Skip to main content

Polaron size and shape effects on oxygen vacancy interactions in lanthanum strontium ferrite

You are here

TitlePolaron size and shape effects on oxygen vacancy interactions in lanthanum strontium ferrite
Publication TypeJournal Article
Year of Publication2017
AuthorsDas, T, Nicholas, JD, Qi, Y
JournalJ. Mater. Chem. A
Volume5
Pagination25031-25043
Abstract

Both aliovalent doping and the charge state of multivalent lattice ions determine the oxygen non-stoichiometry ([small delta]) of mixed ionic and electronic conductors (MIECs). Unfortunately{,} it has been challenging for both modeling and experiments to determine the multivalent ion charge states in MIECs. Here{,} the Fe charge state distribution was determined for various compositions and phases of the MIEC La1-xSrxFeO3-[small delta] (LSF) using the spin-polarized density functional theory (DFT)-predicted magnetic moments on Fe. It was found that electron occupancy and crystal-field-splitting-induced differences between the Fe 3d-orbitals of the square pyramidally coordinated{,} oxygen-vacancy-adjacent Fe atoms and the octahedrally-coordinated{,} oxygen-vacancy-distant-Fe atoms determined whether the excess electrons produced during oxygen vacancy formation remained localized at the first nearest neighbor Fe atoms (resulting in small oxygen vacancy polarons{,} as in LaFeO3) or were distributed to the second-nearest-neighbor Fe atoms (resulting in large oxygen vacancy polarons{,} as in SrFeO3). The progressively larger polaron size and anisotropic shape changes with increasing Sr resulted in increasing oxygen vacancy interactions{,} as indicated by an increase in the oxygen vacancy formation energy above a critical [small delta] threshold. This was consistent with experimental results showing that Sr-rich LSF and highly oxygen deficient compositions are prone to oxygen-vacancy-ordering-induced phase transformations{,} while Sr-poor and oxygen-rich LSF compositions are not. Since oxygen vacancy induced phase transformations cause a decrease in the mobile oxygen vacancy site fraction (X){,} both [small delta] and X were predicted as a function of temperature and oxygen partial pressure{,} for multiple LSF compositions and phases using a combined thermodynamics and DFT approach.

URLhttp://dx.doi.org/10.1039/C7TA06948K
DOI10.1039/C7TA06948K