Neutron powder diffraction study of the nuclear and magnetic structures of the oxygen-deficient perovskite YBaCuCoO5

The nuclear and magnetic structures of the oxygen-deficient perovskite YBaCuCoO5 have been determined by neutron powder diffraction at room temperature. The nuclear structure has the symmetry of space group P 4/mmm and lattice parameters a = 3.8679(1), c = 7.5674(2) Å. Copper and cobalt atoms are completely disordered in this compound and the oxygen vacancies are located on the layer of the yttrium atoms. As a consequence of this configuration, the Co/Cu atoms have fivefold, pyramidal coordination. The pyramids share corners in the (Co/Cu)O2 planes, as well as in the direction of the c axis, thus forming slices of two pyramids with opposite orientation separated by layers of yttrium atoms. The Ba atoms, on the other hand, have 12-fold coordination as in nondefective perovskites, and the Y atoms have eightfold prismatic coordination as in the superconductor YBa2Cu3O6+δ. The magnetic structure is based on a unit cell related to that of the nuclear structure by an axis transformation of matrix (1, -1, 0/1, 1, 0/0, 0, 2). The magnetic origin of the extra reflections observed experimentally was established by polarized neutron diffraction measurements. The reflection conditions of the magnetic intensities are consistent with a model in which magnetic moments of equal modulus have alternately ferromagnetic and antiferromagnetic orientations along the c axis and antiferromagnetic ordering in the planes perpendicular to c. If we also assume that the magnetic structure has tetragonal symmetry, then the magnetic moments have to assume an orientation parallel to the c axis of the unit cell.