Anion Distribution in Superionic Ag₃PO₄–AgI Glasses Revealed by Dipolar Solid-State NMR

The structure of roller-quenched fast ion conductive glasses (FICs) (Ag3PO4)x(AgI)1–x (0.15 ≤ x ≤ 0.50) is investigated by 109Ag and 31P solid-state NMR spectroscopies. Monotonic linear dependences of 109Ag and 31P chemical shifts on x are consistent with a statistical distribution of the phosphate and iodide anions. This conclusion is quantitatively confirmed by a new 31P homonuclear magnetic dipolar recoupling method, termed double-quantum-based dipolar recoupling effects nuclear alignment reduction (DQ-DRENAR), which numerically proves a random spatial distribution of the phosphate anions. Altogether these results give the final answer to a long-standing debate on the structure of silver in AgI-based (FIC) glasses, proving the absence of previously postulated silver iodide cluster domains

Quantification of Short and Medium Range Order in Mixed Network Former Glasses of the System GeO₂–NaPO₃: A Combined NMR and X-ray Photoelectron Spectroscopy Study

Glasses in the system <i>x</i>GeO₂–(1–<i>x</i>)­NaPO₃ (0 ≤ <i>x</i> ≤ 0.50) were prepared by conventional melting–quenching and characterized by thermal analysis, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and ³¹P nuclear magnetic resonance (MAS NMR) techniques. The deconvolution of the latter spectra was aided by homonuclear J-resolved and refocused INADEQUATE techniques. The combined analyses of ³¹P MAS NMR and O-1s XPS lineshapes, taking charge and mass balance considerations into account, yield the detailed quantitative speciations of the phosphorus, germanium, and oxygen atoms and their respective connectivities. An internally consistent description is possible without invoking the formation of higher-coordinated germanium species in these glasses, in agreement with experimental evidence in the literature. The structure can be regarded, to a first approximation, as a network consisting of P⁽²⁾ and P⁽³⁾ tetrahedra linked via four-coordinate germanium. As implied by the appearance of P⁽³⁾ units, there is a moderate extent of network modifier sharing between phosphate and germanate network formers, as expressed by the formal melt reaction P⁽²⁾ + Ge⁽⁴⁾ → P⁽³⁾ + Ge⁽³⁾. The equilibrium constant of this reaction is estimated as <i>K</i> = 0.52 ± 0.11, indicating a preferential attraction of network modifier by the phosphorus component. These conclusions are qualitatively supported by Raman spectroscopy as well as ³¹P­{²³Na} and ³¹P­{²³Na} rotational echo double resonance (REDOR) NMR results. The combined interpretation of O-1s XPS and ³¹P MAS NMR spectra shows further that there are clear deviations from a random connectivity scenario: heteroatomic P–O–Ge linkages are favored over homoatomic P–O–P and Ge–O–Ge linkages

Intermediate Role of Gallium in Oxidic Glasses: Solid State NMR Structural Studies of the Ga₂O₃–NaPO₃ System

