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2017
ACL
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F.Tao, X.Wang, M.Gong, F.Lu, M.Allix, X.Kuang, L.Fang, 'New Eight-Layer Twinned Hexagonal Perovskite Microwave Dielectric Ceramic Ba8NiNb6O24', J. Am. Ceram. Soc. 100 1212-1220 (2017) doi:10.1111/jace.14601
A new eight-layer hexagonal perovskite Ba8NiNb6O24 was synthesized via the high-temperature solid-state reaction and its structure was characterized using selected area electron diffraction, high-resolution transmission electron microscopy, and synchrotron X-ray diffraction. Unlike the eight-layer ordered shifted Ba8CoNb6O24 and Ba8ZnNb6O24, Ba8NiNb6O24 adopts a twinned structure with stacking sequence (ccch)2 for the BaO3 layers and displays more disordered cation and vacancies over the face-sharing octahedral (FSO) sites than the twinned tantalates Ba8MTa6O24 (M = Zn, Ni, Co). The stabilization of twinned structure and cation/vacancy ordering in Ba8NiNb6O24 composition is correlated with the smaller size difference between Ni2+ and Nb5+ in comparison with those between (Zn/Co)2+ and Nb5+ in the shifted Ba8CoNb6O24 and Ba8ZnNb6O24. The Ba8NiNb6O24 pellet exhibits high dielectric permittivity εr ~ 40, modest Qf ~ 41 319 GHz, and large temperature coefficient of resonant frequency τf ~ 60 ppm/°C. The lower Qf value compared with the high-Q Ba8MTa6O24 is ascribed to the reduced short-range B-cationic ordering inside the FSO dimers in Ba8NiNb6O24. These results contribute to understanding the interplay among chemical composition, structure, and dielectric properties of the eight-layer twinned and shifted hexagonal perovskites.
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