8-Layer Shifted Hexagonal Perovskite Ba₈MnNb₆O₂₄: Long-Range Ordering of High-Spin d⁵ Mn²⁺ Layers and Electronic...

A new 8-layer shifted hexagonal perovskite Ba₈MnNb₆O₂₄ has been synthesized in air, featuring unusual long-range B-cation ordering with single octahedral high-spin d⁵ Mn²⁺ layers separated by ∼1.9 nm within the corner-sharing octahedral d⁰ Nb⁵⁺ host, analogous to Ba₈(Zn/Co)Nb₆O₂₄. The large size and charge differences between high-spin Mn²⁺ and Nb⁵⁺, as well as the out-of-center distortion of NbO₆ octahedra associated with the bonding covalence and second-order Jahn-Teller effect of Nb⁵⁺, drive long-range cationic ordering, thus stabilizing Ba₈MnNb₆O₂₄. The Ba₈MnNb₆O₂₄ pellet exhibits a high dielectric permittivity, ε_r ∼ 38, and a large temperature coefficient of resonant frequency, τ_f ∼ 20 ppm/K, but a dielectric loss ( Qf ∼ 987 GHz) and conductivity (∼10^-8-10^-3 S/cm within 473-1173 K) much higher than those of Ba₈ZnNb₆O₂₄. Electronic structures from density functional theory calculations reveal that Ba₈MnNb₆O₂₄ is a Mott insulator in contrast with the charge-transfer insulator nature of Ba₈ZnNb₆O₂₄, and they confirm that the off-center distortion of Nb⁵⁺ contributes to stabilization of the 8-layer ordered shifted structure. The contrast between conductivity and dielectric loss of Ba₈MnNb₆O₂₄ and Ba₈ZnNb₆O₂₄ is understood based on the electronic structure that depends on high-spin d⁵ Mn²⁺ and d¹⁰ Zn²⁺ cations. The hopping of 3d valence electrons in high-spin Mn²⁺ to Nb⁵⁺ 4d conduction bands over a small gap (∼2.0 eV) makes Ba₈MnNb₆O₂₄ more conductive than Ba₈ZnNb₆O₂₄, where the electrons are conducted via the hopping of a lattice O 2p valence electron to the Nb⁵⁺ 4d conduction bands over a larger gap (∼3.9 eV). The high microwave dielectric loss of BMN may be mainly ascribed to the half-filled Mn 3d orbits, which is understood based on the softened infrared modes that increase the lattice vibration anharmonicity as well as the resonant spin excitation of unpaired d electrons.