Neutron-powder-diffraction study of the nuclear and magnetic structures of the substitution compound (Y1-xCax)Ba2Fe3O8+δ (x=0.05, 0.10, and 0.20)

The nuclear and magnetic structures of (Y1-xCax)Ba2Fe3O8+δ with x=0.05, 0.10, and 0.20 have been investigated by neutron-powder diffraction at room temperature. The compound crystallizes with the symmetry of space group P4/mmm and has a structure similar to that of the superconductor YBa2Cu3O7 (1:2:3). There are two types of Fe atoms in the unit cell, one having fivefold pyramidal coordination and the other octahedral coordination. The Ca and Y atoms are randomly distributed over the 1/2,1/2,1/2 position, which are exclusively occupied by Y in the 1:2:3 superconductor. Since the oxygen sites on the basal plane of the structure are fully occupied to achieve octahedral coordination for the iron atoms on this plane, the oxygen content is eight atoms per formula unit, the barium atoms are twelvefold coordinated, and the coordination polyhedron is a cuboctahedron. Extra oxygen atoms corresponding to δ=0.08 and 0.05 are located in the Y/Ca layer. The presence of these atoms produces disorder in the structure resulting in a shift of the oxygen atoms located near the plane of the pyramidal iron atoms. The charge compensation required by the substitution of Y3+ by Ca2+ is achieved in these materials by elimination of the extra oxygen, rather than oxidation of the iron atoms. The magnetic structure is based on a unit cell related to that of the nuclear structure by the transformation of axes (1,-1,0/1,1,0/0,0,2). The magnetic origin of some of the observed diffraction peaks was established by polarized neutron diffraction measurements. As in YBa2Fe3O8, the iron moments are coupled antiferromagnetically within each (FeO2) layer as well as along the c axis of the structure, and they lie in the planes perpendicular to c. The magnetic moments of the two iron atoms are practically identical and have values at room temperature of μ=3.50(3)μB, 3.52(2)μB, and 3.62(2)μB for the three compositions x=0.05, 0.10, and 0.20, respectively.