Local structure and redox state of vanadium in vanadium strontium borate [(V₂O₅)_0.5(SrO)_0.5-y(B ₂O₃)_y] oxide glasses

The local structure of vanadium strontium borate glasses with the nominal composition [(V₂O₅)_0.5(SrO)_0.5-y (B₂O₃)_y], where 0.1≤y≤0.4, as well as the valence state of the vanadium ions have been investigated by x-ray photoelectron spectroscopy (XPS). The core level binding energies of V 2p, Sr 3p, B 1s and O 1s have been measured. It was observed that there is a small increase in binding energies of the doublet peaks attributed to Sr 3p _3/2 and Sr 3p_1/2 in the Sr 3p spectra with an increase in the B₂O₃ content and also shifted by ∼ 0.6 eV towards higher binding energies in comparison to their values in SrO powder. The binding energies of the B 1s peak positions increase with an increase in the B ₂O₃ content, have essentially the same FWHM but, in comparison to B₂O₃ powder, the peak positions have shifted towards lower binding energy side by ∼0.7 eV. The O 1s core level spectra, however, show asymmetry for all glass samples which results from two contributions, one from the presence of oxygen atoms in the V-O-V, V-O-B, B-O-B environment (bridging oxygen BO) and the other from oxygen atoms in V-O-Sr, B-O-Sr, V=O, environment (non-bridging oxygen NBO). The O 1s core level spectra were deconvoluted into two peaks (BO and NBO) and the ratio of NBO to total oxygen was found to decrease with increasing B₂O₃ content. The quantitative ratio, [V⁴⁺/V_total], for each glass sample, has been determined from the analysis of the V 2p core level spectra. The ratio remains practically constant, independent of vanadium concentration within experimental uncertainties.