Structure and red-ox chemistry of tin in SnO center dot 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), P-31-HR-MAS-NMR and Sn-119 Mossbauer spectroscopies. Sn-119 Mossbauer spectra suggests that there is an equilibrium of Sn-II(SnO) and Sn-IV(SnO2) in the glasses in the ratio 40:60 irrespective of the composition. P-31-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 similar 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 Sn-IV 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 Sn-II and Sn-IV 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 O-2(2-) species