Single Sublattice Endotaxial Phase Separation Driven by Charge Frustration in a Complex Oxide

Antoine Demont,Ruth Sayers,Matthew J Rosseinsky,Hongjun Niu,Zengqiang Deng,Maria A Tsiamtsouri,Simon Romani,Philip A Chater,Michael F Thomas,Carlos Martí-Gastaldo,Yohann Bréard,John B Claridge,Zhongling Xu

doi:10.1021/ja403611s

Abstract

Complex transition-metal oxides are important functional materials in areas such as energy and information storage. The cubic ABO3 perovskite is an archetypal example of this class, formed by the occupation of small octahedral B-sites within an AO3 network defined by larger A cations. We show that introduction of chemically mismatched octahedral cations into a cubic perovskite oxide parent phase modifies structure and composition beyond the unit cell length scale on the B sublattice alone. This affords an endotaxial nanocomposite of two cubic perovskite phases with distinct properties. These locally B-site cation-ordered and -disordered phases share a single AO3 network and have enhanced stability against the formation of a competing hexagonal structure over the single-phase parent. Synergic integration of the distinct properties of these phases by the coherent interfaces of the composite produces solid oxide fuel cell cathode performance superior to that expected from the component phases in isolation.

Highlights

Of separate phases within a single bulk grain is important in the production of structural and functional materials: it can determine the mechanical properties of steels[1] and optimize thermoelectric performance.[2]
The two points marked with crosses correspond to the single perovskite (SP) and doubled perovskite (DP) component phases refined for 2. (i) Rietveld refinements of X-ray diffraction (XRD) data and (ii) DF transmission electron microscopy (TEM) images of representative compositions in the three regions of the phase field are shown for (b) z = 0.125 (1, SP), (c) z = 0.375 (2, SP/DP), and (d) z = 0.45 (3, SP/DP + BaMoO4) at m = 4
Reflection markers refer to SP, DP, and BaMoO4; the inset shows the region where the double perovskite superstructure reflection is observed

Summary

Introduction

Of separate phases within a single bulk grain is important in the production of structural and functional materials: it can determine the mechanical properties of steels[1] and optimize thermoelectric performance.[2]. The structures adopted by complex oxides allow fine-tuning of physical behavior by chemical substitution, for example, the use of charge reservoir layers in superconducting copper oxides to remotely control the electron count in the electronically active CuO2 planes.[5] The ABO3 cubic perovskite Ba0.5Sr0.5(Co0.8Fe0.2)O3 has small dn Co and Fe cations disordered on the Bsites of an AO3 network defined by the larger Ba and Sr cations It is a single-phase material because the similar charges and bonding chemistries of Co and Fe can be accommodated on a single site in the average structure, producing a combination of electronic (B-site d electrons) and oxide ion (anion vacancies) transport that makes it an effective solid oxide fuel cell (SOFC) cathode.[6] In this work, we explore the effect of incorporating the highly charged d0 Mo6+ cation onto the B-site of this system. The overall O content of 2 is higher than the 2.75−2.4 range found for Ba0.5Sr0.5(Co0.8Fe0.2)O3−δ.7,10−12

Objectives

Results

Conclusion