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

Abstract The vanadium(IV)-vanadium(V) equilibrium in a 37.5BaO, 5.0Al 2 O 3 , 57.5B 2 O3 mol% + X mol% V 2 O 5 (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% V 2 O 5 (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, VO 2+ ; g | decreased monotonically with increasing vanadium content of these glasses, whereas g β 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 ( ϱ / 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% V 2 O 5 ; with further increase of V 2 O 5 the slope decreased sharply. It has been concluded that the abrupt changes in properties like partial molar volume of V 2 O 5 , 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.