The small-polaron hopping model has been used for several decades for modeling electronic charge transport in oxides. Despite its significance, the model was developed for binary oxides, and its accuracy has not been rigorously tested for higher-order oxides. To investigate this issue, we chose the MnxFe3-xO4 spinel system, which has exciting electrochemical and catalytic properties, and mixed cation oxidation states that enable us to examine the mechanisms of small-polaron transport. Using a combination of experimental results and DFT+U calculations, we find that the charge transport occurs only between like-cations (Fe/Fe or Mn/Mn). And due to asymmetric hopping barriers and formation energies, we find that the (Mn)2+(Oh) polaron is energetically preferred to the (Fe)2+(Oh) polaron, resulting in an asymmetric contribution of the Mn/Mn pathways.
Reference: A. Bhargava, R. Eppstein, J. Sun, M. A. Smeaton, H. Paik, L. F. Kourkoutis, D. G. Scholm, M. Caspary Toroker*, R. D. Robinson*, “Breakdown of the small-polaron hopping model in higher-order spinels”, Adv. Mat., 2004490 (2020).
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