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

Abstract

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 BaMn0.2Co0.8O2.80. 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 [BaO2] as opposed to [BaO3]. The resulting layer structure consists of [MnCo2O12] blocks of three sharing faces octahedra linked by corners to two unconnected [CoO4] 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 BaCoO2.80 (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 BaMn0.2Co0.8O2.8 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.

Structures and Microwave Dielectric Properties of Ba[Ti1−x(Co0.5W0.5)x]O3 (x = 0.40–0.90) Perovskite Ceramics

“…All diffraction spots are sharp and no apparent streaking along the c * axis is observed. No sign of disorder is observed by SAED . Thus, an interesting phase transition from cubic to hexagonal and then to cubic double perovskite, with increasing x , was observed.…”

“…A great deal of works have been done on the structures of cubic perovskites and their relationship with microwave dielectric properties. Recently, hexagonal perovskites have attracted attention for their multiferroic properties 3,4 and various crystal structures, such as 6H, 10H, 12R. [5][6][7][8] Some hexagonal perovskites have good dielectric properties at microwave frequencies, especially the near-zero τ ƒ values, which have been proven to be a type of promising dielectric materials for microwave applications.…”

“…9,10 The ABO 3 perovskite structure can be described as a close-packed arrangement of AO 3 layers with B cations occupying one-quarter of the oxygen octahedral centers. 4,7 The cubic perovskite is formed by a three-dimensional cornersharing octahedral network, whereas the exclusive hexagonal perovskite is composed of one-dimensional infinite extended chains of face-sharing octahedra along the c-axis. 4,7 But most of hexagonal perovskite structures, containing both cornerand face-sharing octahedra, have mixed structures between the exclusive hexagonal and cubic perovskites.…”

Microwave dielectric properties of Ba[Ti0.4Ga0.3Nb0.3(1−x)Sb0.3x]O3 perovskite ceramics

“…And the structurecomposition-property relationships among them have been established. In recent years, some researches have been focused on the hexagonal perovskites for their interesting crystal structures and special physical properties [2,6]. But the formation mechanism of the hexagonal perovskites and the relationships between structures and properties are still not well clear.…”

Synthesis, structure, and magnetic property of hexagonal perovskite Ba5Sn1.1Mn3.9O15

“…New hexagonal perovskite polytypes are generated when Mn is substituted by different cations. Several doped BaMnO 3 systems including Ba 1−x Sr x MnO 3−ı [5,8], BaTi x Mn 1−x O 3 [9], BaCa x Mn 1−x O 3−ı [10,11], BaIr x Mn 1−x O 3−ı [12], BaLn x Mn 1−x O 3−ı (Ln = rare earth element) [13][14][15][16][17], BaIn x Mn 1−x O 3−ı [18,19], BaCo x Mn 1−x O 3−ı [20,21], BaRu x Mn 1−x O 3 [22] and BaFe x Mn 1−x O 3−ı [23] have been explored. Among them, two B-cation ordered hexagonal perovskite structure are found with a formula Ba n+1 XMn n O 3n−ı (n = 2, 3), which contains one layer of corner-sharing XO 6 octahedron and n layers of face-sharing MnO 6 octahedra [13][14][15][16]19].…”