Extended X-ray Absorption Fine Structure (EXAFS) Measurements on Alkali Metal Superatoms of Ta-Atom-Encapsulated Si₁₆ Cage

The silicon cage nanoclusters encapsulating a tantalum atom, termed Ta@Si₁₆, exhibit characteristics of alkali metal “superatoms (SAs)”. Despite this conceptual framework, the precise structures of Ta@Si₁₆ and Ta@Si₁₆⁺ 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@Si₁₆ 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@Si₁₆ SAs, this analysis preserved locally ordered structures. Spectral comparisons between experimental and simulated Ta L₃-edge EXAFS unveil that Ta@Si₁₆ 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.