LOCAL OXYGEN OCTAHEDRA ROTATIONS IN BA1-XKXBIO3 AND BABIO3

We report XAFS investigations of BaBiO3 and Ba0.6K0.4BiO3 in the tempe rature range from 10 K to 300 K extending out to fourth nearest neighb ours. The results show that in both systems the local structure differ s substantially from the average structure determined by neutron diffr action 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 w ithin the experimental accuracy the local rotation components about th e [10 0], [0 1 0] and [0 0 1] axes are equal. The results in supercond ucting Ba0.6K0.4BiO3, are consistent with oxygen octahedra rotations e ither 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 degrees 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 theor etical study of the effect of these random rotations on the supercondu cting properties of Ba1-xKxBiO3 may be of interest. (C) 1997 Published by Elsevier Science Ltd.