Neutron powder diffraction study of the crystal structure of ysr₂alcu₂o₇

The structure of YSr₂AlCu₂O₇has been analyzed by neutron powder diffraction techniques. This compound crystallizes with space group P4/mmm symmetry and with latticeparameters a = 3.8646(1), c = 11.1139(3)Å. The general structural features of YSr₂AlCu₂O₇ are similar to those found for the 123 superconductor YBa₂Cu₃O_6+x, and in particular the sequence of layers is the same in the two materials, with SrO substituting for BaO. The Al ionssubstitute for the copper ions of the chain sites of the 123 parent structure and have tetrahedral coordination. The tetrahedra form chains propagating along the a and b axes on the basal plane of the unit cell. Thus the structure is made of short-range domains within which the local symmetry is orthorhombic. All the oxygen atoms in this compound have a disordered configuration around the specialized positions of the 123 structure. This arrangement results in tilting of the CuO₅ pyramids and in significant distortions of the coordination polyhedra of the other cations.