Deciphering the structural origins of high sulfur solubility in vanadium-containing borosilicate glasses

The addition of V2O5 has been long known to increase the sulfur (as SO42-) solubility in borosilicate glasses. However, the mechanism governing this effect is still unknown. Although several studies have been published in the past two decades attempting to decipher the structural origins of increasing sulfur solubility as a function of V2O5 in borosilicate glasses, most of these studies remain inconclusive. The work presented in this paper attempts to answer the question, "Why does V2O5 increase sulfur solubility in borosilicate glasses?" Accordingly, a series of melt-quenched glasses in the system [30 Na2O - 5 Al2O3 - 15 B2O3 -50 SiO2]((100-x)) - xV(2)O(5), where x varies between 0 - 9 mol.%, have been characterized for their short-to-intermediate range structure and the redox chemistry of vanadium using 11B, 27Al, 51V MAS NMR, Raman, and XPS spectroscopy. The impact of V2O5 on sulfur solubility in glasses has been followed by ICP-OES. The addition of = 5 mol.% V2O5 results in a linear increase in sulfur solubility in the investigated glass system. Based on the results, we hypothesize that adding vanadium to the glasses increases their network connectivity, but reduces the network rigidity by replacing stronger Si-O-Si linkages with weaker Si-O-V linkages and forming (VO3)n-single chains. These modifications to the glass structure increase the flexibility of the network, thus making it possible to accommodate SO42- in their voids/open spaces.