STABILITY, OXYGEN NONSTOICHIOMETRY, AND TRANSFORMATIONS OF THE BI2SR2COO6+DELTA CERAMIC

Thermogravimetry at 20-900 degrees C and 10(-3)-1 atm of oxygen partia l pressure (pO(2)) has been used to establish a region of stability an d oxygen nonstoichiometry of the compound Bi2Sr2CoO6+delta. This phase was found to be unstable below 850 degrees C at high oxygen partial p ressures (air, oxygen), whereas at low oxygen concentrations in the ga s phase (pO(2)=1.5 x 10(-3)-2 x 10(-2) atm) it exhibits stability in t he entire temperature region under investigation. Within, the region o f stability, the oxygen content in Bi2Sr2CoO6+delta (6+delta) has been determined under equilibrium conditions as a function of temperature and oxygen partial pressure and the corresponding defect model has bee n proposed. The annealing of Bi2Sr2CoO6+delta outside the region of st ability is found to result in the formation of Bi2Sr3Co2O9 and the bin ary compounds of the Bi2O3-SrO system. The kinetics of this transforma tion has been studied at 700-850 degrees C in a pure oxygen atmosphere and a possible interpretation has been given. (C) 1998 Academic Press .