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Compositional mapping of LuFe2O4 viewed along the  LuFe2O4 zone axis. (a) HAADF-STEM image showing the repetition of Lu layers separated by with double layers of Fe. The EELS spectroscopic composition image, (b), is an overlay of the Lu-M4,5 (c), Fe-L2,3 (d) and O-K (e) signals. In (b), Lu is plotted in blue, Fe in red, and O in green. The inset shows the bulk LuFe2O4 crystal structure color-coded accordingly. The ADF image recorded simultaneously with the EELS map is shown in (f). Images (b)-(f) are 70 × 186 pixel images with a field of view of 2.8 nm by 7.4 nm and were acquired with 40 ms/pixel.
Local bonding information extracted from the O-K edge. (a) HAADF-STEM image recorded simultaneously with the O concentration map of (b). Two distinct components were present in the O-K edge fine structure, (c), and fit to the spectroscopic image as shown in (d). The inset to (d) shows the crystal structure Fe plotted in red, Lu in blue, and “O1” and “O2” in magenta and turquoise respectively. (a), (b), and (d) are 123 × 91 pixel images with a field of view of 3.4 nm by 2.5 nm and were acquired with 40 ms/pixel.
FEFF calculations of the EELS fine structure. The O p and Lu and Fe d local densities of states are shown for the distinct O1 and O2 sites in (a) and (b), respectively. In both cases, the much weaker O p LDOS tracks the combined Fe and Lu LDOS. The corresponding calculated EELS signal is shown in (c). A cartoon inset to (c) shows the local chemical environment of O1 and O2. Lu is plotted in blue, Fe in red, and the O atom in magenta and turquoise, respectively.
Measurement of the Fe oxidation state down [1 0] zone axis of the film. (a) ADF image on which the Fe valence obtained from an analysis of the Fe-L2,3 edge fine structure is overlaid. Blue is used to indicate pure Fe+2 and red, pure Fe+3. No consistent charge ordering pattern is observed. (b) A histogram of Fe valences from all rows of panel (a) as well as other spectroscopic images with the same acquisition parameters (272 total Fe columns), showing a unimodal Gaussian distribution with a standard deviation of 0.09 e, placing a bound on the detection sensitivity for charge modulations, which should produce a bimodal distribution.
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