Local oxygen octahedra rotations in Ba_1-xK_xBiO₃

We report XAFS analyses results of Ba_0.6K_0.4BiO₃ in the temperature range from 10K to 300K extending out to fourth nearest neighbors. The results show that the local structure differs substantially from the average structure determined by neutron diffraction measurements. The results in superconducting Ba_0.6K_0.4BiO₃, are consistent with oxygen octahedra rotations either around [111] or [110] type axes. The rotation angles are 4.5° up to at least T = 220K if the rotation is assumed to be about [111] type axes. This local rotation exists in spite of the fact that the crystal is on the average cubic at all temperatures. The existence of these local rotations suggests that the charge carrier pairing mechanism may be enhanced through the interaction with the bistable states of the oxygen octahedra as proposed by Plakida[ 4] and Hardy and Flocken[ 5].