Redox and structure of sodium-vanadophosphate glasses

xV2O5-(100−x)NaPO3 glasses were prepared with 0 ⩽ x ⩽ 80. The redox ratio (V(+IV)/Vtotal) was measured by wet chemical analysis, and found to be consistent with magnetic measurements on glasses. It decreases with increasing x values, which is explained by the variation of the acid–base character of the melts. The glass transition temperature first increases with x, then decreases for x values larger than 20%. 31P MAS-NMR indicates that PO4 tetrahedra are connected first to one vanadate, then to two vanadate groups. The assignment is further confirmed by double-quantum filtered 31P MAS-NMR, and also by considering the chemical shift of crystalline references. 51V MAS-NMR spectra show two resonances for x lower than 20%, which are assigned to tetrahedral and octahedral vanadate sites. For x values larger than 20%, only tetrahedral vanadium sites remain, then progressively change into sites similar to those in V2O5. The shift to high field of the 31P chemical shift of PO4 groups isolated in the vanadate network, is attributed to the evolution of the vanadium co-ordination. The non-linear evolution of the glass transition temperatures is interpreted from the structural data: the increase of Tg at low x values is due to the reticulation of the phosphate network by VO6 groups, whereas the decrease of Tg for x larger than 20% is due to the formation of a vanadate network.