We report direct observation of fluorine ion dopants in the recently discovered LaFeAsO1-xFx superconductor, where the doping is critical for the superconductivity. Employing spectroscopic imaging based on scanning transmission electron microscopy, the spatial distribution of the doped fluorine ions, which are invisible in Z-contrast imaging, is clearly visualized with atomic-scale resolution. The observation directly proves that fluorine substitutes into the oxygen site in the superconducting LaFeAsO 1-xFx compound. Our results demonstrate the potential of the present method for revealing dopants undetectable by conventional microscopy imaging.
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