher field strength modifier cations to promote the concentration of negative charges on non-bridging oxygens in their local coordination environment, systematically converting four- to three-coordinated boron.     

Study of the alkaline environment in mixed alkali compositions by multiple-quantum magic angle nuclear magnetic resonance (MQ–MAS NMR)

45S5 Bioglasses of the composition 46.1 SiO₂–2.6 P₂O₅–26.9 CaO–(24.4 ? x) Na₂O–xMe₂O (Me = Li or K) have been investigated using MAS NMR and MQ–MAS NMR methods. The analysis of the ²⁹Si MAS NMR spectrum revealed two lineshapes whose chemical shift is consistent with two silica Q_n=2,3 species. The ³¹P MAS NMR spectrum reveals the effect of both Na and Ca ions. The chemical shift of the observed ³¹P signal is intermediate between those of Na₃PO₄ (near 10 ppm) and Ca₃(PO₄)₂ (near 3–0 ppm) species. The ²³Na MAS NMR spectra were observed in the alkali oxide composition: 24.4 Na₂O, 12.2 Na₂O–12.2 K₂O and 12.2 Na₂O–12.2 Li₂O. The substitution of Na with Li or K was done to determine the extend of alteration of the glass structure. This goal was best accomplished by ²³Na MQ–MAS NMR. The two-dimensional spectra revealed three sites in the 24.4 mol% Na₂O glass. These sites were not resolved in the 1D MAS NMR spectroscopy. In the mixed glasses, only two sites were obtained.     

Solid-state NMR study of metastable immiscibility in alkali borosilicate glasses

In several series of lithium, sodium, and potassium borosilicate glasses whose compositions traverse known regions of liquid-liquid phase separation, we have applied triple-quantum magic-angle spinning (3QMAS) ¹¹B and ¹⁷O NMR to obtain high-resolution information about short-range structure and connections among various network structural units, and their variation with composition and thermal history. Oxygen-17 3QMAS spectra reveal changes in connectivities between silicate and BO₃ (^[3]B) and BO₄ (^[4]B) units, by quantifying populations of bridging oxygens such as B-O-B, Si-O-B and Si-O-Si, and of non-bridging oxygens. ^[3]B-O-Si and ^[4]B-O-Si as well as ^[3]B-O-^[3]B and ^[4]B-O-^[3]B linkages can be distinguished. ¹¹B MAS and 3QMAS at a magnetic field of 14.1 T allow proportions of several borate units to be determined, including ^[3]B in boroxol ring and non-ring sites and ^[4]B with 3 versus 4 Si neighbors. By combining the ¹¹B and ¹⁷O NMR results, detailed information on Si/B mixing in sodium borosilicates can be derived, showing, for example, that ^[4]B and non-ring ^[3]B tend to mix with silicate units, while ring ^[3]B is mainly connected to borate groups. In a preliminary study of the effects of varying alkali cation, potassium-containing glasses are similar to those in the sodium borosilicate system, but a lithium borosilicate seems to exhibit considerably greater chemical heterogeneity. In annealing experiments that converted an optically clear to obviously phase-separated glasses, the ratio of ^[3]B to ^[4]B does not change significantly, but part of the non-ring ^[3]B converts to ring ^[3]B as the degree of unmixing increases.     

O K-edge XANES studies of alkali germanophosphate glasses

Alkali germanophosphate glasses exhibit a ‘germanate anomaly’, which is a maximum or minimum in their density profiles with the addition of alkali oxide. Several structural mechanisms have previously been recognized to affect the density behavior of these glasses: (1) depolymerization of the PO₄ tetrahedral network; (2) conversion of 4- to 3-membered GeO₄ rings and (3) depolymerization of the GeO₄ network through the formation of non-bridging oxygens. A coordination change from 4- to 5-fold Ge has been detected in alkali germanate glasses, which also exhibit the germanate anomaly. In this study, oxygen K-edge XANES of alkali germanophosphate glasses is used to investigate the coordination change of Ge with increasing alkali content and at various Ge to P ratios. The dominant peak at ～538 eV in the spectra undergoes a shift to lower energy of ～0.4 eV, indicating the formation of 5-fold Ge similar to previous studies on alkali germanate glasses. However, the formation of 5-fold Ge does not correlate with the density anomaly compositions of these glasses suggesting that the appearance of higher-coordinated Ge species is not controlling the anomaly behavior.     

