The effect of Na/Si on the structure of sodium silicate and aluminosilicate glasses quenched from melts at high pressure: A multi-nuclear (Al-27, Na-23, O-17) 1D and 2D solid-state NMR study

Abstract

We report multi-nuclear (Na-23 and O-17) solid-state NMR [MAS and triple quantum (3Q) MAS] spectra for sodium tetrasilicate glasses (NS4) quenched from melts at high pressure up to 8 GPa. The results show clear evidence for the pressure-induced structural changes in the glasses, forming oxygen linking [4]Si and [5,6]Si ( [4]Si–O– [5,6]Si) with increasing pressure. Whereas the general trend in the effect of pressure is consistent with that of sodium trisilicate glasses (NS3), detailed pressure-induced structural changes for NS4 are largely different from NS3. These differences include the larger fraction of [4]Si–O– [5,6]Si and smaller fraction of Na–O– [5,6]Si for NS4 than NS3 at isobaric conditions. Topological disorder due to Si–O bond length distribution in [4]Si–O– [4]Si is also larger for more polymerized NS4 than that for NS3, demonstrating the complexity in structural rearrangement with pressure in silicate glasses and melts with composition at elevated pressure. Na-23 MAS NMR spectra at 14.1 T for the NS4 and aluminosilicate glasses (albite composition) reveal distinct differences in Na environments within sodium silicate and aluminosilicate glasses at high pressure. Variation of isotropic chemical shift with pressure for NS4, though minor, may suggest increasing Na–O distance with pressure, possibly due to a slight increase in Na coordination at high pressure. On the other hand, Na-23 isotropic chemical shift for albite composition glass increases with increasing pressure, implying the decrease in average Na–O distance and thus free volume for non-framework cations with pressure. Higher magnetic field, coupled with 2D Al-27 3QMAS NMR, improves the resolution of Al sites for aluminosilicate glasses at high pressure and may suggest two [5]Al sites in partially polymerized aluminosilicate glasses (Na 2O/Al 2O 3/SiO 2 = 15:5:60) quenched from melts at 8 GPa. The current results, together with our previous results, imply that the pressure-induced structural changes and the corresponding changes in properties in silicate melts and glasses are complex function of composition.