Structural Evolution of GdBaCo2O5+δ (δ = 7/18) at Elevated Temperatures

The structural chemistry of the double-layered perovskite-type gadolinium barium cobaltate, GdBaCo2O5+δ (0 3+ and Co2+ in a ratio of 8:1 and is incommensurately modulated in the five-dimensional superspace. The approximant of the modulated structure reveals the presence of island-like charge-ordered square cell domains interleaved with a discommensurate zone. The γ phase underwent a reversible first-order phase transition at ∼380 K to the commensurate phase β, without any change in composition at δ = 7/18. The charge ordering of Co in γ ceased in β, in combination with a redistribution of oxygen atoms (O3) in the [GdOδ] layer and a change in the spin state of Co. Further heating of β induced partial oxygen detachment at ∼700 K, causing a change in δ from 7/18 (∼0.389) to 0.19 in the nitrogen flow atmosphere. The crystal irreversibly turned into a tetragonal prototypal phase α. A peristaltic oxygen transport mechanism in GdBaCo2O5+δ (0 ≤ δ ≤ 1/2) accompanied by electron transfer and resulting in polaronic local structure relaxation is also proposed. This mechanism underpins a usefulness of the compound for application to solid oxide fuel cells as revealed in recent years.双层钙钛矿型钆钡钴氧化物GdBaCo2O5+δ（0 < δ ≤ 1）中，Co³⁺与Co²⁺的摩尔占比为8:1，且该化合物在五维超空间（five-dimensional superspace）中呈现非公度调制结构。其调制结构的近似相（approximant）显示，体系中存在岛状电荷有序四方晶畴（charge-ordered square cell domains）与失配区（discommensurate zone）相互穿插的微观结构。γ相在约380 K下发生可逆一级相变（first-order phase transition），转变为β公度相（commensurate phase），且在δ=7/18时体系成分未发生任何变化。γ相中的Co电荷有序在β相中消失，同时伴随[GdOδ]层内氧原子（O3）的重新分布以及Co离子自旋态的改变。在氮气流动气氛中对β相进一步加热至约700 K时，会引发部分氧脱附，使得δ值从7/18（约0.389）变为0.19，该晶体不可逆地转变为四方原型相（tetragonal prototypal phase）α。本文还提出了GdBaCo2O5+δ（0 ≤ δ ≤ 1/2）体系中的蠕动氧输运机制（peristaltic oxygen transport mechanism），该机制伴随电子转移并引发极化子局域结构弛豫（polaronic local structure relaxation）。正如近年研究揭示的那样，该机制支撑了该化合物在固体氧化物燃料电池（solid oxide fuel cells）领域的应用潜力。