New eight‐layer twinned hexagonal perovskite microwave dielectric ceramic Ba 8 NiNb 6 O 24

A new eight‐layer hexagonal perovskite Ba 8 NiNb 6 O 24 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 8 CoNb 6 O 24 and Ba 8 ZnNb 6 O 24 , Ba 8 NiNb 6 O 24 adopts a twinned structure with stacking sequence ( ccch ) 2 for the BaO 3 layers and displays more disordered cation and vacancies over the face‐sharing octahedral ( FSO ) sites than the twinned tantalates Ba 8 M T a 6 O 24 ( M =Zn, Ni, Co). The stabilization of twinned structure and cation/vacancy ordering in Ba 8 NiNb 6 O 24 composition is correlated with the smaller size difference between Ni 2+ and Nb 5+ in comparison with those between (Zn/Co) 2+ and Nb 5+ in the shifted Ba 8 CoNb 6 O 24 and Ba 8 ZnNb 6 O 24 . The Ba 8 NiNb 6 O 24 pellet exhibits high dielectric permittivity ε r ~ 40, modest Qf ~ 41 319 GHz, and large temperature coefficient of resonant frequency τ f ~ 60 ppm/°C. The lower Qf value compared with the high‐ Q Ba 8 M T a 6 O 24 is ascribed to the reduced short‐range B‐cationic ordering inside the FSO dimers in Ba 8 NiNb 6 O 24 . These results contribute to understanding the interplay among chemical composition, structure, and dielectric properties of the eight‐layer twinned and shifted hexagonal perovskites.