Phosphate speciation in Na2O–CaO–P2O5–SiO2 and Na2O–TiO2–P2O5–SiO2 glasses

P2O5 was added in amounts of 0.4–2.5 mol% to (5.1–41.9)Na2O–(5.3–35.1)CaO–(57.0–66.9)SiO2 and (35.4–38)Na2O–(4.7–9.8)TiO2–(50–58.7)SiO2 glasses. Sodium and calcium ions were also added to maintain the charge balance on the phosphate complexes, without modification of the silicate condensation. 31P magic-angle-spinning nuclear magnetic resonance spectra of Na2O–CaO–P2O5–SiO2 glasses show that the phosphate complexes are monophosphate and diphosphate. The linear dependance of 31P chemical shifts with CaO/(CaO+Na2O) is consistent with a random distribution of sodium and calcium around phosphate complexes, except for diphosphates at CaO/(CaO+Na2O) less than 0.3. The relative proportion of mono- and diphosphate is analyzed in terms of an acid–base reaction involving exchange of O2−. We suggest that the amount of monophosphate larger than predicted by basicity analysis is consistent with calcium monophosphate complexes forming, and shifting the acid–base equilibria. An additional discrete 31P resonance in the Na2O–TiO2–P2O5–SiO2 glass spectra is attributed to titanium bearing phosphate complexes. They reveal the formation of Ti–O–P covalent bonds, owing to the electrostatic field strength of Ti4+ ions.