Redox equilibria and constitution of polyvalent ions in oxide melts and glasses

Abstract

It is shown that redox equilibria of polyvalent ions in glass melts cannot usually be considered without complex-formation reactions and therefore without ligand formation and coordination behaviour. In principle, a distinction should be made between ligand numbers and coordination numbers, as well as between conditions at high temperatures and those at temperatures below Tg. The equations derived and some experiments in literature indicate that most transition metal ion pairs in melts show a stronger tendeney to form complexes for the higher valency states than for the lower ones. In many cases this behaviour coincides with the ligand number of polyvalent cations in the corresponding glasses, viz. the higher valency state leads to a stronger complex formation with network-forming character and thus, to a lower coordination number than is the case with the lower valency state of a polyvalent ion. However, this behaviour often does not occur because of the very common tendeney for coordination to change during cooling from the melt to the glassy state. Those differences between the stable melt at very high temperatures, the metastable melt at intermediate temperatures and the glassy state at room temperature need to be considered and are discussed.