Determination of the Degree of Ordering in the Modifier Cation Clustering in Silicate Glasses by 29Si NMR and Ab Initio Calculations

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The Ohio State University

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New data is presented on mixed modified tetra-silicate glass of composition (1- z)[K2xMg(1−x)O · 4SiO2] · zCuO where x = 1, 0.875, 0.75, 0.625, and 0.5, and z ≈ 0.004. This glass, with compositions of x = 1 and 0.5, was studied using 17O dynamic-angle spinning (DAS) NMR in 1992 by Farnan and coworkers [1] and it was shown that there was considerable order of the non-bridging oxygen sites. The 29Si sites were studied using two-dimensional magic angle flipping (2D MAF) NMR at natural abundance to examine the degree to which this clustering can be observed in a range of glass compositions. A chemical shift anisotropy, ζδ site 1, of -84.7 ± 0.1 ppm for the x = 1 sample and, ζδ site 2 of -60.4 ± 0.2 ppm for the x = 0.5 sample were determined based on a two site simulation of the MAF spectra. The chemical shift anisotropy of the x = 1 sample corresponds to a pure potassium coordination environment, whereas, the chemical shift anisotropy of the x = 0.5 sample corresponds to a mixed potas- sium/magnesium coordination environment, which is proposed as a K2i · Mgi, where the potassium-to-magnesium ratio is believed to be restrictive but the total number of modifier cations near each non-bridging oxygen is not known. A three-site simulation was used to fit the other glass compositions. Two distinct anisotropic lineshapes werefoundwith: ζδ site 1 =-85.5±1.1ppm and ζδ site 2 =-63.1±4.8 ppm for the x=0.875sample, ζδ site 1 =-86.2±0.4 ppm and ζδ site 2 =-58.6±0.3 ppm for the x=0.75sample, and ζδ site 1 =-87.5±0.6 ppm and ζδ site 2 =-61.9±0.3 ppm for the x = 0.625 sample. These sites correspond to the same cation clustering coordina- tion environments from the x = 1 and x = 0.5 samples. Ab initio quantum chemical calculations were performed on a basic silicate tetrahedron built to emulate various Q(n)-species with different Si-NBO bond distances to investigate the experimentally determined trend between the network modifying cation potential and the anisotropy of the clusters. Both the experimental results and theoretical ab initio calculations indicate that as cations with larger cation potentials are incorporated into the glass, the magnitude of the anisotropy increases.



Glasses, Si-29 NMR, natural abundance, Q(n) speciation