J.W. Zwanziger

Multiple boron sites in borate glass detected with dynamic angle spinning nuclear magnetic resonance

Abstract The first high-field boron nuclear magnetic resonance spectra of sufficient resolution to detect multiple sites in borate glass are reported. These results are obtained by using dynamic angle spinning nuclear magnetic resonance spectroscopy, in fields of 4.7, 7.1 and 8.4 Tesla, to measure the boron-11 spectrum in samples enriched in boron-10. Two boron sites, which differ in quadrupole interaction strength by 5%, in chemical shift by 5 ppm, and have relative population of 3:1, were resolved. The more populated site is assigned to boron in boroxol rings, and the less populated site is assigned to non-ring boron trioxide units. The chemical shift difference between these sites suggests that the Bue5f8O bond in the boroxol rings is somewhat longer than that in the boron trioxide groups.

The structure of tellurite glass : a combined NMR, neutron diffraction, and X-ray diffraction study

Models are presented of sodium tellurite glasses in the composition range (Na{sub 2}0){sub x}-(TeO{sub 2}){sub 1{minus}x}. 0.1 < x < 0.3. The models combine self-consistently data from three different and complementary sources: sodium-23 nuclear magnetic resonance (NMR), neutron diffraction, and x-ray diffraction. The models were generated using the Reverse Monte Carlo algorithm, modified to include NMR data in addition to diffraction data. The presence in the models of all five tellurite polyhedra consistent with the Te{sup +4} oxidation state were found to be necessary to achieve agreement with the data. The distribution of polyhedra among these types varied from a predominance of highly bridged species at low sodium content, to polyhedra with one or zero bridging oxygen at high sodium content. The models indicate that the sodium cations themselves form sodium oxide clusters particularly at the x = 0.2 composition.

Design of organic–inorganic solid polymer electrolytes: synthesis, structure, and properties

This paper reports the synthesis, structure, and properties of novel hybrid solid polymer electrolytes (SPEs) consisting of organically modified aluminosilica (OM-AlSi), formed within a poly(ethylene oxide)-in-salt (Li triflate) phase. To alter the structure and properties of these polymer electrolytes, we used functionalized silanes containing poly(ethylene oxide) n (PEO) tails or CN groups. The SPEs described here were studied using differential scanning calorimetry, Raman spectroscopy, X-ray powder diffraction, and AC impedance spectroscopy. The size of the OM-AlSi domains was estimated using transmission electron microscopy and comparing the sizes of AlSi nanoparticles, obtained via calcination of the hybrid SPE. The conductivity enhancement, caused by incorporation of PEO tails or CN groups in the hybrid materials based on 600 Da poly(ethylene glycol), can be ascribed to a decrease of OM-AlSi domain size accompanied by an increase of the OM-AlSi/PEO + LiTf interface. For the CN modifier, increase of this interface increases the amount of CN groups exposed to PEO + LiTf phase, thus increasing the effective dielectric constants of the materials and their conductivity, although this dependence is not linear. In the case of the PEO modifier, different effects are observed for 600 Da PEG and 100 kDa PEO. For 100 kDa PEO, incorporation of the silane with a PEO tail caused a decrease of conductivity. Here, AlSi particle size remains basically unchanged with addition of silane-modifier, and the decrease of conductivity can be attributed to formation of a crystalline phase at the OM-AlSi/PEO + LiTf interface.

Short and intermediate range order in glass : nuclear magnetic resonance probes of site connectivities and distances

Abstract We present results on the spatial distribution and connectivity of phosphate in silver iodideue5f8silver phosphate glasses, as a function of silver iodide and silver oxide content. These results were obtained using several different nuclear magnetic resonance experiments which give selective signals for phosphorous atoms which are in spatial proximity to one another. Our data show that the silver iodide in these glasses acts to spread the phosphate chains apart, and that, in highly modified phosphates, a distribution of chain lengths exists, with chain lengths as short as two phosphate units.

Off-angle correlation spectroscopy applied to spin-1/2 and quadrupolar nuclei.

