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.