title: Nanometer-scale separation of d(10) Zn(2+)-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 d(10) cationic (Zn(2+)) layers within the purely corner-sharing octahedral d(0) cationic (Nb(5+)) host. The long-range ordering of the B-site vacancy and out-of-center distortion of the highly-charged d(0) Nb(5+) that is assisted by the second-order Jahn-Teller effect contribute to this unusual B-cation ordering in Ba8ZnNb6O24. A small amount (∼15%) of d(10) Sb(5+) substitution for Nb(5+) in Ba8ZnNb6-xSbxO24 dramatically transformed the shifted structure to a twinned structure, in contrast with the Ba8ZnNb6-xTaxO24 case requiring 50% d(0) Ta(5+) substitution for Nb(5+) 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).
authors.affiliation: Guangxi Ministry-Province Jointly-Constructed Cultivation Base for State Key Laboratory of Processing for Nonferrous Metal and Featured Materials, Guangxi Universities Key Laboratory of Non-ferrous Metal Oxide Electronic Functional Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, P. R. China. kuangxj@gult.edu.cn.
authors.forename: Fengqi
authors.initials: F
authors.lastname: Lu
authors.forename: Xiaoming
authors.initials: X
authors.lastname: Wang
authors.forename: Zhengwei
authors.initials: Z
authors.lastname: Pan
authors.forename: Fengjuan
authors.initials: F
authors.lastname: Pan
authors.forename: Shiqiang
authors.initials: S
authors.lastname: Chai
authors.forename: Chaolun
authors.initials: C
authors.lastname: Liang
authors.forename: Quanchao
authors.initials: Q
authors.lastname: Wang
authors.forename: Jing
authors.initials: J
authors.lastname: Wang
authors.forename: Liang
authors.initials: L
authors.lastname: Fang
authors.forename: Xiaojun
authors.initials: X
authors.lastname: Kuang
authors.forename: Xiping
authors.initials: X
authors.lastname: Jing