High-resolution double-quantum P-31 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 1 D P-31 MAS and 2D P-31 double-quantum NMR. The distribution of the Q(n) pho sphate 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 sa mples, 2D double-quantum experiments can be used to determine the connectiv ities between Q(n) groups. This allows us to probe the phosphorus connectiv ity 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 quantit ative 1D MAS spectra and 2D double-quantum spectra, we have determined the phosphorus connectivity scheme in the glass structure and its evolution wit h composition.