Visualizing Anionic Electrons in an Electride*

Electrides are an unusual family of materials that feature loosely bonded electrons that sit at special interstitial sites and serve as anions. They are attracting increasing attention because of the wide range of exotic physical and chemical properties that are being discovered. Despite the critical role of the anionic electrons in inducing such properties, their presence has not been directly observed experimentally. Here, we visualize the anionic electron density within a prototype one-dimensional electride, Y₅Si₃, with sub-Ångstrom spatial resolution by utilizing differential phase-contrast imaging in a scanning transmission electron microscope. Our data additionally reveal an unexpected charge variation at different anionic sites. Density-functional-theory simulations show that the presence of trace H impurities, which are challenging to detect by other means, are the cause of the inhomogeneity. The visualization and quantification of charge inhomogeneities in crystals serve as useful input in future theoretical predictions and experimental analysis of exotic properties in electrides and materials beyond.