Role of bismuth and titanium in Na2O–Bi2O3–TiO2–P2O5 glasses and a model of structural units

0.60Na2O–0.40P2O5 and (0.55−z)Na2O–0.05Bi2O3–zTiO2–0.40P2O5 glasses (0≤z≤0.15) were prepared by melting at 1000°C mixtures of Na2CO3, Bi2O3, TiO2 and (NH4)2HPO4. Differential Scanning Calorimetry (DSC) measurements give the variation of glass transition temperature (Tg) from 269°C (for 0.60Na2O–0.40P2O5) to 440°C (for z=0.15). The density measurements increases from 2.25 to 3.01 g/cm3. FTIR spectroscopy shows the evolution of the phosphate skeleton: (PO3)∞ chains for 0.60Na2O–0.40P2O5 to P2O74− groups in the glasses containing Bi2O3 or both Bi2O3 and TiO2. When bismuth oxide and titania are added to sodium phosphate glass, phosphate chains are depolymerized by the incorporation of distorted Bi(6) and Ti(6) units through POBi and POTi bonds. Bi2O3 and TiO2 are assumed to be present as six co-ordinated octahedral [BiO6/2]3−and [TiO6/2]2− units again with shared corners. This is accompanied by the simultaneous conversion of [POO3/2] into [PO4/2]+ units which achieves charge neutrality in the glasses.