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

Graham King; Corey M Thompson; Kun Luo; John E Greedan; Michael A Hayward

doi:10.1039/c6dt04291k

Identifying the local structural units in La_0.5Ba_0.5MnO_2.5 and BaY_0.25Fe_0.75O_2.5 through the neutron pair distribution function

Dalton Trans. 2017 Jan 24;46(4):1145-1152. doi: 10.1039/c6dt04291k.

Authors

Graham King¹, Corey M Thompson², Kun Luo³, John E Greedan⁴, Michael A Hayward³

Affiliations

¹ Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. gmkchemist@gmail.com.
² Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA and Department of Chemistry and Brockhouse Institute of Materials Research, McMaster University, Hamilton, ON L8S 4M1, Canada.
³ Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, UK.
⁴ Department of Chemistry and Brockhouse Institute of Materials Research, McMaster University, Hamilton, ON L8S 4M1, Canada.

PMID: 28054073
DOI: 10.1039/c6dt04291k

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.