Structure Determination of Sr1.25Bi0.75O3 and Sr0.4K0.6BiO3 as a Function of Temperature from Synchrotron X-Ray Powder Diffraction Data

The structures of Sr1.25Bi0.75O3 and superconducting Sr0.4K0.6BiO3 have been determined from synchrotron X-ray powder diffraction data between 4 K and the decomposition temperature, at 973 and 573 K, respectively. The symmetry remains monoclinic (a≈2ap, b≈2ap, c≈2ap, β≈90°, P21/n space group with two Bi sites allowing charge localization) for the undoped compound, and tetragonal (a≈2ap, c≈2ap, I4/mcm space group with a unique Bi site implying a charge delocalization) for the K-doped phase, over the whole temperature range. In both cases the distortion from cubic symmetry decreases as temperature increases. Above 400 K, Sr0.4K0.6BiO3 progressively loses oxygen until it reaches the Sr0.4K0.6BiO2.5 stoichiometry, after which it decomposes.