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Abstract The small‐polaron hopping model has been used for six decades to rationalize electronic charge transport in oxides. The model was developed for binary oxides, and, despite its significance, its accuracy has not been rigorously tested for higher‐order oxides. Here, the small‐polaron transport model is tested by using a spinel system with mixed cation oxidation states (MnxFe3−xO4). Using molecular‐beam epitaxy (MBE), a series of single crystal MnxFe3−xO4thin films with controlled stoichiometry, 0 ≤x ≤ 2.3, and lattice strain are grown, and the cation site‐occupation is determined through X‐ray emission spectroscopy (XES). Density functional theory +Uanalysis shows that charge transport occurs only between like‐cations (Fe/Fe or Mn/Mn). The site‐occupation data and percolation models show that there are limited stoichiometric ranges for transport along Fe and Mn pathways. Furthermore, due to asymmetric hopping barriers and formation energies, the polaron is energetically preferred to the polaron, resulting in an asymmetric contribution of Mn/Mn pathways. All of these findings are not contained in the conventional small‐polaron hopping model, highlighting its inadequacy. To correct the model, new parameters in the nearest‐neighbor hopping equation are introduced to account for percolation, cross‐hopping, and polaron‐distribution, and it is found that a near‐perfect correlation can be made between experiment and theory for the electronic conductivity.
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X-ray emission spectroscopy: an effective route to extract site occupation of cations
Cation site occupation is an important determinant of materials properties, especially in a complex system with multiple cations such as in ternary spinels. Many methods for extracting the cation site information have been explored in the past, including analysis of spectra obtained through K-edge X-ray absorption spectroscopy (XAS). In this work, we measure the effectiveness of X-ray emission spectroscopy (XES) for determining the cation site occupation. As a test system we use spinel phase Co x Mn 3−x O 4 nanoparticles contaminated with CoO phases because Co and Mn can occupy all cation sites and the impurity simulates typical products of oxide syntheses. We take advantage of the spin and oxidation state sensitive Kβ 1,3 peak obtained using XES and demonstrate that XES is a powerful and reliable technique for determining site occupation in ternary spinel systems. Comparison between the extended X-ray absorption fine structure (EXAFS) and XES techniques reveals that XES provides not only the site occupation information as EXAFS, but also additional information on the oxidation states of the cations at each site. We show that the error for EXAFS can be as high as 35% which makes the results obtained ambiguous for certain stoichiometries, whereas for XES, the error determined is consistently smaller than 10%. Thus, we conclude that XES is a superior and a far more accurate method than XAS in extracting cation site occupation in spinel crystal structures.
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- PAR ID:
- 10109513
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 20
- Issue:
- 46
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 28990 to 29000
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c8cp04628j
