Visualizing oxygen anion transport pathways in NdBaCo2O5+δ by in situ neutron diffraction

The layered perovskite NdBaCo2O5+delta (NBCO) was characterized using neutron powder diffraction under in situ conditions from 577-852 degrees 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 <= delta <= 5.11. The tetragonal lattice parameters increased with temperature as expected. However, the a-axis expands while the c-axis contracts with decreasing pO(2) at a given temperature due to increasing vacancy concentration in the Nd layer.