Glass formation in and structural investigation of Li2S + GeS2 + GeO2 composition using Raman and IR spectroscopy

Li₂S + GeS₂ + GeO₂ ternary glasses have been prepared and a wide glass-forming range was obtained. The glass transition temperatures increase with the GeO₂ concentration in the glasses. The vibrational modes of both bridging (Ge–S–Ge) and non-bridging (Ge–S⁻) sulfurs are observed in Raman and IR spectra of binary Li₂S + GeS₂ glasses. Additions of GeO₂ to this binary glass increase the bridging oxygen band (Ge–O–Ge) at the expense of decreasing the bridging sulfur band (Ge–S–Ge), whereas the bands associated with the non-bridging sulfurs (Ge–S⁻) remain constant in intensity up to high GeO₂ concentrations. At higher concentrations of GeO₂ (⩾60%), the non-bridging oxygen band, which is not observed at low and intermediate GeO₂ concentrations, appears and grows stronger. From these observations, it is suggested that the added lithium ions favor the non-bridging sulfur sites over the oxygen sites to form non-bridging sulfurs, whereas the added oxygen prefers the higher field strength Ge⁴⁺ cation to form bridging Ge–O–Ge bonds. The structural groups in the Li₂S + GeS₂ + GeO₂ glasses that are consistent with results of Raman and IR spectra are described and are used to develop a structural model of these glasses.     

Hydrothermal crystallization in the NaOH-GeO2-H2O system at 350°C: Mechanism of self-assembly of subpolyhedral clusters in Na4Ge

The Na,Ge^[6]-germanates of the compositions Na₄Ge ₄ ^[6] Ge ₅ ^[4] O₂₀ and Na₃HGe ₄ ^[6] Ge ₃ ^[4] O₁₆ · 4H₂O are crystallized in the NaOH-GeO₂-H₂O system under a pressure of 0.05 GPa and a temperature of 350°C. The structure of the germanates consists of open frameworks of M octahedra (Ge^[6]) and T tetrahedra (Ge^[4]). With an increase in the NaOH concentration, the crystallization fields change in the sequence R-GeO₂ (rutile structure type) ⇒ R-GeO₂ + Na₄Ge ₄ ^[6] Ge ₅ ^[4] O₂₀ ⇒ Na₄Ge ₄ ^[6] Ge ₅ ^[4] O₂₀ + Na₃HGe ₄ ^[6] O₁₆ · 4H₂O ⇒ Na₃HGe ₄ ^[6] Ge ₃ ^[4] O₁₆ · 4H₂O. The phase formation for Na,Ge^[6] germanates and Na,Ti silicates (Na₄Ti ₄ ^[6] Si ₃ ^[4] O₁₆ · 6H₂O and NaHTi ₂ ^[6] O₇ · 2H₂O) is considered based on the model of the matrix assembly of crystal structures from invariant Ge^[6],Ge^[4] subpolyhedral structural units. The homologous family of phases consisting of Na₄Ge ₄ ^[6] Ge ₅ ^[4] O₂₀ (M ₄ T ₅ framework), Na₃HGe ₄ ^[6] Ge ₃ ^[4] O₁₆ · 4H₂O, Na₄Ti ₄ ^[6] Si ₃ ^[4] O₁₆ · 6H₂O (M ₄ T ₃ framework), and NaHTi ₂ ^[6] Si^[4]O₇ · 2H₂O (M ₄ T ₂ framework) phases with topologically equivalent one-dimensional MT structures is singled out. __________ Translated from Kristallografiya, Vol. 48, No. 5, 2003, pp. 950–958. Original Russian Text Copyright ? 2003 by Ilyushin, Dem’yanets. Dedicated to the 60th Anniversary of the Shubnikov Institute of Crystallography of the Russian Academy of Sciences     

