Identifying the local structural units in La 0.5 Ba 0.5 MnO 2.5 and BaY 0.25 Fe 0.75 O 2.5 through the neutron pair distribution function

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.