Neutron powder diffraction study of TbBaCo2-xFexO5+γ layered oxides

The crystal and magnetic structures of layered perovskites TbBaCo2-xFexO5+γ (0.08≤x≤0.24) were studied by neutron powder diffraction. Increasing iron concentration up to the x values higher than 0.10 leads to the orthorhombic→tetragonal phase transition resulting from the transformation of 2x1x1-type superstructure, formed due to ordering of extra oxygen incorporated into the vacant sites in [TbOγ] layers, into 3x3x1 superlattice. The concentration ranges, where the orthorhombic and tetragonal lattices exist, are separated with a narrow two-phase domain. For the tetragonal phases with 3x3x1 superstructure (space group P4/mmm), the Co/Fe ions are antiferromagnetically coupled, forming G-type spin-ordered configuration. The Co3+ cations located in square-pyramidal sites adopt intermediate spin state, whilst a relatively small magnetic moment of Co3+ ions in the octahedral sublattice indicates that a minor fraction of cobalt is in the low-spin state