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-(51.0-66.9)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. P-31 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 P-31 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 P-31 resonance in the Na2O-TiO2-P2O5-SiO2 glass spectra is attributed to titanium hearing phosphate complexes. They reveal the formation of Ti-O-P covalent bonds, owing to the electrostatic field strength of Ti4- ions. (C) 2000 Elsevier Science B.V. All rights reserved.