Nanometer-scale separation of d10 Zn2+-layers and twin–shift competition in Ba8ZnNb6O24-based 8-layered hexagonal perovskites

Abstract

The 8-layered shifted hexagonal perovskite compound Ba₈ZnNb₆O₂₄ was isolated via controlling the ZnO volatilization, which features long-range B-cation ordering with nanometer-scale separation by ∼1.9 nm of octahedral d¹⁰ cationic (Zn²⁺) layers within the purely corner-sharing octahedral d⁰ cationic (Nb⁵⁺) host. The long-range ordering of the B-site vacancy and out-of-center distortion of the highly-charged d⁰ Nb⁵⁺ that is assisted by the second-order Jahn–Teller effect contribute to this unusual B-cation ordering in Ba₈ZnNb₆O₂₄. A small amount (∼15%) of d¹⁰ Sb⁵⁺ substitution for Nb⁵⁺ in Ba₈ZnNb_6−xSb_xO₂₄ dramatically transformed the shifted structure to a twinned structure, in contrast with the Ba₈ZnNb_6−xTa_xO₂₄ case requiring 50% d⁰ Ta⁵⁺ substitution for Nb⁵⁺ for such a shift-to-twin transformation. Multiple factors including B-cationic sizes, electrostatic repulsion forces, long-range ordering of B-site vacancies, and bonding preferences arising from a covalent contribution to the B–O bonding that includes out-of-center octahedral distortion and the B–O–B bonding angle could subtly contribute to the twin–shift phase competition of B-site deficient 8-layered hexagonal perovskites Ba₈B₇O₂₄. The ceramics of new shifted Ba₈ZnNb₆O₂₄ and twinned Ba₈ZnNb_5.1Sb_0.9O₂₄ compounds exhibited good microwave dielectric properties (ε_r ∼ 35, Qf ∼ 36 200–43 400 GHz and τ_f ∼ 38–44 ppm/°C).