April 23, 2021

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Nuclear Magnetic Resonance

       Hellmut Eckert specializes in the use of high-resolution NMR techniques to probe the short and intermediate range structures of glass and amorphous materials. He has established one of the foremost labs in the world in this area and has developed some of the most capable techniques in using NMR spectroscopy to probe the structure of glass. Eckert’s students use NMR to develop detailed chemical ordering and structural models of the ternary MGF Glasses.

       Owing to its element-selectivity and inherently quantitative character, NMR is a powerful structural characterization method for glassy materials. The structural information stems from the influence of various local spin interactions, which sensitively reflect the local structural environments of the nuclear spin probes used. Using advanced experimental NMR methodology, these interactions can be separated, quantified, and then related to structural information using quantum chemical calculations.

In particular, the use of magic-angle spinning (MAS) in many cases affords a detailed quantification of local bonding configurations. In addition, dipolar techniques such as spin echo double resonance (SEDOR) or rotational echo double resonance (REDOR) have given important quantitative information about short and intermediate range order aspects such as (1) the connectivity of different network former species, (2) the distance correlations between network former and network modifier ions, and (3) the spatial distribution of the network modifier ions.

      In recent years, we have been studying the above issues in a number of binary oxide glass systems consisting of one network former species (SiO2, B2O3, P2O5) and either lithium or sodium oxide as the network modifier. Data are also available on some MGF systems, particularly aluminoborate aluminophosphate, and borophosphate glasses. These studies will serve as an extensive body of reference for understanding the MGFE subject of this proposal. In principle, the MGFE can modify all of the three above-mentioned aspects of medium-range order, and can be studied in detail using the appropriate NMR techniques.

Regarding issue (1), network-former connectivities, four distinct scenarios are possible when considered in relation to the corresponding binary systems:

a)      same network-former clustering and avoidance of heteroatomic connectivities,

b)      statistical connectivities (no specific interaction preferences),

c)      preference for heteroatomic connectivities of those local environments in binary systems,

d)      preference for heteroatomic connectivities of local environments not present in binary systems.

While it can be conceived that type (d) behavior might show the strongest MGFE on the physical properties, this hypothesis remains to be proven by the systematic studies as a function of composition.

       Issue (2), the network former/network modifier correlation, bears upon the competition of both network former species for the modifier ions. Both MAS-NMR as well as REDOR experiments involving the nuclei 7Li and 23Na and constituent nuclei of the network such as 11B, 29Si, and 31P, yield quantitative insights into the extent to which the different network former species attract modifier ions. For example, recent NMR results on MGF glasses in the binary AgPO3+AgBO2 have shown that the phosphate component has a strong tendency to “scavenge” network modifiers to the extent of producing doubly charged pyrophosphate species, while part of the borate network is left unmodified.

       With regard to issue (3), it is clear that the spatial network modifier distribution (randomness or clustering) will have a pronounced influence on the ionic conductivity of the glass. In this connection, a detailed analysis of homo- and hetero-nuclear magnetic dipole-dipole interactions has been carried out to characterize the spatial distribution of the network modifier species in various types of binary glasses.  Based on such experiments we have quantified the tendency of alkaline ions to cluster in lithium and sodium silicate glasses, whereas the cation distributions in borate, phosphate and germanate glasses are shown to be more or less statistical. Mixing of different network former species may have a profound influence on the spatial cation distribution, which can then be studied by the appropriate dipolar NMR techniques.

      To address the MGFE most effectively, compositional series will be investigated in which the ratio of network modifier to network former will be held fixed, whereas the ratio of the two network former species will be systematically varied. In systems 1(a) and (b) (see list of compositions above) 29Si MAS-NMR studies will be able to quantify the distinct populations of the silicate species (Q(n) units) generated by the interaction of silica and alkali oxide. Systematic studies as a function of network composition will give detailed insights into the competition of the network formers silica and germania for the network modifier species (issue 2). Furthermore, dipolar NMR methods will show in which way the interaction between the two network formers silica (cation clustering tendency) and germania or boron oxide (random cation distribution) affects the cation distribution in the MGF system. For system 2(b) complementary information on this question will be available from 11B{23Na} REDOR studies. Analogous information will be available by characterizing the interaction between the silicon sulfide component and the alkali ions using 29Si{23Na} or 29Si{7Li} REDOR experiments, as previously illustrated for silicate glasses.

      In the systems 1(b), 2(b), and 2(c), 11B MAS-NMR affords a sensitive method for discriminating quantitatively between three- and four-coordinated boron atoms on the basis of their different chemical shifts. Furthermore, in system 1(b) 29Si{11B} REDOR experiments will provide detailed insights into the distribution of the individual connectivities of the various silicate Q(n) species with boron, indicating if and to which extent heteroatomic connectivities will be preferred. Analogous information will be obtained in system 2(c), using a combination of 11B and 31P MAS-NMR with 31P{11B} and 11B{31P} REDOR. The analogous oxide system (which is of type b mentioned above) has been studied by us recently, resulting in a detailed quantification of B-O-B, B-O-P, and P-O-P linkages for both sodium and silver borophosphate glasses. The comparison between the results on system 2(c) with this data will thus illustrate the influence of the chalcogen species (oxygen or sulfur), on the manifestations of the MGFE.

       These NMR studies will be conducted in close cooperation with ISU where the samples to be measured will be prepared and characterized. This structural information will be complementary to IR and Raman (ISU), ND (Chalmers) and XRD (CMU) and provide critical input data to the modeling and simulation (Ilmenau and Munster). Once the behavior of the pure oxide and sulfide systems are understood on the basis of our NMR experiments, we will explore the analogous mixed oxy-sulfide systems. Munster will support one graduate student to carry out their NMR research.