Influence of the CaO/MgO ratio on the structure of phase-separated glasses: a solid state 29Si and 31P MAS NMR study

29Si and 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy was used to investigate structural aspects of glasses of the series (mol%) 31SiO2–11P2O5–(58−x)CaO–xMgO, with x ranging from 0 to 32, in which precipitation of amorphous silica occurs. As expected for alkaline-earth silicates, a strong 29Si MAS NMR peak was recorded, shifting to higher frequencies with decreasing CaO/MgO molar ratios. Q2 species predominate in the alkaline-earth rich phase and the observed shift in the 29Si spectra is attributed to a decrease in the average degree of Si polymerization in the matrix. The experimental evidence suggests that Mg2+ replaces Ca2+ and also promotes the disruption of Si–O–Si bonds. Substitution of CaO by MgO increases the amount of dispersed phase. 29Si MAS NMR showed the presence of Q4 species in the dispersed phase and also suggested the presence of Q0 species. A disproportionation reaction 2Q2⇌Q0+Q4 is proposed to balance the silicate charges. 31P MAS NMR showed that phosphorus is present in an orthophosphate environment. The shift of the 31P MAS NMR peak was attributed to the replacement of Ca by Mg.