Active-Metal-Rich Intermetallics as a Platform for Tiered Interstitial Electronic States

Interstitial electronic states, commonly found in F-centers and electrides, have garnered significant attention in materials discovery, and applications spanning catalysis and electronics. Here, we demonstrate that active-metal-rich intermetallics serve as a platform for forming diverse cationic cages that accommodate distinct interstitial electrons. This arises from two key factors: (1) the high atomic density of active metals, promoting their aggregation to form cages; and (2) the high valence electron density, facilitating electron confinement in cage voids. We examine 10 metal-rich RE-T-Al/Mg (RE = rare earth, T = transition metal) ternary compounds as examples. Density functional theory (DFT) calculations reveal that these compounds exhibit electride characteristics, featuring tiered interstitial electrons at distinct Wyckoff sites and energy levels. Detailed analyses identify two types of interstitial electrons: those in RE-rich cages, with band centers at -1.25 eV< E-E_F < 0 eV, ∼1 e^-/site, and negative hydrogen binding energies, exhibiting anionic-like behavior; and those in Al cages, with band centers at E-E_F < -1.8 eV, ∼2 e^-/site, and positive hydrogen binding energies, displaying covalent-like behavior. Our results provide a systematic insight into interstitial electrons in RE-T-Al/Mg ternary compounds, potentially offering a guideline for designing intermetallic compounds with tailored interstitial electronic states.