Local oxygen octahedra rotations in Ba_1-xK_xBiO₃ and BaBiO₃

We report XAFS investigations of BaBiO₃ and Ba_0.6K_0.4BiO₃ in the temperature range from 10 K to 300 K extending out to fourth nearest neighbours. The results show that in both systems the local structure differs substantially from the average structure determined by neutron diffraction measurements. At temperatures up to at least T = 80 K the local oxygen octahedra rotations in BaBiO₃ have in addition to the [1 1 0] component a disordered rotation component about the [0 0 1] axis. To within the experimental accuracy the local rotation components about the [1 0 0], [0 1 0] and [0 0 1] axes are equal. The results in superconducting Ba_0.6K_0.4BiO₃, are consistent with oxygen octahedra rotations either around [1 1 1] or [1 1 0] type axes. If the rotation is assumed to be about [1 1 1] type axes, the rotation angles are 4.5° up to at least T = 220 K. This local rotation exists in spite of the fact that the crystal is on the average cubic at all temperatures. A theoretical study of the effect of these random rotations on the superconducting properties of Ba_1-xK_xBiO₃ may be of interest.