Structural Evolution of GdBaCo₂O_5+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 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.

Effect of A-Site Cation Ordering on Chemical Stability, Oxygen Stoichiometry and Electrical Conductivity in Layered LaBaCo2O5+δ Double Perovskite

“…Tsvetkov et al [ 36 ] reported a similar behavior for PrBaCo 2 O 5+δ where a transition from P4/mmm to Pmmm was observed at low pO 2 , 500 °C and 5.5 in oxygen content. For oxygen contents below 5.5, a larger tetragonal cell with a 3a p × 3a p × 2a p supercell is adopted [ 37 ]. Transition from a tetragonal structure to an orthorhombic structure with a later transition to the original tetragonal structure together with the oxygen content variation is shown in Figure 11 .…”

Relating defect chemistry and electronic transport in the double perovskite Ba1−xGd0.8La0.2+xCo2O6−δ (BGLC)

“…7,8,22 Oxygen vacancies preferentially form on the O3-site, whereas there are indications of some oxygen vacancies in the Co-O layer. 12,15,23,24 The oxidation of oxygen vacancies on O3 sites can then be expressed as:…”

Synthesis and conductivity properties of Gd0.8Ca0.2BaCo2O5+δ double perovskite by sol–gel combustion

“…Among various types of TMOs, GaBaCo 2 O 5?d (GBCO) has attracted a lot of attention due to its certain interesting properties, such as insulator-metal transition (IMT) [12], total conductivity [13], charge ordering [14], magnetic transition [15] and non-stoichiometry in thermogravimetric analysis [16]. GBCO 2 ] stacked sequentially along the c axis [17].…”