We report that the interlayer states common to the compounds AeAlSi (Ae=Ca, Sr, Ba) arise from F-center-like electrons arrayed in periodic cavities. The SrPtSb-type intermetallic phases exhibit electrons localized to columns of the trigonal bipyramidal Ae3Al2 cages running perpendicular to the honeycomb layers. Ab initio calculations in combination with hard/soft x-ray photoemission spectroscopic measurements reveal that these features correspond to the anionic electrons that hybridize with apical Al3pz orbitals from the honeycomb layers above and below. Extra bands with a significant dispersion along the kz direction therefore contribute to the Fermi level in contrast to the apparent two-dimensional connectivity of the bonding in the compounds, and completely account for the presence of interlayer states. Our study demonstrates how the cage centers may serve as electronically important crystallographic sites, and extend the anionic electron concept into honeycomb lattice compounds.
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