STRUCTURE AND CHEMISTRY OF BA0.6K0.4BIOY AT HIGH-TEMPERATURE

A combination of neutron powder diffraction and thermogravimetric analysis has been used to study the structural phase behavior of Ba1-xKxBiOy under conditions that simulate synthesis. On heating in 1% O2, the cubic perovskite Ba0.6K0.4BiO3 decomposes to a mixture of Ba1-xKxBiOy with x < 0.4 and KBiO2. Further increase in temperature causes the reincorporation of potassium into the perovskite at the expense of KBiO2. At 720°C, the sample is again a single-phase cubic perovskite with x = 0.4 containing a large concentration of oxygen vacancies. If this sample is then cooled in argon, no chemical phase decomposition occurs. Instead, the cubic oxygen-deficient perovskite transforms to an orthorhombic oxygen-vacancy-ordered phase. A subsequent low-temperature anneal in pure oxygen fills the oxygen vacancies while retaining the potassium in the lattice, resulting in a transformation back to the cubic perovskite. This work shows that fully oxygenated Ba1-xKxBiO3 for x > 0.1, including the superconducting compositions, is metastable, and supports the hypothesis that the creation of oxygen vacancies is necessary to allow the substitution of K1+ for Ba2+ in the lattice. © 1991.