Structure and chemistry of Ba0.6K0.4BiOy at high temperature

Abstract A combination of neutron powder diffraction and thermogravimetric analysis has been used to study the structural phase behavior of Ba 1− x K x BiO y under conditions that simulate synthesis. On heating in 1% O 2 , the cubic perovskite Ba 0.6 K 0.4 BiO 3 decomposes to a mixture of Ba 1− x K x BiO y with x 2 . Further increase in temperature causes the reincorporation of potassium into the perovskite at the expense of KBiO 2 . 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 Ba 1− x K x BiO 3 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 K 1+ for Ba 2+ in the lattice.