Identifying the local structural units in La0.5Ba0.5MnO2.5 and BaY0.25Fe0.75O2.5 through the neutron pair distribution function

Abstract

Neutron pair distribution function data are used to investigate the local structures of two oxygen deficient perovskites with simple cubic crystal structures. La_0.5Ba_0.5MnO_2.5 is found to have alternating layers of MnO₆ octahedra and MnO₄ tetrahedra, making it similar to other members of the La_1−xBa_xMnO_2.5 series with smaller x values which have brownmillerite crystal structures. Our fitting results suggest that La_0.5Ba_0.5MnO_2.5 is not locally similar to any one brownmillerite structure type but instead has a variety of intra-layer and inter-layer relationships. We propose that this could be due to short range segregation of the much differently sized La³⁺ and Ba²⁺ cations, which creates significantly different inter-layer distances. BaY_0.25Fe_0.75O_2.5 is found not to have a brownmillerite-type local structure but rather seems to consist of structural units which are similar to Ba₃YFe₂O_7.5, an earlier member of the Ba_nYFe_n−1O_2.5n series. The PDF analysis shows that there are never neighboring Y atoms and that the O vacancies lie exclusively between Fe atoms, such that Y is always octahedrally coordinated. The PDF also suggests that there are more tetrahedral Fe than expected, which could be due to the presence of terminal O atoms in Fe centered dimeric units, similar to what is found in Ba₃YFe₂O_7.5.