Spectroscopy,oxidation-reduction behaviour and DC conductivity of a vanadium-containing barium aluminoborate glass

The vanadium(IV)-vanadium(V) equilibrium in a 37.5BaO, 5.0Al₂O₃, 57.5B₂O3 mol% + X mol% V₂O₅ (where X = 0.25?32.5) glass system has been studied as functions of temperature, partial pressure of oxygen and total vanadium concentration of the melt. The vanadium(V)/vanadium(IV) ratio in the melt increased with increasing partial pressure of oxygen, lowering temperature of melting, and with increasing total vanadium content of the melt. With X ? 10, the vanadium(V)/vanadium(IV) ratio became almost independent of the total vanadium content of the melt.With this knowledge of oxidation-reduction behaviour, a series of glasses containing 2.8?32.5 mol% V₂O₅ (at about 4 mol% intervals) and having a constant vanadium(IV)/vanadium(V) ratio (0.17) were prepared. Density, electronic absorption spectrum (both d-d and charge transfer transitions), and ESR of these glasses were measured. Optical and ESR spectra of these glasses indicated the vanadium(IV) to be present as vanadyl ion, VO²⁺; g| decreased monotonically with increasing vanadium content of these glasses, whereas gβ remained unchanged. The charge transfer transition energy due to vanadium(V) decreased, and the extinction coefficient increased by orders of magnitude with increasing vanadium content of the glass; the most striking changes occurred at X ≈ 10 mol%. DC conductivity of these glasses was measured at different temperatures; a plot of log (?/T) versus 1/T produced straight lines. The slope of these lines remained almost constant (39 ± 1 kcal/mol) for the glasses containing up to about 10 mol% V₂O₅; with further increase of V₂O₅ the slope decreased sharply.It has been concluded that the abrupt changes in properties like partial molar volume of V₂O₅, charge transfer spectrum of vanadium(V), activation energy of polaron hopping — all of which occurred around X ≈ 10 mol% — is due to a major change in the nature of vanadate groups rather than vanadium(IV) in these glasses.