ON THE STRUCTURE OF THE ORTHO-II PHASE OF YBA2CU3O6+X - OXYGEN ORDERING AND CORRELATED ATOMIC DISPLACEMENTS

A neutron diffraction study of the YBa2Cu3O6.45 single crystal in the ortho-II phase gives evidence of doubling of the unit cell along the a-axis. For the first time a quantitative intensity analysis of the superstructure reflections has been performed. It is found that, in addition to the formation of alternating Cu (1)-O and Cu (1)-vacancy chains, the apex oxygen O(1) and the cations (Y, Ba, Cu (2) ) are involved in the superstructure. In two adjacent unit cells these atoms have opposite displacements, the magnitude of which is derived.