Solid-state NMR study of mixed network sodium–niobium phosphate glasses

xNb2O5–(100−x)NaPO3 glasses with 0⩽x⩽40 were characterized by 31P, 93Nb and 17O NMR. 31P MAS-NMR indicates that Nb2O5 depolymerises the sodium metaphosphate network with the formation of Q1 and Q0 sites. However, Nb2O5 is only partially dissociated in NaPO3 melt since the amount of Qn sites does not fit the calculated values for a total dissociation. 93Nb NMR spectra show two niobium sites, assigned to corner and edge-shared NbO6 octahedra. The quantity of corner-shared octahedra decreases when the quantity of Nb2O5 increases in the glasses. An 17O MQ-MAS spectrum was recorded at high magnetic field (18.8T) on 17O-enriched 6Nb2O5–94NaPO3 glass. Considering the empirical correlation of quadrupolar constant to M–O bond ionicity, 17O resonances were assigned to oxygen atoms in POP, PONa and PONb sites. No NbONb sites could be detected, because of their small quantity in the enriched sample.