A series of (NaPO₃)_1–<i>x</i>(Ga₂O₃)_<i>x</i> glasses (0 ≤ <i>x</i> ≤ 0.35) prepared by conventional melt-quenching methods has been structurally characterized by various complementary high resolution one-dimensional and two-dimensional (2D) solid state magic angle spinning nuclear magnetic resonance (MAS NMR) techniques, which were validated by corresponding experiments on the crystalline model compounds GaPO₄ (quartz) and Ga­(PO₃)₃. Alloying NaPO₃ glass by Ga₂O₃ results in a marked increase in the glass transition temperature, similar to the effect observed with Al₂O₃. At the atomic level, multiple phosphate species Q^<i>n</i>_<i>m</i>Ga (<i>n</i> = 0, 1, and 2; <i>m</i> = 0, 1, 2, and 3) can be observed. Here <i>n</i> denotes the number of P–O–P and <i>m</i> the number of P–O–Ga linkages, and (<i>m</i> + <i>n</i> ≤ 4). For resolved resonances, the value of <i>n</i> can be quantified by 2D J-resolved spectroscopy, refocused INADEQUATE, and a recently developed homonuclear dipolar recoupling method termed DQ-DRENAR (double-quantum based dipolar recoupling effects nuclear alignment reduction). Ga³⁺ is dominantly found in six-coordination in low-Ga glasses, whereas in glasses with <i>x</i> > 0.15, lower-coordinated Ga environments are increasingly favored. The connectivity between P and Ga can be assessed by heteronuclear ⁷¹Ga/³¹P dipolar recoupling experiments using ⁷¹Ga­{³¹P} rotational echo double resonance (REDOR) and ³¹P {⁷¹Ga} rotational echo adiabatic passage double resonance (READPOR) techniques. Up to <i>x</i> = 0.25, the limiting composition where this is possible, the second coordination sphere of all the gallium atoms is fully dominated by phosphorus atoms. Above <i>x</i> = 0.25, ⁷¹Ga static and MAS NMR as well as REDOR experiments give clear spectroscopic evidence of Ga–O–Ga connectivity. ³¹P/²³Na REDOR and REAPDOR results indicate that gallium has no dispersion effect on sodium ions in these glasses. They also indicate significant differences in the strength of dipolar interactions for distinct Q^<i>n</i>_<i>m</i>Ga species, consistent with bond valence considerations. On the basis of these results, a comprehensive structural model is developed. This model explains the compositional trend of the glass transition temperatures in terms of the concentration of bridging oxygen species (P–O–P, P–O–Ga, and Ga–O–Ga) in these glasses. The results provide new insights into the role of Ga₂O₃ as an intermediate oxide, with features of both network modifier and network former in oxide glasses

High Surface Area Mesoporous GaPO₄–SiO₂ Sol–Gel Glasses: Structural Investigation by Advanced Solid-State NMR

Mesoporous silica–gallium phosphate glasses along the composition line, <i>x</i>GaPO₄–(1 – <i>x</i>)­SiO₂ (<i>x</i> = 0.5, 0.33, 0.20, 0.14, and 0.11, respectively) were prepared via the sol–gel route. This glass-forming range is significantly wider than that accessible by previously reported routes. The glasses exhibit a mesoporous structure with surface areas around 400 m²/g, after calcination at 650 °C. The structural evolution from liquid to gel to glass was analyzed by liquid and advanced solid-state nuclear magnetic resonance techniques. The NMR results indicate that the glasses consist of GaPO₄ and SiO₂ nanodomains. With increasing GaPO₄ content, the sizes of the GaPO₄ domains become larger. Evidence for the connection of both domains at their interfaces by P–O–Si and Ga–O–Si linkages is presented by advanced ⁷¹Ga, ³¹P, and ²⁹Si high-resolution dipolar solid-state NMR methods

Fast Ionic Conducting Glasses in the System 20LiCl–40Li₂O–(80–<i>x</i>)PO_5/2–<i>x</i>MoO₃: The Structural Dependence of Ion Conductivity Studied by Solid-State Nuclear Magnetic Resonance Spectroscopy

Glass samples with the stoichiometric components of 20LiCl–40Li2O–(80–x)­PO5/2–xMoO3 (x = 0, 10, 20, 30, 40, 50, 60, and 70) are synthesized. The dependence of ion conductivity on structure is investigated. The structures of these glasses are investigated by Raman and solid-state nuclear magnetic resonance (NMR) spectroscopy. The phosphorus structure unit Q(n)mMo species (where n represents the number of P–O–P linkages in per phosphorus species, while m denotes the number of P–O–Mo linkages) are identified by 31P 2D J-resolved and magic angle spinning NMR spectra. With the increase of MoO3, the phosphorus chains are broken into dimer phosphorus Q(1)0Mo and orthophosphate Q(0)1Mo species successively. Raman shows that both 4- and 6-coordination molybdenum exists in the glasses. In the glasses with x > 20, 6-coordination molybdenum becomes dominated. With the replacement of P by Mo, a large number of Li+ ions transfer from the phosphorus phases into molybdenum phases, resulting in an enhancement of the ionic conductivity. When the substituted amount (x) is 70, the ionic conductivity can be increased by about 250 times, i.e., reaches 1.05 × 10–5 S·cm–1

