Neutron powder diffraction study of Pb₂Sr₂YCu₃O₈, the prototype of a new family of superconductors

The structure of Pb₂Sr₂YCu₃O₈ has been analyzed with powder neutron diffraction techniques and profile analysis. The results of this study confirm the general structural features determined by X-ray single-crystal methods. The material has pseudo tetragonal symmetry, but is orthorhombic, space group Cmmm with lattice parameters a=5.3933(2), b=5.4311(2), and c=15.7334(6)Å. The orthorhombic distortion is caused by the disordering in the ab plane of the oxygen atoms of the PbO layers over the general position of the space group, with x=0.275(5) and y=0.402(5) Å. The structure of this compound can be derived from that of YBa₂Cu₃O₆ by substituting the Cu layers with blocks of ... PbO.Cu.PbO... . Of the two Cu atoms in the asymmetric unit, one has two fold coordination while the other has five fold pyramidal coordination with the apex elongated along the c-axis.