Redox and structure of sodium-vanadophosphate glasses

xV(2)O(5)-(100-x)NaPO3 glasses were prepared with 0 less than or equal to x less than or equal to 80. The redox ratio (V(+IV)/V-total) 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%. P-31 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 P-31 MAS-NMR, and also by considering the chemical shift of crystalline references. V-51 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 P-31 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 T, at low x values is due to the reticulation of the phosphate network by VO6 groups, whereas the decrease of T-g for x larger than 20% is due to the formation of a vanadate network. (C) 2004 Elsevier B.V. All rights reserved.