Extended X-ray Absorption Fine Structure (EXAFS) Measurements on Alkali Metal Superatoms of Ta-Atom-Encapsulated Si 16 Cage.

The silicon cage nanoclusters encapsulating a tantalum atom, termed Ta@Si 16 , exhibit characteristics of alkali metal "superatoms (SAs)". Despite this conceptual framework, the precise structures of Ta@Si 16 and Ta@Si 16 + remain unclear in quantum calculations due to three energetically close structural isomers: C 3v , T d , and D 4d structures. To identify the geometrical structure of Ta@Si 16 SAs, structural analysis was conducted using extended X-ray absorption fine structure (EXAFS) with a high-intensity monochromatic X-ray source, keeping anaerobic conditions. Focusing on "superordered" films, which constitute amorphous thin films composed solely of Ta@Si 16 SAs, this analysis preserved locally ordered structures. Spectral comparisons between experimental and simulated Ta L 3 -edge EXAFS unveil that Ta@Si 16 SAs on a substrate adopt a C 3v -derived structure, while Si K -edge EXAFS introduces spectral ambiguity in structural identifications, attributed to both intracluster and intercluster scatterings. These findings underscore the significance of locally ordered structure analyses in understanding and characterizing novel nanoscale materials.