Network structure of multi-component sodium borosilicate glasses by neutron diffraction

Neutron diffraction structure study has been performed on multi-component sodium borosilicate based waste glasses with the composition of (65 − x)SiO_2. · xB₂O₃ · 25Na₂O · 5BaO · 5ZrO₂, x = 5–15 mol%. The maximum momentum transfer of the experimental structure factor was 30 Å⁻¹, which made available to determine the distribution function with high r-space resolution. Reverse Monte Carlo modelling was applied to calculate several partial atomic pair correlation functions, nearest neighbor distances and coordination numbers have been revealed. The characteristic features of Si–O and Si–Si distributions are similar for all glassy samples, suggesting that the Si–O network consisting of tetrahedral SiO₄ units is highly stable even in the multi-component glasses. The B–O correlations proved to be fairly complex, two distinct first neighbor distances are present at 1.40 Å and 1.60 Å, the latter equals the Si–O distance. Coordination number distribution analyzes has revealed 3 and four-coordinated boron atoms. The O–O distribution suggests a network configuration consisting of boron rich and silicon rich regions. Our findings are consistent with a structure model where the boron rich network contains mostly trigonal BO₃ units, and the silicon rich network is formed by a mixed continuous network of ^[4]Si–O–Si^[4] with several different ^[4]B–O–Si^[4] and ^[3]B–O–Si^[4] linkages.     

Ca–Mg mixing in aluminosilicate glasses: An investigation using 17O MAS and 3QMAS and 27Al MAS NMR

To better understand non-framework cation mixing in Ca–Mg aluminosilicate glasses, ¹⁷O MAS and 3QMAS NMR studies were done on glasses on the Mg₃Al₂Si₃O₁₂–Ca₃Al₂Si₃O₁₂ join as well as several compositions with lower Al/Si ratios. While not all of the oxygen sites are fully resolved, the non-bridging oxygen associated with two Ca and one Mg cations is under-represented relative to that predicted by a model assuming random Ca/Mg mixing. Therefore, local non-bridging oxygen environments are rich in Mg, and extra Ca must be associated with charged bridging oxygens such as Al–O–Si. The deviation from random mixing increases as the Al/Si ratio decreases and as X_Mg approaches 0.50, suggesting a reduction in configurational entropy that may be compensated for by other sources, including mixing of network forming cations. Al–O–Al is detected, and appears to increase with increasing X_Mg and increasing Al/Si. High field ²⁷Al MAS NMR also shows that these glasses all have substantial concentrations of ^[5]Al, but no detectable ^[6]Al (0.5% detection limit). The amount of ^[5]Al increases non-linearly as X_Mg increases and very slightly as Al/Si increases.     

Raman and NMR study of bioactive Na2O–MgO–CaO–P2O5–SiO2 glasses

The results of a structural study combining NMR and Raman spectroscopy of several melt-derived glasses in the system Na₂O–MgO–CaO–P₂O₅–SiO₂ are presented. The Raman spectra show clear changes in the Si–O–Si vibrational modes (related to the bridging oxygen atoms, BO) and also verify the presence of non-bridging oxygen atoms (NBO), also named terminal oxygens. The intensity of the Si–O–NBO stretching mode depends on the cation concentration. It can be concluded from the NMR studies that the MgO-containing samples have orthophosphate units charge-compensated by Ca²⁺ and Mg²⁺. The silicate matrix also contains both types of two-valent cations and consists of Q² and Q¹ units. Similarly, the Na₂O-containing samples contain isolated orthophosphate units in a silicate matrix (Q² and Q³ units), both charge-compensated by mixed cations Ca²⁺ and Na⁺. These experimental data were compared with theoretical parameters given by the Stevels model, which is a suitable tool for understanding bioactive behavior of these glasses. Furthermore, results of the in vitro tests carried out in simulated body fluids are presented and compared with both Raman and NMR structural data.     

