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Galvanic cells with MF2 solid electrolytes (M = Ca, Sr, and Ba) were used to determine activity-composition diagrams in the BizO3-MO systems. Activities of MO were determined from the measured electromotive force (EMF) values while those of Bi20:~ were estimated by Gibbs-Duhem integration. Equilibrium phases were identified by x-ray diffraction. In the stability range of the rhombohedral phase (~2), the activity of Bi~O~ exhibited a drastic decrease with increasing amount of MO in all three systems. Bi~O:~-SrO system showed the widest rhombohedral phase field and the lowest activity of Bi20:I (0.117) in the SrO-richest composition, (Bi~O:,)o.~(SrO)o,:,, at 630~ Partial molar entropies of dissolution of MO were determined from the tempcraturc dependence of EMF. On the Bi~Q-rich side, partial molar entropies of dissolution of MO were compared with a theoretical calculation of the configuration part of the partial molar entropy. A free energy vs. composition diagram was constructed for the CaO-Bi~O~ system.Bi~O~-based solid solutions are known to exhibit high oxygen ion conductivity over a wide temperature range. These materials can in principle be used, for instance, in oxygen sensors for the regulation of combustion processes, as membranes for the separation of oxygen from air, etc. Additionally, they also are potential candidates as solid electrolytes for sol~d oxide fuel cells (SOFCs). The ionic conductivity of Bi._,O~ stabilized with 20 mole percent (m/o) SrO is 2.2 x 10 1 ~-~-1 cm ~ at 700~ and 6.0 x 10 -:~ ~2 : cm -1 at 500~These values are one to two orders of magnitude higher than those of the widely used stabilized zirconia over the same range of temperatures. Y20:rstabilized cubic zirconia is stable in very low oxygen partial pressures, and oxygen ion conduction is predominant over a wide range of oxygen partial pressures down to 10 -=' atm at 1000~ -~ By contrast, Bi20:,-based materials are easily reduced under low partial pressure of oxygen, P~2. For example, the p,, for BifBi20:~ equilibrium at 700~ is ~ 1.36 x 10 -11 arm. 3 Thus, despite high oxygen ion conductivity, Bi20:~ containing materials have as yet found no applications in SOFCs. Recently, in an attempt to combine the high conductivity of Bi20.~ and the excellent thermodynamic stability of zirconia, investigations were undertaken in the ZrO2-Y203-Bi~O:, ternary system. 4-r' However, no systematic thermodynamic studies have been carried out on Bi203-based ternary systems. Also, it is important to recognize that simply dissolving ZrO2 in Bi~O~ is not expected to significantly influence the thermodynamic stability in a reducing atmosphere since the activity of Bi20.~ is not necessarily altered significantly by dissolving in it a more stable oxide such as ZrO2. For example, if the solution is ideal, the activity will simply be the mole fraction. In such a case, small amount of an additive (e.g., ZrO2, solute) is not expected to lower the activity of Bi~O.~ (solvent) significantly. For the enhancement of thermodynamic stability in a reducing atmosp...

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Many oxygen‐deficient oxide perovskites are oxygen ion conductors in a dry environment. However, in a humid atmosphere, they absorb water to fill up oxygen vacancies, release quasi‐free protons into the structure, and thereby become proton conductors. Absorption of water requires the transport of both protons and oxygen ions. This effectively leads to the transport of H2O under its chemical potential gradient. The present manuscript develops relationships between the chemical diffusion coefficient of H2O and the diffusion coefficients of protons and oxygen vacancies with an emphasis on the role of solution thermodynamics.

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