Mn-Rich BaMn1−xFexO3−δ perovskites revisited: structural, magnetic, and electrical properties of two new 6H′ polytypes
Résumé
The crystal and magnetic structures and electrical properties of two new Mn-rich 6H′-type hexagonal perovskites in the BaMn1−xFexO3−δ system have been investigated. Structural characterization performed by X-ray, electron, and neutron diffraction and high resolution electron microscopy indicates that both BaMn0.85Fe0.15O2.87 and BaMn0.6Fe0.4O2.72 crystallize in the 6H′ hexagonal polytype (P6m2 space group). The structure is formed by tetramers and dimers of face-sharing octahedra that are linked by corners. The anion deficiency is located at random through the hexagonal layers and increases with the Fe-content. In both phases, the central position of the tetramers is fully occupied by Mn, the remaining Mn and Fe cations being randomly distributed over different polyhedra. The Mssbauer spectroscopy data show Fe to be present only as FeIII in octahedral and tetrahedral coordination. The magnetic structure is formed by ferromagnetic sheets with the magnetic moments aligned along the x-axis and stacked antiferromagnetically perpendicular to the c-axis. The electrical properties have been characterized by impedance spectroscopy and reveal both compounds behave as leaky insulators at room temperature with bulk permittivity values <20.