NEUTRON-POWDER-DIFFRACTION STUDY OF THE NUCLEAR AND MAGNETIC-STRUCTURES OF YBA2FE3O8 AT ROOM-TEMPERATURE

The nuclear and magnetic structures of YBa2Fe3O8 have been investigated by powder neutron diffraction at room temperature. The nuclear structure of the compound has the symmetry of space group P4/mmm and lattice parameters a = 3.9170(1) and c = 11.8252(4) angstrom. The configuration of the atoms in the unit cell is very similar to that of the superconductor YBa2Cu3O7, with the exception that the iron ions corresponding to the Cu-chain ions have octahedral coordination, rather than square planar; the octahedra thus are arranged in layers rather than in chains. This significant difference is a consequence of the fact that all possible oxygen sites in these layers are fully occupied, resulting in an oxygen content of eight rather than seven atoms per formula unit. A second consequence of the full occupancy of the oxygen sites is that the Ba ions have a twelvefold cuboctahedral coordination in the iron compound, rather than tenfold. The magnetic structure is based on a unit cell related to that of the nuclear structure by an axis transformation of matrix (110BAR/110/002). The magnetic origin of the extra intensities and the basic spin configuration were determined by polarized-neutron-diffraction measurements. The iron moments are coupled antiferromagnetically within each FeO2 layer, as well as along the c axis. The magnetic moments of all the iron ions are the same with a value of 3.49(2)mu(B). This configuration results in the magnetic symmetry I(c)mm'm.