X-ray diffraction study of Na2OGeO2 melts

The structure of Na₂OGeO₂ melts in the temperature range from 1100 to 1150°C has been investigated with the high temperature X-ray diffraction technique. Comparing the radial distribution functions obtained for the melts with those for the corresponding glasses, the first peak due to the GeO interatomic distance is invariant upon melting, although it becomes broader due to thermal vibration. The second peak for the GeGe interatomic distance for melts shifts toward the large distance, which is explained by broadening of the GeOGe bond angle, not by the thermal expansion of the GeOGe bond. The composition dependences of GeO distances and coordination numbers of the Ge⁴⁺ ion of the melts are found to be almost the same as the corresponding glasses, indicating that even in melts at such high temperatures 6-fold coordinated Ge⁴⁺ ions are present and their content changes with the Na₂O content as in the case of the corresponding glasses.     

XAFS study of six-coordinated silicon in R2O–SiO2–P2O5 (R = Li,Na, K) glasses

The local structures around silicon and phosphorous atoms in R₂O–SiO₂–P₂O₅ (R = Li, Na and K) glasses have been investigated using Si and P K-edge XAFS spectroscopy by transmission mode at BL-4 and BL-3 at the synchrotron facility in Ritsumeikan University. As a result of XANES and EXAFS analyzes, six-coordinated silicon atoms were observed in the glasses. The fraction of six-coordinated silicon atom changed with increasing of the concentration of alkali oxide and P₂O₅. For the change of concentration of alkali oxide, it takes maximum values which are 60% in Li₂O, 90% in Na₂O and 85% in K₂O system at 20 mol% alkali oxide. It gradually increased up for the increase of the concentration of P₂O₅ to 55% in Li₂O, 80% in Na₂O and 90% in K₂O system. The Si–O inter-atomic distance in the glasses changes from 1.63 to 1.79 Å with increasing the fraction of six-coordinated silicon atom. On the other hand, it was not observed the local structural change around the phosphorous atom.     

Validation of the mechanism of nitrogen/oxygen substitution in Li–Na–Pb–P–O–N oxynitride phosphate glasses

As evidenced by O_1s X-ray photoelectron spectroscopy, there are three kinds of oxygen atoms in Li_0.25Na_0.25Pb_0.25PO_3−3x/2N_x oxynitride phosphate glasses (0 < x ⩽ 0.67): (i) bridging oxygens (BO) which interconnect the P(O, N)₄ tetrahedral units of the glass network (ii) non-bridging oxygens (NBO) which coordinate the modifier cations and (iii) oxygen atoms, also non-bridging, which belong to PO₄ tetrahedra involved in the coordination sphere of Pb²⁺. The variation of their relative proportions as the nitrogen content increases is explained according to the nitridation mechanism previously proposed for this series of glasses. Moreover, the variation of the BO/NBO ratio confirms that the nitrogen/oxygen substitution taking place during nitridation obeys the simple chemical equivalences already stated.     

X-ray diffraction studies of glasses in the systems Na2OMeOSiO2 (MeMg,Zn, Ca,Ba)

X-ray diffraction studies of glasses in the following ternary systems have been made: Na₂OMgOSiO₂, Na₂OZnOSiO₂, Na₂OCaOSiO₂ and Na₂OBaOSiO₂. The following heavy atom substitutions have been used: Ag for Na and Ge for Si. The changes in the electron radial distribution curves resulting from AgNa replacement can be explained as amplifications of relatively well-defined NaSi distances, which are nearly the same in all the glasses investigated. The GeSi substitution causes changes which can be explained on the basis of isostructural GeSi substitutions.     

