Trigonal-Planar Low-Spin Co2+ in a Layered Mixed-Polyhedral Network from Topotactic Reduction

Oxygen Migration Pathways in Layered LnBaCo2O6-δ (Ln = La - Y) Perovskites

Layered LnBaCo2O6-δ perovskites are important mixed ionic-electronic conductors, exhibiting outstanding catalytic properties for the oxygen evolution/reduction reaction. These phases exhibit considerable structural complexity, in particular, near room temperature, where a number of oxygen vacancy ordered superstructures are found. This study uses bond valence site energy calculations to demonstrate the key underlying structural features that favor facile ionic migration. BVSE calculations show that the 1D vacancy ordering for Ln = Sm-Tb could be beneficial at low temperatures as new pathways with reduced barriers emerge. By contrast, the 2D vacancy ordering for Ln = Dy and Y is not beneficial for ionic transport with the basic layered parent material having lower migration barriers. Overall, the key criterion for low migration barriers is an expanded ab plane, supported by Ba, coupled to a small Ln size. Hence, Ln = Y should be the best composition, but this is stymied by the low temperature 2D vacancy ordering and moderate temperature stability. The evolution of the oxygen cycling capability of these materials is also reported.

Stability and Evolution of the Crystal Structure of TbBaCo2O6-δ During Thermal Oxygen Release/Uptake

A-site ordered double perovskites with the general formula LnBaCo₂O_6-δ (where Ln is a lanthanide element) present electrical and electrocatalytic properties that make them attractive as possible ceramic electrode materials for solid oxide cells or alkaline electrolyzers. The properties are highly influenced by the anion vacancy concentration, which is strongly related to the Co-oxidation state, and their location in the structure. Awareness of the stable phases is essential to synthesize, evaluate, and optimize the properties of LnBaCo₂O_6-δ oxides at operating conditions in different applications. TbBaCo₂O_6-δ are representative oxides of these layered perovskite systems. The present article reports a study of TbBaCo₂O_6-δ by electron diffraction, high-resolution electron microscopy, and powder neutron diffraction experiments at different temperatures. The synthesis of TbBaCo₂O_6-δ in air and slow cooling to room temperature (RT) at 5 °C h^-1 leads to samples formed by distinct phases with different oxygen contents and crystal structures. The 122 and 112 phases (with a_p × 2a_p × 2a_p and a_p × a_p × 2a_p unit cells, respectively, with a_p being the lattice parameter of the simple cubic perovskite structure) are predominant in quasi-equilibrium prepared samples (cooled at RT at 1 °C h^-1) or prepared in Ar flow and quenched to RT. The evolution of the crystal structure of TbBaCo₂O_6-δ during thermal oxygen release/uptaking consists of modulation from the 122 phase to the 112 phase (or vice versa during uptaking) by creation/occupation of anion vacancies within the TbO_1-δ planes. Anion vacancies are not detected in the oxygen crystallographic position different from those located within the TbO_1-δ planes even at the highest temperatures, supporting the 2D character of the high anion conduction of the LnBaCo₂O_6-δ oxides.

High-pressure Synthesis of Ba2CoO2Ag2Te2 with Extended CoO2 Planes

Using a high-pressure synthesis method, we prepared the layered oxychalcogenide Ba₂CoO₂Ag₂Te₂ (space group: I4/mmm) with alternating stacks of CoO₂ and Ag₂Te₂ layers, separated by Ba atoms. The CoO₂ plane is greatly extended (Co-O = 2.19 Å on average) due to tensile strain from adjacent Ag₂Te₂ layers, causing displacement of oxide anions. Layered cobaltates with trans-CoO₄X₂ (X = chalcogen, halogen) coordination feature large spin-orbit coupling, which is linearly scaled by the tetrahedral factor of d_Co-X/d_Co-O. However, applying this relation to Ba₂CoO₂Ag₂Te₂ yields a magnetic moment of ∼4 μ_B, which is nearly twice the experimentally observed value of 1.87(17) μ_B. This result, along with a reduced Néel temperature (T_N = 60 K), originates from the off-centered position of otherwise under-bonded oxide anions, which changes the crystal field splitting of Co d orbitals.