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

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

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

Percolative Channels for Superionic Conduction in an Amorphous Conductor

All-solid-state batteries greatly rely on high-performance solid electrolytes. However, the bottlenecks in solid electrolytes are their low ionic conductivity and stability. Here we report a new series of amorphous xAgI·(1–x)Ag3PS4 (x = 0∼0.8 with interval of 0.1) conductors, among which the sample with x = 0.8 exhibits the highest ionic conductivity (about 1.1 × 10–2 S cm-1) and ultrahigh chemical stability. We discovered the existence of mixed disordered Ag3PS4 and AgI clusters in the amorphous conductors using solid-state nuclear magnetic resonance spectroscopy. The high ionic conductivity was ascribed to the formation of the interconnecting AgI clusters, i.e., the percolative channels for superionic conduction. The composition dependence of the ionic conductivity for this series of amorphous conductors was clarified by a continuum percolation model. These findings provide fundamental guidance for designing and fabricating high-performance amorphous solid electrolytes for all-solid-state batteries

Percolative Channels for Superionic Conduction in an Amorphous Conductor

All-solid-state batteries greatly rely on high-performance solid electrolytes. However, the bottlenecks in solid electrolytes are their low ionic conductivity and stability. Here we report a new series of amorphous xAgI·(1–x)Ag3PS4 (x = 0∼0.8 with interval of 0.1) conductors, among which the sample with x = 0.8 exhibits the highest ionic conductivity (about 1.1 × 10–2 S cm-1) and ultrahigh chemical stability. We discovered the existence of mixed disordered Ag3PS4 and AgI clusters in the amorphous conductors using solid-state nuclear magnetic resonance spectroscopy. The high ionic conductivity was ascribed to the formation of the interconnecting AgI clusters, i.e., the percolative channels for superionic conduction. The composition dependence of the ionic conductivity for this series of amorphous conductors was clarified by a continuum percolation model. These findings provide fundamental guidance for designing and fabricating high-performance amorphous solid electrolytes for all-solid-state batteries

Glass-to-Crystal Transition in Li_1+<i>x</i>Al_<i>x</i>Ge_2–<i>x</i>(PO₄)₃: Structural Aspects Studied by Solid State NMR

The structural aspects of the glass-to-crystal transition in the technologically important ion conducting glass ceramic system Li_1+<i>x</i>Al_<i>x</i>Ge_2–<i>x</i>(PO₄)₃ (0 ≤ <i>x</i> ≤ 0.75) have been examined by complementary multinuclear solid state nuclear magnetic single and double-resonance experiments. In the crystalline state, the materials form solid solutions in the NASICON structure, with additional nanocrystalline AlPO₄ present at <i>x</i> values ≥0.5. Substitution of Al in the octahedral Ge sites results in a binomial distribution of multiple phosphate species, which differ in the number P–O–Al and P–O–Ge linkages and can be differentiated by ³¹P chemical shift and ³¹P­{²⁷Al} rotational echo adiabatic passage double resonance (REAPDOR) spectroscopies. The detailed quantitative analysis of these data, of complementary ²⁷Al­{³¹P} rotational echo double resonance (REDOR) and of homonuclear ³¹P–³¹P double quantum NMR studies suggest that the AlO₆ coordination polyhedra are noticeably expanded compared to the GeO₆ sites in the NASICON-type LiGe₂(PO₄)₃ (LGP) structure. While the glassy state is characterized by a significantly larger degree of disorder concerning the local coordination of germanium and aluminum, dipolar solid state NMR studies clearly indicate that their medium range structure is comparable to that in NASICON, indicating the dominance of P–O–Al and P–O–Ge over P–O–P and Al–O–Ge connectivities

Understanding Enhanced Upconversion Luminescence in Oxyfluoride Glass-Ceramics Based on Local Structure Characterizations and Molecular Dynamics Simulations

In this Article, large enhancement in upconversion (UC) luminescence was verified in a transparent aluminosilicate glass-ceramics (GCs) containing CaF₂ nanocrystals (NCs) codoped with Er³⁺ and Yb³⁺ ions. On the basis of the joint spectroscopic and structural characterizations, we suggest that the precipitation of fluoride NCs is correlated with the pre-existence of the fluoride-rich domains in the as-melt glass, which is supported by scanning transmission electron microscopy (STEM) and reproduced by molecular dynamics (MD) simulation. The precipitation of the fluoride NCs starts from a phase-separated as-melt glass consisting of fluorine-rich and oxygen-rich domains, while the spatial distribution of rare earth (RE) ions and the vibration energies of the bonds connecting RE ions remain almost unchanged after crystallization. In the GCs, both the fluoride domain and the oxygen-containing polyhedrons surrounding RE ions experience significant ordering, which may affect the UC emission for both glasses and GCs. We therefore attribute the enhanced UC emissions of the GCs to the long-range structural ordering and the change of site symmetry surrounding RE ions, rather than the preference of RE ions in migrating from fluoride-rich phase to the fluoride NCs. Our results may have strong implications for a better understanding of the enhanced UC emission in similar oxyfluoride GCs

Formation Mechanism of Crystalline Phase during Corrosion of Aluminum Phosphate Glasses

The formation mechanism of crystalline phases within the corrosion layer of glasses has attracted considerable attention, but research on the microscopic chemical process of their formation has rarely been studied. This study focuses on investigating potassium aluminum phosphate glass with a nominal molar composition of 41.6K2O–16.7Al2O3–41.7P2O5. Liquid- and solid-state nuclear magnetic resonance (NMR) techniques are employed to investigate the evolution of the aluminum species and phosphorus units of the corroded glasses, leachates, and sediments derived from immersing the glass for various durations. Our findings provide compelling evidence that the formation of the crystalline phases during the phosphate glass immersion process is a result of leached glass elements saturating in the solution and subsequently precipitation onto the glass surface. Furthermore, we have identified two distinct dissolution modes in this process, which include the overall dissolution of large molecular units presented in the initial stage and the continuous dissolution of small molecular units that persists throughout the entire corrosion process. The coexistence of these two dissolution modes leads to the formation of crystalline phases on the glass surface even before both the glass and the solution have fully reached dissolution saturation. This study sheds light on the glass corrosion mechanism at the molecular level, providing new insight into comprehending the corrosion process of glass