Neutron and X-Ray Powder Diffraction Study ofRBa2Fe3O8+wPhases

Compounds of compositionRBa2Fe3O8+w(R=La, Nd, Sm, Gd, Dy, Er, Yb, Lu, and Y) with variable oxygen content have been synthesized using the liquid mixing technique and have been analyzed by powder X-ray and neutron diffraction methods. A triple perovskite-type structure with ordered Ba andRcations and having the symmetry of space groupP4/mmmwas obtained only forR=Y, Dy, and Er, whereas the largerRatoms gave the atomic arrangement of disordered, defective perovskites with average symmetryPm3m. No perovskite-type phases were obtained when Yb and Lu were tried. The oxygen content of the oxygen-saturated phases was found to increase with increasing size of the cationRfromw=0.07 for Er tow=0.83 for La. The Néel temperature (~650 K) and the magnitude of the Fe magnetic moment (~3.3μBat room temperature) are highest when trivalent iron is involved (w=0), but these quantities are rather insensitive to the nature ofRand to slight variations of oxygen content in the triple perovskite-type structure of the Y, Dy, and Er compounds. In the case of the cubic structures, however, both quantities depend strongly on the oxygen stoichiometry. In the magnetically ordered state, nearest-neighbor iron moments are coupled antiferromagnetically along the three crystallographic directions in all samples, resulting in magnetic structures with symmetryImm′mand magnetic unit cells related to those of the corresponding nuclear structures by the transformation matrix (110/110/002).