Study of the structure of alkali–silica reaction gel by high-resolution NMR spectroscopy

The alkali–silica reaction is a deleterious chemical process that can occur in concrete. The product of the reaction is an amorphous silicate material with gel characteristics, whose high expansion properties may cause cracking in the matrix and in the discrete aggregate of particles, leading to severe deterioration of the concrete structure. Structural information of this gel at the atomic scale can provide critical information on how to develop appropriate repair of the affected structure. Samples of this gel, produced under in-service conditions in a large concrete dam, were studied by ²⁹Si and ²³Na high-resolution nuclear magnetic resonance spectroscopy, triple quantum magic angle spinning ²³Na nuclear magnetic resonance, scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. The short-range atomic structure of the compound was determined as an amorphous potassium–hydroxide–silicate glass, with a Q³-like dominant silicate connectivity, having a silicate speciation highly disproportioned when compared with potassium–silicate glasses with the same K₂O content. Sodium ions are mostly segregated from the bulk amorphous silicate network, forming crystal domains attributed to the trona compound (sodium sesquicarbonate, Na₂CO₃ · NaHCO₃ · 2H₂O). The structural picture at atomic scale obtained in this study gives support for double-layer models of the expansive properties of the alkali–silica reaction gel.     

Effect of quenching rate and annealing on ESR of Cu2+ and γ-induced Cd+ in low alkali borate glasses

A new ESR hyperfine structure (hfs) of Cu²⁺ was detected in moderately (cooling rate ≈ 3 K/s) and rapidly (10³–10⁵K/s) quenched borate glasses with Na₂O content smaller than 5 mol% in addition to the already reported three patterns, spectra I, II and III, for the moderately quenched borate glasses with 5 ? [Na₂O]?13, 20 ? [Na₂O] ?37 and 55 ? [Na₂O] ? 75, respectively. The ESR parameters of this spectrum were much the same as those of II, but a significant difference was observed between thermal stabilities of these spectra. The newly observed spectrum was named spectrum II′. A superposition of spectra II′ and I was seen in low alkali borate glasses ([Na₂O] ? 5 mol%). The relative intensity of II′ to I increased remarkably with increasing quenching rate of the melts and with decreasing alkali content under the same quenching condition. In addition, spectrum II′ was found to relax into I upon thermal annealing of the glasses. Similar changes were observed also in ESR of the γ-induced Cd⁺ with 5s¹ electronic configuration. A tentative model to account for the spectral changes was proposed on the basis of the characteristics of B—O bonding.     

Network connectivity in aluminoborosilicate glasses: A high-resolution 11B, 27Al and 17O NMR study

O-17 enriched calcium and potassium aluminoborosilicate glasses with compositions similar (or analogous) to commercial E-glass were made and studied by ¹¹B, ²⁷Al and ¹⁷O NMR in order to explore the network speciation and mixing. Fractions of non-bridging oxygens associated with silicon and boron can be obtained simply from ¹⁷O magic angle spinning (MAS) NMR, while bridging oxygen populations result from complete peak assignment of high-resolution ¹⁷O triple quantum magic angle spinning (3QMAS) NMR spectra. Dramatic differences between the two samples in boron and oxygen speciation demonstrate a large effect of the charge of the modifier cation on mixing behavior and on the stabilization of non-bridging oxygens. The observed oxygen speciation is compared with that calculated from two models: random mixing of Si, B, and Al, and mixing with avoidance of linkages between tetrahedral aluminum and tetrahedral boron groups. Mixing of B and Al in the K-containing glass tends to follow the latter, while the network speciation in the Ca-containing glass is closer to the random model. The effects of Al and Si on boron speciation are discussed, and indicate that the maximum fractions of four-coordinated boron observed in a wide range of glass compositions are closely related to tetrahedral B and Al ‘avoidance’. A modified ‘Dell and Bray’ model is proposed which seems to accurately approximate boron speciation in many alkali aluminoborosilicate glasses. The higher field strength modifier cation (Ca²⁺ vs. K⁺) promotes the formation of non-bridging oxygen, trigonal over tetrahedral boron, and at least minor amounts of five-coordinated aluminum.     

