Stabilising elements in high oxygen coordination numbers: topo-structural implication on glass strength

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Glass topology usually describes the short-to-intermediate arrangement of structural elements in the glass structure, and recent advances in the constraint theory have been successfully used to predict mechanical properties of certain glass systems. Here, we will characterise the actual topology of the glass network employing a combination of methods: State of the art solid-state NMR spectroscopy will be used to characterize the glass topology on short and intermediate length scales. While high resolution (Multiple Quantum) Magic Angle Spinning NMR experiments provide decisive information about short range structural motifs (1 - 2 Å) and thus allows to identify the nature of the network polyhedra, the employment of the full inventory of modern homo- and heteronuclear dipolar based NMR experiments, tracing structural motifs on an intermediate length scale (2 - 8 Å) will enable us to elucidate the interconnection of the identified network polyhedra towards an extended glass network. Results from complementary techniques such as X-ray absorption near edge structure (XANES) spectroscopy and infra-red and Raman spectroscopy will help to refine the description of glass topology .Glass systems with low concentrations of non-bridging oxygens and, thus, consisting of mostly covalent bonds are considered the compositions of choice to achieve stronger glasses. The combination with locally increased coordination numbers may offer advantages owing to increased packing densities, but this may depend on the bonding situation of the atom in high coordination being of a more ionic (Al) or a more covalent character (Si). The topo-structural findings on short and intermediate length scales will be correlated with mechanical data obtained from micro- and nanoindentation experiments, Brillouin spectroscopy and (in collaboration) in situ mechanical testing. The aim is not only to understand how Young's modulus, crack resistance, fracture toughness and hardness are controlled by the glass topology on various length scales, but to pave the way towards glasses with significantly increased strength.