A multinuclear solid state NMR study of the sol-gel formation of amorphous Nb2O5-SiO2 materials

Multinuclear H-1, C-13, O-17, Si-29 MAS and Nb-93 static NMR is reported from a series of sol-gel prepared (Nb2O5)(x)(SiO2)(1-x) materials with x = 0.03, 0.075 or 0.30. C-13 NM R shows that by 500 degreesC the organic precursor fragments have been removed although some residual carbon remains as a separate phase. The Si-29 NMR typically shows three Q-species (Q(2,3,4)) in the initial gels, and that with increasing heat treatment the average n of the Q(n)-species increases as the organic fragments and hydroxyl groups are removed. 170 shows unequivocally that the x = 0.03 and 0.075 samples are not phase separated, while at the much higher niobia-content of x = 0.30 Nb-O-Nb signals are readily detected, a definite indication of the atomic scale phase separation of Nb2O5. The x = 0.03 and 0.075 samples heated to 750degreesC are thus representative of amorphous niobium silicates. Comparison is made to other sol-gel prepared metal silicates especially with another Group Va metal tantalum. The effects of tantalum and niobium on the silica network are very different and it is suggested here that most of the niobium is present as NbO4, forming part of the silicate network.