Phosphate speciation in sodium borosilicate glasses studied by nuclear magnetic resonance

The structure of RNa₂O · B₂O₃ · KSiO₂ · xP₂O₅ (0.5 < R < 2; 0.86 < K < 3) borosilicate glasses has been studied by nuclear magnetic resonance (NMR). ³¹P magic angle spinning (MAS), double quantum-magic angle spinning (DQ-MAS) and ³¹P–¹¹B transfer of populations under double resonance magic angle spinning (TRAPDOR MAS) NMR were used to determine the phosphate speciation in the glasses and their connectivity with the borosilicate network. The structure of the glass network was characterized with ¹¹B, ²⁹Si and ²³Na MAS NMR. Ab initio calculations of the ³¹P chemical shielding were carried out in order to confirm the connectivity between phosphorus and the structural units of the borosilicate glass network. Na₃PO₄ (monophosphate), Na₄P₂O₇ (diphosphate) and P–O–B species (mono- and diphosphate groups with borate units as the next nearest neighbors) are found all along the compositional range studied. The proportion of the P–O–B groups increases as the glass optical basicity decreases, while the proportions of mono- and diphosphate species decrease. The change in the glass transition temperature of the phospho-borosilicate glasses with respect to that of the borosilicate ones is discussed in terms of the structural characterization. The formation of phosphate species gives rise to the increase in T_g, which is attributed to the re-polymerization of the silicate network, while the formation of P–O–B bonds weakens the glass network and produces a decrease in the glass transition temperature.     

A combined 77Se NMR and Raman spectroscopic study of the structure of GexSe1-x glasses: Towards a self consistent structural model

The short-range structures of stoichiometric and Se-deficient binary Ge_xSe_100 ?x glasses with 42 ≥ x ≥ 33.33 have been investigated using a combination of Raman and ⁷⁷Se Car–Purcell–Meiboom–Gill (CPMG) spikelet nuclear magnetic resonance (NMR) spectroscopy. When taken together, these spectroscopic results allow for self-consistent assignment of average ⁷⁷Se NMR isotropic chemical shifts to Se atoms in various coordination environments in Ge_xSe_100 ?x glasses. Analysis of the compositional variation of the relative concentrations of these Se environments indicates considerable violation of chemical order in the nearest-neighbor coordination environments of the constituent atoms in the stoichiometric Ge_33.33Se_66.67 glass. On the other hand, the presence of a random distribution of Ge―Ge bonds can be inferred in the Se-deficient glasses. Simulations of the previously published ⁷⁷Se NMR line shapes of Se-excess glasses on the basis of the revised structural assignments of ⁷⁷Se NMR chemical shifts obtained in this study conclusively indicate that the structure of these glasses is intermediate between a randomly connected and a fully clustered network of GeSe₄ tetrahedra and Se chains.     

O nuclear magnetic resonance study of Na2O-P2O5 glasses

The preparation and structural investigation of ¹⁷O-enriched xNa₂O-(100−x)P₂O₅ glasses (46.5?x?62.8) by nuclear magnetic resonance (NMR) is described. Enriched phosphoric acid was prepared by hydrolysis of PCl₅ with ¹⁷O-enriched water and neutralized with sodium carbonate. The sodium metaphosphate was then melted at 800 °C for 15 h and quenched. Polyphosphate and ultraphosphate glass compositions were prepared by remelting the metaphosphate with sodium carbonate and phosphorus pentoxide, respectively. ³¹P magic angle sample spinning (MAS) NMR was used to determine the Na₂O/P₂O₅ content in the glasses. ¹⁷O NMR spectra (quadrupole echo for non-rotating samples and multiple-quantum excitation for rotating samples (MQMAS)) show two oxygen sites in the samples with large quadrupolar coupling constants (4.7 and 7.7 MHz), in accordance with the high phosphorus electronegativity. According to the correlation of ¹⁷O quadrupolar constants with bond ionicity, these two components are attributed to bridging P-O-P and non-bridging P-O?Na oxygens. The average P-O-P bond angle is estimated with the quadrupolar asymmetry derived from the fit of the static echo spectra. The MQMAS spectrum shows a distribution of non-bridging oxygen chemical shifts, attributed to a variation of bond length and angle.