Study of oxygen ion diffusion in (Ba0.5Sr0.5)(Co0.8Fe0.2)O2.33–δ through in-situ neutron diffractions at 300 and 720 K

In this study, we analyze the mechanism of oxygen ion diffusion in (Ba0.5Sr0.5)(Co0.8Fe0.2)O2.33–δ by using the Rietveld refinement, the maximum entropy method (MEM) analysis, and MEM-based pattern fitting (MPF) with in-situ neutron diffractions at 300 and 720 K. We show that at 300 K, the anisotropic atomic displacement parameter (Uaniso) of the O2(8d) site indicates anisotropy toward the O1(4c) site. The Uaniso of the O2(8d) site keeps the anisotropy at 720 K, and the Uaniso with the isotropy of the O1(4c) site, which has a large number of vacancies at 300 K, metamorphoses into Uaniso with anisotropy directed toward the O1(4c) and O2(8d) sites by the Rietveld refinement at 720 K. Through MEM analysis, we show that the neutron scattering length density (NSLD) of the O1(4c) site expands toward the O1(4c) and O2(8d) sites at 720 K. We also show the large thermal motion using Uaniso and the NSLD of the O1(4c) site with a large number of vacancies at 720 K. From these results, we speculate that when Uaniso and NSLD of O1(4c) with a large number of vacancies metamorphose into that with anisotropy directed toward the O1(4c) and O2(8d) sites at 720 K, the oxygen ions diffuse along the paths between O1(4c) and O1(4c), and O1(4c) and O2(8d).