Interplay between A-site and oxygen-vacancy ordering, and mixed electron/oxide-ion conductivity in n = 1 Ruddlesden–Popper perovskite Sr2Nd2Zn2O7

Abstract

Oxygen vacancies in Ruddlesden–Popper (RP) perovskites (PV) [AO][ABO₃]_n play a pivotal role in engineering functional properties and thus understanding the relationship between oxygen-vacancy distribution and physical properties can open up new strategies for fine manipulation of structure-driven functionalities. However, the structural origin of preferential distribution for oxygen vacancies in RP structures is not well understood, notably in the single-layer (n = 1) RP-structure. Herein, the n = 1 RP phase Sr₂Nd₂Zn₂O₇ was rationally designed and structurally characterized by combining three-dimensional (3D) electron diffraction and neutron powder diffraction. Sr₂Nd₂Zn₂O₇ adopts a novel 2-fold n = 1 RP-type Pmmn-superstructure due to the concurrence of A-site column ordering and oxygen-vacancy array ordering. These two ordering models are inextricably linked, and disrupting one would thus destroy the other. Oxygen vacancies are structurally confined to occupy the equatorial sites of “BO₆”-octahedra, in stark contrast to the preferential occupation of the inner apical sites in n ≥ 2 structures. Such a layer-dependent oxygen-vacancy distribution in RP structures is in fact dictated by the reduction of the cationic A–A/B repulsion. Moreover, the intrinsic oxygen vacancies can capture atmospheric O₂, consequently resulting in a mixed oxide ion and p-type electrical conductivity of 1.0 × 10⁻⁴ S cm⁻¹ at temperatures > 800 °C. This value could be further enhanced to > 1.0 × 10⁻³ S cm⁻¹ by creating additional oxygen vacancies on the equatorial sites through acceptor doping. Bond valence site energy analysis indicates that the oxide ion conduction in Sr₂Nd₂Zn₂O₇ is predominated by the one-dimensional pathways along the [Zn₂O₇] ladders and is triggered by the gate-control-like migration of the equatorial bridging oxygens to the oxygen-vacant sites. Our results demonstrate that control of anion and cation ordering in RP perovskites opens a new path toward innovative structure-driven property design.