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

• Published in: Journal of non-crystalline solids 2000-03, Vol.263-264, p.312-317
• Main Authors: Grussaute, H, Montagne, L, Palavit, G, Bernard, J.L
• Format: Article
• Language: English
• ISSN: 0022-3093
• DOI: 10.1016/S0022-3093(99)00643-2

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. Na and Ca ions were also added to maintain the charge balance on the phosphate complexes, without modification of the silicate condensation. 31P MAS NMR 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 Na and Ca around phosphate complexes, except for diphosphates at CaO/(CaO+Na2O) < 0.3. The relative proportion of mono- and diphosphate is analyzed in terms of an acid-base reaction involving exchange of O2-. Authors 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 Ti bearing phosphate complexes. They reveal the formation of Ti-O-P covalent bonds, owing to the electrostatic field strength of Ti4+ ions. 16 refs.