Research ArticleMATERIALS SCIENCE

Direct visualization of anionic electrons in an electride reveals inhomogeneities

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Science Advances  07 Apr 2021:
Vol. 7, no. 15, eabe6819
DOI: 10.1126/sciadv.abe6819
  • Fig. 1 The crystal structure of electride Y5Si3.

    (A) Schematic of the Y5Si3 structure in the [001] projection. Black solid boxes indicate unit cells. Inset circles: Projected and side views of a hexagonal Y ring, with the center forming a column of electrons. (B) HAADF image of Y5Si3 with overlaid atoms defining an interstitial electron column. Scale bar, 5 Å.

  • Fig. 2 Differential phase contrast in Y5Si3.

    (A) HAADF image of an interstitial column in Y5Si3. (B) Schematics for beam interaction with positive and negative charge densities generating CoM shifts. (C) Projected magnitude and direction of the electrostatic field in the sample as obtained from the CoM shifts. (D) Charge density derived from the divergence of the CoM shifts. The charge density color scale displays positive values on a white-to-red scale, negative values on a white-to-blue scale, and net zero charges as white. Note that the bonding electrons between Y and Si atoms are not discernible in (D) because of the broadened nuclear charges. The interstitial electrons, which are the focus of this work, are seen in blue in the column at the center of the map.

  • Fig. 3 Quantitative comparison to theory.

    The total charge density in an interstitial unit cell of Y5Si3 as determined by (A) DPC and (B) DFT showing a strong qualitative match for the entire unit cell charge profile. (C) Line profiles [across the anionic column, from the regions highlighted with dashed lines in (A) and (B)] for both methods. The average charge density is calculated for the center (1-Å radius) region (marked by the gray area) of each interstitial column, demonstrating an excellent quantitative match between theory and experiment.

  • Fig. 4 Inhomogeneity of anionic charge density of Y5Si3.

    (A) Charge density map of a large area of Y5Si3 showing an inconsistent profile across the center of the interstitial columns. (B to E) Zoomed-in views of columns having both the uniform negative electron density of the electride (B and C) and an inhomogeneity in the anionic electrons observed across the entire dataset (defined by a sharp peak of close to neutral charge density at interstitial column center) (D and E). (F to I) Line profiles across the column centers compared to the DFT line profile from Fig. 3, demonstrating that columns that do not show the inhomogeneity peak match quantitatively with the predicted DFT charge density (F and G), and the ones that have the inhomogeneity show the sharpest difference directly at the column center (H and I).

  • Fig. 5 Quantitative charge density maps of Y5Si3 with various H integrations.

    (A to F) DFT calculated charge density maps (after probe convolution) with the integrations of 0, 0.1, 0.5, 1, and 2 H atoms per interstitial layer. (G) Line profiles across the column centers in (A) to (F), showing the dependence of charge density on the concentration of H at the anionic site. (H) Plot of average charge density within the central region (1-Å radius) of the interstitial column versus H concentrations based on (A) to (F). A nominal polynomial fit of the calculated charge density data (blue dots) is shown by a dashed blue curve. The experimental interstitial cells highlighted in Figs. 3 and 4 are plotted as green squares on this nominal line. The red triangle symbol represents the average H concentration in Y5Si3 based on the experimental DPC of Fig. 4A, and its error bar is presented by the SD of all analyzed interstitial cells.

  • Fig. 6 Inelastic neutron scattering spectroscopy.

    Comparison of the measured INS spectrum with the simulated spectra for Y5Si3 and Y5Si3H0.5. au, arbitrary unit.

Supplementary Materials

  • Supplementary Materials

    Direct visualization of anionic electrons in an electride reveals inhomogeneities

    Qiang Zheng, Tianli Feng, Jordan A. Hachtel, Ryo Ishikawa, Yongqiang Cheng, Luke Daemen, Jie Xing, Juan Carlos Idrobo, Jiaqiang Yan, Naoya Shibata, Yuichi Ikuhara, Brian C. Sales, Sokrates T. Pantelides, Miaofang Chi

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