High-resolution double-quantum 31P MAS NMR study of the intermediate-range order in crystalline and glass lead phosphates

We have investigated the structure of binary lead phosphate glasses using 1D P-31 MAS and 2D P-31 double-quantum NMR. The distribution of the Q(n) phosphate species, determined from the simulation of 1D MAS spectra, indicates a significant disproportionation reaction (2Q(n) <-> Q(n+1) + Q(n-1)) near the pyrophosphate composition. As demonstrated on crystalline reference samples, 2D double-quantum experiments can be used to determine the connectivities between Q(n) groups. This allows us to probe the phosphorus connectivity scheme in the glass network and to show that the Q(n) chemical shift is influenced by the type of the bonded PO4 tetrahedron. By analyzing quantitative 1D MAS spectra and 2D double-quantum spectra, we have determined the phosphorus connectivity scheme in the glass structure and its evolution with composition.