Structure of ten-layer orthorhombic Ba5Fe5O14 (BaFeO2.8) determined from single crystal X-ray diffraction

The structure of the mixed-valent Ba5Fe5O14 (BaFeO2.8), prepared using a molten KOH-Ba(OH)2 flux, has been determined using single crystal X-ray diffraction. Ba5Fe5O14 forms twinned crystals with the orthorhombic space group Cmcm, a=5.7615(8), b=9.9792(14) and c=24.347(3) A, Z=4. The structure, which is closely related to the 10H BaFeO2.65 perovskite, is composed of two oxygen-deficient BaO2 layers and eight BaO3 layers with a (hchhc)2 stacking sequence, where h and c denote hexagonal or cubic layers. A displacement of barium and oxygen atoms in the BaO2 layers from hexagonal special positions lowers the symmetry from hexagonal to orthorhombic. This combination of stacking and vacancies creates trimers of face-sharing FeO6 octahedra pillared by dimers of corner-sharing FeO4 tetrahedra. The Fe4+ atoms are located in the center of the trimer and in the tetrahedral sites. The magnetism of Ba5Fe5O14, investigated using SQUID magnetometry, is characteristic of a strongly coupled antiferromagnet