8-Layer Shifted Hexagonal Perovskite Ba 8 MnNb 6 O 24 : Long-Range Ordering of High-Spin d 5 Mn 2+ Layers and Electronic Structure.

A new 8-layer shifted hexagonal perovskite Ba8 MnNb6 O24 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)Nb6 O24 . 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 Ba8 MnNb6 O24 . The Ba8 MnNb6 O24 pellet exhibits a high dielectric permittivity, εr ∼ 38, and a large temperature coefficient of resonant frequency, τf ∼ 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 Ba8 ZnNb6 O24 . Electronic structures from density functional theory calculations reveal that Ba8 MnNb6 O24 is a Mott insulator in contrast with the charge-transfer insulator nature of Ba8 ZnNb6 O24 , 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 Ba8 MnNb6 O24 and Ba8 ZnNb6 O24 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 Ba8 MnNb6 O24 more conductive than Ba8 ZnNb6 O24 , 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.