Chemical Basis of the Structural Modification in Sodium Borovanadate Glasses. Thermal and Spectroscopic Studies

Sodium borovanadate glasses have been investigated using infrared spectroscopy (IR) and magic angle spinning nuclear magnetic resonance (MAS NMR) studies. It is found that modification of borovanadate glass structure leads to the formation of tetrahedral boron and the ratio of tetrahedral $(B_4)$ to trigonal boron $(B_3)$ exhibits a maximum value of unity in these systems. A structural model has been proposed for these glasses keeping in view this observation. The model, which envisages formation of two isomorphous diborovanadate species whose structures are similar to that of diborate units, is able to account for the variation of $B_4/B_3$ ratio over the entire composition range. Variation of the glass transition temperature of these glasses has been investigated using the structural model in conjunction with cluster-tissue model of glass transition. It is emphasized throughout that electronegativities of structural units are of vital importance in understanding local oxide ion reactions which results in structural modification