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 Ba8ZnNb6−xSbxO24 dramatically transformed the shifted structure to a twinned structure, in contrast with the Ba8ZnNb6−xTaxO24 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 Ba8ZnNb5.1Sb0.9O24 compounds exhibited good microwave dielectric properties (εr ∼ 35, Qf ∼ 36 200–43 400 GHz and τf ∼ 38–44 ppm/°C).