Insights into Oxygen Migration in LaBaCo₂O_6−δ Perovskites from In Situ Neutron Powder Diffraction and Bond Valence Site Energy Calculations

Abstract

Layered cobalt oxide perovskites are important mixed ionic and electronic conductors. Here, we investigate LaBaCo 2 O 6−δ using in situ neutron powder diffraction. This composition is unique because it can be prepared in cubic, layered, and vacancy-ordered forms. Thermogravimetric analysis and diffraction reveal that layered and disordered samples have near-identical oxygen cycling capacities. Migration barriers for oxide ion conduction calculated using the bond valence site energy approach vary from E b ∼ 2.8 eV for the cubic perovskite to E b ∼ 1.5 eV for 2D transport in the layered system. Vacancy-ordered superstructures were observed at low temperatures, 350–400 °C for δ = 0.25 and δ = 0.5. The vacancy ordering at δ = 0.5 is different from the widely reported structure and involves oxygen sites in both CoO 2 and LaO planes. Vacancy ordering leads to the emergence of additional migration pathways with low-energy barriers, for example, 1D channels with E b = 0.5 eV and 3D channels with E b = 2.2 eV. The emergence of these channels is caused by the strong orthorhombic distortion of the crystal structure. These results demonstrate that there is potential scope to manipulate ionic transport in vacancy-ordered LnBaCo 2 O 6−δ perovskites with reduced symmetry.

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

“…The sample does not regain oxygen during cooling (unlike during the TGA experiment, using the same N 2 gas) and keeps the basic layered tetragonal structure and an unchanged (δ = 1) composition at 300 °C (Table ). This discrepancy results from a tighter vacuum in the neutron sample environment and was also noted for our earlier investigation into the LaBaCo 2 O 6‑δ system . The oxygen contents from TGA and NPD are in excellent agreement with the NPD data collected upon heating.…”

“…3D transport involving O1 sites in the Ba−O layers carries a very large energy penalty and is strongly disfavored, leading to 2D oxide ion transport at all practical application temperatures. 23,28,31 BVSE calculations are a computationally inexpensive way to calculate migration barriers (E b ) for ionic transport directly from the unit cell structure. 36,37 They use potentials derived from self-consistent bond valence parameters that are available for many ionic pairs.…”

“…Another reason for the discrepancy between different reports is that oxide-ion transport rates are limited by either the surface-exchange or oxide-ion diffusion coefficient. ,, Hence, variations in microstructure between samples can cause differences in EIS and IEDP. Simulations, − diffraction, ,, and total scattering experiments have been used to establish the mechanism for ionic conductivity. These studies show that ionic motion is predominantly between the O3 sites in the Ln-O layers and the O2 sites in the Co–O 2 layers.…”

Magnocolumbites Mg1–xMxNb2O6−δ (x = 0, 0.1, and 0.2; M = Li and Cu) as New Oxygen Ion Conductors: Theoretical Assessment and Experiment

“…47 The DFT-derived MgNb 2 O 6 migration energy is higher than its experimental migration energy, which is typical for simulation of oxygen ion conductors, for example, La 2 NiO 4+δ (E m (DFT) = 1.20 eV 48 vs E a,σ = 1.02 eV 49 ) and UO 2 (E m (DFT) = 0.93 eV vs E a,σ = 0.88 eV 50 ). The oxygen migration energies from the BVSE calculation are lower than the experimental activation energy, which is also common, for example, PrBaCa 2 O 6−δ (E m (BVSE) = 0.90 eV 51 vs E a,σ = 1.40 eV 52 ), LaSrGa 3 O 7 (E m (BVSE) = 0.58 eV 51 vs E a,σ = 0.85 eV 53 ), and CaGa 6 O 14 (E m (BVSE) = 0.76 eV 51 vs E a,σ = 1.09 eV 54 ). Note that the BVSE method does not take into account the Coulombic interactions between the working ions and structural relaxation while DFT calculations are carried out for ideal crystals without any defects/impurities.…”

Recent developments in oxide ion conductors: focusing on Dion–Jacobson phases

“…Layered perovskites have layered structures with AB O 3 perovskite or AB O 3 perovskite-like layers. Layered perovskite-type oxides such as double perovskite, 104–109 BIMEVOX, 110–112 Aurivillius phase, 49,113,114 Ruddlesden–Popper phase, 14,15,115–120 brownmillerite, 121,122 and hexagonal perovskite derivative, 25,123–126 BaNdInO 4 -based oxides 127–130 exhibit high oxide-ion conductivities.…”