Title | Polaron size and shape effects on oxygen vacancy interactions in lanthanum strontium ferrite |
Publication Type | Journal Article |
Year of Publication | 2017 |
Authors | Das, T, Nicholas, JD, Qi, Y |
Journal | J. Mater. Chem. A |
Volume | 5 |
Pagination | 25031-25043 |
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. |
URL | http://dx.doi.org/10.1039/C7TA06948K |
DOI | 10.1039/C7TA06948K |