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

Abstract

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 Sn^II(SnO) and Sn^IV(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 127 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 O^2- to the phosphate matrix from the dissolved O₂^2- species.