A two-dimensional correlation experiment is described, in which homonuclear dipolar couplings are used to realize through-space magnetization exchange on spin-1/2 (31P) and on quadrupolar nuclei (23Na and 11B). In the detection period, Magic Angle Spinning is applied to enhance resolution, and the dipole couplings are re-introduced in the mixing period by spinning off the Magic Angle. The dependency of the exchange rates on the mixing time and the spinning angle is investigated. The influence of strong spin-locking during mixing is discussed, and shown in the spin-1/2 case to remove the dependence on chemical shift offset effects. For quadrupolar spins, the experiment yields information on the relative tensor orientations of the coupled quadrupoles. Applications to crystalline sodium aluminum diphosphate, sodium sulphite, and potassium borate glasses are shown.

The ring structure of boron trioxide glass

Abstract A new experiment, sensitive to the dihedral angle distribution of neighboring structural units, was applied to boron trioxide glass. The experiment consisted of correlating the NMR frequencies of neighboring boron nuclei. The frequencies are identical if the boron oxide polyhedra containing the two borons are co-planar, as found in ring systems, but differ for non-co-planar geometries. Through the rate of growth of signal intensity arising from the non-co-planar case, as compared to that obtained in a boron trioxide crystal of known structure, the fraction of boron contained in rings in the glass was determined to be 0.70±0.08 .

Multi-nuclear and multi-dimensional nuclear magnetic resonance investigation of silver iodide-silver phosphate fast ion conducting glasses

The results of a multi-nuclear nuclear magnetic resonance (NMR) study of (AgI)x(Ag2O)y(P2O5)1-x-y glasses are reported. Using the two-dimensional variable-angle correlation spectroscopy experiment, the isotropic and anisotropic chemical shift interactions of phosphorus were determined as a function of silver iodide and silver oxide composition. From these measurements we determine the average conformation of the phosphate groups. In addition, the 109Ag NMR spectra were recorded, as a function of both composition and temperature. At high silver oxide concentrations, interaction between the silver and phosphate groups has been detected, but in glasses in the series the (AgPO3)x(AgI)1-x 31P NMR is strikingly independent on the silver iodide composition. The temperature dependence of the silver NMR linewidths in these systems shows clearly the silver mobility, and at lower temperatures no evidence for multiple distinct silver binding sites was observed. The silver chemical shift is strongly dependent on both composition and temperature. The former effect is interpreted in terms of the influence on the chemical shift of binding to oxygen versus iodine.

Interpreting nuclear magnetic resonance spectra of disordered materials: direct inversion of powder patterns

The nuclear magnetic resonance (NMR) spectra of disordered materials are often interpreted by assuming distributions of the interaction parameters and fitting the spectra under these assumptions. Here we illustrate methods to extract the distributions directly from the spectra, making no such prior assumptions. The inhomogeneously broadened powder pattern observed in the NMR spectrum of a disordered solid is expressed as an integral over the powder patterns for individual sites weighted by the population distribution of the different sites. The resulting integral equation is solved for the underlying probability distribution of sites, both by singular value decomposition and by a regularization method. Results are shown for model and real one-dimensional and two-dimensional NMR experiments, with and without noise.

The glass forming ability of tellurites: a rigid polytope approach

The rigid polytope approach to understanding glass formation of Cooper and Gupta is modified to include lone-pair electron vertices as a geometrical constraint, a modification that is supported by ab initio calculations. The extended model provides an explanation for the very different glass forming abilities of silica, telluria and their alkali-modified forms with only minimal assumptions about their atomic structure.

A comparison of strategies for obtaining high-resolution NMR spectra of quadrupolar nuclei

Abstract A comparison of high-resolution NMR methods for quadrupolar nuclei is presented. The samples studied are pure and modified boron oxide glasses, and the boron NMR spectra are recorded using three different experiments capable of high resolution: Double Rotation, Dynamic Angle Spinning, and Multiple-Quantum Magic Angle Spinning. Resolution of the similar ring and nonring boron sites in these glasses, in the presence of disorder, provides a realistic study of the strengths and limitations of these experiments. Conclusions about the relative applicability of these experiments in different situations are presented.