Visualizing oxygen anion transport pathways in NdBaCo2O5+δ by in situneutron diffraction

Abstract

The layered perovskite NdBaCo₂O_5+δ (NBCO) was characterized using neutron powder diffraction under in situ conditions from 577–852 °C and in 10⁻¹ to 10⁻⁴ atm oxygen. The best fit to the data was obtained in the tetragonal (P4/mmm) space group. No oxygen atom vacancy ordering was observed that warranted a lowering of the symmetry to orthorhombic (Pmmm). Two P4/mmm structural models were investigated: Model 1 (no split sites) and Model 2 (split Nd and O2 sites). Transport of oxygen through the material via the vacancy hopping mechanism likely involves the nearest-neighbor oxygen atom sites in the Nd layer. Total oxygen stoichiometry values were in the range 5.51 ≤ δ ≤ 5.11. The tetragonal lattice parameters increased with temperature as expected. However, the a-axis expands while the c-axis contracts with decreasing pO₂ at a given temperature due to increasing vacancy concentration in the Nd layer.