Malfait challenges our XPS results by arguing that the XPS spectroscopic technique cannot be used to determine small amounts of O2– in potassium silicate glasses. Instead, he claims that there is no free oxide (O2–) in potassium silicate glasses based primarily on his 29Si MAS NMR spectroscopic results. In this rebuttal, we demonstrate that O1s XPS and well-resolved 2D 29Si MAF NMR spectral results of potassium disilicate (K2Si2O5) glass are consistent with each other and that both techniques indicate the presence of a few mol% of O2– in the glass. Neither of these techniques, however, supports the interpretation of the 29Si MAS NMR results presented in the comments of Malfait. The major difficulty relates to the low resolution of the 29Si MAS NMR spectra, which does not reveal the Q4 signal beneath a strong Q3 peak in these spectra. The proof is provided by the 2D 29Si MAF NMR spectrum of potassium disilicate glass in which both Q3 and Q4 peaks are revealed; the 2D 29Si MAF NMR results for potassium disilicate glass are far more informative than 29Si MAS NMR spectra. It demonstrates that the potassium disilicate glass (K2Si2O5) contains greater Q4 intensity, is more polymerized than previously considered, and that O2– is present at ∼2 (±1) mol% in the potassic glass. This O2– value confirms our O1s XPS results. Specific points raised by Malfait are rebutted in Appendix A .
Anionic speciation in sodium and potassium silicate glasses near the metasilicate ([Na,K]2SiO3) composition: 29Si, 17O, and 23Na MAS NMR
