Nuclear magnetic resonance studies on vanadium phosphate glasses

Abstract

The structures of a single V₂O₅, binary CdO-V₂O₅ and ternary CdO-V₂O₅-P₂O₅ glass systems have been investigated by means of ⁵¹V and ³¹P MAS-NMR spectroscopy. NMR spectrum of pure V₂O₅ glass has showed that the network structure is made up of VO₅ groups, as in crystalline V₂O₅. This means that only 5-fold coordination of the vanadium atom, V(5), is the predominant type in this glass. Substitution of V₂O₅ with CdO in the glass network leads to destruction of the layered VO₅ structure and as a result VO₄ tetrahedral units are formed. Vanadium is located in this situation at V(5) and V(4) sites. The difference between the feature of NMR spectra of the sample containing 30 mil % CdO and the glass of pure V₂O₅ is considered to be due to formation of both V(4) and V(5). On the other hand, cadmium orthovanadate Cd₃(VO₄)₂ building units are the main constituent of CdO rich glass (60 mol % CdO). The local structure of the ternary CdO-P₂O₅-V₂O₅ glasses contains three types of vanadium sites, which are pentavalent vanadate units (V5), tetrahedral (V4) and mixed vanadate and phosphate units (V4-O-P4). The latter represents V atoms coordinated with P atoms as the second neighbor (VO₄)P. ³¹P NMR results have showed that the concentration of nonbridging oxygen atoms (NBOs) is increased in the phosphate network with increasing CdO contents.