Structure and red–ox chemistry of tin in SnO·NaPO3 pseudo-binary glasses

Thermal and spectroscopic investigations have been carried out on a number of binary SnO–NaPO3 glasses over a wide range of compositions; SnO:NaPO3 from 0:100 to 45:55. Structures of the glasses have been investigated using Raman, Fourier transform infrared (FTIR), 31P-HR–MAS–NMR and 119Sn Mössbauer spectroscopies. 119Sn Mössbauer spectra suggests that there is an equilibrium of SnII(SnO) and SnIV(SnO2) in the glasses in the ratio 40:60 irrespective of the composition. 31P-MAS–NMR, IR and Raman spectra confirm the partial oxidation of SnO to SnO2. The unoxidized part of SnO in the glass acts as a modifier up to ∼27 mol% of nominal composition. Above this concentration, SnO acts as a glass former. SnO2 however, is always found to behave as a glass network former. A structural model has been proposed, which envisages SnIV as playing a key role in preserving part of the metaphosphate units in the structure so that SnO changes its role from a modifier to former above 27 mol% (SnO) concentration. The model is consistent with the behaviour of glass transition temperatures, molar volumes and compositional dependence of infrared (IR) and Raman spectroscopic features. The equilibration of SnII and SnIV in the phosphate glasses is rationalized on the basis of a kinetic approach, which visualizes the presence of a pre-equilibrium situation where the slow step is the transfer of O2− to the phosphate matrix from the dissolved O22− species.