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

Abstract 0.60Na 2 O–0.40P 2 O 5 and (0.55− z )Na 2 O–0.05Bi 2 O 3 – z TiO 2 –0.40P 2 O 5 glasses (0≤ z ≤0.15) were prepared by melting at 1000°C mixtures of Na 2 CO 3 , Bi 2 O 3 , TiO 2 and (NH 4 ) 2 HPO 4 . Differential Scanning Calorimetry (DSC) measurements give the variation of glass transition temperature ( T g ) from 269°C (for 0.60Na 2 O–0.40P 2 O 5 ) to 440°C (for z =0.15). The density measurements increases from 2.25 to 3.01 g/cm 3 . FTIR spectroscopy shows the evolution of the phosphate skeleton: (PO 3 ) ∞ chains for 0.60Na 2 O–0.40P 2 O 5 to P 2 O 7 4− groups in the glasses containing Bi 2 O 3 or both Bi 2 O 3 and TiO 2 . 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. Bi 2 O 3 and TiO 2 are assumed to be present as six co-ordinated octahedral [BiO 6/2 ] 3− and [TiO 6/2 ] 2− units again with shared corners. This is accompanied by the simultaneous conversion of [POO 3/2 ] into [PO 4/2 ] + units which achieves charge neutrality in the glasses.