Neutron-powder-diffraction study of the nuclear and magnetic structures of ${\mathrm{YBa}}_2$${\mathrm{Fe}}_3$${\mathrm{O}}_8$ at room temperature

The nuclear and magnetic structures of ${\mathrm{YBa}}_2$ ${\mathrm{Fe}}_3$ ${\mathrm{O}}_8$ 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) Å. The configuration of the atoms in the unit cell is very similar to that of the superconductor ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$, 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 (11¯0/110/002). 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 ${\mathrm{FeO}}_2$ 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)${\mathrm{\mu }}_{\mathit{B}}$. This configuration results in the magnetic symmetry ${\mathit{I}}_{\mathit{c}}$mm’m.