Anionic speciation in sodium and potassium silicate glasses near the metasilicate ([Na,K]2SiO3) composition: 29Si, 17O, and 23Na MAS NMR

Alkali silicate glasses near to the metasilicate composition provide sensitive tests of anionic speciation. However, reports on 29Si NMR data vary considerably in their derived proportions of silicate species. In some cases, an apparent deficit in the content of non-bridging oxygen has been attributed to up to 5 to 10% “free” oxide ions. Here we provide new NMR data on a series of sodium silicate glasses ranging from 56 to 45 mol% silica with accurately analyzed compositions. High signal-to-noise data for a 29Si-enriched glass provide improved constraints on fitting models for spectra, and suggests a slight asymmetry in the main Q2 component. Data are consistent with conventional models in which “free” oxide contents are below detection limits of about 0.5 to 1%. Spectra for an 17O-enriched K-silicate glass with about 53% silica confirm that there is no obvious direct signature for “free” oxide species in this compositional range.