The Bismuth-Barium Oxides

Phase relations have been determined for the Bi-Ba oxide pseudobinary up to 50 cat % Ba in 1 atm of oxygen at 640°-1000°C. The low-temperature α-Bi2O3 polymorph does not dissolve appreciable BaO. All other phases in the system have significant ranges of solution. The δ-Bi2O3 polymorph, stable from 730°C to 825°C is an end-member of a face-centered cubic solid solution (FCCss) that dissolves up to 2.7 % Ba. Ba-saturated FCCss and Bi-saturated rhombohedral (ß) solid solution (6.3 % Ba) melt at a eutectic at 753 °C. Less Bi is needed to saturate the ß phase at lower temperatures so α-Bi2O3 coexists with a ß phase containing 11.5 % Ba at 646°C.The amount of Ba required to saturate the ß phase depends less strongly on temperature. Ba-saturated ß phase contains 19 % Ba at 700°C. These ß materials are in equilibrium with an oxide near Bi3BaO5.5 that undergoes two polymorphic transformations: low-temperature cubic (<700°C); orthorhombic (700-730°C); high-temperature cubic (Cht). There is a eutectic between the ß and Cht, at 775±6°C. At T<700°C, 26.5 % Ba saturates the latter but it can take in up to 29.5 % Ba (at 812°C). At T<815°C the coexisting phase is BiBaO3. A tetragonal (T) phase forms by reaction of Ch, and BiBaO3 and has ~35% BaO at 815°C. The composition span of T widens as temperature increases. Cht, melts incongruently at 820°C to a liquid and T with 29.8 % Ba. Above that temperature the Bi-saturated and Ba-saturated T phases both become more Ba-rich as temperature is elevated. T melts incongruently to liquid and BiBaO3.The δ-Bi2O3 and ß, both anion conductors, have structures based on that of fluorite. The other oxides have perovskite-like structures. Half of the Bi in BiBaO3 is pentavalent and half is trivalent. The other oxides appear to have all their Bi in the 3+ state.