Structural distortion in superconducting Ba1-xKxBiO3

The structure of superconducting Ba1-xKxBiO3 (BKBO) has been studied by neutron and synchrotron diffraction on single crystalline and on powder samples. In contrast to the phase diagram reported by several groups we find a long range structural distortion persisting in the K-concentration range of superconductivity. The distortion is characterized by a tilt of the BiO6 octahedra around the cubic [001] direction, space group 14/mcm, so far not observed for any BKBO compound. The mixing of light monovalent K atoms with the heavier two-valent Ba atoms causes strong structural disorder, in particular for the octahedron tilt. This disorder is simulated within a harmonic lattice dynamical model.