Study of the Structural, Magnetic, and Electrical Properties of the 5H Hexagonal-Type Perovskite BaMn_0.2Co_0.8O_2.80

A combination of X-ray, neutron and electron diffraction, and high-resolution electron microscopy have been used to establish the 5H structural type of a new hexagonal-type perovskite BaMn 0.2 Co 0.8 O 2.80 . The structure can be described as a (cc′chh) 5H hexagonal polytype with ordered oxygen vacancies where the cubic c′ layer corresponds to a composition of [BaO 2 ] as opposed to [BaO 3 ]. The resulting layer structure consists of [MnCo 2 O 12 ] blocks of three sharing faces octahedra linked by corners to two unconnected [CoO 4 ] tetrahedra. Electron Energy Switch order Loss Spectroscopy shows Mn to be present only as Mn(+IV) and therefore Co is present as mixed +III/+IV. Mn(+IV) and Co(+III) ions are distributed over the face sharing octahedral sites whereas Co(+IV) ions are located on the tetrahedral sites. The magnetic behavior is more complex than is observed for BaCoO 2.80 (a ferromagnet with T c = 47 K) and can be described by a Stoner–Wohlfarth model of random-anisotropic, noninteracting monodomain ferromagnetic clusters. The ferromagnetic clusters occur below ∼35 K and are assigned to groups of Co ions in octahedral and/or tetrahedral sites; however, incorporation of Mn ions in the octahedral sites disrupts the transition into long-range three-dimensional ferromagnetic order. Impedance Spectroscopy data reveals semiconducting grain conductivity at room temperature (∼1 × 10 −2 S cm −1 ); however, subambient data reveal an unusual temperature dependence with a smooth changeover from a thermally activated process (∼0.07 eV) in the range 40–300 K to a low-temperature state below 40 K with a near-zero activation energy. The data cannot be described by conventional Arrhenius or variable-range hopping conduction models and the conduction mechanism(s) remain unresolved. Several possible suggestions for the conductivity behavior are made, including Anderson localization, anisotropic conduction associated with the 5H crystal structure or some complex correlated mechanism between the magnetic and electronic transport properties. The electrical microstructure of BaMn 0.2 Co 0.8 O 2.8 ceramics consist of semiconducting grains and constrictive grain boundaries and therefore exhibit internal barrier layer capacitor (IBLC) behavior, with a high and temperature-stable apparent permittivity of ∼10 000 (at 10 kHz) above 100 K.