New eight-layer twinned hexagonal perovskite microwave dielectric ceramic Ba sub(8)NiNb sub(6)O sub(2 4)

A new eight-layer hexagonal perovskite Ba sub(8)NiNb sub(6)O sub(2 4) 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 Ba sub(8)CoNb sub(6)O sub(2 4) and Ba sub(8)ZnNb sub(6)O sub(2 4), Ba sub(8)NiNb sub(6)O sub(2 4) adopts a twinned structure with stacking sequence (ccch) sub(2) for the BaO sub(3) layers and displays more disordered cation and vacancies over the face-sharing octahedral (FSO) sites than the twinned tantalates Ba sub(8)MTa sub(6)O sub(24) (M=Zn, Ni, Co). The stabilization of twinned structure and cation/vacancy ordering in Ba sub(8)NiNb sub(6)O sub(2 4) composition is correlated with the smaller size difference between Ni super(2+) and Nb super(5+) in comparison with those between (Zn/Co) super(2+) and Nb super(5+) in the shifted Ba sub(8)CoNb sub(6)O sub(2 4) and Ba sub(8)ZnNb sub(6)O sub(2 4). The Ba sub(8)NiNb sub(6)O sub(2 4) pellet exhibits high dielectric permittivity epsilon sub(r) ~ 4 0, modest Qf ~ 41 319 GHz, and large temperature coefficient of resonant frequency tau sub(f) ~ 6 0 ppm/ degree C. The lower Qf value compared with the high-Q Ba sub(8)MTa sub(6)O sub(24) is ascribed to the reduced short-range B-cationic ordering inside the FSO dimers in Ba sub(8)NiNb sub(6)O sub(2 4). These results contribute to understanding the interplay among chemical composition, structure, and dielectric properties of the eight-layer twinned and shifted hexagonal perovskites.