Role of bismuth and titanium in Na_2O-Bi_2O_3-TiO_2-P_2O_5 glasses and a model of structural units

摘要

0.60Na_2O-0.40P_2O_5 and (0.55-z)Na_2O-0.05Bi_2O_3-zTiO_2-0.40P_2O5 glasses (0 ≤ z ≤ 0.15) were prepared by melting at 1000℃ mixtures of Na_2CO_3, Bi_2O_3, TiO_2 and (NH_4)_2HPO_4. Differential Scanning Calorimetry (DSC) measurements give the variation of glass transition temperature (T_g) from 269℃ (for 0.60Na_2O-0.40P_2O_5) to 440℃ (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_2O-0.40P_2O_5 to P_2O_7~(4-) groups in the glasses containing Bi_2O_3 or both Bi_2O_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_2O_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.