Conditions Extrêmes et Matériaux : Haute Température et Irradiation

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F.Tao, C.Liang, X.Wang, X.Li, F.Porcher, M.Allix, F.Lu, H.Gong, L.Liu, X.Kuang, 'New 8-Layer Shifted Hexagonal Perovskite Ba8MnNb6O24: Nano-Scale Ordering of High-Spin d5 Mn2+ Layers and Electronic Structure', Inorg. Chem. 57 5732-5742 (2018) doi:10.1021/acs.inorgchem.7b03023

A new 8-layer shifted hexagonal perovskite Ba8MnNb6O24 has been synthesized in air atmosphere, featuring unusual nanometer-scale B-cation ordering with single octahedral high-spin d5 Mn2+ layers separated by ~ 1.9 nm within the corner-sharing octahedral d0 Nb5+ host. The large cationic size and charge differences between high-spin Mn2+ and Nb5+, as well as the second-order Jahn-Teller (SOJT) out-of-center distortion of NbO6 octahedra drive long-range cationic ordering in Ba8MnNb6O24. The Ba8MnNb6O24 ceramic pellet exhibits a high dielectric permittivity εr ~ 38, large temperature coefficient of resonant frequency τ f ~ 20 ppm/K, but a much higher dielectric loss (Qf ~ 987 GHz at 5.31 GHz) and conductivity (~ 10-8-10-3 S/cm within 473-1173 K) than the Ba8ZnNb6O24 analogue material. Electronic structures from density functional theory (DFT) calculations reveal that Ba8MnNb6O24 is a Mott insulator in contrast with the charge transfer insulator nature of Ba8ZnNb6O24 and confirm that the SOJT effect of Nb5+ stabilizes the 8-layer ordered shifted structure. The contrast between conductivity and dielectric loss of Ba8MnNb6O24 and Ba8ZnNb6O24 is understood as 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 (~ 2.52 eV) makes Ba8MnNb6O24 more conductive than Ba8ZnNb6O24 where the electrons are conducted via hopping of lattice O-2p valence electron to Nb5+-4d conduction bands over a larger gap (~ 3.94 eV). The high dielectric loss of BMN may be mainly ascribed to the half-filled Mn 3d orbitals, 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.