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

Abstract

The structural chemistry of the double-layered perovskite-type gadolinium barium cobaltate, GdBaCo2O5+δ (0 < δ < 1/2), is not well-known, in comparison to the representative end-member phases α at δ = 0 and β at δ = 1/2. This study unveils the structural evolution of the room-temperature stable phase γ at δ = 7/18, using in situ single-crystal X-ray diffraction at elevated temperatures and electron microscopy. The γ phase is essentially charge-ordered with Co3+ 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...