NEUTRON AND X-RAY-POWDER DIFFRACTION STUDY OF RBA2FE3O8+W PHASES

Compounds of composition RBa2Fe3O8+w (R = La, Nd, Sm, Gd, Dy, Er, Yb, Lu, and Y) with variable oxygen content have been synthesized using th e liquid mixing technique and have been analyzed by powder X-ray and n eutron diffraction methods. A triple perovskite-type structure with or dered Ba and R cations and having the symmetry of space group P4/mmm w as obtained only for R = Y, Dy, and Er, whereas the larger R atoms gav e the atomic arrangement of disordered, defective perovskites with ave rage symmetry Pm (3) over bar m. No perovskite-type phases were obtain ed when Yb and Lu were tried. The oxygen content of the oxygen-saturat ed phases was found to increase with increasing size of the cation R f rom w = 0.07 for Er to w = 0.83 for La. The Neel temperature (similar to 650 K) and the magnitude of the Fe magnetic moment (similar to 3.3 mu(B) at room temperature) are highest when trivalent iron is involved (w = 0), but these quantities are rather insensitive to the nature of R and to slight variations of oxygen content in the triple perovskite -type structure of the Y, Dy, and Er compounds. In the case of the cub ic structures, however, both quantities depend strongly on the oxygen stoichiometry. In the magnetically ordered state, nearest-neighbor iro n moments are coupled antiferromagnetically along the three crystallog raphic directions in all samples, resulting in magnetic structures wit h symmetry Imm'm and magnetic unit cells related to those of the corre sponding nuclear structures by the transformation matrix(1 (1) over ba r 0/110/002). (C) 1998 Academic Press.