Oxygen nonstoichiometry of BaBiO 3−δ

Dependence of oxygen deficiency of BaBiO 3−δ on temperature and oxygen partial pressure at a state of equilibrium was investigated by thermogravimetry. Two phases, one with 0.00≤ δ <0.03 (phase I) and the other with 0.43< δ ≤0.50 (phase II), and the first order phase transition between them was observed below 700°C. Besides the two phases, another phase (phase III) with 0.10< δ <0.20, which could be regarded as stable at high temperatures, appeared at 750°C. It was observed that the phase transitions of I⇔III and II⇔III are also first order. By increasing the temperature up to 800°C, the δ range of phase III increased to 0.06< δ <0.25. The eutectoid point, at which the three phases coexisted, was observed at a temperature of ∼710°C and an oxygen partial pressure of ∼10 −3 atm in the relationship between the temperature and the oxygen partial pressure at the two phase region. From the relationship, variations of enthalpy of phase transitions were calculated to be 170±35 kJ mol −1, 190±20 kJ mol −1 and 80±5 kJ mol −1 for I→II, I→III and III→II, respectively.