8-Layer Shifted Hexagonal Perovskite BaMnNbO: Long-Range Ordering of High-Spin d Mn Layers and Electronic Structure.

A new 8-layer shifted hexagonal perovskite BaMnNbO has been synthesized in air, featuring unusual long-range B-cation ordering with single octahedral high-spin d Mn layers separated by ∼1.9 nm within the corner-sharing octahedral d Nb host, analogous to Ba(Zn/Co)NbO. The large size and charge differences between high-spin Mn and Nb, as well as the out-of-center distortion of NbO octahedra associated with the bonding covalence and second-order Jahn-Teller effect of Nb, drive long-range cationic ordering, thus stabilizing BaMnNbO. The BaMnNbO pellet exhibits a high dielectric permittivity, ε ∼ 38, and a large temperature coefficient of resonant frequency, τ ∼ 20 ppm/K, but a dielectric loss ( Qf ∼ 987 GHz) and conductivity (∼10-10 S/cm within 473-1173 K) much higher than those of BaZnNbO. Electronic structures from density functional theory calculations reveal that BaMnNbO is a Mott insulator in contrast with the charge-transfer insulator nature of BaZnNbO, and they confirm that the off-center distortion of Nb contributes to stabilization of the 8-layer ordered shifted structure. The contrast between conductivity and dielectric loss of BaMnNbO and BaZnNbO is understood based on the electronic structure that depends on high-spin d Mn and d Zn cations. The hopping of 3d valence electrons in high-spin Mn to Nb 4d conduction bands over a small gap (∼2.0 eV) makes BaMnNbO more conductive than BaZnNbO, where the electrons are conducted via the hopping of a lattice O 2p valence electron to the Nb 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.