Structural distortion in superconducting Ba1-xKxBiO3

The structure of superconducting Ba1-xKxBiO3 (BKBO) has been studied by neu tron and synchrotron diffraction on single crystalline and on powder sample s. In contrast to the phase diagram reported by several groups we find a lo ng range structural distortion persisting in the K-concentration range of s uperconductivity. The distortion is characterized by a tilt of the BiO6 oct ahedra around the cubic [001] direction, space group 14/mcm, so far not obs erved for any BKBO compound. The mixing of light monovalent K atoms with th e heavier two-valent Ba atoms causes strong structural disorder, in particu lar for the octahedron tilt. This disorder is simulated within a harmonic l attice dynamical model.