New 2D NMR experiments for determining the structure of phosphate glasses: a review

Double quantum nuclear magnetic resonance measurements exploiting the dipole coupling between the phosphorus atoms as well as a novel two-dimensional exchange experiment using the scalar coupling are described for measuring the connectivities of phosphate tetrahedra in glasses. It is shown that bonding scenarios of up to four connected tetrahedra can be determined in phosphates and phosphate glasses and that estimations of the chain length distributions are possible in glasses having shorter chain fragments. Nuclear magnetic resonance (NMR) is also capable of measuring local order in extruded phosphate glasses using rotorsynchronized excitation in combination with magic angle sample spinning. Finally, we show that the relative orientations of chemical shift tensors can be determined by two-dimensional nuclear magnetic resonance. This determination provides an additional verification of connectivities and can possibly enable access to bonding angle data in the future.