Partitioning and structural role of Mn and Fe ions in ionic sulfophosphate glasses

Ionic sulfophosphate liquids of the type ZnO-Na2O-Na2SO4-P2O5 exhibit surprising glass forming ability, even at slow or moderate cooling rate. As a concept, they also provide high solubility of transition metal ions which could act as cross-linking sites between the sulfate and phosphate entities. It is therefore investigated how the replacement of ZnO by MnO and/or FeO affects the glass structure and the glass properties. Increasing manganese levels are found to result in a monotonic increase of the transition temperature Tg and most of the mechanical properties. This trend is attributed to the change of metal-ion coordination from four-fold around Zn(2+) to six-fold around Mn(2+) ions. The higher coordination facilitates cross-linking of the ionic structural entities and subsequently increases Tg. Raman and infrared spectroscopy show that the structure of these glasses involves only SO4(2-) and PO4 (3-) monomers as well as P2O7(4-) dimers. Replacement of ZnO by MnO is found to favour PO4(3-) over P2O7(4-) species, a trend which is enhanced by co-doping with FeO. Both transition metal ions show, like Zn(2+), a preference to selectively coordinate to phosphate anionic species, as opposed to sodium ions which coordinate mainly to sulfate anions. EPR spectroscopy finally shows that divalent Mn(2+) ions are present primarily in MnO6-clusters, which, in the studied sulfophosphate glasses, convert upon increasing MnO content from corner-sharing to edge-sharing entities.