Phase Equilibria of the 1/2Bi2O3‐CaO System in Oxygen at 1 atm Pressure

Abstract

Phase equilibria in the Bi2O3‐CaO system have been examined over the temperature range of 650°‐1050°C in oxygen at a pressure of 1 atm. Direct electron microprobe analysis has been used to determine the compositions of several phases with fixed Bi:Ca ratios and to determine the extents of compositional ranges of the three solid solutions in the system. The face‐centered‐cubic solid solution (FCCss) phase extends from pure Bi2O3 to a maximum CaO content of 27 mol% at 816°± 4°C. In the bismuth‐rich portion of the diagram, the FCCss phase is replaced by monoclinic Bi2O3 (which dissolves essentially no CaO) and a rhombohedral solid solution (Rhss) phase with 12 mol% CaO. The eutectoid is located at 6 mol% CaO and 684°± 3°C. A calcium‐rich FCCss phase also coexists at a eutectoid with a Rhss phase (23.5 mol% CaO) and Bi2CaO4 at 766°± 3°C. There are two two‐phase regions between the FCCss and Rhss phases that have a closure temperature that is estimated to be ∼840°C. The Rhss phase exists between these two‐phase regions. The minimum CaO content of this solid solution is 12 mol%, and its maximum CaO content is 23.5 mol%; however, the extent of solid solution is highly temperature dependent at >700°C. All liquids with 0‐23 mol% CaO may coexist with a FCCss phase. The stability field of the rhombohedral phase does not reach the solidus, and there is a thermal maximum on the liquidus that is estimated at ∼880°C at which a FCCss phase and a liquid phase, each with 17 mol% CaO, coexist. Liquid with 23 mol% CaO, a FCCss phase, and a body‐centered‐cubic solid solution (BCCss) phase with 27 mol% CaO coexist at a eutectic near 866°C. The BCCss phase has a maximum composition range of 27‐42 mol% CaO, although the range is dependent on temperature. At temperatures below 816°± 4°C, the BCCss phase is replaced by a FCCss phase and Bi6Ca4O13. At 970 ± 3°C, the BCCss phase with 42 mol% CaO melts incongruently to CaO and a liquid. Thus, only three phases‐FCCss, BCCss, and CaO‐coexist with liquids in the Bi2O3‐CaO system. However, several other phases are identified in the subsolidus region: Bi14Ca5O26, which is replaced by a Rhss phase and Bi2CaO4 at 745°± 5°C; Bi2CaO4, which decomposes to a FCCss phase and Bi6Ca4O13 at 776°± 3°C; Bi6Ca4O13, which is replaced by a BCCss phase and Bi2Ca2O5 at ∼878°± 3°C; and Bi2Ca2O5, which decomposes to a BCCss phase and CaO at 931°± 5°C.