Medium-Range Order in Sol–Gel Prepared Al₂O₃–SiO₂ Glasses: New Results from Solid-State NMR

The medium-range order of 0.5Al₂O₃–<i>x</i>SiO₂ glasses (1 ≤ <i>x</i> ≤ 6) prepared via a new sol–gel route from the Al lactate precursor has been studied by ²⁹Si and ²⁷Al single- and double-resonance solid-state NMR techniques. For high-alumina samples Si–O–Al connectivities are detected by ²⁹Si MAS NMR as well as by ²⁹Si­{²⁷Al} rotational echo adiabatic passage double-resonance (REAPDOR) spectroscopy. To boost the signal-to-noise ratio, the REAPDOR experiment was combined with a Carr–Purcell–Meiboom–Gill (CPMG) echo train acquisition. While all five silicon units Q⁽⁴⁾_<i>m</i>Al (0 ≤ <i>m</i> ≤ 4) are detectable in appreciable concentrations for <i>x</i> = 1, the spectra indicate that the average number of Al species bound to silicon, ⟨<i>m</i>_Al⟩, gradually decreases toward higher <i>x</i> values, as expected. The ²⁷Al MAS NMR spectra reveal four-, five-, and six-coordinated aluminum in these glasses. For <i>x</i> ≥ 3, the Al species detected are essentially independent of sample composition indicating a constant structural environment of Al. In contrast, for <i>x</i> = 1 and 2, an increase in the ²⁷Al isotropic chemical shifts suggests an increasing number of Al···Al proximities. Consistent with this finding, two-dimensional ²⁷Al–²⁷Al double-quantum/single-quantum correlation spectroscopy reveals spatial proximities among and between all types of aluminum species present. On the basis of the complementary evidence from these single- and double-resonance experiments, a model for the medium-range order of these glasses is developed

Fluorophosphate Upconversion-Luminescent Glass-Ceramics Containing Ba₂LaF₇:Er³⁺ Nanocrystals: An Advanced Solid-State Nuclear Magnetic Resonance Study

Fluorophosphate glass-ceramics containing Ba2LaF7:Er3+ nanocrystal upconversion-luminescent materials were prepared via a melt-quenching method, followed by heat treatment. The glass structure evolution, which was induced by the change in composition, as well as heat treatment, was characterized via solid-state nuclear magnetic resonance (SSNMR) spectroscopy. Multiple QnmLa phosphorous species (n and m represent the numbers of P–O–P and P–O–La bonds, respectively) were proved via multiple 31P magic-angle spinning (MAS). Three fluorine species [P–F···Na, Ba–F···Na, and (Ba, La)–F···Na] were resolved using 19F MAS spectra. Unlike Y3+, La3+ cannot attract F to form an La–F···Na linkage in phosphate glasses. Furthermore, dissimilar to the competition behavior between the positive ions to attract F–, the 19F NMR results demonstrates that Ba2+ and La3+ combined preferentially to attract F–, thus inducing the preferential formation of the (Ba, La)–F···Na species, followed by the ultimate precipitation of the Ba2LaF7 crystal after the heat treatment. The paramagnetic effects of Er3+ on 19F and 31P indicates that there are still many Er3+ ions in the glass phase after crystallization, even though the Er3+ ions dominated the Ba2LaF7 crystal. The evolution of the 31P spectrum via crystallization indicates that the precipitation of Ba2LaF7 facilitates the polymerization of the phosphorus glass network. Finally, a structural evolution model is developed