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 bee n determined from synchrotron X-ray powder diffraction data between 4 K and the decomposition temperature, at 973 and 573 K, respectively. The symmetr y remains monoclinic(a approximate to root 2a(p), b approximate to root 2a( p), c approximate to 2a(p), beta approximate to 90 degrees, P2(1)/n space g roup with two Bi sites allowing charge localization) for the undoped compou nd, and tetragonal (a approximate to root 2a(p), c approximate to 2a(p), I4 /mcm space group with a unique Bi site implying a charge delocalization) fo r the K-doped phase, over the whole temperature range. In both cases the di stortion from cubic symmetry decreases as temperature increases. Above 400 K, Sr0.4K0.6BiO3 progressively loses oxygen until it reaches the Sr0.4K0.6B iO2.5 stoichiometry, after which it decomposes. (C) 2000 Academic Press.