Short and Medium Range Order in Ion-Conducting Glasses Studied by Modern Solid State NMR Techniques

2014

J. Phys. Chem. C

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The structure of mixed-network former glasses in the system (M2O)0.33[(Ge2O4) x (P2O5)1–x ]0.67.(M = Na, K) has been studied by 31P and 23Na high-resolution and dipolar solid state nuclear magnetic resonance (NMR) techniques, O-1s X-ray photoelectron spectroscopy, and Raman spectroscopy. Using an iterative fitting procedure, a quantitative structural model has been developed that is consistent with all of the experimental data and which provides a detailed description of network connectivities, network modification processes, and spatial cation distributions. Formation of heteroatomic P–O–Ge linkages is generally preferred over homoatomic P–O–P and Ge–O–Ge linkages, as shown by a detailed comparison with a random linkage model. An exception occurs in glasses with low germanium contents (x = 0.2) where a pronounced nonlinear dependence of the glass transition temperature on x can be related to a cross-linking of the sodium ultraphosphate network by fully polymerized germanium species, possibly including also 5- and 6-fold coordination states. At higher x values, the Ge component is modified as well, however, the fraction of anionic nonbridging oxygen atoms bound to germanium is always lower than expected for proportional modifier sharing between both network formers. Rather, the phosphate component is preferentially modified by the cations, leading to the formation of P(1) units at high x values. Consequently, the local coordination of the cations is dominated by phosphorus, as is clearly evident from the 23Na{31P} rotational echo double resonance (REDOR) results. This preferred association, combined with the formation of P(1) units, results in partially clustered cation distributions, which can be detected by 23Na spin echo decay spectroscopy. Finally, the joint interpretation of all of the data from NMR, Raman, XPS, and thermal analysis measurements offers indirect evidence for the formation of higher germanium coordination states in this glass system.

Section: Na{mentioning

confidence: 53%

Mixed Network Former Effects in Oxide Glasses: Spectroscopic Studies in the System (M₂O)_1/3[(Ge₂O₄)_x(P₂O₅)_1–x]_2/3

Behrends

2014

J. Phys. Chem. C

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“…Similar to the situation in alkali aluminophosphate glasses, 15−18 gallium prefers six-coordination in meta-and pyrophosphate glasses (O/P ≤ 3.5), while at O/P ratios ≥3.5, four-coordinate gallium is increasingly favored. A close analogy also exists regarding the dependence of the isotropic 71 Ga and 27 Al chemical shifts on coordination number, 19 when considering sets of closely related crystalline aluminum and gallium compounds. 20−23 Almost all the past research involving gallium oxide-based glass systems has been completely focused on the local structural environments probed by magic angle spinning NMR, while more advanced dipolar MAS-based NMR techniques probing site connectivities and heteronuclear correlations 24−27 to date have not been applied to gallium-containing systems.…”

Section: ■ Introductionmentioning

confidence: 89%

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

Ren

2014

J. Phys. Chem. C

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A series of (NaPO 3 ) 1−x (Ga 2 O 3 ) x glasses (0 ≤ x ≤ 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 GaPO 4 (quartz) and Ga(PO 3 ) 3 . Alloying NaPO 3 glass by Ga 2 O 3 results in a marked increase in the glass transition temperature, similar to the effect observed with Al 2 O 3 . At the atomic level, multiple phosphate species Q n mGa (n = 0, 1, and 2; m = 0, 1, 2, and 3) can be observed. Here n denotes the number of P−O−P and m the number of P−O−Ga linkages, and (m + n ≤ 4). For resolved resonances, the value of n 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). Ga 3+ is dominantly found in six-coordination in low-Ga glasses, whereas in glasses with x > 0.15, lower-coordinated Ga environments are increasingly favored. The connectivity between P and Ga can be assessed by heteronuclear 71 Ga/ 31 P dipolar recoupling experiments using 71 Ga{ 31 P} rotational echo double resonance (REDOR) and 31 P { 71 Ga} rotational echo adiabatic passage double resonance (READPOR) techniques. Up to x = 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 x = 0.25, 71 Ga static and MAS NMR as well as REDOR experiments give clear spectroscopic evidence of Ga−O−Ga connectivity. 31 P/ 23 Na 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 n mGa 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 Ga 2 O 3 as an intermediate oxide, with features of both network modifier and network former in oxide glasses. ■ INTRODUCTION Na 2 O−M 2 O 3 −P 2 O 5 (B, Al, and Ga) glasses have received widespread attention because of their interesting physical and chemical properties. They have applications in nonlinear optics, glass seals, and low-melting glass solders and electrolytes in solid state electrochemical cells for fast ion conduction. 1Recently biomedical applications of gallium-containing glasses have come into focus. 2,3 Gallium (Ga 3+ ) has an ionic radius nearly identical to that of Fe 3+ and can function as a "Trojan horse" for treating hypercalcemia of malignancy, 4 and its antibacterial properties may be us...

“…subambient temperatures are generally required to suppress any potential influence of sodium ion dynamics. Figure summarizes the M 2(Na–Na) values as a function of number density in sodium silicate and borate glasses . Note in particular the strikingly different compositional dependences observed.…”

Section: The Current State Of the Art: New Insight Into Medium‐range mentioning

confidence: 99%

Spying with spins on messy materials: 60 Years of glass structure elucidation byNMRspectroscopy

Int J of Appl Glass Sci

2018

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Glasses remain a focus of attraction to fundamental researchers and materials engineers alike. The desire of controlling physical property combinations by compositional design inspires the search for fundamental structural concepts describing the short‐ and medium‐range order of the glassy state. From its early beginnings more than 60 years ago, solid‐state nuclear magnetic resonance (NMR) spectroscopy has been making significant contributions toward this objective. Being element‐selective, inherently quantitative as well as selective to the local environment, NMR in many ways presents an ideal experimental tool of structural investigation of glasses. Over the years, substantial NMR methods development, along with advances in the theoretical interpretation of NMR parameters, have significantly enhanced our fundamental knowledge of medium‐range order and of composition‐structure‐function relationships. As we are approaching the 60th anniversary of the publication of the first article dealing with this topic (A.H. Silver and P.J. Bray, The Journal of Chemical Physics 1958, 29, 984), it is time to look back at the amazing scientific trajectory this field of investigation has taken, from the early beginnings to the present state of the art. As such, this overview does not strive to be comprehensive, but adopts a personal perspective, highlighting what in the view of the author have been influential developments and important insights obtained during the various phases of scientific inquiry in this research field.